Applied Atmospheric
Dynamics
Amanda H. Lynch
Monash University
John J. Cassano
The University of Colorado
Applied Atmospheric Dynamics
Applied Atmospheric
Dynamics
Amanda H. Lynch
Monash University
John J. Cassano
The University of Colorado
Copyright © 2006
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Library of Congress Cataloging in Publication Data
Lynch, Amanda H.
Applied atmospheric dynamics / Amanda H. Lynch and John J. Cassano.
p. cm.
Includes bibliographical references and index.
ISBN-13: 978-0-470-86172-1 (acid-free paper)
ISBN-10: 0-470-86172-X (acid-free paper)
ISBN-13: 978-0-470-86173-8 (pbk.: acid-free paper)
ISBN-10: 0-470-86173-8 (pbk.: acid-free paper)
1. Atmospheric physics. 2. Dynamics—Mathematics. I. Cassano, John J. II. Title.
QC861.3.L957 2006
551.5—dc22
2006016133
British Library Cataloguing in Publication Data
A catalogue record for this book is available from the British Library
ISBN-13: 978-0-470-86172-1 (HB) 978-0-470-86173-8 (PB)
ISBN-10: 0-470-86172-X (HB) 0-470-86173-8 (PB)
Typeset in 10.5/12.5pt Times by Integra Software Services Pvt. Ltd, Pondicherry, India
Printed and bound in Great Britain by Antony Rowe Ltd, Chippenham, Wiltshire
This book is printed on acid-free paper responsibly manufactured from sustainable forestry
in which at least two trees are planted for each one used for paper production.
Contents
Preface
Part I
ix
Anatomy of a cyclone
1 Anatomy of a cyclone
1.1
1.2
1.3
1.4
A ‘typical’ extra-tropical cyclone
Describing the atmosphere
Air masses and fronts
The structure of a typical extra-tropical cyclone
Review questions
2 Mathematical methods in fluid dynamics
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
Scalars and vectors
The algebra of vectors
Scalar and vector fields
Coordinate systems on the Earth
Gradients of vectors
Line and surface integrals
Eulerian and Lagrangian frames of reference
Advection
Review questions
3 Properties of fluids
3.1
3.2
3.3
3.4
3.5
3.6
3.7
Solids, liquids, and gases
Thermodynamic properties of air
Composition of the atmosphere
Static stability
The continuum hypothesis
Practical assumptions
Continuity equation
Review questions
4 Fundamental forces
4.1 Newton’s second law: F = ma
4.2 Body, surface, and line forces
1
3
3
4
9
14
20
23
23
23
27
27
28
31
34
35
38
41
41
42
43
46
50
50
51
53
57
57
57
vi
CONTENTS
4.3 Forces in an inertial reference frame
4.4 Forces in a rotating reference frame
4.5 The Navier–Stokes equations
Review questions
5 Scale analysis
5.1
5.2
5.3
5.4
5.5
Dimensional homogeneity
Scales
Non-dimensional parameters
Scale analysis
The geostrophic approximation
Review questions
6 Simple steady motion
6.1
6.2
6.3
6.4
6.5
Natural coordinate system
Balanced flow
The Boussinesq approximation
The thermal wind
Departures from balance
Review questions
7 Circulation and vorticity
7.1
7.2
7.3
7.4
Circulation
Vorticity
Conservation of potential vorticity
An introduction to the vorticity equation
Review questions
8 Simple wave motions
8.1 Properties of waves
8.2 Perturbation analysis
8.3 Planetary waves
Review questions
9 Extra-tropical weather systems
9.1 Fronts
9.2 Frontal cyclones
9.3 Baroclinic instability
Review questions
Part II Atmospheric phenomena
10 Boundary layers
10.1
10.2
10.3
10.4
Turbulence
Reynolds decomposition
Generation of turbulence
Closure assumptions
Review questions
58
66
72
74
79
79
80
80
84
87
91
93
93
95
104
105
108
114
119
119
124
126
130
132
135
135
138
140
147
149
149
152
161
162
165
167
168
169
172
173
181
CONTENTS
11 Clouds and severe weather
11.1
11.2
11.3
11.4
11.5
Moist processes in the atmosphere
Air mass thunderstorms
Multi-cell thunderstorms
Supercell thunderstorms and tornadoes
Mesoscale convective systems
Review questions
12 Tropical weather
12.1 Scales of motion
12.2 Atmospheric oscillations
12.3 Tropical cyclones
Review questions
13 Mountain weather
13.1 Internal gravity waves
13.2 Flow over mountains
13.3 Downslope windstorms
Review questions
14 Polar weather
14.1 Katabatic winds
14.2 Barrier winds
14.3 Polar lows
Review questions
15 Epilogue: the general circulation
15.1
15.2
15.3
15.4
15.5
Fueled by the Sun
Radiative–convective equilibrium
The zonal mean circulation
The angular momentum budget
The energy cycle
vii
183
183
191
193
194
196
197
199
199
203
205
208
209
209
217
226
230
233
233
242
247
249
251
251
253
254
259
261
Appendix A – symbols
265
Appendix B – constants and units
271
Bibliography
273
Index
277
Preface
Many can brook the weather that love not the wind.
W. Shakespeare, “Love’s Labour’s Lost ”, Act IV, Scene II
During mid-February 2003, one of the biggest winter storms on record cut a swathe
through the mid-western and eastern United States. Low temperature and snowfall
records were set all along the eastern seaboard. Tornadoes, extreme hail, flooding,
and mudslides were all generated by the ‘beast in the east’. When all was said and
done, 45 people lost their lives, and 122 people were injured, as a direct result of the
weather.
The weather can be a cause of disruption, despair, and even danger everywhere
around the world at one time or another. Even when benign, it is a source of constant
fascination for many people. Yet connecting this interest with the underpinnings of
fluid mechanics has remained beyond reach for many. It is our hope with this book
to make the intriguing ways in which the atmosphere moves accessible to a broader
range of students and general readers. We have done this by linking real physical
events with theoretical models at every possible juncture. The storm of February 2003
provides a valuable illustration of many of the important concepts in atmospheric
dynamics, and we have used many other dramatic weather events as well, from the
devastating Hurricane Katrina to the strong katabatic flows of Antarctica. The level
of mathematics required, though not rudimentary, is pre-vector calculus, and the
emphasis is always on the phenomenology.
Part I takes the reader through all of the basic concepts required to understand the
development and decay of mid-latitude low-pressure systems. These concepts include
balanced and unbalanced flows, vorticity, and waves. In Part II, a broader range of
phenomena are considered, ranging from the tropics to the poles. These later chapters
can be considered in any order. For each of the 14 chapters, review questions to
test understanding and to provide practice are posed, the worked solutions of which
are available. The book ends with a discussion of the role of weather systems in
maintaining the global circulation.
The accompanying CD-ROM includes all of the illustrations in the book in JPEG
format, and many more besides. Animations and videos of important processes,
satellite pictures of interesting events, and weather maps of all varieties are collected
x
PREFACE
on the CD-ROM. All of the available data for the storm of February 2003 is in a
searchable database, suitable for a range of investigations. In addition, four possible
research projects are included, on Atlantic hurricanes, southern hemisphere cyclones,
polar lows, and tornadoes.
While only two authors are listed on the front of this book, many others contributed.
Elizabeth Cassano stepped up whenever asked, to prepare content for the CD-ROM,
to frame project material, and to find that last data point. Christopher Takeuchi, Mark
Seefeldt, and Petteri Uotila provided a number of figures for the book and CD-ROM,
which Henry Johnson and Casey Tonkin tested to breaking point. Michael Shaw
provided invaluable assistance in proofreading the book from a student’s perspective
and solving all of the review questions. David Underwood created a number of figures
and animations for the book and CD-ROM. Barbara Lynch’s advice, scientific and
literary, was priceless.
AHL and JJC
I want to thank the many students of atmospheric dynamics that I have taught over
the years – without their enthusiasm, excellence, and well-targeted criticism, this
book would not have been written. They also served as guinea pigs for many of the
approaches we have used here. I also wish to thank my parents, for their boundless
faith in me; my husband, for his most practical support; and my daughters, for
bringing me down from the clouds.
AHL, Melbourne, Australia
I must also thank my family for their unending support in all of my life’s endeavors,
and in particular my parents Emilia and Vito for giving me the opportunity to pursue
an educational and professional path that perhaps was not the one they had hoped
I would follow. Finally, I must thank my wife Elizabeth for her love, support, and
patience. Not only did she help with numerous tasks related to the writing of this
book, she also provided me with the perspective to realize that there is more to life
than work and weather.
JJC, Boulder, Colorado
November 2005
Part I Anatomy of a cyclone
1
Anatomy of a cyclone
1.1 A ‘typical’ extra-tropical cyclone
A snow emergency was declared in Boston, Massachusetts as a record snowfall
paralyzed the city. Airports in Washington, DC and New York City were closed,
and trains and buses were cancelled. The blizzard, dubbed ‘the beast in the east’ by
the media, dumped heavy snow in a broad swathe from Iowa to New England on the
President’s Day holiday weekend of 2003 (15–17 February).
This storm had been traversing the United States for a week – leaving disaster in
its wake in some places, and hardly being noticed in others. Flooding and mudslides
occurred in southern California. Ten tornadoes were reported in the south-eastern
United States. In Delaware, over 500 people were evacuated from a townhouse
complex after water from melting snow began leaking into the electrical system.
Meanwhile, hail and strong winds were reported from the southern Plains to the
south-eastern United States, downing trees and power lines and removing roofs.
During the period that this storm crossed the country, 45 people were killed and 122
people injured as a direct result of the weather (Angel 2003). Over US$144 million
in damage was reported.
These diverse weather events were the result of a low-pressure system outside
the tropics, an extra-tropical cyclone, that crossed the United States from 11 to
19 February 2003.
The system first approached the southern California coast on 11 February. Over
the next two days it crossed the mountainous western portion of the United States,
looking relatively weak and sometimes disappearing from surface maps. Then, on
13 February, the surface low-pressure center redeveloped over south-eastern Colorado, as such systems often do when crossing the Rocky Mountains during the winter
months. This redeveloped low-pressure center began moving slowly east across the
southern Great Plains and the lower Mississippi River Valley during the next two
days. By 16 February it was located west of the southern end of the Appalachian
Mountains and had started to decay once more. However, at the same time low
pressure was developing just off the coast of North Carolina, along the frontal system
of the original low-pressure center. The coastal low developed to become the main
system, and moved north and east just offshore, slowly enough to allow large amounts
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
4
ANATOMY OF A CYCLONE
of snow to fall. All-time record snowfalls were reported in the cities of Baltimore,
Maryland (71.6 cm) and Boston, Massachusetts (69.9 cm).
What were the atmospheric processes responsible for the formation and evolution
of this storm as it passed over the United States? Why were such diverse severe
weather events as heavy snow and tornadoes reported with this one system? The
study of atmospheric dynamics provides us with the tools to answer these questions,
and more broadly helps us understand the processes that shape the circulation of
the atmosphere. This book will use this storm as a guide in our exploration of the
processes and forces that shape the circulation of the atmosphere. Weather maps that
depict the evolution of the storm are provided on the accompanying CD-ROM.
1.2 Describing the atmosphere
We can consider the air that surrounds the Earth to be made up of ‘columns’, rising
vertically from each location. As we move up through the column, the properties
of the air change – its temperature, moisture, cloudiness, chemical constituents, and
density all vary. The portion of the atmosphere of most interest in this book is the
troposphere, the zone between the surface and around 10 km altitude. This is the part
of the column in which ‘weather’ happens, and certainly where we experience it.
Before we begin our exploration of the effects of forces upon the atmosphere,
the dynamics, we need to define some of its basic properties. These properties are
essential for describing and understanding the atmosphere and can be thought of as
the basic vocabulary of atmospheric dynamics.
The most familiar properties are wind speed, temperature, and density. As with all
physical quantities, these are quantified using SI (Système International) or metric
units. The metric system is founded upon a set of base units, of which we will use
five: meter (m), kilogram (kg), second (s), kelvin (K), and mole (mol). Other units,
such as for acceleration or force for example, can be derived from these basic units.
There are two important quantities whose units are not in everyday use, and these
require some further explanation: temperature and pressure.
1.2.1 Atmospheric temperature
Most commonly used around the world today is the Celsius scale ( C) for temperature, which was devised by Swedish astronomer Anders Celsius in 1742. The scale
designates 0 C as the freezing point of water, and 100 C as the boiling point of water
at sea level. The scale is also known as the ‘centigrade’ scale because it divides the
interval between freezing and boiling into 100 equal units. However, this was not the
scale adopted in the Système International.
In fact, in the early eighteenth century, there were many competing temperature
scales. A second scale that has survived to the present day in the United States, and
unofficially in many other English-speaking countries, is the Fahrenheit scale ( F).
This temperature scale is named after the German physicist who suggested it in 1724.
DESCRIBING THE ATMOSPHERE
5
The two references Fahrenheit chose were 0 F for the lowest temperature he could
achieve and 100 F for body temperature. However, there is some dispute as to how
he arrived at these reference points, and some time later the scale was recalibrated.
On the present-day Fahrenheit scale, the freezing point of pure water is 32 F and the
boiling point (at sea level) is 212 F.
The Kelvin temperature scale that was adopted in the Système International was
designed by British scientist William Thomson (later Lord Kelvin) based on a single
reference point, absolute zero, the hypothetically lowest temperature possible. This
temperature was derived from an extrapolation of a graph showing the relationship
between volume and temperature of an ideal gas, which suggested that the gas volume
would become zero at a temperature of −27316 C. The scale is often called the
absolute temperature. Each unit on the Kelvin scale is the same magnitude as one
degree on the Celsius scale, and hence the freezing point of water is around 273 K
and the boiling point is around 373 K.
As we will see in Chapter 5, the physical equations we derive for application to the
atmosphere assume SI units for all quantities. However, because the measurement and
study of the weather has an important role across society, SI units are not always used
in the reporting of the atmospheric state. This is particularly true for temperature, but
also for wind speed and atmospheric pressure. Hence, it is important to be familiar
with all of the units in general use, and to be able to convert between them (see
Appendix B).
1.2.2 Atmospheric pressure
Of all of the properties of the atmosphere, atmospheric pressure is one of the most
important. Horizontal and vertical variations in pressure give rise to the atmospheric
motions which are the focus of the study of atmospheric dynamics.
Consider some air in a container. The pressure of the air on the walls of the
container derives from the momentum of individual molecules as they impact the
walls in their random molecular motion. If we add more molecules to the container,
and that container happens to be a balloon, the difference in pressure between the
interior and the exterior of the balloon will cause it to expand until a new equilibrium
is reached (see Figure 1.1). Similarly, one infinitesimal volume parcel of air exerts
pressure on its neighbor, and vice versa, and this force is always perpendicular to the
interface between the parcels.
Hence, the pressure depends not only on the force imparted by the molecules, but
also upon the area over which the force is acting. Thus, the pressure is expressed as
a force per unit area, or N m−2 , which has the SI derived unit of the pascal, or Pa.
Since 1 Pa is a small pressure in comparison to pressures commonly observed in the
atmosphere, the unit hectopascal (symbol hPa) is more widely used. The unit millibar
(symbol mb) is also in common use, particularly on weather maps:
1 hPa = 100 Pa = 1 mb
6
ANATOMY OF A CYCLONE
At equilibrium
Higher pressure exerted
from within
Figure 1.1 The balloon on the left has as many molecules exerting pressure from within as from
without and hence is at equilibrium. The balloon on the right contains more molecules in the same
volume and so (given the same temperature) is exerting more pressure from within. This balloon’s
walls will expand under the action of this force
z
Leadville
2990 m
700 hPa
Taos
1926 m
800 hPa
Rapid City
965 m
900 hPa
Seattle
87 m
US city
standard
altitude
x
1000 hPa
Figure 1.2 The weight of air in the column above any altitude applies a pressure on the surface
below it. The axis on the left shows the standard altitude for the pressure level, and a representative
US city at that elevation, for comparison
The atmospheric pressure at a point on the surface of the Earth is the pressure
caused by the weight of air above that point (this is discussed in greater detail in
Section 4.3.4). This implies that the atmospheric pressure must drop as one moves
to higher altitudes (Figure 1.2) since there is less air above to exert its weight on a
given point. This was first noted in 1648 by Blaise Pascal, after whom the unit of
pressure is named, who made measurements of atmospheric pressure from the base
to the top of the 1465 m Puy de Dôme mountain in France.
Because of this strong variation with altitude, pressure reports from meteorological
stations are generally normalized to a common altitude, usually the mean sea level.
This allows us to distinguish the smaller, but more important, horizontal variations
7
DESCRIBING THE ATMOSPHERE
1020
A
1016
B
1012
Figure 1.3 Idealized example of isobars, labeled in hPa, on a surface weather map
in surface pressure which are the ultimate cause of most motions observed in the
atmosphere. The correction from surface pressure to mean sea level pressure (MSLP)
is performed using the hypsometric equation, which is discussed in Section 4.3.4.
A line drawn on a weather map connecting points of equal pressure is called
an isobar. Isobars are generated from reports of the pressure at various locations.
Figure 1.3 depicts three isobars that map the observed MSLP in a particular region.
At every point along the middle isobar, the pressure is taken to be 1016 hPa, while
at every point along the bottom isobar, the pressure is 1012 hPa. Point A and all
other points above the 1016 mb isobar have a higher pressure than 1016 hPa and
points below that isobar have a lower pressure. At point B, and any other point lying
in between these two isobars, the pressure must be between 1012 and 1016 hPa.
Surface pressure reports are available as often as every hour in some countries,
and usually every 3 or 6 hours. Isobar maps are updated with every new set of
observations.
1.2.3 Station reports
For all surface weather maps around the world, the weather observations at a given
location are reported using a common station model, an example of which is shown
in Figure 1.4. The basic station model provides information on the temperature, dew
point temperature, sea level pressure, wind speed and direction, cloud cover, and
current significant weather. The dew point temperature (Td ) is used by operational
meteorologists to indicate the amount of moisture contained in the atmosphere. It is
defined as the temperature to which a small volume (or parcel) of air must be cooled
at constant pressure in order for that air parcel to become saturated. If air near the
surface of the Earth is cooled to its dew point temperature, dew will form on the
surface. Higher values of dew point temperature indicate air that contains more water
vapor than air with a lower dew point temperature.
The wind speed symbols are additive, and can be used to represent any wind speed,
often reported in units of knots 1 kt = 051 m s−1 . A short wind barb indicates a
wind speed of 5 kts, a long barb indicates a wind speed of 10 kts, and a pennant
indicates a wind speed of 50 kts. It should be noted that the actual wind may be within
±2 kts of the plotted wind speed. For example, if a station model is plotted with one
8
ANATOMY OF A CYCLONE
Wind Speed
Station Model
Temperature
28
Significant
Weather
229
Coded
Sea Level
Pressure
Calm Wind
1–2 kts
Cloud Cover
26
Dew Point
Temperature
5 kts
Wind Speed
and Direction
10 kts
50 kts
Cloud Cover
Significant Weather
Clear
Light Rain
Light Snow
Drizzle
Scattered
Moderate Rain
Moderate Snow
Freezing Drizzle
Broken
Heavy Rain
Heavy Snow
Freezing Rain
Rain Shower
Snow Shower
Sleet
Thunderstorm
Fog
Drifting Snow
Overcast
Obscured
Figure 1.4 Station model for surface weather map conventions
short and one long wind barb the wind speed would be noted as 15 kts, but the actual
wind speed could be anywhere from 13 to 17 kts. Similarly, a station model plotted
with one pennant and two long barbs would have a nominal wind speed of 70 kts,
but the actual wind speed could be anywhere from 68 to 72 kts. The wind direction
is given by the angle of the line that anchors the barbs; the direction in which the
line points is the direction the wind is coming from.
Sea level pressure is plotted in a coded format. To decode the sea level pressure
report, either a 10 or 9 must be placed in front of the three-digit value, and then a
decimal point added between the last two digits. If the coded sea level pressure is
more than 500, a 9 is placed in front of the coded value. Otherwise, a 10 is placed in
front of the coded value. The decoded sea level pressure is given in units of hPa, and
using this coding system, it must range from 950.0 to 1050.0 hPa. In intense extratropical cyclones or tropical storms the sea level pressure may be less than 950 hPa.
Similarly, strong high-pressure systems (anticyclones) may occasionally have sea
level pressure values greater than 1050 hPa. For these situations care must be taken
when decoding sea level pressure values given on weather maps.
Example What is the weather being reported in the station model shown in
Figure 1.4?
First, we can see that the temperature is 28 F (−2 C) and the dew point temperature
is 26 F (−3 C), corresponding to a relative humidity of around 92%. Why do we
AIR MASSES AND FRONTS
9
assume this is a US model? We can do so because the significant weather icon tells
us that moderate snow is falling, and this is highly unlikely at 28 C (82 F)!
We also see that the wind is blowing from the south at around 10 kts, a brisk
southerly. The skies are overcast, as we would expect while it is snowing. The coded
sea level pressure is 229; this is less than 500 and so we place a 10 at the start
to get 10229, and then add a decimal point between the last two digits to give us
1022.9 hPa. In total, a chilly and gray day is being reported.
As well as conforming to a standard set of observations, stations around the world also
conform to a standard description of the time the observation was taken: Universal
Time (UTC). This time, also known as Greenwich Mean Time (GMT), or simply Z,
is the time at Greenwich in England, and is expressed using a 24 hour clock rather
than a.m and p.m. UTC is used regardless of the local time zone of the observation.
1.3 Air masses and fronts
In the early and middle parts of the last century, weather forecasters began to understand the systematic nature underlying the apparent chaos of the weather, and to
use it to advantage in weather forecasting. The foundation of this understanding
was the dual concepts of air masses and fronts. Air masses, and the fronts between
them, are defined by their thermodynamic properties; that is, their temperature, density, and pressure, and the amount of moisture they contain. The critical idea that
developed was that weather conditions are not randomly distributed over the globe.
Rather, variations in weather elements, such as temperature, in the extra-tropics tend
to be concentrated in narrow bands called fronts. Between these bands, weather elements change very gradually, and these broad, nearly uniform regions of the lower
atmosphere are called air masses.
1.3.1 Air masses
The best conditions for the formation of air masses are large areas where air can be
in relatively constant conditions long enough to take on quite uniform characteristics.
The warm, moist tropical oceans and the cold, dry polar land masses are excellent
source regions. An air mass can be relatively shallow, 1–2 km deep, or may be as
deep as the entire troposphere, and is often not of uniform depth across its entire
horizontal extent.
A commonly used classification of air masses, especially in the Northern Hemisphere, is that of Tor Bergeron of the Norwegian School (see Section 1.4), who in 1928
denoted four primary air masses – continental polar (cP), continental tropical (cT), maritime polar (mP), and maritime tropical (mT). Often, additional air masses such as the
Arctic (A), Antarctic (AA), and equatorial (E) are added to this list. Figure 1.5 shows
the source region and typical locations impacted by these different air masses. These
air masses are identified either subjectively or objectively using a range of criteria,
10
ANATOMY OF A CYCLONE
A
A
mP
A
A
A
mP
cP
mP
cP
cP
cT
mT
cT
mT
cT
mT
E
E
E
E
E
mP
mP
mP
mT
cT
mT
mT
mT
mP
mP
mP
AA
A : Arctic – very cold and dry
cP : continental polar – cold and dry
mP : maritime polar – cold and moist
E : equatorial – very warm and moist
AA
AA
AA
mT : maritime tropical – warm and moist
cT : continental tropical – warm and dry
AA : Antarctic – very cold and dry
Figure 1.5 Typical distribution of air masses around the globe
but are typically identified based on near surface values of temperature and humidity. A qualitative description of the typical temperature and humidity of the different
air mass types is also given in Figure 1.5. Since air masses are three-dimensional features of the atmosphere they may also be identified using atmospheric observations
from balloons with instruments attached, called radiosondes, or by aircraft.
At any one time there are at least several dozen distinct air masses globally. Most
cover thousands of square kilometers of surface area. Some have just formed, some
are in the process of being modified, and some are essentially stationary.
Once an air mass moves out of its source region, it begins to be modified as
it encounters surface conditions different from those found in the source region.
From the point of view of atmospheric dynamics, air masses are generally of interest
primarily because of the interfaces between them: the fronts.
1.3.2 Fronts
Fronts are the boundaries that separate different air masses, and are defined by
thermodynamic differences across the boundary, and the direction of movement of
AIR MASSES AND FRONTS
11
Cold Front
Warm Front
Stationary Front
Occluded Front
Figure 1.6 Symbols used to represent fronts on weather maps. The side on which the symbols
are drawn is the direction in which the front is traveling, except for the stationary front, which is
not moving
the boundary. Meteorologists define four basic types of fronts, although other types
of fronts can also be identified. These four basic types of fronts are referred to as
cold, warm, stationary, and occluded fronts. The symbols used to represent these
fronts on weather maps are shown in Figure 1.6.
Typically fronts separate warm and cold air masses. If the cold air mass is advancing
and the warm air mass is retreating the boundary is called a cold front. If the opposite
occurs, with warm air advancing and cold air retreating, the boundary is called a
warm front. Sometimes the boundary between the two air masses is nearly stationary
and this type of front is called a stationary front. An occluded front separates air
masses that do not have as large a temperature contrast as is found for cold or warm
fronts, and typically separates cold and cool air masses. The processes that lead to
the formation of an occluded front are discussed below, and are important in the life
cycle of extra-tropical cyclones.
Even though fronts are most commonly seen only on surface weather maps it is
important to remember that fronts are the boundaries that separate three-dimensional
volumes of the atmosphere known as air masses, and can be identified both at the
surface and aloft. An example of a warm, cold, and stationary front associated with
the February 2003 cyclone (Section 1.1) is shown in Figure 1.7. Note how the warm
front and the cold front meet at the low-pressure center.
This is a surface weather map of the type frequently used in the United States,
showing isobars, fronts, and station reports, which indicate temperature and dew
point temperature in F, wind speed observations in knots, coded sea level pressure,
and cloud cover. Weather maps used in other parts of the world typically report the
temperature and dew point temperature in C, but are otherwise equivalent. You can
find examples of these types of weather maps on the companion CD-ROM.
Example Identify the warm and the cold air masses in the map shown in Figure 1.7.
Consider the station observation just to the south-east of the center of the low. The
wind is 5 kts from the SSE, the weather is clear, and the temperature is 60 F (16 C).
The surface pressure is 1001.1 hPa. Just to the north of this station, there is a station
on the other side of the warm front. Here, the weather is overcast and foggy, and
the wind is from the NE at 5 kts. The temperature is 44 F (7 C). The dew point
12
ANATOMY OF A CYCLONE
L
Figure 1.7 Surface weather map valid at 00 UTC 15 Feb 2003
depression, the temperature minus dew point temperature, is zero, meaning that the
air is saturated. It is clear that south of the low, between the cold front and the warm
front, is the warm air mass. North, and to either side of the fronts, is the cold air mass.
1.3.3 Upper level weather maps
Meteorologists around the world also use standard format maps to depict atmospheric
features higher in the troposphere. By convention, these upper level maps are plotted
on constant pressure surfaces rather than at specific altitudes, and hence are called
isobaric (iso=constant; baric=pressure) maps. A convenient upper level map to look
at when examining an extra-tropical cyclone is the 500 hPa map, which represents
atmospheric conditions at an altitude of approximately 5.5 km. This is roughly in the
middle of the troposphere, and the winds at this level are often thought to ‘steer’
weather disturbances.
An example of a 500 hPa map, for the same day and time as the map in Figure 1.7
is shown in Figure 1.8. The station reports plotted on this map replace the pressure
code with the height (that is, the altitude) of the 500 hPa surface. For the 500 hPa
constant pressure level the coded height is given in decameters (tens of meters), but
the coding method varies from one constant pressure level to another. On upper level
AIR MASSES AND FRONTS
13
Figure 1.8 The 500 hPa weather map valid at 00 UTC 15 Feb 2003. Height contours (decameters)
are shown as solid lines, and temperature contours ( C) are shown as dashed lines
maps, the temperature is always plotted with units of C throughout the world, and
as for surface weather maps is plotted in the upper left corner of the station model.
The dew point depression, rather than the dew point temperature, is plotted in the
lower left corner of the station model. The central circle of the station model is filled
if the dew point depression is less than 5 C, since a small dew point depression is
indicative of areas that may be cloudy. Otherwise, the circle is left unfilled.
From Figure 1.8 we see that the height of the 500 hPa surface varies, with lower
heights located to the north and higher heights found to the south. There are a number
of ripples, or waves, superimposed on this general decrease of height from south to
north. The height contours on the 500 hPa map can be thought of as being similar to
the elevation contours found on topographic maps of the Earth’s surface. Locations
where low heights are found are referred to as troughs since they represent a dip in
the elevation of the 500 hPa surface. Ridges are regions where high heights are found
on the 500 hPa map. Small ripples in the 500 hPa heights are known as short waves
while larger ripples, that extend across a whole continent for example, are known
as long waves or planetary waves. In this example, a ridge is present over the west
coast of the United States, while a long-wave trough is found over the center of the
country.
The height of the 500 hPa surface is related to several properties of the atmospheric
column. For example, as the temperature of the column decreases, the height of the
500 hPa surface would also tend to decrease (see Section 4.3.4 for more details). That
is, troughs are cold, and ridges are warm, in general. This leads to lower 500 hPa
heights being found near the poles and higher heights being found near the tropics.
14
ANATOMY OF A CYCLONE
The wind at this level generally flows roughly parallel to the height contours.
Higher wind speeds are found in areas where the height contours are closely spaced
and weaker wind speeds are found in areas where the height contours are more widely
spaced. We will see why this is the case in Section 5.5. In the Northern Hemisphere,
the winds blow with lower heights to the left of the wind direction and higher heights
to the right of the wind direction. In the Southern Hemisphere, the winds blow with
lower heights to the right of the wind direction. Because the 500 hPa surface slopes
downward from the equator toward the poles, this means that a large component of
the wind is blowing from the west almost all the time in both hemispheres. These
winds are aptly called the mid-latitude westerlies.
In addition to the height contours on the 500 hPa map shown in Figure 1.8,
contours of constant temperature, or isotherms, are shown. As would be expected,
lower temperatures are found closer to the poles while higher temperatures are found
closer to the tropics. An atmosphere in which the temperature varies across a pressure
surface is referred to as being baroclinic. In a baroclinic atmosphere the density of
air depends upon both temperature and pressure (see Section 3.2 and Equation (3.1)
for more details). An atmospheric state in which temperature does not vary on a
constant pressure surface is referred to as barotropic, and in this case density is only
a function of pressure. It is a situation that occurs in the atmosphere only rarely and
approximately.
1.4 The structure of a typical extra-tropical cyclone
The cyclones of middle and high latitudes are called extra-tropical cyclones. They
differ from cyclones in the tropics in a number of ways, but most prominent is the
fact that extra-tropical cyclones contain frontal systems while tropical cyclones do
not. Consequently, extra-tropical cyclones are also known as frontal cyclones. In this
chapter we will discuss only cyclones from the middle latitudes, reserving discussion
of tropical cyclones for Chapter 12 and polar cyclones for Chapter 14.
No two cyclones are identical, but an idealized model was developed during World
War I in Norway which embodies many of the important features of a frontal cyclone.
See Bjerknes and Solberg (1922) in the Bibliography for the original description,
which is drawn upon in this discussion. This model is known as the Norwegian model,
or the Bjerknes model. The model was not widely accepted in recent decades, until
research since the 1990s confirmed that the model remains a useful tool to describe
the weather associated with extra-tropical cyclones. In this section, and throughout
this book, we will use this model to describe the extra-tropical cyclone that traversed
the United States from 11 to 19 February 2003. However, as with all conceptual
models, the Norwegian model does not always apply.
Frontal cyclones are large traveling atmospheric vortices (rotating air), up to 2000
kilometers in diameter, with centers of low atmospheric pressure. An intense cyclone
in middle to high latitudes may have a surface pressure as low as 970 hPa, compared
to an average sea level pressure of 1013 hPa.
15
THE STRUCTURE OF A TYPICAL EXTRA-TROPICAL CYCLONE
Frontal cyclones are the dominant weather event of the Earth’s middle latitudes, and
are the environment in which smaller, more intense circulations are often embedded.
In the middle latitudes, they are the result of the dynamic interaction of warm tropical
and cold polar air masses. The boundary between these two air masses is called the
polar front. This larger frontal zone is distinct from the individual cold and warm
fronts associated with a particular cyclone. We will now look at the idealized life
cycle of an extra-tropical cyclone, as portrayed in the Norwegian model. We will need
to consider not just the surface, but the middle and upper regions of the troposphere,
since cyclones are three-dimensional features of the atmosphere.
1.4.1 Stages in the life cycle of an extra-tropical cyclone
As discussed in Section 1.2.2, the pressure at any location on the surface of the
Earth is determined by the mass of air in the atmospheric column above. For surface
pressure to decrease, air must be removed from the atmospheric column. Therefore the
formation, intensification, and decay of the surface low-pressure centers that define
cyclones are intimately tied to processes that add atmospheric mass (convergence) or
remove atmospheric mass (divergence) from a column of the atmosphere.
According to the Norwegian school, extra-tropical cyclones form along the polar
front. In this idealized model, the polar front initially runs west to east in a more
or less straight line until a small perturbation in the form of a wave in the upper
levels of the troposphere disturbs it (Figure 1.9). Divergence occurs in the vicinity of
this upper level short-wave trough (Figure 1.9a). This divergence promotes the initial
formation of the surface cyclone.
In the Northern Hemisphere, the circulation around this area of low pressure is
counterclockwise and results in cold air moving southward on the west side of the
surface cyclone, with the leading edge of the cold air marked by a cold front. East of
the surface cyclone warm air is moving northward, with the leading edge of the warm
air marked by a warm front (Figure 1.9b). The result is that the small wave along the
polar front amplifies, and the cyclone is referred to as being an open wave cyclone.
(a)
Cold
(b)
(c)
Cold
L
Cool
L
L
Warm
Cold
Warm
Figure 1.9 Idealized life cycle of a mid-latitude cyclone. L marks the position of the surface
low-pressure center and arrows indicate the surface air circulation. Fronts are shown using standard
symbols. A representative 500 hPa height contour is shown as a dashed line
16
ANATOMY OF A CYCLONE
The wedge of air between the advancing cold and warm fronts (south and east of
the surface cyclone in Figure 1.9b) is known as the warm sector of the cyclone. We
discovered this sector in the example at the end of Section 1.3.2. The air mass in
this sector is characterized by temperature and humidity values that are larger than
behind the cold front or ahead of the advancing warm front.
At upper levels the heights of the constant pressure surfaces begin to respond to
the movement of the cold and warm air masses. The height of the constant pressure
surface begins to decrease in regions where cold air is advancing, forming a deeper
upper level trough to the west of the surface cyclone. Conversely the height of the
constant pressure surface begins to rise in regions where warm air is advancing,
resulting in the formation of an upper level ridge ahead of the surface cyclone. The
cyclone shows a pronounced tilt toward the west with increasing height at this time,
with the upper level trough west of the surface cyclone position. This is the mature
stage of the cyclone life cycle, and may last up to a few days. Figure 1.10 shows
an idealized representation of a mature extra-tropical cyclone, and can be compared
with the mid-latitude cyclone shown in Figure 1.7. The balance between upper level
divergence and lower level convergence will determine how quickly the cyclone
intensifies. During this time the cyclone tends to move in a direction that is parallel
to the 500 hPa winds, and so the cyclone in Figure 1.9b will move north-eastward.
An animation of the three-dimensional circulation around a mature extra-tropical
cyclone is provided on the companion CD-ROM. In this animation both the horizontal
and vertical motion associated with the cyclone are shown, and the tilt of the low
pressure towards the west with increasing height can be seen. Rising motion occurs
near the center of the surface cyclone and ahead of the warm front, leading to
the formation of clouds and precipitation. Clouds tend to be low along and just
ahead of the warm front. As one moves further ahead of the warm front the clouds
become higher and thinner and the precipitation decreases in intensity and eventually
stops. Along the cold front there is a narrow band of rising motion associated with
the cold air undercutting the air in the warm sector of the cyclone (not shown on
the animation), and a narrow band of clouds and precipitation can be expected along
the cold front.
The cold and warm fronts will continue to advance in response to the circulation
around the surface low-pressure center as it deepens. The cold front often advances
more quickly than the warm front, with the cold air undercutting the warm air in the
warm sector of the storm. As the warm front advances, the warm air rises up over
the cold air to the north of the warm front, and thus the warm front tends to advance
more slowly. The result is that the cold front eventually catches up to the warm front
at the surface (Figure 1.9c). Where the cold front has caught up to the warm front,
the warm air is forced aloft and is no longer found at the surface. The boundary that
was the leading edge of the cold front now separates two regions of cold air at the
surface, and is called an occluded front. The cyclone is now said to be occluded and
is often referred to as a cold core cyclone. In response to the change in the surface air
temperature distribution the 500 hPa trough is now centered over the surface cyclone.
The cyclone has now reached the end of its life cycle, and will slowly decay.
17
THE STRUCTURE OF A TYPICAL EXTRA-TROPICAL CYCLONE
Vertical cross-section from A to B
Warm
Cold air
Cold air
A
B
B
A
LOW
1004 mb
C
–10 °C
D
1008 mb
1012 mb
Precipitation
–5 °C
0 °C
5 °C
10 °C
Overcast
Warm
15 °C
Cold air
Vertical cross-section from C to D
Cold air
C
Warm
Cold air
D
Figure 1.10 An idealized representation of a mature extra-tropical cyclone (adapted from the
work of Bjerknes and Solberg 1922)
As with all models, this idealized description of the typical life cycle of an extratropical cyclone does not always match what is observed, and has been modified
over time. Additionally, not all cyclones go through the stages illustrated in this
idealized model. Some cyclones remain as small frontal waves which propagate
(travel) rapidly eastward, while other cyclones grow rapidly but then decay before
attaining the cold core occlusion stage. In some cases the cyclones become massive
storms which are accompanied by changes in the entire long-wave pattern aloft.
Other conceptual models of mid-latitude cyclones are in use by meteorologists and
atmospheric scientists.
18
ANATOMY OF A CYCLONE
1.4.2 A mature cyclone – 00 UTC 15 Feb 2003
Figures 1.7 and 1.8 show the surface and 500 hPa weather maps at 00 UTC 15 Feb
2003 for this cyclone during the mature stage of its life cycle. The surface lowpressure center is located in south-eastern Kansas, with a warm front extending to
the east and a cold front extending to the south-west. The warm sector of the storm is
characterized by air temperatures in the upper 60s and lower 70s F (around 20 C),
with dew point temperatures in the upper 50s and lower 60s F (around 15 C). The
surface winds in the warm sector are primarily from the south or south-east.
North of the warm front the air temperature and dew point temperature are lower
than in the warm sector, and the winds are from the east or north-east. Overcast skies
extend from the warm front north to central Minnesota and North Dakota, associated
with rising motion along and ahead of the warm front. Light rain is falling just north
of the warm front, with light to moderate snow falling further north.
The air and dew point temperatures drop abruptly across the cold front, while the
winds shift to the north and become strong and gusty. Along the eastern edge of the
Rocky Mountains the cold front has become stationary, as is typical when a shallow
cold front encounters the high elevation of the Rocky Mountains.
At the 500 hPa level a broad long-wave trough is located west of the surface
cyclone position, generating south-westerly flow aloft above the surface cyclone. The
positions of the surface lower and upper level troughs indicate that the cyclone is in
a favorable position for further development at this time, but the cyclone maintains a
nearly constant intensity over the next 48 hours, indicating that divergence at upper
levels is nearly balanced by convergence of air near the surface.
1.4.3 An occluded cyclone – 00 UTC 17 Feb 2003
By 00 UTC 17 Feb 2003 the surface cyclone that had been located in south-eastern
Kansas is now located on the Kentucky/Tennessee border just west of the Appalachian
Mountains (Figure 1.11). A weak occluded front extends southward from the surface
low-pressure center and the low is surrounded by cool air at the surface. This low
has become occluded, and will slowly weaken and drift north-eastward over the next
two days.
Two new surface low-pressure centers are beginning to form, with one along the
Georgia/Alabama border and the other just offshore from North Carolina. Both of
these lows have formed along the frontal boundaries associated with the original lowpressure center. A broad area of cloudiness and precipitation is associated with the
occluded low and the frontal boundaries. North of the surface low pressure offshore
of North Carolina winds are from the east, bringing moisture from the Atlantic Ocean,
resulting in light snow.
At 500 hPa, a long-wave trough is located nearly overhead of the occluded surface
low (Figure 1.12), as we would expect from the Norwegian cyclone model. The
surface low off of the North Carolina coast is in a favorable location for further
development, being located to the east of the 500 hPa trough. Further, the contrast
THE STRUCTURE OF A TYPICAL EXTRA-TROPICAL CYCLONE
19
Figure 1.11 Surface weather map valid at 00 UTC 17 Feb 2003
Figure 1.12 The 500 hPa weather map valid at 00 UTC 17 Feb 2003. Height contours (decameters)
are shown as solid lines, and temperature contours ( C) are shown as dashed lines
20
ANATOMY OF A CYCLONE
between the relatively warm water of the Atlantic Ocean and the cold continental
air will serve to increase the temperature contrast across the fronts associated with
this new surface cyclone. In response to these factors that favor intensification of the
surface low pressure, this low does deepen over the next two days bringing heavy
snow to the mid-Atlantic and the north-eastern United States.
Review questions
1.1 In preparing a surface weather map over the Antarctic continent, station models
for all available surface observations must be constructed. At 21 UTC 25 Sep
2005 the surface weather observation at McMurdo Station (779 S 1667 E)
indicated a temperature of −36 F, a dew point temperature of −45 F, a sea level
pressure of 988.4 hPa, and calm winds. At the same time the surface weather
observation from Dumont D’Urville (667 S 1400 E) indicated a temperature
of −11 F, a dew point temperature of −19 F, a sea level pressure of 982.9 hPa,
and a wind from the south-east at 10 kts. Draw the station models for these two
sets of observations using the surface station model plotting conventions for US
surface weather maps. Draw the same station models using international plotting
conventions.
1.2 Examine the map of surface observations shown in Figure 1.13 on the CD-ROM.
One way of identifying air masses is to look for regions of warmer and colder temperatures. Use only the temperature field to draw a line on the map that marks the
boundary between the continental polar air mass and the maritime tropical air mass.
Now use the additional information available in the surface observations to
redefine the air masses using a second copy of the map. What information did
you use? Are there any differences between your first analysis and your second
analysis? If so, explain why.
1.3 What type of air mass would be responsible for the weather conditions listed as
follows?
(a) hot, muggy summer weather in the Midwest of the United States;
(b) refreshing, cool, dry breezes after a long summer hot spell in central Europe;
(c) drought with high temperatures in the Sahel region of Africa.
1.4 Consider the wintertime climates of Rochester, New York and Rochester, Minnesota in the United States (Table 1.1).
Table 1.1 The wintertime climates of Rochester, MN and Rochester, NY
Rochester, MN
Rochester, NY
January average
maximum ( C)
January average
minimum ( C)
Annual
snowfall (m)
Latitude
Elevation (m)
−7
−3
−17
−9
1.17
2.29
44 01′ 18′′ N
43 09′ 17′′ N
1317
50
REVIEW QUESTIONS
21
Find the two cities on a map. What are the causes of the differences between
the winters experienced by people in these two cities?
1.5 Consult the weather map shown in Figure 1.14 on the CD-ROM – it shows an
occluded low-pressure system in the Greenland Sea. An ocean report in the cold
air mass, west of the occluded front, shows overcast skies and drizzle. Identify
this station report and decode the temperature, dew point temperature, sea level
pressure, wind speed and wind direction.
1.6 Using the surface weather map for 00 UTC 16 Feb 2003 on the CD-ROM decode
the surface station models for:
(a) Dallas, Texas
(b) Atlanta, Georgia
(c) Dyersberg, Tennessee.
1.7 Print a copy of the surface weather map for 00 UTC 16 Feb 2003 from the
CD-ROM.
(a) Draw isotherms every 10 F on this map.
(b) Indicate the position of the low-pressure center by marking a letter ‘L’ on
your map.
(c) Draw cold and warm fronts using all of the data displayed on this map.
(d) Where are the weather stations used in question 1.6 located relative to the
fronts you identified in question 1.7(c)? Are the weather observations you
decoded for question 1.6 consistent with your expectations of the weather
associated with the passage of the fronts?
1.8 (a) Why is the dew point always lower than the actual temperature?
(b) Why does a small dew point depression indicate areas that are likely to be
cloudy?
1.9 Decode the upper air station models at Denver, Colorado and Atlanta, Georgia
plotted on the 500 hPa map for 00 UTC 16 Feb 2003 provided on the CD-ROM.
1.10 Identify the position of the troughs and ridges on the 500 hPa map for 00 UTC
16 Feb 2003 on the CD-ROM.
(a) Where is the surface low-pressure center identified in question 1.7(b) located
relative to the troughs and ridges at 500 hPa?
(b) Is this location favorable for the low-pressure system to intensify? Why or
why not?
(c) How has the intensity of the low-pressure system changed by 12 UTC 16
Feb? Is this change in intensity consistent with your answer to question
1.10(b)?
2
Mathematical methods in
fluid dynamics
This chapter presents several useful mathematical definitions, particularly associated
with vectors and functions in multiple dimensions. Many of these definitions
should be familiar to the student, and hence will be reviewed briefly and without
detailed proofs. The important concepts of Eulerian and Lagrangian frameworks, and
the associated process of advection, will be discussed in greater detail.
2.1 Scalars and vectors
A scalar is any quantity which can be fully specified by a single number, its magnitude.
Examples include time, temperature, wind speed, and surface pressure.
Some quantities in nature require both a magnitude and a direction to be fully
specified. Wind velocity is such a quantity, which has a magnitude (the wind speed)
and a direction. Together this description forms a vector quantity. There are two
typical notations for a vector:
• Graphically: A vector pointing from location A to location B is written as the
vector AB. The vector has a magnitude given by the distance between A and B,
and a direction given by the direction of the line in a given coordinate system.
⇀
⇀
⇀
• In terms of components: The vector u
= ux i + uy j is a vector with two compo⇀
nents, one in the x or i unit vector direction with magnitude ux , and one in the
⇀
y or j unit vector direction with magnitude
uy , in a given coordinate system.
⇀
The magnitude, denoted by u, is given by
vector is tan−1 uy /ux .
u2x + u2y , and the direction of the
2.2 The algebra of vectors
In any mathematical theory, we should begin by defining what is meant by addition,
subtraction, and multiplication of our new quantities, vectors. We can define these
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
24
MATHEMATICAL METHODS IN FLUID DYNAMICS
D
D
B
C
B
C
A
A
Addition
Subtraction
Figure 2.1 The addition and subtraction of vectors AB and CD
operations graphically or computationally, and it is useful to consider both of these
approaches.
2.2.1 Addition and subtraction
To add the vector AB to vector CD, we can place CD so that location C, the starting
point of CD, falls on location B, the end point of AB (Figure 2.1). Then the sum of
AB and CD is the vector AD. Vector addition is both commutative and associative.
This graphical definition of a vector sum can be very useful when we are considering
how various forces combine to produce the motion of a parcel of air. However,
sometimes we need to compute a sum, or we need to perform an addition in more
than two dimensions. In such cases, it is more straightforward to write, for example,
⇀
⇀
⇀
⇀
and
u = ux i + uy j + uz k
⇀
⇀
v = vx i + vy j + vz k
⇀
⇀
⇀
⇀
⇀
⇀
⇀
⇒ u + v = ux + vx i + uy + vy j + uz + vz k
⇀
⇀
(2.1)
⇀
for any two vectors with three components in the i j and k unit vector or x, y, z
axis directions.
Conversely, to subtract one vector, CD, from another, AB, we simply add the
negative of CD, which is DC, to AB (Figure 2.1). Computationally, this is simply
⇀
⇀
⇀
⇀
⇀
u − v = ux − vx i + uy − vy j + uz − vz k
(2.2)
2.2.2 Multiplication by scalars
If AB is a vector and c is a scalar, then the product cAB is a vector whose length is
c times the length of AB. This new vector has the direction AB if c is positive, and
has the direction BA if c is negative (Figure 2.2). If c equals zero then cAB is the
zero vector.
25
THE ALGEBRA OF VECTORS
B
AB
2AB
–2.5AB
0.5AB
A
Figure 2.2 The multiplication of vector AB by scalars c = 2 −25, and 0.5
Writing this computationally in terms of components,
⇀
⇀
⇀
⇀
cu = cux i + cuy j + cuz k
(2.3)
2.2.3 Multiplication of two vectors
There are two different ways to formulate the multiplication of two vectors, both of
which are relevant in atmospheric dynamics. The first is called the scalar product or
dot product of two vectors, and this process results in a scalar. The second is called
the vector product or cross product, the result being a vector. Again, we can define
these functions both graphically and computationally. Let us first consider the scalar
product, which is written
⇀
⇀
⇀⇀
u • v = ux vx + uy vy + uz vz = uv cos
(2.4)
where is the angle between the two vectors. Notice that if the two vectors are
perpendicular, then cos = 0 and the scalar product between the vectors is also zero.
Conversely, if the scalar product between two vectors is zero, and we know that the
two vectors themselves are nonzero, then we know that they are perpendicular.
Example The scalar product of the two vectors
⇀
⇀
⇀
⇀
⇀
⇀
⇀
⇀
u = 4 i − 2 j + 3k
v = 2 i − 2 j − 4k
is
⇀
⇀
u • v = 4 × 2 + −2 × −2 + 3 × −4
= 8 + 4 − 12
=0
26
MATHEMATICAL METHODS IN FLUID DYNAMICS
and hence the two vectors are perpendicular. We can also see this graphically
(Figure 2.3)
Computationally, the vector product is written
⇀
⇀
⇀
⇀
⇀
u × v = uy vz − uz vy i − ux vz − uz vx j + ux vy − uy vx k
(2.5)
This definition does not appear to be very intuitive, but it results in a vector product
⇀
⇀
which is perpendicular to both u and v, and in fact forms a right-handed
set with
⇀⇀
these two vectors (Figure 2.4). The magnitude of this vector product is uv sin .
⇀
⇀
This tells us that if u and v are parallel, their vector product will be zero. For the
student who is familiar with the use of determinants, we can use an alternative form
for this product
⇀
i
⇀
⇀
u × v = ux
v
x
⇀
j
uy
vy
⇀
k
uz
vz
(2.6)
⇀
⇀
⇀
= uy vz − uz vy i − ux vz − uz vx j + ux vy − uy vx k
which gives the same result.
z
y
1
u
+3
+2
+4
x
–2
–4
1
v
Figure 2.3 The scalar product of these two vectors must be zero
27
COORDINATE SYSTEMS ON THE EARTH
z
1
1
u ×v
y
1
v
θ
x
1
u
Figure 2.4 The vector product of two vectors is perpendicular to the plane containing the two
vectors
2.3 Scalar and vector fields
A field is a quantity which is continuously defined over a given coordinate space. For
example, in the map shown in Figure 1.7, a value for the surface pressure is known,
or can be deduced, everywhere on the map.
In a scalar field, a scalar (or number) is assigned to every point in the space; the
scalar may change from point to point and with time. In fact, the scalar is a function
of the three coordinates of position and the one coordinate of time. Examples of
scalar fields are temperature and pressure. Scalar fields can be shown physically on
a map by lines joining points of equal value such as in lines of equal temperature
(isotherms) or lines of equal pressure (isobars).
In a vector field, a vector is assigned to every point in the space; the vector may
change from point to point and with time, so that the vector is a function of the three
coordinates of position and one coordinate of time. Examples of vector fields include
the wind and forces such as gravity and the pressure gradient force (Chapter 4).
Vector fields are typically represented by arrows, where the length of each arrow
indicates the magnitude. Meteorological maps will often represent vector fields with
wind barbs as discussed in Chapter 1 (for example, Figure 1.8).
2.4 Coordinate systems on the Earth
A field of a scalar quantity, like temperature, does not change when the coordinate
system is changed. Because of this property, scalar quantities are particularly easy to
28
MATHEMATICAL METHODS IN FLUID DYNAMICS
y
1
u
y′
x′
1
j
α
x
α
1
i
Figure 2.5 Effect of a coordinate system rotation on the components of a vector. The original
coordinate system is denoted x–y and the rotated coordinate system is denoted x ′ –y ′
manipulate mathematically. However, for a vector quantity, the components change
⇀
under the rotation of the coordinate system. For example, consider the vector u =
⇀
⇀
ux i + uy j. Now, consider the vector in a rotated coordinate system as shown in
Figure 2.5.
In the new coordinate system, the components of the vector become
⇀
⇀
⇀
u′ = ux cos + uy sin i ′ + uy cos − ux sin j ′
(2.7)
where is the angle between the original and new coordinate systems as shown in
Figure 2.5.
Since many of the quantities we will be interested in are vectors, it is important that we define our location and coordinate system precisely. In atmospheric
dynamics, it is traditional to define the coordinate system relative to the Earth, even
though the Earth is moving through space. In fact, the Earth is not just moving
through space, it is accelerating, because its motion involves changes in direction
even though the speed does not change on the time scales we are considering. This
is termed a non-inertial frame of reference, and will have consequences for how we
understand motion relative to the Earth. We will examine this in Chapter 4. The way
in which we define this coordinate system is shown in Figure 2.6.
Thus, at any point on the Earth we define direction according to our departures
from due east (the x direction), due north (the y direction), and the local vertical (the
z direction).
2.5 Gradients of vectors
⇀
Consider a three-dimensional vector ut which represents the wind speed and
direction in space at some time t. Then we can differentiate the vector function just
29
GRADIENTS OF VECTORS
z, k, locally "up"
l at
it u
de
y, j, NORTH, meridional
longit
ude
x, i, EAST, zonal
Figure 2.6 Coordinate system anchored on the Earth
as we differentiate a function in one-dimensional space. It is most straightforward to
write the velocity vector in terms of its components:
⇀
⇀
⇀
⇀
ut = ft i + gt j + htk
(2.8)
Assuming that all of the component scalar functions are continuous and differentiable
(and hence the velocity vector function itself is continuous and differentiable) we can
write
⇀
dut df ⇀ dg ⇀ dh ⇀
=
u t =
i+
j+
k
dt
dt
dt
dt
⇀′
(2.9)
Example For the wind velocity vector defined by
⇀
⇀
⇀
ut = t3 + 2t i + 4 j + cos t k
⇀
the acceleration vector, that is the derivative of the velocity vector with respect to
time, is
⇀
⇀
⇀
du
= 3t2 + 2 i − sin t k
dt
However, we are quite unlikely to come across a wind field that is uniform in space
even as it changes in time. For example, as we move out from the shelter of a building
on a windy day, the wind changes in both magnitude and direction. So, in general,
we do need to consider wind fields (and other vector fields) that are functions of
space (x y z) as well as time t; that is, three-dimensional vectors that are functions
of four variables!
Just as in our earlier example, the derivative of a vector gives the rate of change
of that vector. We can simplify our approach by only considering partial derivatives;
that is, the derivative with respect to one independent variable while holding the
30
MATHEMATICAL METHODS IN FLUID DYNAMICS
other independent variables constant. Hence, for the x component of the wind vector
function f = fx y z t, to find only the variation of f with respect to t, we write
the partial derivative as
f
t
So, taking the x, y, and z components of the wind velocity vector into consideration,
the partial derivative of the entire wind vector with respect to time is written
⇀
u f ⇀ g ⇀ h ⇀
i+ j+ k
=
t
t
t
t
(2.10)
Example For the wind velocity vector
⇀
⇀
⇀
ux y z t = t3 xz i + 4 sin xt + 5y3 j + 2xy2 k
⇀
The acceleration vector, that is the derivative of the velocity vector with respect to
time, is
⇀
⇀
⇀
u
= 3t2 xz i + 4x cos xt j
t
Higher order partial derivatives are defined in the same way, so that for example
2 f
2 f
f
f
and
=
=
x2
x x
xt x t
We can assume for our purposes that the order in which the partial derivatives are
taken is generally unimportant.
It is useful at this point to consider graphically what partial derivatives in different
spatial directions may mean. To simplify our picture, let us assume that we have a
wind velocity vector which has a component in the x direction, but no components
on the y or z directions. This means we have a westerly wind (a wind coming from
the west to the east, flowing in the positive x direction as seen in Figure 2.6). This
wind vector would be written as
⇀
⇀
⇀
⇀
ux y z t = fx y z t i + 0 j + 0k
(2.11)
Then the partial derivatives with respect to the x and y spatial dimensions are as
⇀
shown graphically in Figure 2.7. As can be seen, if u/x is positive, and all other
partial derivatives are zero, then the wind speed will increase as one moves to the
⇀
east, but not change as one moves to the north (or up). Conversely, if u/y is
positive and all other partial derivatives are zero, then the westerly wind speed will
increase as one moves north.
31
LINE AND SURFACE INTEGRALS
y
y
x
x
1
1
1
1
∂u
∂u
> 0,
=0
∂y
∂x
∂u
∂u
>0
= 0,
∂x
∂y
⇀
Figure 2.7 Partial derivatives of u with respect to x and y, shown only in two dimensions for
clarity
2.6 Line and surface integrals
The integral of a function of a single variable is defined over some interval of the
x axis such as in
b
fx dx
a
A natural extension to this definition is to replace this segment of the straight x axis
by a general curve in space. Such an integral is called a line integral. Similarly, a
double integral of a function of two variables is defined over some region of the xy
plane, and it is possible to replace this region with a surface in three-dimensional
space. Such an integral is called an area or surface integral. Here, we briefly introduce
these two concepts.
2.6.1 Line integrals
Consider a vector field, composed of well-defined scalar functions, that represents
⇀
a field of forces F x y acting on the air due to the presence of a gradient in
atmospheric pressure (see Chapter 4). For simplicity we initially restrict ourselves to
two dimensions.
Suppose now that we have a parcel of air moving along some smooth directed
curve given by the function
⇀
⇀
⇀
St = ft i + gt j
(2.12)
which is lying in the force field (see Figure 2.8). As the parcel of air moves a short
distance along the curve, we can consider that segment of the curve to be a straight
⇀
⇀
line. Hence, the amount of work done on the air parcel is approximately F • S. That
⇀
is, only the component of F x y directed along the curve is doing work on the air
parcel, and hence we use the scalar product. This allows us then to define the total
32
MATHEMATICAL METHODS IN FLUID DYNAMICS
S
δS
Figure 2.8 Path of an air parcel through a pressure gradient force field
amount of work done on the air parcel by the pressure gradient force, and in so doing
we define the line integral:
work =
⇀
⇀
(2.13)
F • dS
⇀
S
As can be seen from this expression, the value of a line integral depends not only
on the function being integrated, but also on the path taken through the space. In
fact, reversing the direction of travel along the curve reverses the sign of the integral.
In the case where the curve is a closed curve (that is, the starting and end points
coincide), the notation is
⇀ ⇀
F • dS
⇀
s
For such curves, the convention is to consider the counterclockwise direction, or the
direction in which the bounded region is to the left, to be the positive direction.
To extend this definition of the line integral to a more general statement in threedimensional space, we can write
⇀ ⇀
⇀
⇀
⇀
⇀
⇀
⇀
F • dS =
fx y z t i + gx y z t j + hx y z tk • dx i + dy j + dzk
⇀
⇀
S
⇀
S
S
⇀
⇀
F • dS =
(2.14)
fdx + gdy + hdz
⇀
S
The expression fx y z tdx + gx y z tdy + hx y z tdz is called an exact differential if there is a differentiable scalar function x y z t such that
= fx y z t
x
= gx y z t
y
= hx y z t
z
33
LINE AND SURFACE INTEGRALS
⇀
When such a function exists it is called a potential function, or a potential of F .
It turns out that when such an exact differential is being integrated, the line integral
in Equation (2.14) is independent of the path taken through the space, and in fact
⇀ ⇀
F • dS = B − A
(2.15)
⇀
S
where A is the starting point of the curve and B is the end point of the curve. This
has the corollary that the line integral around a closed curve of an exact differential
is identically zero.
Example For the pressure gradient force field
2
⇀
⇀
2yz ⇀
y ⇀
F x y z t = 2C1 xy sin C2 t i + C1 x2 sin C2 t +
j+
k
C3
C3
where C1 = 0 2 N m−3 C2 = /86 400 C3 = 2 m3 N−1 , the work done on an air
parcel traveling from location A(10,8,6) at time t = 3 h to location B(14,10,3) at time
t = 6 h is
⇀ ⇀
work = F • dS = fdx + gdy + hdz
⇀
⇀
S
=
14
10
S
3 y2
10
2yz
2
dy +
dz
2C1 xy sin C2 tdx +
C1 x sin C2 t +
C3
C3
8
6
However, it can be seen that this forms an exact differential, since we can define a
scalar differentiable function such that
x y z t = C1 x2 y sin C2 t +
y2 z
C3
and so we can write
work = B − A
= C1 x2 y sin C2 t +
y2 z
C3
− C1 x2 y sin C2 t +
1410321 600
= 0 2 × 196 × 10 × 0 71 +
y2 z
C3
108610 800
384
300
− 0 2 × 100 × 8 × 0 38 +
2
2
= 428 3 − 252 8
= 175 N m
Noting that C1 is given to one significant figure this gives an answer of 200 N m.
34
MATHEMATICAL METHODS IN FLUID DYNAMICS
2.6.2 Converting line integrals to area integrals
Returning to the case of a function in two dimensions, we may choose to convert a
line integral taken over the closed curve boundary of an area to a double integral of
a suitable function over that area. Without showing a proof of such a relationship,
we assert that it can be shown that
⇀ ⇀
g f
dA
(2.16)
−
F • dS = fdx + gdy ≡
x y
⇀
A
⇀
S
S
This is known as Green’s theorem, the proof of which can be found in most beginning
calculus texts.
2.7 Eulerian and Lagrangian frames of reference
There are two ways we can describe the motion (or flow) of a fluid such as
the Earth’s atmosphere: the Eulerian and the Lagrangian. In an Eulerian frame
of reference, the flow quantities such as temperature or velocity are defined as
functions of position in space and time (for example, Figure 2.9). The primary
flow quantity is the velocity vector field, but the complete description includes
the spatial distribution of other quantities of interest such as temperature, pressure,
and density. A flow variable is written as a function of position and time, F(x, y,
z, t), and the partial derivative gives only the local rate of change at a particular
location and time. So, for example, in Figure 2.9, the temperature of the smoke
flowing from a chimney can be expressed using an Eulerian frame of reference as a
function T(x, y, z, t), and the temperature at location O will be TO = TxO yO zO t.
Parcel A
T = TA(t)
Location O
T = T(xO, yO, zO, t)
z
y
x
Figure 2.9 Eulerian (O) and Lagrangian (A) descriptions of the temperature of smoke from a
chimney
ADVECTION
35
Typically, in the Eulerian description, the components of the wind field are desig⇀
⇀
⇀
⇀
⇀
nated u = ux y z t i + vx y z t j + wx y z tk or simply u = u v w. This
standard usage for wind components will be used in the rest of this book.
The Lagrangian specification makes use of the fact that, as in particle mechanics,
some of the dynamical and physical quantities refer not only to certain positions in
space but also (and more fundamentally) to identifiable pieces of matter. The flow
quantities here are defined as functions of time and the choice of the piece of matter,
or parcel, and thus describe the dynamical history of the selected parcel. In this
description then, any flow variable (including the location of a parcel) is expressed
as a function of time only, Ft. Since parcels change shape as they move, parcels
must be chosen such that they are considered to be ‘small’, and that ‘smallness’
must continue throughout time. So, for example, in Figure 2.9, the temperature of
a parcel of smoke flowing from the chimney can be expressed using a Lagrangian
specification as a function of time Tt, and the temperature of parcel A will be TA t.
The Lagrangian description is useful in some contexts, such as the tracking of air
pollution, and may appear to be simpler. However, it can become cumbersome when
there are many parcels to be tracked, such as within a large cyclone.
2.8 Advection
The way we describe the changes and motions occurring in the atmosphere is through
conservation laws:
• conservation of mass;
• conservation of momentum;
• conservation of energy.
These conservation laws apply to parcels of air in the same way that they apply to
individual bodies or particles. However, in order to apply these conservation laws in
an Eulerian frame, we must determine how forces (like gravity) and processes (like
heating from condensation or radiation) change the values of quantities such as wind
speed and direction or temperature at fixed points in space. In the Eulerian frame,
quantities do not change only as a result of forces and processes acting on the parcel;
change at a given point can occur simply because one air parcel has moved on from
that point in the frame and another air parcel has replaced it.
Consider an air parcel with a temperature T experiencing a steady southerly wind
as shown in Figure 2.10 in the left hand panel. Let us assume for now that there is
no heating or cooling and hence in Lagrangian terms
DT
=0
Dt
(2.17)
since there are no processes acting on the air parcels that can change the temperature.
36
MATHEMATICAL METHODS IN FLUID DYNAMICS
cooler
wind
x
cooler
T increases
rapidly
wind
x
cooler
T decreases
slowly
wind
x
T unchanged
warmer
warmer
warmer
Strong warm advection
Weak cold advection
No advection
Figure 2.10 Temperature change at a point in the presence of a strong southerly, a weak
northerly, and a strong westerly wind
However, in Eulerian terms there will be a change in temperature irrespective of
heating or cooling, since at this location, warmer air is replacing the cooler air that is
flowing northward. This is called warm advection, and is what you experience when,
in the Northern Hemisphere, you feel a warm southerly wind, for example.
If the wind were flowing in the other direction, as shown in Figure 2.10 in the
center panel, then the temperature would decrease as cooler air parcels replaced
warmer ones. This is known as cold advection.
The rate at which this advection occurs must depend on two things – the speed of the
wind that is transporting the air parcels with differing temperatures, and the strength
of the temperature gradient. Also important is the orientation of the temperature
gradient with respect to the wind direction – if the wind is flowing along isotherms
(lines of constant temperature) then no advection will take place (Figure 2.10, right
hand panel).
Any quantity can be advected, although it is most easily understood in the case
of temperature. Mass (in the form of density), momentum (in the form of the wind
velocity vector), and water vapor can all be modified at a particular location by this
process.
Now we turn to the mathematical formulation of advection. Deriving the advective
rate of change of any quantity requires a relationship between:
• the rate of change of the quantity at a fixed point, the local or Eulerian derivative
/t, and
• the rate of change of the quantity following the motion, the substantial, material,
or Lagrangian derivative D/Dt.
Consider again the temperature. For a given air parcel, the Lagrangian rate of change
of temperature DT/Dt is only a function of time. However, at a particular point in the
37
ADVECTION
Eulerian frame, the change in temperature is no longer just a function of time, but
also of position in the fluid:
T
DT
= lim
t→0
Dt
t
T
T
T
T
t+
x+
y+
z
T=
t
x
y
z
DT
T T Dx T Dy T Dz
⇒
=
+
+
+
Dt
t
x Dt y Dt
z Dt
T
T
T
T
=
+u
+v
+w
t
x
y
z
T
DT
T
T
T
=
− u
+v
+w
t
Dt
x
y
z
(2.18)
This relationship gives the rate of advective temperature change − uT /x+
vT /y + wT /z. The sign for the temperature advection makes physical sense,
since the wind must be blowing down the temperature gradient for there to be a temperature increase. For example, in Figure 2.10, left panel, the wind vector is positive
in the y direction, and the temperature gradient is negative (that is, it gets colder) in
the y direction. This configuration yields an advective temperature increase.
Example The air at a point 50 km north of a station is 3 C cooler than at the station
(Figure 2.11). If the wind is blowing from the north-east at 20 m s−1 and the air at
the station is being heated by radiation at the rate of 1 C h−1 , what is the temperature
change at the station?
The temperature change at the station is
T
T
T
=
+
t station
t heating
t advection
T
T
DT
T
+v
+w
=
− u
Dt
x
y
z
y
X
T–3 °C
20 ms–1
x
20sin 45°
50 km
1 °C/ hour
= 2.8 × 10–4 °C/second
20cos 45°
X
T
Figure 2.11 Station with temperature T undergoing radiative heating and advective cooling
38
MATHEMATICAL METHODS IN FLUID DYNAMICS
1 C
DT
=
= 3 × 10−4 C s−1
Dt
3600 s
⇀
⇀
⇀
u = −14 i − 14 j
T
=0
x
T
−3 C
=
= −6 × 10−5 Cm−1
y
50 km
T
=0
z
T
= 3 × 10−4 − −14 × −6 × 10−5
t station
= −5 × 10−4 C s−1
Hence, in this case, the cooling by advection overwhelms the warming from the Sun,
and the temperature at the station gets lower.
Review questions
2.1 To an observer on the ground, the wind speed experienced as a hurricane passes
overhead is equal to the wind speed due to the circulation of air around the hurricane plus the speed of movement of the hurricane. Consider a Northern Hemisphere hurricane with a wind speed of 50 m s−1 blowing in a counterclockwise
direction around the center of the hurricane at a distance of 10 km from the center.
Assume that the center of the hurricane is moving towards the north at 5 m s−1 .
(a) Sketch the wind vectors associated with the circulation of the hurricane only,
at two points 10 km due east and due west from the center of the hurricane.
(b) Sketch the vector that represents the motion of the center of the hurricane.
(c) What is the wind speed experienced by an observer who is 10 km due east
of the center of the hurricane? What is the wind speed 10 km due west of
the center of the hurricane? Determine your answers graphically.
(d) Write the three vectors you sketched in parts (a) and (b) in component form,
using the coordinate system shown in Figure 2.6.
(e) Use Equations (2.1) and (2.2) to calculate the wind speed experienced by an
observer who is 10 km due east of the center of the hurricane and one who is
10 km due west of the center of the hurricane. Do these results match your
graphic solutions in part (c)?
⇀
⇀
⇀
⇀
2.2 Calculate the vector that results from the scalar product of u = 10 i + 3 j − 6k
and c = 1 5. Sketch the original and new vectors.
2.3 Calculate the dot product and vector product of:
(a) an east wind blowing at 10 m s−1 and a south wind blowing at 2 m s−1 ;
(b) a south wind blowing at 4 m s−1 and a north wind blowing at 6 m s−1 .
39
REVIEW QUESTIONS
2.4 Numerical weather prediction models that are used to forecast the weather often
use coordinate systems that are rotated relative to the one shown in Figure 2.6.
⇀
⇀
⇀
Given the wind vector u = ux i + uy j, where ux = 10 m s−1 and uy = 0 m s−1 ,
determine the wind vector in a coordinate system rotated:
(a) 30 counterclockwise
(b) 30 clockwise
(c) 90 counterclockwise
(d) 180
relative to the coordinate system used by meteorologists.
2.5 Calculate the partial derivatives in the x, y, and z directions of the vector
⇀
⇀
⇀
ux y z t = t3 xz i + 4 sin xt + 5y3 j + 2xy2 k
⇀
which was used in the example in Section 2.5.
2.6 Using the horizontal wind vector
⇀
ux y z t = C sin
⇀
C
2
x +
y i
L
4000
where C = 10 m s−1 and L = 1 × 106 m:
⇀
⇀
(a) Plot the i component of u for −1000 km ≤ x ≤ 1000 km at y = −500, 0, and
500 km.
⇀
(b) Calculate u/x.
⇀
(c) Plot u/x for −1000 km ≤ x ≤ 1000 km.
⇀
⇀
(d) Describe the relationship between u and u/x shown in the plots from
parts (a) and (c). Is this consistent with your understanding of the physical
⇀
meaning of u/x?
2.7 Using the surface weather map in Figure 1.7 calculate the rate of change of
temperature due to advection halfway between Holdrege, Nebraska and Dodge
City, Kansas. Assume that the wind speed and direction at this point are equal
to the average wind speed and direction reported at Holdrege and Dodge City.
(Hint: the distance along the western border of Kansas is equal to 333 km.)
3
Properties of fluids
3.1 Solids, liquids, and gases
Imagine that you are holding a brick in front of you between your palms, and you
try to move your right hand away from you and your left hand toward you. The
forces you are exerting with your hands are transmitted through the brick from
one layer of atoms to the next, in a different direction on each side of layer. In
this way, you are imposing a shear stress on the brick. Stress is a measure of the
internal forces in a body between its constituent particles, as they resist tension and
compression (normal stress) or sliding (shear stress) in response to externally applied
forces.
However, a brick is a solid material – it possesses the property of rigidity. When
you apply a moderate shear stress, the brick will deform only slightly, and return to
its original shape when the stress is removed. If you are a body builder and can apply
a large enough stress to the brick, it will shatter rather than deform.
Now, apply this same stress to a brick made of, say, modeling clay. This is a plastic
material, which means it will deform continuously and irreversibly. What if the brick
was made of rubber? This is an elastic substance that will also deform continuously
but will then return to its original shape. Of course, all of these materials also possess
some amount of rigidity – they maintain their identity under the application of the
stress.
In contrast, the defining property of a fluid is that it is a substance which deforms
continuously when acted on by a shearing stress of any magnitude, however small.
So an ideal fluid is a material with no rigidity at all. Liquids and gases are both
considered to be fluids because they possess this defining property. The molecular
mechanism by which a liquid resists deformation is not the same as that in a gas,
although the governing equation determining the rate of change of deformation has the
same form. A fluid which transmits shear stresses internally is called a viscous fluid.
Solids and fluids (both liquids and gases) also differ in the way they respond to
normal stresses, that is stresses at right angles to the surface of the material. While
a solid can support both tensile and compressive stress, a fluid usually supports only
compression.
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
42
PROPERTIES OF FLUIDS
Table 3.1 Some properties of materials
Preferred shape
Density
Intermolecular forces
Differential motion
Gas
Liquid
Solid
No
Lowest
Weak
Yes, with minimum
force
Minimal
Medium
Medium
Yes, with moderate
force
Yes
Highest
Strong
Minimal, with great
force
Thixotropic substances
Solids in
plastic state
Viscoelastic solids
No memory
Compressible
Partial memory
Elastic solids
Perfect memory
Incompressible
As illustrated in Table 3.1, there are many odd substances that do not neatly fit into
the categories of liquids or solids, but lie on the boundary between them. For example, thixotropic substances like jelly or paint behave as an elastic solid until subjected
to severe distortion such as shaking, upon which they behave more like a liquid. Viscoelastic substances like egg white have partial memory, halfway between an elastic solid and a viscous fluid. Other substances, like slurries such as toothpaste, will
behave as a solid if the shear stress is small, but will behave as a fluid once a critical stress level is reached. Sea ice is considered to be ‘elastic plastic’ material but is
often represented in computer models, moderately successfully, as a ‘viscous plastic’
material. The study of the properties of such unusual materials is called rheology.
3.2 Thermodynamic properties of air
If two samples of air can exist in contact with each other without a change in properties, the two samples are said to have the same state. These properties include the
temperature, density, chemical composition, pressure, among others. The state of such
a system can be defined completely by some subset of such properties. For an ideal
gas this subset, known as the state variables, comprises any two state variables that
do not depend on the mass of the system (which are called intensive variables).
Equations of state relate properties of state to one another. Such equations are
usually empirical (that is, determined from the results of experiments). The equation
of state for an ideal gas, the Ideal Gas Law, is given by
p
= R a constant
(3.1)
T
where p is the pressure, is the density, and T is the absolute temperature. The
volume occupied by a mass of gas is also proportional to that mass, and hence
COMPOSITION OF THE ATMOSPHERE
43
we expect the constant R to be specific to a given chemical species. Experiment
shows that at low enough densities, R is a ratio between a universal constant
R∗ = 8314 J mol−1 K−1 and the molecular mass M of the gas (R = R∗ /M).
3.3 Composition of the atmosphere
The atmosphere consists of a number of different gases, each with different properties. The percentage of a volume of the atmosphere occupied by most of these
gases is nearly constant as one moves horizontally through the lower portion of the
atmosphere. One important exception to this is water vapor, which can vary dramatically in both the horizontal and vertical direction. Therefore it is often easiest to
consider first the dry atmosphere, excluding water vapor, before looking at the moist
atmosphere.
3.3.1 The dry atmosphere
The major constituents of the dry atmosphere are nitrogen and oxygen, which together
make up 99% of the lower atmosphere by volume. Trace amounts of carbon dioxide,
argon, ozone, and methane make up much of the remainder.
While the atmosphere is a mixture of various gases, its state can nevertheless
be approximated using the ideal gas law. To do this, first note that the individual
constituents of air obey this law. Hence,
pn = n Rn T
(3.2)
where n represents the nth constituent.
In addition, the mixture of air obeys Dalton’s law, which states that the total
pressure exerted by a mixture of gases that do not interact chemically is simply
the sum of the partial pressures of each constituent. The partial pressure of each
constituent is the pressure it would exert if it was present alone and occupied the
same volume as the whole mixture. Dalton’s law can be expressed mathematically as
n Rn
(3.3)
p=T
n
If we define a weighted average molecular mass for dry air to be that based on measurements of the average composition of the dry atmosphere, Md = 28966 g mol−1 ,
the gas constant for dry air is given by
n R∗ /Mn
n Rn
R∗
n
n
=
=
= 28704 J kg−1 K −1
(3.4)
Rd =
Md
This yields an ideal gas law for the mixture of gases that make up dry air:
p = Rd T
(3.5)
44
PROPERTIES OF FLUIDS
Example Calculate the pressure exerted by dry air that has a density of 12 kg m−3
at a temperature of 20 C. Remember that all units should be SI, so the temperature
needs to be converted from C to K.
p = Rd T
= 12 kg m−3 × 28704 J kg−1 K−1 × 20 + 273K
= 100 923 J m−3 = 100 923 N m−2
= 1009 hPa
3.3.2 The moist atmosphere
Unlike the other gases that make up the atmosphere, water vapor makes up a highly
variable percentage by volume of the atmosphere because of the range of conditions
within it. Indeed, the amount of water vapor can range from close to zero to 4% of the
total volume. Additionally water vapor can undergo phase changes to both the liquid
and solid state, and water can exist simultaneously as a gas, liquid, and solid in a
given volume of the atmosphere. Changes of phase of water in the atmosphere release
or absorb large quantities of heat which can be important in driving the dynamics of
the atmosphere.
Given the unique, and important, properties of water in the atmosphere, it should
come as no surprise that atmospheric scientists use a number of different variables
to describe the amount of water vapor contained in the atmosphere. Some of these
moisture variables include vapor pressure, specific humidity, water vapor mixing
ratio, and dew point temperature.
The partial pressure of the water vapor in a given volume of the atmosphere
is referred to as the vapor pressure (e, and is simply the pressure exerted by
the water vapor molecules present in the volume of the atmosphere that is being
considered.
Specific humidity and water vapor mixing ratio are closely related, and are often
confused by both students and more experienced atmospheric scientists. Specific
humidity (q) is defined as the mass of water vapor contained in a unit mass of
air, while the water vapor mixing ratio (r) is defined as the mass of water vapor
contained in a unit mass of dry air. Since the mass of water vapor contained in
a unit mass of air is small (ranging from close to zero in the polar regions or in
the upper troposphere to approximately 0.04 kg in tropical locations) the numerical values of specific humidity and mixing ratio rarely differ by more than a few
percent.
Just as in the case of a dry atmosphere, we can apply the ideal gas law to
water vapor. Of course water vapor has a lower molecular weight, and hence a
larger gas constant, than dry air. Also, since the composition of water vapor varies
considerably throughout the atmosphere, the approach of defining an average gas
constant based on weighted average molecular weights is likely to yield inaccurate
COMPOSITION OF THE ATMOSPHERE
45
results for moist air. An alternative approach is to retain the gas constant for dry air as
follows:
p = d Rd + v Rv T
d v Rv
+
T
= Rd
Rd
R
= Rd 1 − q + q v T
Rd
p = Rd Tv
(3.6)
where we have defined the new variable Tv , called the virtual temperature, and have
used the definition of specific humidity, q, to replace v /. The virtual temperature
is a fictitious temperature that by definition gives the temperature dry air would have
if it had the same density as the moist air in question at the same pressure. It can be
calculated from
Rv
T
Tv = 1 − q + q
Rd
(3.7)
Since moist air is less dense than dry air, the virtual temperature is always greater
than the actual temperature. However, even for very warm, moist air, the difference
between the actual and the virtual temperature is just a few percentage points. Thus,
for most applications, it is sufficient to use the ideal gas equation as expressed in
Equation (3.5).
Example Calculate the virtual temperature of 25 C air when the water vapor mixing
ratio is 19 g kg−1 .
For this example we will make the assumption that the specific humidity is equal to
the water vapor mixing ratio; that is, we are assuming the mass of water vapor per
unit mass of air is small. Then, converting all values to SI units,
Rv
T
Tv = 1 − q + q
Rd
−1 −1
K
−1
−1 461 J kg
= 1 − 0019 kg kg + 0019 kg kg
298 K
−1 −1
287 J kg K
= 10115 × 298
= 301 K
Tv = 28 C
46
PROPERTIES OF FLUIDS
3.4 Static stability
Consider a parcel of air with temperature T and pressure p. Suppose that it is given
a small amount of heat dQ per unit mass, and as a consequence its temperature and
pressure change by amounts dT and dp respectively. That is, the parcel reacts to the
heating though a combination of temperature rise and expansion. The heating can
be imparted through conduction, convection (that is, hot air rising), or latent heat
release. Then, using the ideal gas law:
p = Rd T
p = Rd T
⇒p
d
dp
dT
+
= Rd
dt
dt
dt
where = 1/ is called the specific volume of the parcel of air. Using the fact that
the gas constant for dry air can be shown to be given by the difference between the
specific heat capacities at constant pressure and volume respectively, the relationship
Rd = cp − cv gives
dp
dT
dT
d
+
= cp
− cv
dt
dt
dt
dt
d
dT
dT
dp
p
+ cv
= cp
−
dt
dt
dt
dt
p
Since heating is manifest as a combination of expansion at constant temperature and
increase of temperature at constant volume,
dT
dp
dQ
= cp
−
dt
dt
dt
(3.8)
This is the first law of thermodynamics as applied to a parcel of air.
As we introduced in Section 2.8, the atmosphere is most usefully described using
a set of conservation laws, which require the identification of conserved quantities,
such as mass and momentum. In this case, we can use the first law of thermodynamics
to derive a new thermodynamic variable which is conserved under certain conditions.
This variable is known as the potential temperature, which is the temperature a
parcel of air would possess if it were brought to a given reference pressure with no
heat exchange with the surroundings. Such a process is called an adiabatic process
and the reference pressure is typically taken to be 1000 hPa. For adiabatic motions,
that is, motions wherein no heat is exchanged with the surrounding environment,
potential temperature is a conserved quantity. Temperature may not be conserved in
such motions.
First, we use the Ideal Gas Law to eliminate the specific volume from Equation (3.8),
dQ = cp dT −
Rd T
dp
p
47
STATIC STABILITY
In an adiabatic process, dQ = 0, and hence we can integrate the above expression,
cp
1
dT − dp
Rd T
p
cp
C=
ln T − ln p
Rd
0=
where C is a constant of integration. Applying the boundary condition that T is equal
to the potential temperature, denoted , when p0 = 1000 hPa,
cp
ln − ln p0
Rd
cp
cp
ln − ln p0 =
ln T − ln p
Rd
Rd
cp /Rd
p0
=
ln
ln
p
T cp /Rd
Rd /cp
p0
⇒=T
p
C=
(3.9)
Example Calculate the potential temperature of an air parcel that has a temperature
of 10 C and a pressure of 850 hPa.
For this example we will use p0 = 1000 hPa Rd = 287 J kg−1 K −1 , and cp =
1004 J kg−1 K−1 . The temperature must be converted to absolute units K.
1000 × 102
= 283
850 × 102
= 297 K
287/1004
= 23 C
In atmospheric science conserved quantities such as potential temperature are useful
since they allow us to track the movement of air, while other non-conserved properties
of the air are changing. From Equation (3.9) we note that an air parcel that is displaced
vertically in an adiabatic process will experience a temperature decrease as the air
parcel is lifted (p decreases while remains constant), and this cooling is referred
to as adiabatic cooling. Conversely, an air parcel that is forced to sink adiabatically
(p increases) will warm in a process referred to as adiabatic warming.
Example What will the temperature of the air parcel from the example above be if
the air parcel is now lifted from 850 hPa to 500 hPa, assuming that no phase change
of water occurs as the parcel is lifted?
48
PROPERTIES OF FLUIDS
From the previous example we know that the potential temperature of this air parcel
is 297 K or 23 C. Rearranging Equation (3.9) gives the following:
p Rd /cp
T =
p0
287/1004
500 × 102
= 297
1000 × 102
= 244 K
T = −29 C
The vertical derivative of the atmospheric potential temperature profile is also a
fundamental quantity for characterizing the stability of air parcels undergoing vertical
motion. Consider an air parcel with volume V that starts at height z and is lifted
adiabatically to height specified to be z + dz. The initial potential temperature of the
air parcel and the environment are equal at height z. Therefore the temperature of
both the air parcel and the environment at height z will also be equal and will be
given by
Rd /cp
pz
Tz = z
p0
As the air parcel is lifted to height z + dz its temperature will now be given by
pz+dz Rd /cp
Tparcelz+dz = z
p0
Note that the potential temperature of the air parcel has not changed between heights
z and z + dz since the air parcel is being lifted adiabatically and potential temperature
is a conserved quantity for adiabatic processes.
The temperature of the air parcel’s environment at height z + dz will be given by
pz+dz Rd /cp
Tz+dz = z+dz
p0
where the potential temperatures of the environment at heights z and z + dz are not
assumed to be equal and are given by z and z+dz respectively.
The buoyancy force or upward thrust per unit mass F experienced by the parcel at
z + dz due to the air is
weight of air displaced − weight of air parcel
mass of parcel
gz+dz V − gparcelz+dz V
=
parcelz+dz V
F=
49
STATIC STABILITY
Canceling V and using the ideal gas law, this expression becomes
p/Rd Tz+dz − p/Rd Tparcelz+dz
F =g
p/Rd Tparcelz+dz
=g
1/Tz+dz − 1/Tparcelz+dz
1/Tparcelz+dz
=g
Tparcelz+dz − Tz+dz
Tz+dz
=g
parcelz+dz − z+dz
z+dz
Using the fact that the potential temperatures of the air parcel at heights z and z + dz
are equal we can replace parcelz+dz with z to give
− z+dz
F =g z
z+dz
=−
g d
dz
dz
= −N 2 dz
where we have defined the buoyancy frequency, N , which is also known as the
Brunt–Väisälä frequency. This is the frequency at which a parcel will oscillate if
displaced vertically and acted upon by the restoring force arising from the buoyancy
of the parcel.
If the potential temperature of the environment is uniform with height, the displaced
parcel experiences no buoyancy force and will remain at its location. Such a layer
of air is said to be neutrally stable. If the potential temperature of the environment
increases with height, a parcel displaced upward experiences a negative restoring
force, and vice versa, and hence will tend to return to its equilibrium level. Thus
d/dz > 0 of the environment characterizes a stable layer of air. In contrast, if the
potential temperature of the environment were to decrease with height, a displaced
parcel would experience a force in the direction of the displacement and continue to
accelerate in the direction of the displacement, clearly an unstable situation.
The above discussion assumes that no phase change of water occurs in the air
parcel as it is displaced, and this type of process is referred to as a dry adiabatic
process. It is important to note that a dry adiabatic process does not refer to a process
in which water is not present, only that the water that is present (in any of the vapor,
liquid, or solid phases) does not change phase during the process.
If the water in an air parcel (vapor, liquid, or solid) experiences a phase change as
the parcel is displaced, latent heat will be released or absorbed by the water during the
phase change resulting in a change in the potential temperature of the air parcel. For
this situation the air parcel is said to experience a moist adiabatic process. The special
50
PROPERTIES OF FLUIDS
situation of moist adiabatic processes will be discussed further in Chapter 11 in the
discussion of convective systems.
3.5 The continuum hypothesis
The science of fluid dynamics is concerned with behavior on a macroscopic scale;
that is, a scale that is large compared to the distance between molecules. Because of
this, it can be assumed that a fluid is perfectly continuous in structure, and physical
quantities such as mass and temperature are spread uniformly over the volume of
fluid. Hence, the fluctuations arising from the different properties of molecules can
be assumed to have no effect on the observations at meteorological space and time
scales. This is known as the continuum hypothesis.
What does it mean for the fluid properties to vary smoothly?
Consider a laboratory experiment in which an instrument is inserted in a fluid.
The sensitive volume of the instrument, that is the volume over which the instrument
can detect variations, must be small enough to measure the ‘local’ property (that is,
no significant variation in that property within the volume). At the same time, the
sensitive volume must be large compared to the molecular scale. In this example, let
us assume that our instrument is very sensitive, with a sensitive volume of 10−5 m3 .
At normal temperatures and pressures, this volume of air would contain about 3×1010
molecules. This is large enough for an average over the molecules to be independent
of their number. Further, the mean free path of the molecules is about 5 × 10−8 m,
which is small enough to be contained within the volume. Hence, we know that our
instrument will measure the macroscopic behavior of our fluid.
In fact, problems generally occur only when the number of molecules in a given
volume is very low (such as the atmosphere at the altitude of a satellite) or if the
variation across the volume is large (such as in a shock wave). Thus, the hypothesis
implies that we can attach a definite and real meaning to the idea of fluid properties
‘at a point’, and these properties are continuous functions of position in the fluid and
of time.
3.6 Practical assumptions
Most practical applications of fluid dynamics concern water or air, or fluids which
closely resemble one of these (leaving aside such fluids as molten polymers, liquid
crystals, or plasmas in a magnetic field). In this book, we are considering air, which is
a fluid composed of a mixture of gases, including nitrogen, oxygen, and water vapor,
among many others. For such a fluid we can make certain practical assumptions:
1. The atmosphere is an isotropic fluid. This means that the properties of the fluid
and their spatial derivatives do not depend on direction.
2. The atmosphere is a Newtonian fluid. Strictly speaking, this means that the fluid
obeys a linear relationship between shear stress and rate of deformation. We
51
CONTINUITY EQUATION
will not address the details of this relationship in this book. Suffice to say that
this gives us a simple way of treating viscous forces (that is, friction) in the
atmosphere.
3. The atmosphere is a classical fluid. This means that motions in the atmosphere
are governed by classical mechanics and thermodynamics, not quantum physics!
3.7 Continuity equation
In most situations important to atmospheric dynamics, mass is not created or
destroyed – it is always conserved. This idea is often termed continuity. However,
even if the mass remains constant, the volume may change – air can expand or flow
outward (a process called divergence, as introduced in Section 1.4.1), or compress
(convergence). Hence, the approach to describing this mass conservation requirement
is to consider how the density changes in the presence of divergence or convergence
to keep the total mass constant.
Consider a small volume of air V at some fixed point in our Eulerian frame
of reference. The mass of air in that volume at any instant is simply the density
multiplied by the volume. However, we cannot assume that the density is constant
throughout the volume, so we express this mass as an integral of density over the
volume:
Mair =
(3.10)
dxdydz
For the purposes of our argument, we will assume that our volume is a box as shown
in Figure 3.1. We must assume that air is flowing through this box, which is fixed in
space, all the time, so that the mass of air in the box accumulates at a rate equal to
the total inflow minus the total outflow at each face of the box.
u(x)
u(x + dx)
z
y
x
Mass in at x
ρ(x)u(x)dydz
Mass out at x + dx
ρ(x + dx)u(x + dx)dydz
Figure 3.1 Air flowing into and out of a box fixed in space
52
PROPERTIES OF FLUIDS
Hence, we can express the rate of change of mass in the box as follows:
Mair
=
t
t
dxdydz =
ux −ux+dx dydz
+
vy −vy+dy dxdz
+
wz −wz+dz dxdy
This can then be simplified to give the continuity equation:
t
dxdydz = −
v
w
u
+
+
dxdydz
x
y
z
u
v
w
+
+
+
=0
t
x
y
z
(3.11)
The terms u, v, and w are called mass fluxes, that is density multiplied by speed
of flow. Hence, this form of the continuity equation is called the ‘flux form’ because
it is making use of the gradients of the mass fluxes. We can expand the derivatives
of the mass fluxes to arrive at a different form of the continuity equation:
u
v
w
+
=0
+ u +v +w
+ +
t
x
y
z
x
y
z
1
2
3
(3.12)
Term (2) represents the net transport of mass into and out of the volume. This
term accounts for the variation in density across the volume while holding the flow
constant. Hence, terms (1) and (2) together represent the Lagrangian rate of change
in density, which is made up of the local (Eulerian) rate of change at a point, minus
the advection of density (see Section 2.7). Term (3) represents the divergence of the
flow while holding density constant. For example, considering only the x component
of the divergence, if u/ x>0, then the flow is getting faster as we move to the east.
Hence, air is leaving the volume faster than it is entering, contributing to a decrease
in mass in the volume (see also Figure 2.7). The form of the continuity equation
shown in Equation (3.12) is also called the Eulerian form of the equation.
Consider an air column where the density is not changing at all with time, so that
=0
t
⇒
u
v
w
=0
+
+
z
x
y
REVIEW QUESTIONS
53
Figure 3.2 An illustration of Dines’ compensation, showing convergence at a low level and
divergence at an upper level of a column. This flow is typical of the low-level circulation in
cyclone
If we also assume that the density is approximately constant in space (clearly a very
strong assumption), then we have
v
w
u
=0
+ +
x
y
z
u
v
w
+
=−
⇒
x
y
z
This relationship, known as Dines’ compensation, suggests that in order that there
be no net change in mass in the column, any horizontal or vertical divergence of air
in the column is replaced by the convergence of air at other levels in the column
(Figure 3.2). In the real atmosphere, convergence does not always exactly balance
divergence, but, in fact, departures from this balance are generally quite small.
Review questions
3.1 Using the ideal gas law for dry air calculate the density of air when
(a) the temperature is 0 C and the pressure is (i) 1000 hPa, (ii) 500 hPa, and (iii)
300 hPa; and
(b) the pressure is 1000 hPa and the temperature is (i) −30 C, (ii) 0 C, and (iii)
30 C.
(c) Based on your answers to parts (a) and (b), are the changes in density greater
for changes in pressure at constant temperature or for changes in temperature
at a constant pressure?
54
PROPERTIES OF FLUIDS
3.2 Typically, do changes in temperature or changes in pressure contribute more to
observed changes in density in the mid-latitudes? To guide your answer, assume
that a sea level location is experiencing an observed range of pressure of 980 to
1040 hPa and an annual range of temperature of −30 C to +30 C.
3.3 A weather report for Melbourne, Australia indicates a temperature of 15 C, a
water vapor mixing ratio of 0003 kg kg−1 , and a pressure of 1000 hPa. What is
the specific humidity for this location?
3.4 Consider the weather observations listed in Table 3.2.
(a) What is the specific humidity at each location?
(b) What is the percentage difference between the mixing ratio and specific
humidity at each location?
3.5 Consider the radiosonde observations from Denver, Colorado at 12 UTC 29 Sep
2005, listed in Table 3.3.
(a) What is the specific humidity at each pressure level?
(b) What is the percentage difference between the mixing ratio and the specific
humidity at each of these pressure levels?
3.6 Calculate the virtual temperature for each location listed in Table 3.2. What is
the percentage difference between the temperature and the virtual temperature at
each of these locations?
3.7 Calculate the potential temperature at each pressure level listed in Table 3.3.
Table 3.2 Meteorological observations for calculations required in
question 3.4
Location
Belem, Brazil
New York, NY
Barrow, AK
Temperature
Mixing ratio
Pressure
31 C
23 C
−3 C
22 g kg−1
10 g kg−1
25 g kg−1
1009 hPa
1005 hPa
1011 hPa
Table 3.3 Meteorological observations for calculations required in
question 3.5
Pressure level
843 hPa
500 hPa
300 hPa
100 hPa
Height
Temperature
Mixing ratio
1625 m
5840 m
9570 m
16520 m
5 C
− 9 C
−38 C
−64 C
62 g kg−1
08 g kg−1
03 g kg−1
001 g kg−1
REVIEW QUESTIONS
55
3.8 Calculate the Brunt–Väisälä frequency using the data in Table 3.3 for the layers:
(a) 843 to 500 hPa
(b) 500 to 300 hPa
(c) 300 hPa to 100 hPa.
3.9 An air parcel that has a temperature of 17 C at the 850 hPa standard level is lifted
dry adiabatically. What is the density of the parcel when it reaches 500 hPa?
3.10 Consider an air parcel with an initial pressure of 1000 hPa and a temperature of
20 C. This air parcel rises dry adiabatically over a mountain range to a pressure
of 700 hPa and then descends on the downwind side of the mountain range to a
pressure of 850 hPa.
(a) What is the temperature, potential temperature, and density of this air parcel
at the initial position, at the top of the mountain range, and on the downwind
side of the mountain range?
(b) How will the temperature, potential temperature, and density of this air differ
from that found in part (a) if condensation of water vapor occurs as the air
parcel rises to the top of the mountain range. (Hint: a qualitative answer for
this question is acceptable.)
3.11 (a) How will the potential temperature of an air parcel change if, as it ascends
adiabatically, the water vapor condenses to form a cloud?
(b) In this situation, is the criterion d/dz > 0 sufficient to identify a stable
layer of the atmosphere? Why or why not?
3.12 Surface weather observations across an east–west-oriented cold front indicate
winds from the north at 10 m s−1 on the north side of the cold front and winds from
the south at 5 m s−1 on the south side of the front. These weather observations
are made at locations that are 100 km apart. Assume that these winds occur over
the lowest 1 km of the atmosphere and that the vertical velocity at the surface
is 0 m s−1 . Calculate the vertical velocity at a height of 1 km assuming that the
density of the air does not vary in time or space.
4
Fundamental forces
4.1 Newton’s second law: F = ma
Because we have made the assumptions in Chapter 3 that the atmosphere is a
classical Newtonian fluid, continuous in structure and physical properties at the
scales of interest, we can simply use Newton’s second law of motion to describe
⇀
the acceleration a of the atmosphere. However, our approach will differ from that of
the classical mechanics of solid bodies in that we must achieve this description for a
continuous field of matter with varying density, rather than for discrete objects with
constant mass. Hence, it is most straightforward to write this description as
force ⇀
=a
mass
(4.1)
By using this relationship and predicting the distribution of density based on the continuity equation (Equation (3.3)), we can in theory determine the field of acceleration
for any situation. So, in order to examine the motion of the atmosphere, we need to
consider all forces that are acting upon it.
4.2 Body, surface, and line forces
The forces that act upon a fluid in general can be split into two types:
• body or volume forces, which act at a distance and affect the entire fluid
volume; and
• surface forces, which act locally upon a part of a fluid.
Body forces include gravity, electromagnetic forces, and forces that arise due to the
acceleration of the frame of reference. In all of these cases, the medium is in contact
with a force field of some sort.
A surface force is one in which there is direct mechanical contact between two
bodies. In a fluid, that is a gas or a liquid, the situation is more complex than in the case
of a solid. In a gas this contact is due predominantly to transport of momentum across
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
58
FUNDAMENTAL FORCES
the boundary by migrating molecules. In a liquid there are additional contributions
arising from forces between molecules on either side of the boundary. The primary
surface force of interest in the study of atmospheric dynamics is stress. The boundary
across which the stress is acting need not be the physical interface between two
fluids – internal boundaries within a single fluid can also transmit stress.
Because it is a surface force, stress acts only in a thin layer adjacent to the
boundary, and the total force acting is proportional to the surface area of the plane
that describes the boundary. The forces of this type that are of interest are pressure
and friction. Pressure is a ‘normal stress’, and friction is a ‘shear stress’.
There is a third type of force, called a line force, because it acts along a line.
Surface tension is an example of a line force. Surface tension acts at fluid interfaces
and so becomes important only when we consider boundaries such as that between
the atmosphere and the ocean. In this book, we will neglect surface tension.
Let us consider each of the above forces in turn. First, we will consider the forces
that are acting on a fluid regardless of the motion of the frame of reference. Recall
that a non-accelerating frame of reference is called an inertial frame.
4.3 Forces in an inertial reference frame
4.3.1 Gravity
The force due to gravity originates from the mutual attraction between any object
or fluid parcel and the Earth. Detailed observations of periodic motions in the
solar system confirm the law, developed by Newton and published in his Principia
in 1687, that every particle in the Universe attracts every other particle with a
force proportional to the product of the masses and inversely proportional to the
square of the distance between them. The universal constant of proportionality is
G = 6673 × 10−11 N m2 kg−2 . Strictly speaking, the inverse square law, which is
expressed in Table 4.1, holds only for point masses, but for spheres of uniform
density it is correct to consider the mass, however great, to be concentrated at the
Table 4.1 Forces acting on a fluid in an inertial frame
Force
Gravity
Pressure gradient force
Viscous force
Mathematical expression
⇀
⇀∗
r
GM
g =−
r2
r
⇀
1 p ⇀ p ⇀ p ⇀
j +
Pg = −
i +
k
x
y
z
2
2
2
2
⇀
v 2 v 2 v ⇀
u u u ⇀
j
+
+
+
+
+
i
Fr =
x2 y2 z2
x2 y2 z2
2
w
2 w 2 w ⇀
+
+
d
+
k
x2
y2
z2
FORCES IN AN INERTIAL REFERENCE FRAME
59
center. Of course, the Earth is neither spherical nor of uniform density; nevertheless,
the relationship holds to a sufficient level of accuracy for our purposes. Hence, by
substituting for the mass and mean radius of the Earth (Appendix B), we arrive at a
value of the gravitational force per unit mass acting at the surface of the Earth, g0∗ ,
of 983 m s−2 .
Gravity is a conservative force; that is, by definition, the work done by gravity is
independent of the path, and indeed the work done by gravity around a closed path is
identically zero. We know from the discussion in Chapter 2 that this implies that we
can write the gravity vector g ∗ as the gradient of a scalar potential function. In the
case of gravity, this function is known as the geopotential and because we assume,
for now, that the gravity vector has a component only in the z direction, this function
varies only with z. The geopotential is designated by the symbol :
z =
z
g ∗ z dz
(4.2)
0
Though small, the variations in gravity can be accounted for by using the geopotential.
Since a surface of constant geopotential represents a surface along which all objects
of the same mass have the same potential energy, if gravity were constant, a geopotential surface would also have the same altitude everywhere. Since gravity is not
constant, a geopotential surface will have varying altitude. The geopotential height is
defined as
z
1 ∗
Z = ∗ = ∗ g dz
g0
g0
(4.3)
0
Because the variations in gravity are small, the geopotential height is very close
to the actual (geometric) height, particularly close to the surface. Hence, in many
situations we can use a constant value of g and the geometric height. Nevertheless,
it is important to be aware of the approximation.
4.3.2 Pressure gradient force
A pressure force can only be exerted within a fluid if there is a difference in pressure
from one parcel to another; otherwise the parcels are exerting an equal and opposite
force on each other, and there is no net force. Therefore, the force associated with
pressure arises from the pressure gradient, not the pressure itself.
To derive the mathematical form of the pressure gradient force, we consider an
infinitesimal parcel of air in the presence of a pressure gradient (Figure 4.1). The
force due to atmospheric pressure acting on the left hand face of the parcel is
pA = p y z
60
FUNDAMENTAL FORCES
p
z
A
A
p + δp
y
δx
x
Figure 4.1 The x component of the pressure gradient forcing acting over an area A on the surface
of an infinitesimal parcel of air
and on the right hand face is simply
− p + p
p
y z = − p+
x y z
x
if the dimensions of the parcel are small compared to the scale of the pressure
changes. Hence, the x component of the net force exerted by this pressure field on
the parcel is simply the sum of the forces acting on either side of the parcel:
−
p
x y z
x
In order to write this in terms of a force per unit mass, we must divide the expression
by the mass, x y z, to get
⇀
⇀
Pg • i = −
1 p
x
(4.4)
The negative sign shows that the force is directed from high pressure to low pressure,
as we would expect. The other components of the force as shown in Table 4.1 can
be derived in an analogous manner.
Example The pressure gradient force at any location can be easily calculated using
station reports or a mean sea level pressure map. Consider our case study storm at
12 UTC on Friday, 14 February 2003 (Figure 4.2). The low is situated over western
Kansas at this time and there is a sea level pressure report of 1000.3 hPa from Dodge
City, near the center of the low. The temperature is reported to be 48 F 9 C at
Dodge City. Around 500 km away, in Limon, Colorado, the sea level pressure is
61
FORCES IN AN INERTIAL REFERENCE FRAME
08
10
35 045
1004
10
32
08
04
10
38 067
36
40 009 024
37
36
26 040 27 38
48 003 46
23
47 004 48
46
42 072
32
62 044
40
1008
54 031 54 043
1004
40
61
43 60 065 65 062
64
53
08
10
Figure 4.2 Sea level pressure and station reports over Oklahoma and surrounding states on 14
February 2003 at 12 UTC
reported to be 1000.9 hPa. From this information and using the ideal gas equation,
we can calculate the pressure gradient force acting:
=
100 030
p
≈
Rd T
287 × 9 + 273
≈ 12 kg m−3
⇀
1 p ⇀
n
n
1 100 090 − 100 030 ⇀
≈−
n
12
500 × 103
Pg = −
⇀
≈ −97 × 10−5 n
⇀
where we have assumed a coordinate system with unit vector n directed outward
from the center of the low, for simplicity. The resulting pressure gradient force of
97 × 10−5 N kg−1 is then directed toward the center of the low, as expected. We have
also allowed the density at Dodge City to represent the density of air in the entire
region – this will introduce a small error.
4.3.3 Viscous force
The effect of viscosity, or friction, within a fluid, results in a surface force like
pressure, and its basis is expressed in the same way, namely as a force per unit
area. In order to illustrate how the viscous force behaves, we can consider a classic
62
FUNDAMENTAL FORCES
Area A
d
V
fluid
u(z)
z
x
Figure 4.3 Flow of a fluid through a channel bounded by two plates, one moving with speed V
(top), and one stationary
example: the steady state flow of a fluid in a channel between two solid plates
(Figure 4.3). The upper plate is moving at some slow speed V and the lower plate
is stationary. There is no variation of the resulting flow with time or in the x or y
directions.
Recall that in Chapter 3 we characterized a fluid as a material that yields continuously
to any shear stress, no matter how small. Although it is not self-evident, from empirical
evidence it turns out that the relative velocity of a fluid is exactly zero at the boundary
with a solid. That is, the fluid molecules adjacent to the solid move with the same velocity
as the solid. This is because the solid surface is exerting a shear stress on the fluid. In the
example of the channel, this means that the fluid adjacent to the upper plate is moving at
speed V and the fluid adjacent to the lower plate is stationary. Measurements from this
experiment show that the magnitude of the shear stress = F /A exerted on the fluid by
the upper plate is proportional to the speed of the upper plate and inversely proportional
to the distance d between the plates:
V
F
∝
A
d
The constant of proportionality can also be measured, and from such measurements
it is known that this constant, designated the coefficient of shear viscosity , is
dependent upon the fluid in question. That is, this coefficient depends on the molecular
properties of the fluid constituents. This relationship holds regardless of the distance
d between the plates, and hence we can write the x component of the shear stress
acting on the uppermost fluid layer in the channel as
⇀
⇀
•i =
u
z
(4.5)
The moving molecules in the fluid exert a shear stress on the layer of fluid below
them, causing them to move also, but this layer is also being acted upon by the layer
below that, exerting a retarding stress, and so on, down through the layers, creating
a linear profile of fluid flow as shown in Figure 4.3.
FORCES IN AN INERTIAL REFERENCE FRAME
63
We have defined our example to be steady state; that is, although there is motion,
there is no acceleration. Hence, there can be no net viscous force acting. Thus, just
as in the case of pressure, the mere presence of shear stress (internal friction) in a
fluid does not cause a net force. In order for this to occur, there must be a gradient
in the shear stress. This may be derived in an analogous manner to the derivation
of the pressure gradient force in the previous section, resulting in an expression of
the form
2 u
1
=
z
z2
but with one important distinction. The shear stress must be considered to be a vector
quantity, rather than a scalar like pressure, and hence a more general expression of
the x component of the viscous force (or ‘shear stress gradient force’) in a truly
three-dimensional example is
2
⇀
⇀
u 2 u 2 u
+
+
(4.6)
Fr • i =
x2 y2 z2
We can replace the coefficient of shear viscosity in Equation (4.6) by a new quantity
=
(4.7)
which is known as the kinematic viscosity coefficient. For standard conditions at sea
level the kinematic viscosity of the atmosphere is around 15 × 10−5 m2 s−1 , and of
the ocean is around 10 × 10−6 m2 s−1 .
4.3.4 Hydrostatic balance
Consider the atmosphere at rest. There is a downward force acting at all times, which
is simply the weight of the air under the influence of the gravitational force. For
the atmosphere to be at rest, there can be no net force present and hence there must
be a balancing upward force. This is provided by the pressure gradient force in the
vertical, directed from higher pressure near the surface to lower pressure aloft (see
Figure 1.2). This state of balance between the two forces is known as hydrostatic
balance, and is expressed mathematically as
1 dp
= g∗
dz
dp
= −g ∗
⇒
dz
−
(4.8)
Note that we do not use the partial derivative of pressure in this expression, since it
is only the variations of pressure with height that are permitted (horizontal variations
64
FUNDAMENTAL FORCES
could give rise to motion). This equilibrium is in fact a good approximation even
when the atmosphere is in motion, because, as we will see, motions that cause
departures from this balance tend to be comparatively small.
In practice, the measurement of density in the atmosphere is more difficult than the
measurement of pressure or temperature. We can eliminate this quantity by combining
Equation (4.8) with the ideal gas equation (Equation (3.1)) to give
dp = −g ∗ dz
p ∗
g dz
=−
Rd T
dp
g∗
=−
dz
p
Rd T
z g∗
p
=−
dz
ln
p0
Rd T
(4.9)
0
If we assume that the integrand is approximately a constant, by using a layer average
temperature (denoted by angle brackets below), and we define that integrand to be a
quantity known as the scale height,
Rd T
g∗
(4.10)
p ≈ p0 e−z/H
(4.11)
H=
we can write
Hence, the altitude is proportional to the logarithm of the pressure. Equation (4.11)
is one form of what is known as the hypsometric equation, which relates the pressure
and altitude. In the Earth’s atmosphere, the scale height, at which the pressure drops
by a factor of 1/e, is around 8 km.
In Section 4.3.1 we defined the geopotential, allowing us to take account of the
variation in g. So if we assume instead that the integrand in Equation (4.9) is not
constant, we can write
g∗
dp
=−
dz
p
Rd T
dp
d = −Rd T
p
= −Rd
p
Td ln p
p0
p
Rd
⇒ Z = − ∗ Td ln p
g0
p0
(4.12)
FORCES IN AN INERTIAL REFERENCE FRAME
65
The difference in geopotential height between two pressure levels derived here is
also called the thickness of the layer, and is proportional to the mean temperature in
the layer. Thus, we can see that as a layer of air warms, it expands and its thickness
increases.
Example Figure 4.4 shows a vertical profile of temperature and dew point temperature
at Upton, NY on 00 UTC 16 Feb 2003. This data is plotted on a skew T thermodynamic
diagram. On skew T diagrams the temperature axis is ‘skewed’ such that lines of constant
temperature slope from the lower left to the upper right of the diagram. The vertical
axis on the diagram is logarithmic in pressure, with pressure decreasing from the bottom
to the top of the diagram. The uses of the diagram are explored further in Chapter 11.
Using this diagram we can determine the thickness of the layer between 500 and
400 hPa, using Equation (4.12) to calculate Z.
Figure 4.4 ‘Skew T ’ atmospheric sounding from 00 UTC 16 Feb 2003 at Upton, NY showing
temperature (right curve) and dew point temperature (left curve) expressed in C against pressure
(hPa). On this diagram pressure is indicated by the vertical scale on the left side of the diagram.
Lines of constant temperature are ‘skewed’ and slope from the lower left to the upper right and
are labeled along the top and right sides of the diagram
66
FUNDAMENTAL FORCES
Using the layer-averaged temperature will simplify the calculation. From
Figure 4.4, the temperature at 500 hPa is −20 C and the temperature at 400 hPa is
−31 C. This gives an average temperature (since the temperature variation is assumed
to be roughly linear throughout this layer) of −255 C. Converting the temperature
from C to K and using SI units gives
p
Rd
Z = − ∗ Td ln p
g0
p0
=
40 000
−287
2475d ln p
985
50 000
−287
40 000
× 2475 × ln
985
50 000
= 1609 m
=
Had this layer of the atmosphere had an average temperature of 277.5 K (30 K warmer
than was actually observed) the layer thickness would have been 1804 m. Conversely
a colder layer-averaged temperature would have resulted in a smaller thickness.
Other items to note on this sounding are that the temperature generally decreases with
decreasing pressure (increasing altitude). The exceptions to this are the layers between
950 and 800 hPa where temperature increases with increasing altitude (referred to
as an inversion) and in the layer from 240 to 150 hPa where the temperature is
nearly constant with increasing altitude (referred to as an isothermal layer). The
base of this layer is the tropopause and marks the top boundary of the troposphere
(Section 1.2).
4.4 Forces in a rotating reference frame
The Earth is rotating about its axis. Since it is convenient to adopt a frame of reference
fixed to the Earth, rather than space, we need to develop equations of motion that are
appropriate for a rotating coordinate system. Such a frame of reference is known as
a non-inertial reference frame because the frame itself imparts an acceleration to the
motion. The angular velocity of rotation of the Earth, , is small:
=
2
2
=
= 7292 × 10−5 s−1
1 day 86 400 s
and hence for many phenomena, the effects of the rotation of the reference frame
are negligible. However, on the space and time scales of some atmospheric motions,
the effect is important and must be included. There are two aspects to this effect,
which we will deal with in turn: forces on an object or parcel at rest, and forces on
a moving parcel.
FORCES IN A ROTATING REFERENCE FRAME
67
4.4.1 Centrifugal force
Consider a student standing at the center of a table that is rotating with angular
velocity in a laboratory (Figure 4.5). He conducts an experiment in which he
places an object of mass m at some distance r from him on the table. He finds that,
if the forces of friction between the object and the table are negligible, the object
moves radially away from him. Then, the student attaches a string of length r to the
object, and measures the force required to hold the object in place. Using a series of
measurements using different values of m and r, the student reports that the force
required to hold the object in place acts radially toward to him and can be described
by the expression mr2 . Since the object is at rest from the point of view of the
student, he concludes that, in order for there to be no net force, there must be an
equal and opposite force acting radially outward. This outward radial force is called
the centrifugal force.
Meanwhile, a second student is observing the experiment while standing on the
floor beside the table. Her observations are quite different. She notes that the object,
while being held by the string, is in fact accelerating because it is rotating at the
same rate as the table. Hence, in her frame of reference, there is a net force acting
on the object, exerted by the student on the table. This force acting radially inward
from the object is called the centripetal force and can be described by the expression
mr2 .
Hence, the centrifugal force arises only if the observation is taken in the rotating
frame of reference, and derives directly from the acceleration of the frame. This is
why the centrifugal force is often called an apparent force. However, to take account
of the rotating frame of reference, this force must be treated as any other force acting
on an object.
If the Earth were a smooth sphere, then an object placed at any point on the Earth
would have, as in the case of the rotating table, a tendency to move away from
the center of rotation, as shown in Figure 4.6(a). This means that there would be a
ω
s1
r
centripetal force
s2
m
centrifugal force
Figure 4.5 Laboratory experiment in which two students (s1 and s2) observe the motion of an
object held on a string by s1 on a spinning frictionless table
68
(a)
FUNDAMENTAL FORCES
(b)
Ω
Rcosφ
mRΩ2cosφ
z
y
Rφ
mR φ Ω2
mg*
mg*
φ
Ω
z
y
mg
mg
R
equator
φ
R
equator
Figure 4.6 Centrifugal and gravitational forces acting on an object of mass m at rest at latitude
on (a) a perfectly spherical Earth and (b) the actual Earth (not to scale). Note that the centrifugal
force is in fact almost three orders of magnitude smaller than the gravitational force
component of the centrifugal force in the positive z direction, and a component in
the negative y or equatorward direction.
The centrifugal force is a body force; that is, it acts through the center of mass of the
object. Hence, it can be combined with the gravity force to create a composite body
force. This results in a quantity known as the effective gravity, whose value at sea
level is 981 m s−2 , slightly reduced from the value calculated in Section 4.3.1 because
of the centrifugal force component in the positive z direction. The component of the
centrifugal force in the negative y direction results in a change in the direction of the
effective gravity force relative to the gravity force, and thus the effective gravity force
is not directed at the center of the Earth. The effective gravity is denoted g, and can
replace g ∗ in all of the previous equations in this chapter, including Equation (4.8)
above, which expresses the hydrostatic balance.
However, the Earth is not a perfect sphere. Consider the distance between two
points on the equator that are on the ‘opposite sides of the Earth’ from each other:
Quito, Ecuador and Pekanbaru, Indonesia provide an approximate example. If we
could measure this distance directly though the center of the Earth, we would find
that it is 12 756 272 m. If the Earth were a sphere, the distance between the North
Pole and the South Pole through the center of the Earth would also be 12 756 272 m,
but in fact this distance is 12 713 504 m, around 43 km or 0.34% smaller. The reason
for this ‘equatorial bulge’ is, in fact, the equatorward component of the centrifugal
force: as the Earth formed, it cooled from a liquid state as it spun, and the mass
distribution adjusted until there was no net equatorward component in the net force
represented by the effective gravity (Figure 4.6b). The result is that the effective
gravity is everywhere normal to the Earth’s surface (neglecting topography) with a
magnitude of 981 m s−2 , and objects at rest on the surface of the Earth experience
no net equatorward force.
69
FORCES IN A ROTATING REFERENCE FRAME
4.4.2 Coriolis force
The second influence of the rotating frame becomes evident when the object in
question starts to move relative to the frame of reference; for example, a parcel
of air moving on the rotating Earth. This results in apparent acceleration and an
associated force, the Coriolis force, which is named after the nineteenth-century
French mathematician Gaspard Gustave de Coriolis. There are two aspects to the
Coriolis force. The first is due to an additional centrifugal force that arises from the
relative motion. The second is due to changes in the relative angular momentum of
the object. We will now look at each of these aspects of the Coriolis force.
First, consider a situation where an object (such as a parcel of air) located at
some latitude and longitude is started impulsively from rest in the eastward direction.
Assuming the absence of friction and the pressure gradient force (apart from that
imparting hydrostatic balance in the vertical), the object subsequently experiences
⇀
no forces, and so moves with a constant zonal velocity u = u 0 0 . However, now
its rate of rotation has increased relative to that of the Earth, from to + u/R .
This increases the centrifugal force which appears to act in this frame of reference.
This alters the balance between the gravitational force and the centrifugal force, and
so from the frame of reference of the Earth, we will observe an additional, radially
outward, force.
We can express the total centrifugal force mathematically as follows:
u 2⇀
R
CFtotal = m +
R
⇀
= m2 R +
2mu ⇀
u2 ⇀
R + m 2 R
R
R
where the notation is consistent with Figure 4.6(b). Since R ≫ u for the wind speeds
we typical experience on the Earth, u2 /R2 ≪ 1, and we can neglect the final term
in the expression. The first term is simply the centrifugal force for a parcel at rest,
and can be incorporated into the gravity term to yield the effective gravity as before.
This leaves a remaining component of centrifugal force which is due to the motion
of the object. We can decompose this into components based on the geometry shown
in Figure 4.6, to give
⇀
⇀
CFmotion = −2mu sin j + 2mu cos k
(4.13)
So, in the inertial reference frame, an observer would conclude that the centripetal
force provided by effective gravity was not in balance with the vertical component
of the pressure gradient force, and would lead to additional relative motion of the
object causing the object to move upward and equatorward.
However, this centrifugal force is only observed while on the rotating frame
in the presence of zonal motion. Meridional and vertical motion relative to the
surface of the Earth do not give rise to an additional component of the centrifugal
70
FUNDAMENTAL FORCES
force, but these motions also give rise to apparent forces on our rotating frame of
reference on the Earth. We will now look at this response to meridional and vertical
motion.
Once again, consider an object which is moving at a constant northward velocity,
subject to no net forces. At a given moment, the object passes through a latitude
circle , and possesses an angular momentum given by
I = 21 mR2
where I is the moment of inertia of the object. As the object continues northward, it
will conserve its angular momentum even as it moves to a new latitude circle which,
being further north, is a smaller distance from the center of rotation. We can write
this conservation equation as
1
u
1
2
2
+
mR = m R + R
2
2
R + R
R2 = R2 + 2R R +
R2 u
R + R
(4.14)
where we have neglected second-order terms, and made the assumption that the
angular momentum will be balanced at the new location by an increase in the angular
velocity of the object. Solving this equation for u, further neglecting second-order
terms, and noting that R = −v sin t, we find
u = 2v sin t
Taking the limit as t → 0 we can write this as
du
= 2v sin
dt
(4.15)
Hence, the requirement for the conservation of angular momentum results in a zonal
acceleration of the object which is proportional to the meridional speed of the object.
Now, consider the conservation of angular momentum for an object which is
moving with a constant vertical velocity. In this case, the latitude is not changing,
but the distance to the center of rotation is changing due to the vertical motion. In
this case, we again solve for u in Equation (4.14) but note that R = w cos t.
Taking the limit as before, we find that
du
= −2w cos
dt
(4.16)
Finally, we can note that for an object moving with a constant zonal velocity there
is no change in angular momentum, since both the angular velocity of rotation, ,
and the distance to the center of rotation, R, are not changing.
71
FORCES IN A ROTATING REFERENCE FRAME
Hence, we have derived, using two different perspectives, the acceleration that
is apparent due to relative motion in a rotating frame of reference in the zonal,
meridional, and vertical directions. All of these accelerations are typically grouped
in a single apparent vector force known as the Coriolis force, which can be written
in total as
⇀
⇀
⇀
FCoriolis
= 2v sin − 2w cos i − 2u sin j + 2u cos k
m
(4.17)
We can, in fact, write this as the combination of two vectors, the wind vector and
⇀
⇀
⇀
the Earth’s rotation vector, which in components is = cos j + sin k (see
Figure 4.6). Using this, we can write
⇀
FCoriolis
⇀
= −2 × u
m
(4.18)
Example The Coriolis force per unit mass at any location can be easily calculated
using station reports. Consider our case study storm at 12 UTC on Friday 14 February
2003 (see Figure 4.2). In Limon, the wind report is 5 kts, from the north. The latitude
at Limon is 39 44′ N. From this, we find
wind speed = 5 kts = 26 m s−1
u = 0 v = −26
= 39 44′ = 3973
= 7292 × 10−5 s−1
⇀
⇀
FCoriolis
= 2v sin − 2w cos i − 2u sin j
m
⇀
+ 2u cos k
⇀
⇀
⇀
= 146 × 10−4 −26 × sin 3973 i + 0 j + 0k
⇀
⇀
⇀
= − 24 × 10−4 i + 0 j + 0k
The Coriolis force per unit mass is directed toward the east; that is, to the right of
the wind velocity. It is of a similar order of magnitude to the pressure gradient force
of 97 × 10−5 N kg−1 , which is directed toward the center of the low (calculated for
this location in Section 4.3.2).
In general, then, the Coriolis force does no work, but deflects the flow of air parcels
to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
This force cannot initiate motion.
72
FUNDAMENTAL FORCES
4.5 The Navier–Stokes equations
Forces cause acceleration. When there are no forces, or when all the forces cancel each
other, there is no acceleration. We have seen that the primary forces in the vertical
direction are gravity and the centrifugal force acting downward and the pressure
gradient force acting upward. There is also a small vertical component in the Coriolis
force. The forces acting in the horizontal direction are the pressure gradient force,
the viscous force, and the Coriolis force. We can use this information to construct
an equation that calculates the net force acting on a parcel of air, and hence its
acceleration. If we can perform this calculation at every point in the atmosphere,
then, in theory, we can determine the winds anywhere on the Earth. To construct the
equation, we simply sum all of the forces acting in each vector direction, and equate
this sum to the acceleration of the parcel. Note that forces affect fluid parcels, and
hence the acceleration is represented by the material derivative of wind with respect
to time (see Section 2.8):
2
1 p
u 2 u 2 u
+
+
+
+ 2v sin − 2w cos
x
x2 y2 z2
2
1 p
v 2 v 2 v
+
+
= −
+
− 2u sin
y
x2 y2 z2
2
1 p
w 2 w 2 w
+
+
= −g −
+
+ 2u cos
z
x2
y2
z2
Du
=
Dt
Dv
Dt
Dw
Dt
−
acceleration
of air parcel
pressure
gradient
gravity force
viscous force
(4.19)
Coriolis force
This set of equations is usually called the Navier–Stokes equations for the conservation of momentum. The equations are named after Claude Louis Navier (1785–1836)
and George Gabriel Stokes (1819–1903), who both contributed to the development
of the equations. Navier developed the form of these equations for an incompressible
(that is, constant density) fluid in 1821. In 1822, he published a further refinement
for viscous fluids, despite not having access to a physical theory for shear stress in a
fluid. Nevertheless, he arrived at the proper form for these equations. Twenty years
later, Stokes published papers on incompressible fluid motion, and then continued
his investigations by addressing the problem of internal friction in fluids in motion
from a more theoretical perspective.
We can write the set of equations in some alternative forms. First, since we expect
to be solving these equations in an Eulerian framework, it is often appropriate to
THE NAVIER–STOKES EQUATIONS
73
replace the material derivative with the local derivative. Based on the derivation in
Section 2.8, then, we can write
2
u
u
u
1 p
u 2 u 2 u
u
+
+
+u +v +w = −
+
t
x
y
z
x
x2 y2 z2
v
v
v
v
+u +v +w
t
x
y
z
w
w
w
w
+u
+v
+w
t
x
y
z
+ 2v sin − 2w cos
2
1 p
v 2 v 2 v
+
+
=−
+
− 2u sin
y
x2 y2 z2
2
1 p
w 2 w 2 w
= −g −
+
+ 2 + 2 + 2u cos
z
x2
y
z
Clearly, this is a very long-winded way of writing the equation set, and hence typically
the material derivative form is written.
In addition, we can write this set of equations as a single vector equation that solves
⇀
for Du/Dt, rather than three components. Such notation is much more efficient, and
allows the interpretation of the physical terms in the equation in a more straightforward way. However, for the full Navier–Stokes equations this form requires the use
of vector calculus operators, which is beyond the scope of this book. Hence, in this
case we will retain the component equations form.
4.5.1 Perturbation pressure
The variations in pressure that give rise to the pressure gradient force are quite small
compared to the background pressure field that is in balance with the effective gravity
force field. From the perspective of atmospheric motions, it is the pressure gradient
that is of primary importance. Hence, one way to alter the perspective provided by
the Navier–Stokes equations is to subtract a reference pressure field from the pressure
gradient force term.
Recall that the pressure distribution in the vertical for the atmosphere at rest can
be represented in part by the hydrostatic pressure, denoted p0 :
dp0
= −0 g
dz
For this application, we have also defined a corresponding reference density 0 .
This hydrostatic pressure is a function only of height, and so we can define a new
quantity by
p = p0 z + pd
where pd is called the perturbation pressure or dynamic pressure. It is generally true
in the study of atmospheric motion that most of the vertical pressure gradient is
balanced hydrostatically, and vertical motions result from very small departures from
that balance.
74
FUNDAMENTAL FORCES
Using this definition in the z component of the Navier–Stokes equations, we get
2
w 2 w 2 w
Dw
= −g − p0 z + pd +
+ 2 + 2 + 2u cos
Dt
z
x2
y
z
2
2
2
p
w w w
= −g + 0 g − d +
+ 2 + 2 + 2u cos
z
x2
y
z
2
2
− 0
1 pd
w w 2 w
Dw
+ 2 + 2 + 2u cos
=−
g−
+
Dt
z
x2
y
z
where now the equation is in terms of the perturbation pressure, and the effective
gravitational force is replaced by a buoyancy force.
It is important to note that the definition of reference pressure and density is not
unique. Hence, while the total force acting on a parcel of air is independent of the
form of the equation, the partition of this unique force field between the perturbation
pressure gradient and the buoyancy force is not unique.
For example, using the background hydrostatic pressure profile of the environment
as our reference pressure and density, we would conclude that the updrafts in severe
thunderstorms are negatively buoyant at the cloud base (that is, they cause sinking
motion) and do not become positively buoyant until some kilometers higher in the
cloud. In this perspective, the updraft is actually driven by a large perturbation
pressure gradient that compensates for the negative buoyancy. However, if we used
the thunderstorm rather than the environment as our reference profile for pressure
and density, we would conclude a forcing that consists of zero or positive buoyancy
and a smaller perturbation pressure gradient.
Review questions
⇀∗
4.1 Calculate the magnitude of g at:
(a) sea level at the equator and at the North Pole;
(b) at the equator at altitudes of 0, 10, 100, and 1000 km above sea level.
4.2 Using the surface weather map shown in Figure 1.7 calculate the magnitude of
the pressure gradient force between
(a) Minneapolis, Minnesota and Des Moines, Iowa
(b) Charleston, South Carolina and Atlanta, Georgia.
(c) How does the spacing of the sea level pressure isobars differ between these
cities and how does this spacing relate to the magnitude of the pressure
gradient force calculated for parts (a) and (b) of this question?
4.3 As a thunderstorm passes over an area, typically the air temperature will decrease
and the surface pressure will increase. The increase in pressure is a hydrostatic
REVIEW QUESTIONS
75
response to the changing temperature (and density) of the air. On 6 May 2005 a
weather station in Boulder, Colorado reported a decrease in temperature from 74 F
to 59 F (23 C to 15 C) and an increase in pressure from 826 hPa to 827.3 hPa as a
thunderstorm passed over the station. Assume that the cooling of the air was uniform
over some depth of the atmosphere. Calculate the depth of air that would need to be
cooled by the observed amount to produce the observed change in pressure.
4.4 (a) Write an equation for the geopotential height at a given pressure level for
an atmosphere in which the vertical temperature profile is given by T z =
T0 + z, where T0 is the temperature at an elevation of 0 m and = −dT /dz
is the lapse rate.
(b) Calculate the height of the 300 hPa pressure surface for = 6 C km−1 and
T0 = −30, 0, and 30 C.
4.5 At 00 UTC 15 Feb 2003, the radiosonde data from Davenport, Iowa indicated
a temperature of −29 C at 850 hPa and −163 C at 500 hPa. At the same time
the radiosonde at Little Rock, Arkansas indicated a temperature of 104 C at
850 hPa and −145 C at 500 hPa. Using this data, calculate the thickness of the
850 to 500 hPa layer at Davenport and at Little Rock. Discuss the difference in
thickness of this layer at these two cities, based on the information shown on the
surface weather map in Figure 1.7.
4.6 Isolines of 1000 to 500 hPa thickness are often drawn on surface weather maps
using a contour interval of 60 m. What is the corresponding layer mean temperature interval?
4.7 Consider the sounding shown in Figure 4.7, taken at McGrath, Alaska at 12 UTC
26 Aug 2002.
(a) Which curve is dew point temperature and which curve is temperature? Why?
(b) There are two significant temperature inversions. At what levels do they
begin?
(c) Calculate the thickness of the 600 to 300 hPa layer in this sounding.
4.8 Show that Equations (4.17) and (4.18) are equivalent.
4.9 Consider the map of sea level pressure shown in Figure 4.8, which shows a
low-pressure system that occurred in August 2000 in the Chukchi Sea north
of Alaska. This system caused record-breaking high winds along the Alaskan
north coast.
(a) Calculate the total horizontal pressure gradient force between the center of
the low and the center of the high-pressure system to its west, assuming an air
density of 1 kg m−3 . At this latitude, one degree of longitude is approximately
equivalent to 28 km.
76
FUNDAMENTAL FORCES
100 16200 m
13540 m
200 11650 m
10220 m
300 9060 m
400 7130 m
500 5540 m
600
700 3001 m
800
1456 m
900 762 m
114 m
–40
1
0.4
–30
2
–20
–10
10
0
10
16
20
24 320 g/kg
30
40
Figure 4.7 Radiosonde sounding from McGrath, AK on 12 UTC 22 Aug 2002
140 E
150 E
160 E 170 E
180 170 W 160 W 150 W 140 W
130 W
1016
1008
1000
1016
L
987
H
1020
992
1008
70 N
1016
H
1021
1008
1016
60 N
1008
1016
1024
Figure 4.8 Sea level pressure over Alaska during August 2000
REVIEW QUESTIONS
77
(b) A 1 kg parcel of air starts from rest at 175 W, 75 N. In what direction
would it be accelerated if it was under the influence alone of the above
pressure field? How many hours does it take for the parcel to achieve a
speed of 35 m s−1 ?
(c) Suppose the high-pressure system was displaced 50 km further to the west.
Without performing an additional calculation, would the parcel take more
or less time to reach this speed? Why?
4.10 (a) Use the surface weather observations from 00 UTC 17 Feb 2003 (Figure 1.11)
to calculate the horizontal components of the pressure gradient force and
Coriolis force at Davenport, Iowa. (Hint: use the sea level pressure observations from Chicago, Illinois (ORD), St. Louis, Missouri (STL), Des Moines,
Iowa (DSM), and Madison, Wisconsin (MSN) to calculate the pressure gradient force.)
(b) What additional force would be required to have no net acceleration of the
air at Davenport?
4.11 (a) While skiing in Colorado, at a latitude of 40 N, I become airborne after
jumping off the lip at the top of a ski run at a Colorado resort that faces north.
I commence my flight at 10 m s−1 traveling parallel to the 42 slope, and
make contact with the snow 50 m down the slope. Calculate in centimeters
to three significant figures my displacement due to the Coriolis force by the
time I touch the surface. Assume that the Coriolis force remains constant
during the flight at the initial value at lift-off.
(b) Without recalculating, what is the displacement if the speed is 20 m s−1 ? Is
the Coriolis force larger or smaller? Why?
5
Scale analysis
5.1 Dimensional homogeneity
Consider the following empirical relationship, used as a rule of thumb by ocean
weather forecasters to estimate the speed of travel of ocean waves, Cp , as a function
of the wavelength (the distance between wave crests), l:
√
Cp = 134 l
(5.1)
where l is measured in feet to yield a wave speed in knots. For this relationship to
be useful as a forecasting tool it is crucial to know the requirements for the units,
particularly in this case where they are not conventional SI units. A better formulation
would be to write
√
Cp = l
knots
where
= 134
where
= 038 m /2 s−1
1
feet /2
(5.2)
or in conventional SI units
√
Cp = l
1
(5.3)
The form of the relationship expressed in Equations (5.2) and (5.3) has the advantage
that it can be tested easily for the property of dimensional homogeneity; that is, that
all terms in an equation have the same dimensions. Such a property is a prerequisite
for any physical equation, so that the numerical equality does not depend on the units
of measurement as long as appropriate unit conversions are made.
In our example, we can see that wave speed has the dimensions, regardless
of
choice of units, of length [L] divided by time [T ], which can be denoted LT −1 .
In Equation (5.1), if we initially assume that the number 1.34 hasno units, then the
1
dimensions of the right hand side of the equation are simply L /2 , which is clearly
an unphysical result. If, on the other hand, we consider Equation (5.2) or (5.3), the
term on the right hand side has the dimensions
1
1
L /2 T −1 L /2 = LT −1
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
80
SCALE ANALYSIS
and so the equation is dimensionally homogeneous. This is a basic check which
can be applied to any equation, but is especially useful in the analysis of complex
equations that have no analytic solutions.
5.2 Scales
The atmosphere is a complex system capable of supporting many different types of
motion from small turbulent eddies between two buildings to large weather systems.
In order to make the theoretical study of a particular system easier, we can simplify
our approach by knowing which scales are important in the driving mechanisms of
that system. This allows us to determine whether we can ignore processes that are
happening at different scales. Time scale is important as is length scale.
How do we determine these scales? For a particular type of system, we measure
the magnitudes of the important variables, such as pressure, wind speed, and moisture
mixing ratio. From these we determine the amplitudes and typical distances over
which fluctuations occur. If we are looking at a wave-like phenomenon, we can
measure variables like frequency and wavelength. From a series of measurements of
many different examples of the same type of system, we can determine typical values
for many of these quantities, and hence determine the appropriate scale. Some scales
of some common phenomena are shown in Table 5.1.
Note that motion with a small spatial scale tends to have a short time scale, and
vice versa. This is often, although not always, true.
5.3 Non-dimensional parameters
If the governing equation for a particular situation is known, the principal of dimensional homogeneity can be used to derive useful non-dimensional parameters. For
Table 5.1 Scales for some typical atmospheric phenomena. A planetary wave is a type of wave
in the atmosphere that encircles the entire Earth (see Chapter 8). A foehn or a chinook is a strong
wind that is created when air flows downhill from a high elevation (see Section 13.3) – such winds
often have names associated with a particular mountain range
Type of motion
Cold front
Tornado
Mid-latitude weather system
Cumulus cloud
Surf
Planetary wave
European Alps foehn or Rocky
Mountain chinook
Horizontal length scale
50 km
100 m
1000 km
1 km
10 m
10 000 km
10 km
Time scale
1–2 days
Minutes
Several days
Tens of minutes
Seconds
Weeks–months
Hours
NON-DIMENSIONAL PARAMETERS
81
example, consider the x component of the Navier–Stokes equations, which are
applicable to any general atmospheric flow:
2
u
u
u
1 pd
u 2 u 2 u
u
+
+
+u +v +w =−
+
t
x
y
z
x
x2 y2 z2
+ 2 v sin − 2 w cos
Since this physical equation is dimensionally homogeneous, all of the terms will have
the same dimensions, which we can derive most easily from the first term:
−1
LT
L
U
=
=
T2
T
T
where [U ] is the dimension for wind
Thus, we can expect that all terms in the
speed.
equation will have the dimension L/T 2 . A corollary of this is that the ratio of any
two terms in the equation will be a non-dimensional number. Further, the magnitude
of this number, based on the scales of the system of interest, can tell us something
about the relative contributions of the two terms to the equation for that type of
system. For example, let us consider the specific case of the ratio of the acceleration
of the wind (often called the inertial term) and the Coriolis term. We can write this
ratio as follows:
u/t
2 v sin − 2 w cos
∼
L/T 2
2 L/T
where the relationship ∼ may be interpreted as ‘scales as’, and L T , and denote
the orders of magnitude of the scales of motion. These are to be distinguished from
the dimensions, which are written L T, and [ ]. Note that the trigonometric
quantities have no dimension and an order of magnitude of 1. It is often the case
that it is easier to measure the wind speed than to measure a time scale associated
with the wind speed. Hence, it is more common to express such ratios in terms of
the scales U and L, rather than L and T . So, we note that T has dimension L/U
and write the ratio as
u/t
2 v sin − 2 w cos
∼
L/T 2
2 L/T
1
2 T
U
Ro ∼
2 L
∼
(5.4)
This ratio is known as the Rossby number Ro, named after Carl-Gustav Rossby
(1898–1957), a scientist who made many important and fundamental contributions
to the study of the atmosphere and ocean dynamics. When Ro is evaluated for a
particular type of system it can tell us the importance of the Coriolis force in the
development of the wind field.
82
SCALE ANALYSIS
Example Consider our case study storm at 12 UTC on Friday, 14 February 2003
(see Figure 4.2). In Limon, the wind report is 5 kts or 257 m s−1 , from the north.
We estimate that the size of the system is around 1000 km across, and we know that
= 7292 × 10−5 s−1 . In order to determine the Rossby number for this situation,
we only need orders of magnitude for each of the dimensions in the expression, and
we can see from a comparison to Table 5.1 that this system is similar to the typical
scales for mid-latitude weather systems, as we might expect. Hence we can write
L ∼ 106 m
T ∼ several days ∼ 105 s
⇒ U ∼ 10 m s−1
Such a system is called a synoptic scale system. We can also write
2
∼ 10−4
The Rossby number is then
Ro ∼
U
2 L
10
∼ −4
10 × 106
Ro ∼ 01
Since the Rossby number is small (less than 1), this tells us that the Coriolis force,
arising from the rotation of the frame of reference, is large compared to the acceleration of the wind, and hence must be important in the development of the flow
around this system at this time. This is consistent with the result in the example in
Section 4.4.2, which found that the Coriolis force was of the same order of magnitude
as the pressure gradient force for this situation. However, our calculation here was
much simpler to perform.
Let us consider some other illustrations of Rossby numbers. For example, the large,
long-lived planetary waves that encircle the globe (Table 5.1) yield the following:
L ∼ 107 m
T ∼ weeks to months ∼ 106 s
⇒ U ∼ 10 m s−1
Ro ∼ 10−2
NON-DIMENSIONAL PARAMETERS
83
This means that, even though the wind speed in a planetary wave is comparable to
that in a mid-latitude weather system, the Coriolis force is comparatively even more
important in the force balance that governs the planetary wave.
A foehn, a strong mountain lee wind, has very different scales:
L ∼ 104 m
T ∼ hour ∼ 103 s
⇒ U ∼ 10 m s−1
Ro ∼ 10
In this case, the wind speed scale is again 10 m s−1 , but because of the smaller spatial
scales, we see that the Coriolis force is not particularly important in the generation
of such winds.
Finally, let us consider the role of the Coriolis force in the flow of water out
of a bath:
L ∼ 1m
T ∼ 10 s
⇒ U ∼ 10−1 m s−1
Ro ∼ 103
So we see that the Coriolis force has an extremely small influence on the flow of water
as it leaves the bath. This means that the hemisphere in which the bath is situated,
which determines the direction of deflection induced by the Coriolis force, will have
a negligible influence on the sense of rotation observed in the water flow. This is in
contrast to mid-latitude weather systems, which have distinctive flows depending on
the hemisphere – low-pressure systems rotate in the clockwise sense in the Southern
Hemisphere and the counterclockwise sense in the Northern Hemisphere.
Other non-dimensional numbers that we may use in the course of our study include
Reynolds number: Re ∼
∼
Froude number: Fr ∼
inertial
u/t
∼
viscous
2 u/x2 + 2 u/y2 + 2 u/z2
U 2 /L
UL
∼
U/L2
inertial
U 2 /L U 2
u/t
∼
∼
∼
gravity
g
g
gL
Thus, non-dimensional parameters like the Rossby, Reynolds, and Froude numbers
allow an efficient measure of the relative importance of various terms in the dynamical
equations, and aid the process of approximation and simplification. These parameters
are also linked to the concept of dynamic similarity, in which flows at different spatial
84
SCALE ANALYSIS
scales or with different fluid properties can be determined to be dynamically similar
if all of their relevant non-dimensional parameters have the same magnitude. In such
a situation, conclusions about one type of flow can be extended to other dynamically
similar flows. Such a concept is indispensable when designing laboratory models that
are used for understanding the behavior of atmospheric phenomena such as tornadoes.
5.4 Scale analysis
The process of simplifying the equations of motion using typical scales for the
phenomenon of interest is called scale analysis. As an illustration of the technique,
we will perform a scale analysis of the Navier–Stokes equations for a typical midlatitude system. However, first we will make one prior simplification of the equation
by examining the properties of the Coriolis force term.
5.4.1 Coriolis parameter
Recall the form of the Coriolis force (Equation (4.17)):
FCoriolis
= 2 v sin − 2 w cos
m
⇀
⇀
⇀
i − 2 u sin j + 2 u cos k
We will define the Coriolis parameter, f , which arises from the fact that the important
effects of the Earth’s rotation in the middle latitudes arise mainly from the local
⇀
vertical component of the rotation vector • k = sin . We will see this is true
once we perform a scale analysis (Section 5.4.2). Hence, we define
f = 2 sin
(5.5)
For scaling purposes, we use the value of f at 45 latitude:
f0 = 2 sin 45 = 103 × 10−4
(5.6)
Thus, the Rossby number (Equation (5.4)) is typically written
Ro ∼
U
f0 L
(5.7)
which does not change the analyses we performed in Section 5.3.
5.4.2 Scale analysis for a mid-latitude weather system
The Navier–Stokes equations are highly nonlinear, and cannot be solved analytically
without considerable simplification. A nonlinear differential equation is one that
includes products of terms involving the dependent variable – in this case, the
85
SCALE ANALYSIS
nonlinear terms are the advection terms, uu/x and vu/y for example. In nonlinear
systems, different solutions cannot be superposed (added together) to form new
solutions. This makes solving the equations much more difficult than linear systems.
Hence, our approach in this book will be to simplify the equations in order to
solve them.
Since much of the significant weather in middle latitudes is associated with cyclones
and, to a lesser extent, anticyclones, we will derive a simpler form of the equation
specifically for these systems. Typical values for the appropriate scales are shown in
Table 5.2.
Then, the process of scale analysis is conducted as follows:
u
u
u
+ u +v
t
x
y
v
v
v
+ u +v
y-eqn
t
x
y
U2
U2
scale
L
L
magnitude 10−4
10−4
x-eqn
2
u 2 u
u
1 pd
2 u
+
=−
+
+ 2 +2 v sin
2
2
z
x
y
z
x2
v 2 v
v
1 pd
2 v
+
+w
=−
+
+
−2 u sin
z
y
x2 y2
z2
UW
p
U
U
f0 U
H
L
L2
H2
10−5
10−3
10−16
10−12
10−3
+w
−2 w cos
≈ f0 W
10−6
Based on these magnitudes, we can see that the viscous term makes a very small
contribution to the force balance experienced in a typical mid-latitude cyclone. So
we can ignore that term with very little impact on the precision of our results. In fact,
to a good degree of accuracy, we can also discard the Coriolis deflection caused by
the vertical motion in the system 2 w cos , and the vertical advection term wu/z.
Hence, we can rewrite the horizontal component of the Navier–Stokes equations in
an approximate form:
u
1 pd
u
u
+ 2 v sin
+u +v = −
t
x
y
x
v
v
v
1 pd
+u +v = −
− 2 u sin
t
x
y
y
Table 5.2 Typical scales for a mid-latitude weather system
Scale
Horizontal wind scale
Vertical wind scale
Horizontal length scale
Vertical length scale
(depth of troposphere)
Time scale (L/U )
Kinematic viscosity
Dynamic pressure scale
Total pressure scale
Gravity
Density variation scale
Symbol
Magnitude
U
W
L
H
10 m s−1
10−2 m s−1
106 m
104 m
T
p/
P/
g
/
105 s
10−5 m2 s−1
103 m2 s−2
105 m2 s−2
10 m s−2
10−2
(5.8)
86
SCALE ANALYSIS
We can⇀ write
this in a shorter vector form if we define the horizontal wind to be
⇀
⇀
uh = u i + v j and the horizontal material derivative to be
Dh
= +u +v
Dt
t
x
y
Then, we can write
1 pd
Dh u
=−
+ 2 v sin
Dt
x
1 pd
Dh v
=−
− 2 u sin
Dt
y
⇀
⇀
Dh uh
1 pd ⇀ pd ⇀
⇀
i+
j − 2 × uh
⇒
=−
Dt
x
y
or
⇀
Dh uh
1
=−
Dt
⇀
pd ⇀ pd ⇀
⇀
j − f k × uh
i+
x
y
(5.9)
Turning to the vertical momentum equation, and using the total pressure form:
2
w 2 w
w
w
1 p
2 w
w
w
+w
=−
+
+u
+v
+ 2 + 2 −g +2 u cos
2
t
x
y
z
z
x
y
z
UW
UW
W2
P
W
W
g
f0 U
scale
L
L
H
H
L2
H2
magnitude 10−7
10−7
10−8
10
10−19
10−15 10
10−3
z-eqn
Thus, the atmosphere in motion is strongly hydrostatic on the synoptic scale, since
to an excellent degree of accuracy, we can write
1 p
= −g
z
Are the disturbances themselves hydrostatic? We find that if we do the same scale
analysis using the dynamic pressure and the buoyancy term, there is a similar balance:
2
w
w 2 w
w
w
1 pd
2 w
− 0
w
+u
+v
+
g +2 u cos
+w
=−
+
+ 2 −
2
2
t
x
y
z
z
x
y
z
2
UW
UW
W
p
W
W
g
scale
f0 U
L
L
H
H
L2
H2
−7
−7
−8
−1
−19
−15
−1
10
10
10
10
10
10
10−3
magnitude 10
z-eqn
So the answer is yes: synoptic scale disturbances themselves are strongly hydrostatic.
This means that vertical velocities in such systems tend to be relatively small and are
excited by departures from hydrostatic balance. One must always be careful to distinguish between the hydrostatic equation for an atmosphere at rest (Section 4.3.4) and
the hydrostatic approximation for synoptic scale atmospheric motions derived here.
THE GEOSTROPHIC APPROXIMATION
87
An important attribute of scale analysis is that in the process of deriving approximate equations for the system of interest, the level of accuracy is immediately
quantified. Hence, one can choose different levels of approximation depending on the
application. This choice may depend, for example, on the accuracy of the observations
to be used, or on the need to retain certain terms in the equation.
5.5 The geostrophic approximation
In Section 5.3, we saw that the ratio of the inertial term and the Coriolis term in the
Navier–Stokes equations is characterized by the Rossby number. We will now look
at flows in which this ratio is very small. All flows in which Ro → 0 are termed
geostrophic; however, the definition of ‘smallness’ may vary with application. In
our case we will assume that a 10% error is acceptable, and hence the case of a
mid-latitude cyclone satisfies this criterion:
Ro = U /f0 L = 10−1
Then the horizontal momentum Equation (5.8) reduces to
1 pd
+ 2 v sin
x
1 pd
−
− 2 u sin
y
−
=0
(5.10)
=0
Since time does not appear in this set of equations, this is a diagnostic relationship
which cannot predict the evolution of the velocity field. We call this set of equations
the geostrophic approximation for mid-latitude weather systems. In fact, for any flow,
it is possible to define a horizontal velocity field called the geostrophic wind, which
is a component of the total horizontal velocity field that satisfies exactly the force
balance between the pressure gradient force and the Coriolis force. Such a geostrophic
wind can be written, using Equation (5.5), as
1 pd
f y
1 pd
vg =
f x
ug = −
(5.11)
We can see the effect of this force balance in the presence of a simple meridional
pressure gradient in Figure 5.1. It is clear from this diagram that the geostrophic
wind must increase in magnitude in the presence of an increased pressure gradient,
but that the Coriolis force will then increase concomitantly and thus maintain the
geostrophic force balance. If such an increase occurs in time, of course it is not the
geostrophic wind that can accelerate, since the equation governing this wind allows
no time dependence.
88
SCALE ANALYSIS
(a) Northern Hemisphere: Ω, f >0
(b) Southern Hemisphere: Ω, f <0
isobar
low pressure
pressure gradient force: – 1 dpd >0
ρ dy
isobar
low pressure
pressure gradient force: – 1 dpd >0
ρ dy
ug
ug
Coriolis force: –2Ωugsinφ = –fug <0
Coriolis force: –2Ωugsinφ = –fug <0
isobar
high pressure
y
isobar
high pressure
x
Figure 5.1 Geostrophic force balance and corresponding zonal wind in the case of a simple
meridional pressure gradient for (a) the Northern and (b) the Southern Hemisphere
08
COR
10
1004
10
08
Example Recall the example explored in Sections 4.3.2 and 4.4.2, concerning the
calculation of the pressure gradient force and Coriolis force in the vicinity of our case
study low-pressure system. The map is reproduced in Figure 5.2 with the calculated
forces illustrated by arrows. In this case, the pressure gradient force between Dodge
City and Limon is directed from the high to the low pressure with a magnitude of
97 × 10−5 N kg−1 . The horizontal Coriolis force at Limon is directed to the right of
the wind velocity vector at Limon with a magnitude of 24 × 10−4 N kg−1 . Hence,
we can see that while there is a very approximate balance in the magnitudes of
the forces, the directions of the forces do not suggest a strict geostrophic balance.
009
PGF
1000
10
04
003
100
8
1008
Figure 5.2 Sea level pressure over Oklahoma and surrounding states on 14 February 2003 at 12
UTC, showing the pressure and wind reports from Denver and Dodge City, and the geostrophic force
balance
89
THE GEOSTROPHIC APPROXIMATION
In part, this is due to the fact that we are using a wind field very close to the
surface, where it is not accurate to assume that viscous forces are small (Chapter 10
discusses this in more detail). In the presence of viscous forces, surface winds
are typically smaller in magnitude and changed in direction toward low pressure
(typically around 20 ) compared to the geostrophic wind. Further, if the system
in question is propagating, as this one is to the east, there will be a component
of wind in the direction of movement. Hence, in this real application it is clear
that only a component of the wind field can be described as being in geostrophic
balance.
Despite these shortcomings of the geostrophic approximation, the resulting force
balance does explain an important attribute of flow around cyclones in the real
atmosphere: that the wind flows in a generally counterclockwise direction in the
Northern Hemisphere and a clockwise direction in the Southern Hemisphere. This
direction is called cyclonic so that the hemisphere need not be specified. Conversely,
the wind flow tends to be clockwise around an anticyclone in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, and such a flow is termed
anticyclonic.
The geostrophic approximation also helps to explain why weather systems move
(most of the time) from west to east. Consider two columns of air – one in a polar air
mass and the other in a maritime tropical air mass (Section 1.3.1). Assuming the same
surface pressure, the pressure at a given altitude in the cold polar air mass will be
less than that in the warm tropical column, because the thickness of the warm column
is greater than that of the cold column (Figure 5.3). This creates a net horizontal
pressure gradient force toward the cold air mass at this altitude, even though there
is no net pressure gradient force at the surface. Then, regardless of whether we
are considering the Northern Hemisphere (where the cold air mass will be to the
north) or the Southern Hemisphere (where it will be to the south), a geostrophic
(a)
90°
(b)
Warm
L
PGF
H
700 hPa
850 hPa
Vg
180°– Warm
NORTH
POLE
LO
Cold
Vg
Vg
Warm 0°
Vg
1000 hPa
Warm
90°
Figure 5.3 Geostrophic flow between a warm air mass and a cold air mass (a) leads to a westerly
flow around the pole (b)
90
SCALE ANALYSIS
wind will be induced from the west. When we consider the entire polar region,
rather than just one column, we see that the result is a large low-pressure area at
some altitude above the surface, centered near the pole, with cyclonic westerly flow
encircling it. This is the polar vortex and it is associated with a prevailing westerly
flow. This upper level flow acts to steer disturbances, as we will see in upcoming
chapters.
Example Calculate the thickness of the 1000 hPa to 700 hPa layer of the atmosphere
in a polar air mass and in a maritime tropical air mass. Assume that the temperature
in the polar air mass is −20 C and that the temperature in the maritime tropical air
mass is 20 C.
We can use Equation (4.12) to calculate the thickness. As always, we must convert
the temperature units to K.
For the polar air mass we find
R
Z = − ∗d
g0
p
Td ln p
p0
−287
=
981
70 000
253d ln p
100 000
70 000
−287
× 253 × ln
=
981
100 000
= 2640 m
And for the tropical air mass we find
R
Z = − ∗d
g0
p
Td ln p
p0
−287
=
981
70 000
293d ln p
100 000
70 000
−287
× 293 × ln
=
981
100 000
= 3057 m
Assuming that the 1000 hPa surface does not change elevation between the polar
and tropical air masses, which is consistent with the surface pressure being the
same in both locations, we find that the 700 hPa surface decreases in elevation by
417 m between the tropical and polar air masses. Assuming that these air masses are
REVIEW QUESTIONS
91
located at approximately 30 N and 60 N (or 3000 km apart) this gives a slope of
417 m/3 × 106 m = 139 × 10−4 .
Review questions
5.1 Verify that the Navier–Stokes equations (4.19) are dimensionally homogeneous.
5.2 Calculate the Rossby number for each type of motion listed in Table 5.1. You
may assume a mid-latitude location for your calculations.
5.3 When cold air overlies a sloped ground surface (such as in the mountains) the
cold air will flow down the slope under the influence of gravity. This downslope
flow is known as a katabatic wind. Katabatic winds are observed over the world’s
great ice sheets in Greenland and Antarctica, as well as throughout smaller
mountain ranges. Over the large ice sheets, the katabatic wind speed is of the
order of 10 m s−1 and the horizontal scale of the flow is of the order of 1000 km.
Over individual mountains katabatic wind speeds are of the order of 1 m s−1 and
have a horizontal scale of 1 km.
(a) Calculate the Rossby number for both types of katabatic flow.
(b) Discuss the implications of the difference in Rossby number for the forces
of relevance in each flow.
5.4 (a) What is the range of values for the Reynolds number for the types of motion
listed in Table 5.1?
(b) What does this imply about the role of the viscous force for these types
of motion?
(c) Under what conditions would the viscous force be important?
5.5 What physical conditions would result in a large Froude number and a small
Froude number?
5.6 Assume that the scales given in Table 5.1 for mid-latitude weather systems are
appropriate for tropical and polar weather systems as well.
(a) Calculate the Rossby number at latitudes 0 , 10 , 30 , 45 , and 90 .
(b) Based on your results for part (a), where can the geostrophic approximation
be applied and with what accuracy?
5.7 For mid-latitude synoptic scale systems, what terms can be neglected in the
horizontal momentum equation to give an accuracy of:
(a) 10%
(b) 1%
(c) 0.1%?
92
SCALE ANALYSIS
5.8 For mid-latitude synoptic scale systems, what terms can be neglected in the
vertical momentum equation to give an accuracy of:
(a) 10%
(b) 1%
(c) 0.1%?
5.9 (a) Estimate the geostrophic wind speed at Little Rock, Arkansas at 00 UTC
15 Feb 2003 at the surface, 700 mb, 500 mb, and 300 mb levels using the
station reports on the weather maps provided on the CD-ROM.
(b) How does the geostrophic wind speed compare to the observed winds at
each of these levels?
5.10 Calculate the geostrophic wind at an altitude of 9.5 km and a latitude of 40 N for:
(a) Winter conditions with a surface temperature of 20 C at 30 N and −10 C
at 50 N.
(b) Summer conditions with a surface temperature of 30 C at 30 N and 20 C
at 50 N.
You may assume that the pressure at sea level is 1000 hPa and that the lapse
rate is 6 C km−1 .
6
Simple steady motion
6.1 Natural coordinate system
Typically, we use the latitude–longitude coordinate system that we introduced in
Section 2.4 when describing atmospheric motions. However, for some classes of
atmospheric flow it is more convenient to use another coordinate system. There are
many choices of coordinate system on the Earth, including systems that use pressure
instead of height as a vertical coordinate, and spherical coordinate systems that
assume the Earth is a perfect sphere. In this chapter, we will make use of the natural
coordinate system, which is a coordinate system in which one axis is everywhere
⇀
tangent to the horizontal wind, represented by the unit vector , and a second axis is
⇀
normal to and to the left of the wind, represented by unit vector (Figure 6.1). We
assume that the flow has a vertical component that is negligible or zero, and hence
⇀
retain unit vector k as our vertical axis. This coordinate system has the potential to
change in orientation with both time and space, but it follows that for all times and
⇀
locations the wind field vector has a single component, directed in the direction.
⇀
⇀
sn z as our coordinate locations,
We denote this wind field V = V , and define
⇀ ⇀ ⇀
which correspond to locations on the axes k .
In order to write our Navier–Stokes equations in this coordinate system, we must
first determine the form of the material derivative in this coordinate system, which
means we must determine the form of
⇀
⇀
D
DV ⇀ DV
=
+V
Dt
Dt
Dt
Based on the geometry shown in Figure 6.2, we can write
⇀
⇀
s
=
=
and =
R
s
⇀
⇒ =
R
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
94
SIMPLE STEADY MOTION
τ
η
η
η
τ
τ
j
i
⇀ ⇀ ⇀
Figure 6.1 Natural coordinate system k in a hypothetical wind field. The wind field every⇀
⇀
where is, by definition, V = V
δτ
τ + δτ
δψ
δs
δψ
τ
R
η
Figure 6.2 Change in orientation of the natural coordinate axes following a circular path with
radius R. Adapted from Holton (1992)
⇀
⇀
Since, as s → 0, is parallel to ,
⇀
⇀
d
=
ds
R
⇀
⇀
D Ds
D
=
Dt
Ds Dt
⇀
= V
R
⇀
DV ⇀ DV ⇀ V 2
⇒
=
+
Dt
Dt
R
BALANCED FLOW
95
Hence the acceleration of the parcel in this coordinate system is made up of two
components, the rate of change of the parcel speed along the path of the flow, and the
centripetal acceleration of the parcel if it follows a curved path. For this equation R
⇀
will be positive when the center of curvature is in the positive direction (to the left
⇀
of the wind direction) and negative when the center of curvature is in the negative
direction (to the right of the wind direction).
Now we must consider the other terms in the Navier–Stokes equations. We know
that the Coriolis force always acts normal to the direction of motion, and to the right
⇀
in the Northern Hemisphere. Hence, it can be expressed simply as −fV , since f is
positive in the Northern Hemisphere. In the Southern Hemisphere f is negative (due
to the fact that is negative in the Southern Hemisphere). This results in a Coriolis
⇀
term (−fV that is positive in the Southern Hemisphere and directed to left of the
wind as expected. The pressure gradient force, in general, will have components in
⇀
⇀
the and the directions, and hence can be written as
pd ⇀
pd ⇀
1
+
−
s
n
We can also express the viscous force in the same way. However, for this chapter
we will assume that we are considering motions that are far enough from the surface
of the Earth that friction plays little role. The form of the Navier–Stokes equations
that omits viscous forces is often called the Euler equation. In the natural coordinate
system then, the horizontal momentum equations are
s component:
1 pd
DV
=−
Dt
s
n component:
V2
1 pd
= −fV −
R
n
(6.1)
Since we have assumed a priori that there is no vertical motion, there is no vertical
momentum equation and a simple hydrostatic balance pertains.
6.2 Balanced flow
A special case of purely horizontal frictionless flows, such as those governed by
Equation (6.1), is known as balanced flow, which has the further restriction of
being steady state – that is, all time derivatives are zero. It should be apparent that
the geostrophic flow derived through scaling in the previous chapter is a type of
balanced flow, since it is horizontal, does not change in time, is hydrostatic in the
vertical, and experiences no viscous forces. With the assumption of steady state flow,
Equation (6.1) reduces to
s component:
DV
1 pd
=−
=0
Dt
s
96
SIMPLE STEADY MOTION
n component:
1 pd
V2
= −fV −
R
n
This necessarily implies that all flows must be parallel to the isobars, since
pd / s = 0; that is, there is no change in pressure along the direction of flow. The
single equation governing all balanced flows is then simply
1 pd
V2
+ fV = −
R
n
(6.2)
If we know something about the atmospheric system of interest, we can make this
equation even simpler. We consider all the possibilities below.
6.2.1 Inertial oscillations
Consider a flow in which there is no significant pressure gradient present, or alternatively, there is a small pressure gradient that is balanced by viscous forces. In either
case, such a flow can be treated as though there are no external forces acting on the
air parcels, and they are simply under the influence of the rotating frame of reference.
Thus, this kind of flow is as fundamental as straight motion at uniform velocity in
a stationary frame of reference. Such flows, called inertial oscillations, are regularly
observed in the lower atmosphere above the influence of the surface where frictional
forces are significant.
The equation that governs inertial oscillations is then
V2
+ fV = 0
R
(6.3)
which allows us to solve for the speed of the air parcels as a function of the radius
of curvature R and the latitude:
V = −fR
The period of the oscillation is then
2 R 2
2
=
T =
=
V
f
2 sin
=
1 day
2 sin
(6.4)
since the Earth undergoes one rotation (2 ) per day. Hence, there is a characteristic
period which can be identified in observations, an example of which is shown in
Figure 6.3. The atmospheric motion was measured using a radar wind profiler, and is
shown in the form of a hodograph, which plots u velocity on the x axis and v velocity
on the y axis, as a function, in this case, of time. Such a graph would show a pure
inertial oscillation as a perfect circle. The latitude of Whitewater is around 38 N,
and hence the inertial period (time to complete one revolution) is around 19.5 hours.
While a complete inertial circle was not observed, the oscillation in this observational
period lasted around 17 hours.
97
BALANCED FLOW
20
18
v [ms–1]
16
2200
14
0300
12
0800
1700
10
8
6
–4
–2
0
2
4
u [ms–1]
6
8
10
Figure 6.3 Hodograph showing an inertial oscillation of amplitude 419 m s−1 observed at Whitewater, Kansas (around 38 N) at an elevation of 192 m. The hours shown are local time. Reprinted
from Dynamics of Atmospheres and Oceans, 33, W. Blumen and J. K. Lundquist, ‘Spin-up and Spindown in rotating fluid exhibiting inertial oscillations and frontogenesis, 219–237, © 2001, with
permission from Elsevier
6.2.2 Cyclostrophic flow
Consider now a flow where the horizontal scale is sufficiently small that we can
neglect the Coriolis force. That is, we are considering flows in which the Rossby
number is large. In fact, we can determine the Rossby number for such a flow by
calculating the ratio of the first two terms in Equation (6.2):
2
V
V
Ro = fV =
R
fR
A large cyclostrophic Rossby number implies a situation characterized by high wind
speeds and tight rotation, such as a tornado or the even smaller phenomenon, a
whirlwind (also known as a dust devil or willy willy). In this case, Equation (6.2)
becomes
1 pd
V2
=−
R
n
(6.5)
and hence we expect a force balance between the centrifugal force and the pressure
gradient force. Theoretically, there is no limit to the strength of a system governed
by this force balance, since no matter how strong the pressure gradient, the wind can
increase to maintain the balance. In reality, however, frictional forces will tend to
limit the intensity.
Example The most damaging tornado recorded in the United States was the socalled ‘Tri-State Tornado’ of 18 March 1925. It passed from Missouri to Illinois
98
SIMPLE STEADY MOTION
and thence to Indiana, causing the deaths of 695 people in just 4 hours. The tornado
traveled at an estimated speed of 326 m s−1 , which is still considered a record. If
it was a single tornado rather than a family of tornadoes, it would have been about
1200 m wide, based on the width of the path of destruction it left. It measured F5
on the Fujita–Pearson scale,1 which is a scale used to rate tornado intensity based on
the type of damage observed. An F5 tornado is estimated to have wind speeds in the
range 117–142 m s−1 (or 420–512 km h−1 . Using this information, we can estimate
a cyclostrophic Rossby number for this tornado:
R ∼ 600 m
V ∼ 130 m s−1
f ∼ 10−4 s−1
⇒ Ro ∼ 22 × 103
This is certainly large enough to justify the neglect of the Coriolis term. The pressure
gradient required to balance the estimated centrifugal force would be very large
indeed:
V2
pd
=−
n
R
1302
∼ −1 ×
600
∼ −28 Pa m−1
Given the difficulty of measuring the pressure gradient inside of a tornado such a large
horizontal pressure gradient has never been observed in the atmosphere. Assuming
that the pressure at the edge of the tornado was approximately 1000 hPa, this pressure
gradient implies a central pressure in the tornado of 832 hPa! However, such a large
pressure gradient is probably an overestimate, even for such an unusual event.
6.2.3 The geostrophic approximation
Now we consider the case where the Rossby number is very small. In that case,
the ratio of the first two terms in Equation (6.2) suggests that we can neglect
the centrifugal term in favor of the Coriolis term, and the governing equation
reduces to
1 pd
fV = −
(6.6)
n
This is simply geostrophic balance (Section 5.5) expressed in the natural coordinate
system.
1
Named after T. Theodore Fujita (1920–1998), a renowned tornado researcher at the University of
Chicago, and Allen Pearson, director (now retired) of the National Severe Storms Forecast Center.
99
BALANCED FLOW
6.2.4 The gradient wind approximation
In situations where the Rossby number is close to 1, we must consider the three-way
balance between Coriolis, centrifugal, and pressure gradient forces. Like geostrophic
flow, it is always possible to define a component of the observed flow that satisfies
Equation (6.2) – such a flow is called the gradient wind. We can determine the
gradient wind in any situation by solving the quadratic equation for V :
1 pd
V2
+ fV +
=0
R
n
1
2 2
fR
R pd 2
f R
⇒V =−
±
−
2
4
n
This can also be written in terms of the geostrophic wind (using Equation (6.6) and
denoting the geostrophic wind as Vg :
1
2 2
2
f R
fR
V =−
±
+ fRVg
2
4
(6.7)
This equation clearly has several possible solutions depending on the signs of the
radius of curvature and the pressure gradient, and the chosen sign of the root.
However, we can use physical requirements to narrow the range of solutions. First,
we can note that both the gradient wind and the geostrophic wind in the natural
coordinate system must be positive definite. Tables 6.1 and 6.2 list the sign and
magnitude for selected terms in the gradient wind equation for Northern and Southern
Hemisphere flows respectively, for clockwise (CW) and counterclockwise (CCW)
flow around areas of low (L) and high (H) pressure. From these tables we can find
all of the possible physical solutions for this equation. Second, we can perform
a Taylor series expansion on the expression given in Equation (6.7) – only those
solutions in which V → Vg as the radius of curvature increases indefinitely have any
physical meaning. We find that the solutions which conform to this requirement in
the Northern Hemisphere are
1
2 2
R pd 2
f R
fR
+
−
cyclonic (CCW) flow around L R > 0 V = −
2
4
n
1
2 2
fR
R pd 2
f R
anticyclonic (CW) flow around H R < 0 V = −
−
−
2
4
n
and in the Southern Hemisphere are
1
2 2
fR
R pd 2
f R
+
−
cyclonic (CW)flow around L R < 0 V = −
2
4
n
1
2 2
fR
R pd 2
f R
anticyclonic (CCW) flow around H R > 0 V = −
−
−
2
4
n
100
SIMPLE STEADY MOTION
Table 6.1 Sign and magnitude of terms in gradient wind equation for all possible flow
regimes in the Northern Hemisphere
Term
f
R
p
n
1
2 2
R p 2
f R
−
4
n
fR
2
V positive for:
−
Northern Hemisphere
Cyclonic
(CCW) flow
around L
Anticyclonic
(CW) flow
around H
Anticyclonic
(CW) flow
around L
Cyclonic (CCW)
flow around H
+
+
+
−
+
−
+
+
−
−
+
+
Always >
fR
2
fR
or
2
imaginary for
f 2 R2
R p
<
4
n
<
Always >
fR
2
fR
or imaginary
2
2 2
f R
R p
for
<
4
n
<
−
+
+
−
+ root only
Either root but
R p
f 2 R2
>
4
n
+ root only
never +
Table 6.2 Sign and magnitude of terms in gradient wind equation for all possible flow
regimes in the Southern Hemisphere
Term
f
R
p
n
1
2 2
R p 2
f R
−
4
n
fR
2
V positive for:
−
Southern Hemisphere
Anticyclonic
(CCW) flow
around L
Cyclonic
(CW) flow
around H
Cyclonic
(CW) flow
around L
−
+
−
−
−
−
−
+
−
−
+
+
Always >
fR
2
fR
or
2
imaginary for
f 2 R2
R p
<
4
n
<
Always >
fR
2
+
−
−
+ root only
never +
+ root only
Anticyclonic
(CCW) flow
around H
fR
or imaginary
2
2 2
f R
R p
<
for
4
n
<
+
Either root but
R p
f 2 R2
>
4
n
101
BALANCED FLOW
The force balance represented by the Northern Hemisphere solutions is shown in
Figure 6.4.
The cyclonic cases above always yield a real solution, but the anticyclonic cases
require a restriction for real solutions to result:
R pd
f 2 R2
>
4
n
Hence, wind speeds near the center of anticyclones (high-pressure system) are generally light, whereas wind speeds associated with cyclones can be quite strong. This
is because, in an anticyclone, the Coriolis force must balance the sum of the pressure
gradient force and centrifugal force. The centrifugal force increases as V 2 but the
Coriolis force increases only as V , which gives a physical reason for the upper limit
in V . In a cyclone, the Coriolis and centrifugal forces act together to balance the
pressure gradient, and hence the deeper the low, the faster the wind speed to create
the balancing forces. In fact, the wind speed may become arbitrarily large in this
model, although in reality surface frictional forces act to limit the intensity of the
system, just as in the case of a tornado.
Example Consider our case study storm at 12 UTC on Friday 14 February 2003
(Figure 6.5). We can use the station observations to test the accuracy of the
geostrophic and gradient wind models in this case. We start by estimating R, V , and
the pressure gradient. The surface station report at Oklahoma City, Oklahoma gives a
sea level pressure of 1004.4 hPa and a wind speed of 13 kts (from the actual weather
observation at this time rather than the station model, which shows 15 kts on the
surface weather map). This wind speed is then 67 m s−1 and is from the south-southwest. The sea level pressure reported near the center of the low is 1000.0 hPa, and at
VNH
(a)
(b)
VNH
LO
HI
PG
CO
PG
CE
CO
CE
Figure 6.4 Force balance represented by the gradient wind (VNH ) model in the case of (a) a
low-pressure system LO and (b) a high-pressure system HI in the Northern Hemisphere. The forces
shown are the pressure gradient force (PG), the Coriolis force (CO), and the centrifugal force (CE)
102
SIMPLE STEADY MOTION
Figure 6.5 Surface weather map valid at 12 UTC 14 Feb 2003
this time the low is approximately 450 km to the northwest of Oklahoma City. From
this, we can write
R = 45 × 105 m
p 10000 − 10044 × 102
=
n
45 × 105
= −98 × 10−4 Pa m−1
Vobs = 67 m s−1
Oklahoma City is at a latitude of 352 N, which results in f = 84 × 10−5 .
From this information, we can then calculate the geostrophic and gradient winds:
Geostrophic
1 pd
Vg = −
f n
1
84 × 10−5 × 12
= 97 m s−1
Vg = −
Gradient
1
2 2
fR
R pd 2
f R
V =−
+
−
2
4
n
× −98 × 10−4 = −189 + 357 − 38×105 ×−98×10−4
= 81 m s−1
1
2
103
BALANCED FLOW
Hence, we can see that the gradient wind model gives a more reasonable result than
the geostrophic wind model by taking account of the curvature of the isobars, which
is significant so close to the center of the low. However, both models overestimate
the observed wind speed due to the absence of frictional effects in the models. This
serves to reduce the actual wind speed and turn the wind toward the center of the low.
This is because friction reduces the wind speed sufficiently to reduce the Coriolis and
centrifugal forces, so that they are no longer in balance with the pressure gradient
force (Figure 6.6.) Hence, the actual wind will turn towards low pressure and may
also accelerate. This, of course, represents a departure from balance.
It is possible to represent the relationship between the gradient wind and the
geostrophic wind as a function of the Rossby number:
V2
1 pd
+ fV +
=0
R
n
V2
+ fV − fVg = 0
R
V
⇒ Vg = V 1 +
fR
∴ Vg = V 1 + Ro
(6.8)
For mid-latitude synoptic systems, the difference between the gradient and
geostrophic wind speeds generally does not exceed 10–20% (recall that the Rossby
number for such systems is of order 10−1 . For tropical systems, in which the Rossby
number is in the range 1–10, the gradient wind should be used in preference to
the geostrophic approximation. In cyclones, the geostrophic wind overestimates the
gradient wind (R > 0) and in anticyclones the geostrophic wind underestimates the
gradient wind (R < 0).
resultant
wind
LO
PG
resultant
net force
CE
CO
FR
Figure 6.6 Force balance represented in the case of a low-pressure system in the presence of
friction FR. All other forces indicated as in Figure 6.4
104
SIMPLE STEADY MOTION
6.3 The Boussinesq approximation
One aspect of balanced flow that we have yet to consider is the appropriate form
of the continuity equation. Let us consider specifically the geostrophic model for
simplicity. The full continuity equation is
t
+ u
x
+v
y
+w
z
+
u
v
w
+ +
x
y
z
=0
If we perform a scale analysis on this equation using the scales in Table 5.2, we find
that to an accuracy of the order of the Rossby number, Dines’ compensation (see
Section 3.7) holds:
v
w
u
+ +
=0
x
y
z
and it can be assumed that density is approximately constant in space and time.
However, it can be shown that such a severe restriction results in a model for the
wind field that does not allow any variation as we move up the atmospheric column –
this is known as the Taylor–Proudman theorem. That is, the distribution of wind
near the surface is also the distribution in the upper atmosphere. This is clearly not
typically the case (see Figures 1.7 and 1.8, for example). Such a situation is known
as having zero wind shear, which is expressed mathematically as
v
u
=
=0
z
z
Note that this condition says nothing about the vertical wind, only the variation of the
horizontal wind in the vertical direction. Hence, we seek a form of the equations that
allows the wind field to vary from the lower atmosphere to the upper atmosphere,
but without allowing vertical motion or changes with time, which would complicate
the picture unnecessarily. We can do this by allowing variations in density to give
rise to buoyancy forces but have no impact on the horizontal force balance – this
approach is known as the Boussinesq approximation, named after French physicist
Valentin Boussinesq (1842–1929). It is constructed as follows.
First, we define a constant reference density 00 and a vertically varying density
consistent with the hydrostatic pressure profile 0 (z) as we did in Section 4.5.1. Then,
we define a horizontally varying density x y consistent with the horizontally
varying dynamic pressure pd . Note that we continue to allow no variations with time.
Using this notation and recalling that we will not allow variations in density to impact
the horizontal force balance, the geostrophic equation becomes
fVg = −
1
00
pd
n
THE THERMAL WIND
105
Because, in applying the Boussinesq approximation, we wish to determine how the
wind field changes in space, it is more appropriate to return to a coordinate system
which is not tied to the wind field. So we write instead
1 pd
1 pd
fug = −
fvg =
(6.9)
y
00
00 x
The vertical momentum equation in this approximation is of course the hydrostatic
relationship which we will write in terms of the dynamic pressure and buoyancy
force (see Section 5.4.2):
−
−
0
g−
00
1
pd
=0
z
00
(6.10)
where we have used the reference density so that variations in density only affect the
calculation of the buoyancy force. The buoyancy force, denoted , can be expressed
using the ideal gas equation in terms of temperature:
=−
−
0 z
g=
00
T − T0 z
g
T00
(6.11)
Combining Equations (6.10) and (6.11), we write
=
1
00
pd
z
(6.12)
The Boussinesq approximation is an excellent one in the oceans where relative density
differences nowhere exceed more than 1 or 2%. However, it is not strictly valid in
the atmosphere. The reason is that air is compressible under its own weight to a
degree that the density at the height of the tropopause (the boundary between the
troposphere and the stratosphere) is only about one-quarter the density at sea level.
At any particular height, however, departures of from 0 z are small and so we
can often make use of the Boussinesq approximation for shallow motions.
In this case an appropriate form of the continuity equation is no longer strictly
incompressible:
u
v
w
0
=0
+ 0
+ +
w
z
x
y
z
Note that this retains the simplicity of a diagnostic rather than a prognostic system.
Also, as we are about to discover, the flow is no longer strictly two dimensional.
6.4 The thermal wind
By taking the horizontal derivatives of Equation (6.12) and substituting these into the
vertical derivatives of Equations (6.9), a new equation can be created which describes
the variation of the geostrophic wind with height:
ug
1
=−
z
f y
vg
1
=
z
f x
106
SIMPLE STEADY MOTION
or, by using Equation (6.11),
ug
g T
=−
z
fT00 y
vg
g T
=
z
fT00 x
(6.13)
This is known as the thermal wind relationship, and it connects the wind shear of the
geostrophic wind (the vertical variation of the horizontal geostrophic wind) with the
horizontal variation in temperature. It can also be understood as a balance between
the buoyancy and the net Coriolis force. This relationship is illustrated as a movie
on the accompanying CD-ROM.
Let us now consider a flow which, for the sake of illustration, is in the Northern
Hemisphere and in a westerly direction, and where temperatures get progressively
colder as we move north. The flow configuration is illustrated schematically in
Figure 6.7 and for the real atmosphere in Figure 15.4. Thus, the relationship implied
by Equation (6.13) correctly predicts the increase of zonal winds with height in
the troposphere which is related to the decrease of temperature with latitude. The
temperature gradient is reversed in the zone above the troposphere, known as the
stratosphere: the polar stratosphere is actually warmer than the equatorial stratosphere,
and the winds decrease with height in this layer, as predicted by the thermal wind
relationship.
The thermal wind equation is, like the geostrophic equation, a diagnostic relation.
As such it is useful in checking analyses of the observed winds and temperature
fields for consistency. The thermal wind constraint is also important in ocean current
systems wherever there are horizontal density contrasts.
6.4.1 Thermal advection
In the flow described in Figure 6.7, the geostrophic wind (and hence the isobars) are
parallel to the isotherms. However, this is generally not the case. Consider the more
typical situation in which the geostrophic wind blows at an angle to the horizontal
isotherms. Suppose, for example, that the geostrophic wind at a particular height z
blows from a region of high-temperature air to a region of low-temperature air – such
z
stratosphere
cold
z
~10 km
u
y
∂T < 0
∂y
x
hot
troposphere
u(z)
Figure 6.7 Illustration of the temperature–wind relationship embodied by the thermal wind
equation
107
THE THERMAL WIND
a situation is known as warm advection (Figure 6.8a). Then, the geostrophic wind at
height z + z, where z is assumed to be small, can be written
v⇀
u⇀
⇀
⇀
i + j z + O z2
u z + z = u z +
z
z
g T ⇀
⇀
z i
≈ u z −
fT00 y
because there is no temperature gradient in the x direction. Note that T / y in
this example is negative, and hence we add a vector in the positive x direction to
determine the geostrophic wind at height z + z. Thus it follows, and this is true
in general, that in the case of warm advection the geostrophic wind turns clockwise
with height. We say that the wind veers with height. Conversely, the geostrophic
wind turns counterclockwise, or backs, with height in the case of cold advection
(Figure 6.8b). Note that the change in the wind vector is the same as in the warm
advection case because the temperature gradient is the same: all that has changed is
the direction of the geostrophic wind, which depends only on the orientation of the
isobars.
In the Southern Hemisphere, these directions are of course reversed. A confusion
that often occurs is that while the terms cyclonic and anticyclonic change sense with
hemisphere (cyclonic is clockwise in the Southern Hemisphere and counterclockwise
in the Northern Hemisphere), the terms veering and backing do not. Veering always
means turning clockwise and backing anticlockwise, regardless of hemisphere. It is
easier to remember that, regardless of hemisphere, warm advection leads to anticyclonic turning with height and cold advection leads to cyclonic turning with height
of the geostrophic wind.
Example Figure 6.9 shows the air temperature distribution at 925 hPa for the case
study storm at 00 UTC on 15 February 2003. This corresponds to the surface map
shown in Figure 1.7. Also shown are the surface wind reports for two stations –
Dodge City, Kansas and Nashville, Tennessee. At which station would one expect
veering, and at which station would one expect backing?
p – ∆p
p
p + ∆p
T
T
∆ug
y
ug(z)
ug(z + ∆z)
z
T + ∆T
p + ∆p
p
p – ∆p
ug(z + ∆z)
x
ug(z)
∆ug
T + ∆T
(a) warm advection
(b) cold advection
Figure 6.8 The temperature–wind relationship when the isotherms and isobars are not parallel
108
SIMPLE STEADY MOTION
Figure 6.9 Contours of 925 hPa air temperature (in C) valid at 00 UTC 15 Feb 2003. Also shown
are the surface wind reports for Dodge City, Kansas and Nashville, Tennessee. The surface map
valid at this time, which shows the full station reports for Dodge City and Nashville, is shown in
Figure 1.7
The map shows a strong horizontal temperature gradient with temperature decreasing
to the north. The isotherms are particularly close together in the vicinity of the
fronts, as would be expected. Referring also to Figure 1.7, it is clear that at this
time, Dodge City is located behind the advancing cold front, in the cold air mass.
The wind is from the north at 5 kts, or 26 m s−1 . Since the wind is blowing toward
warmer temperatures, this is a location of cold advection, and hence we would expect
backing, or counterclockwise turning of the wind with height. Conversely, at this
time Nashville is located ahead of the advancing warm front, and is experiencing a
wind of the same magnitude but from the south. This results in warm advection and
a veering of the horizontal wind with height. These conclusions can be verified using
the upper air station reports from these locations, an exercise left for the student in
the review questions.
6.5 Departures from balance
In the example in Section 6.2.4, the departure from the gradient wind balance due
to the action of friction was discussed, and the resulting net force was illustrated
(Figure 6.6). Departures from balanced flow can result in quite complex motions,
particularly under the influence of surface friction, but for large-scale motions well
above the surface, departures from balance are often very small since a process of
adjustment tends to return the system to a balanced state. One example of this type
of motion is called quasi-geostrophic flow, so named because it represents a small
departure from the balanced case of geostrophic flow.
DEPARTURES FROM BALANCE
109
In quasi-geostrophic flow, the Rossby number is small, but finite, and both time
evolution and vertical motion are allowed. We commence our analysis with the
assumption that we are considering synoptic scale motions, and so we can use the
synoptically scaled Equation (5.9):
⇀
⇀
pd ⇀
1
pd ⇀
Dh uh
⇀
i+
j − f k × uh
=−
Dt
x
y
as our starting point to derive a simplified horizontal momentum equation. With this
scaling, we can also assume that the vertical momentum equation is simply the hydrostatic approximation – the appropriateness of this choice will be confirmed below.
The above equation is still rather complex, and we seek to simplify it based on
our assumption that departures from geostrophy (and hence the Rossby number)
are small. To do this, we can decompose the wind field into two components, the
geostrophic flow and the departure from it, called the ageostrophic wind:
⇀
⇀
⇀
uh = ug + ua
and further, we define that
⇀
ua
∼ Ro ≪ 1
⇀
ug
We can then use the continuity equation to confirm the scaling for the allowed vertical
motion. First we assume that variations in density are negligibly small; this has the
implication, based on the derivation of the thermal wind equation, that any motion
that begins from a state in which the geostrophic wind is independent of height will
remain so. The appropriate continuity equation is
ug
vg
v
w
uh
+ h+
= 0 and
+
=0
x
y
z
x
y
v
w
u
=0
⇒ a+ a+
x
y
z
w
v
ua
=−
+ a
z
x
y
(6.14)
Thus, despite the fact that density variations are ignored, this model allows flow that
is divergent in the horizontal. Consider the scales for the terms in this equation:
ug ∼ U
ua ∼ RoU
w ∼ W
x y ∼ L
z∼H
Then Equation (6.14) implies that
W
RoU
≤
H
L
110
SIMPLE STEADY MOTION
We use ≤ because the right hand side of Equation (6.14) is a summation of terms that
may be of different signs. Since for large-scale motions in the atmosphere Ro ∼ 01
or less, and H/L ∼ 001, it is clear that the vertical velocity scale W is many orders
of magnitude less than the horizontal velocity scale U . Thus, small departures from
geostrophy permit only very small vertical velocities, as we would expect.
Finally, we can use our scaling of the ageostrophic wind to derive a simplified
form of the horizontal momentum equation. We will consider only the x component
for simplicity:
u
1 pd
=−
+ fv
z
x
u
= −fvg + fv
z
ug + ua
ug
ua
+
+ ug + ua
+ vg + va
t
t
x
u
+u
t
u
+u
t
u
+v
x
u
+v
x
u
+w
y
u
+w
y
ug + ua
ug + ua
+w
= fva
y
z
Discarding terms of order Ro and smaller,
ug
ug
ug
+ ug
+ vg
= fva
t
x
y
We retain the Coriolis force arising from the ageostophic wind since
fva ∼ fRoU ∼
U2
fU 2
∼
fL
L
like the geostrophic advection terms. Thus, the governing equation for quasigeostrophic flow is
⇀
⇀
Dg ug
⇀
= −f k × ua
Dt
(6.15)
6.5.1 Ageostrophic flow
As we have seen, the ageostrophic wind is the component of the wind that is
present above and beyond the geostrophic wind, which flows parallel to the isobars.
Equation (6.15) allows us to determine the magnitude of the cross-isobaric flow due
to both changes in time and changes in space. Writing this equation as an expression
to determine the ageostrophic flow,
vg
vg
1 vg 1
ug
−
+ vg
ua = −
f t
f
x
y
ug
ug
1 ug 1
va =
ug
+
+ vg
f t
f
x
y
DEPARTURES FROM BALANCE
111
From this, we can see that in the Northern Hemisphere, the ageostrophic wind blows
⇀
to the left of the acceleration vector Dg ug /Dt. Substituting Equation (6.9) into this
gives the ageostrophic components in terms of the pressure gradient:
2
2
p
1
p
ug 2 + vg
2
x
x y
00 f
2
2
2
1
p
p
p
−
ug
+ vg 2
2
y t
x y
y
00 f
2
p
1
ua = −
−
2
x t
00 f
va = −
1
2
00 f
If the largest component of change in a particular flow is the change with time, then
ua = −
2
1
p
2
x t
00 f
2
p
1
va = −
2
y t
00 f
(6.16)
which is known as the isallobaric wind, which flows normal to the isallobars, or
lines of constant pd / t. This concept was first introduced by Brunt and Douglas
(1928), who suggested that isallobaric charts may be useful for determining regions
of convergence and divergence.
In regions where the flow is approximately stationary in time, the cross-isobaric
flow is represented by the advection term:
2
2
vg
vg
1
p
p
1
ug
=−
+
v
+ vg
u
g
g
2
f
x
y
x2
x y
00 f
2
2
ug
ug
1
1
p
p
va =
ug
=−
+ vg
u
+
v
g
g
2
f
x
y
x y
y2
00 f
ua = −
(6.17)
and the ageostrophic wind is perpendicular to the advective acceleration.
Example Figure 6.10 illustrates these two components of the ageostrophic wind. In
Figure 6.10(a) a map of isallobars (lines of constant pressure tendency) is shown. For
this simple case we see that
2
p
p
=
<0
x t
x t
and
2
p
p
=
=0
y t
y t
112
SIMPLE STEADY MOTION
(a)
(b)
∂p
∂t
= 0 hPa/h
–2 hPa/h
– 4 hPa/h
0
100
Low
va
ug
ug
ua
ug
va
High
Pressure steady
falling
10
10
falling
rapidly
Figure 6.10 Zonal ageostrophic wind resulting from changes in pressure witn (a) time and (b)
space
Using this information in Equation (6.16), and assuming a Northern Hemisphere
location (f > 0, we find that ua > 0 and va = 0.
Figure 6.10(b) illustrates the ageostrophic component of the wind due to advective acceleration for a situation with isobars that are coming together (confluence)
on the left side of the figure and isobars spreading apart (diffluence) on the right
side of the figure. With this pressure pattern, the largest zonal geostrophic wind
must be at the center of the figure, with weaker zonal geostrophic winds on the
left and right sides, and ug > 0 at all locations. It follows then that ug / x > 0 on
the left side of the figure and ug / x < 0 on the right side of the figure. Applying this to Equation (6.17), again assuming a Northern Hemisphere location and
neglecting meridional variations and vg , we find that ua = 0 and va > 0 on the
left side of the diagram and ua = 0 and va < 0 on the right side of the diagram.
This cross-isobaric flow creates regions of convergence (upper left and lower right
corners of Figure 6.10b) and divergence (lower left and upper right corners of
Figure 6.10b) and thus vertical motion as diagnosed from Equation (6.14). We will
see the importance of this small but crucial flow when we consider cyclogenesis in
Chapter 9.
6.5.2 The maintenance of balance
As can be deduced from Figure 6.11, the Coriolis force associated with the crossisobaric flow toward the surface low-pressure center is directed toward the west; that
is, in the same direction as the geostrophic wind. In general, an ageostrophic wind
directed toward low pressure will always tend to accelerate the wind in the direction
of geostrophic flow. This is also true of the isallobaric component of the ageostrophic
wind – it is the component of the flow that accelerates or decelerates to take up the
geostrophic wind velocity consistent with the evolving pressure field. Hence, when
a changing pressure field, either in space or in time, creates an imbalance of forces,
113
DEPARTURES FROM BALANCE
L
ua
L
ua
H
COLD
WARM
ug
L
z
ua
H
ug
W
N
Figure 6.11 Vertical motion and subsequent circulation between the surface and the 500 hPa
level induced by a thermal wind imbalance. Heavy dashed lines show the geostrophic wind and
heavy solid lines show the ageostrophic wind. Vertical motion is indicated by fine dashed lines.
Note that north is directed toward the right in this figure
a cross-isobaric flow is generated which will act to restore geostrophic balance. It is
for this reason that, in the absence of other forcing, the departures from geostrophy in
large-scale flow tend to be small. Such a process of restoring the flow to geostrophic
balance is termed an adjustment.
A similar adjustment process takes place to maintain thermal wind balance. Consider flows occurring on two levels in the atmosphere, illustrated by our case study
storm at 12 UTC on Monday, 17 February 2003, shown in Figure 6.11 as a schematic
of the sea level pressure and 500 hPa height fields. Assume we are in a situation in
which the flow is initially slower than required by geostrophic balance, and the wind
shear is insufficient to maintain thermal wind balance with the meridional temperature
gradient. The resulting cross-isobaric flow accelerates the geostrophic flow at both
levels and increases the vertical wind shear. At the same time, the associated rising
motion produces adiabatic cooling in the region of warm temperature and sinking
motion produces adiabatic warming in the region of cold temperature, thus reducing
the temperature gradient and hence the wind shear required to maintain thermal wind
balance. In concert, these effects act to bring the flow back into balance with the
pressure and temperature fields.
The rising motion in the warm air mass and the sinking motion in the cold air
mass results in what is known as a thermally direct circulation. This process is highly
efficient in the maintenance of thermal wind balance in large-scale motions in the
atmosphere. In addition, the process itself acts to keep the ageostrophic flow small
in comparison to the geostrophic flow.
114
SIMPLE STEADY MOTION
Review questions
6.1 (a) What is the period of an inertial oscillation at the South Pole (latitude 90 S),
Dunedin, New Zealand (latitude 46 S), Cape Town, South Africa (latitude
34 S), and Quito, Ecuador (latitude 0 )?
(b) What is the period of an inertial oscillation at latitude 46 N? How does this
compare to the period of an inertial oscillation at Dunedin, New Zealand?
6.2 (a) Calculate the Rossby number for the atmospheric flows listed in Table 6.3.
Assume a latitude of 40 N for all of the atmospheric flows considered.
(b) Calculate the change in pressure from the center to the edge of circulation
for each of the atmospheric flows listed in Table 6.3 based on the equations
for cyclostrophic flow, geostrophic balance, and gradient wind balance.
(c) Which balanced flow approximation is most appropriate for each type of
flow and why?
(d) Describe physically why the change in pressure estimated in part (b) differs
for each type of balanced flow.
6.3 What flow conditions (wind speed, radius of curvature, and Coriolis parameter)
are required for the geostrophic approximation to provide a reasonable representation of the flow? (A qualitative answer is sufficient.)
6.4 At 15 UTC 28 Aug 2005 Hurricane Katrina was located at latitude 260 N
and longitude 881 W in the Gulf of Mexico. Katrina was the third strongest
hurricane on record in the Atlantic Basin, with a central pressure of 907 hPa. At
this time reconnaissance aircraft measured a maximum wind speed at the surface
of 75 m s−1 at a distance of 20 km from the center of the hurricane.
(a) Calculate the change in pressure between the center of the hurricane and
the location of maximum wind speed assuming (i) cyclostrophic flow, (ii)
geostrophic balance, and (iii) gradient wind balance.
(b) Which of the answers in part (a) is most likely to be closest to the actual
change in pressure observed at this time? Why?
6.5 Draw a figure analogous to Figure 6.4 for a Southern Hemisphere location.
Table 6.3 Typical wind speed, radius, and direction of rotation for atmospheric flows in
the Northern Hemisphere
Atmospheric flow
Wind speed (m s−1 )
Radius (m)
Dust devil
Tornado
Hurricane
Synoptic cyclone
10
100
50
10
10
500
1 × 104
1 × 106
Direction of rotation
CW or CCW
CW or CCW
CCW
CCW
REVIEW QUESTIONS
115
6.6 Consider a balanced flow at 40 N where the magnitude of the pressure gradient
force on a horizontally moving air parcel, expressed in natural coordinates, is
15 × 10−3 m s−2 .
(a) Find the geostrophic wind speed in m s−1 .
(b) Find the gradient wind velocity for a cyclone of this strength with radius of
curvature 750 km.
(c) Find the maximum absolute value of the pressure gradient force for a balanced
flow to exist in an anticyclone with a radius of curvature of 750 km. What
is the corresponding geostrophic wind speed?
6.7 (a) Assume that the wind speeds given in Table 6.3 are equal to the gradient
wind speed. Calculate the geostrophic wind speed for each type of flow using
the gradient wind speed and the Rossby number.
(b) In which case does the geostrophic wind speed differ most from the gradient
wind speed given in the table? In which case does it differ the least?
(c) Explain the physical reason for your answers to part (b).
6.8 Derive Equation (6.13) using Equations (6.9) and (6.12).
6.9 In a low-pressure system at 45 N, the geostrophic wind at the surface is 20 m s−1
from the south-west, and the geostrophic wind at 500 hPa is 25 m s−1 from the
west. Is the wind veering or backing with height? What does this imply about
thermal advection?
6.10 (a) Using the surface weather maps for the storm of 2003 on the CD-ROM, calculate the horizontal temperature gradient at the surface at Davenport, Iowa at
00 UTC 16 Feb 2003. (Hint: use the temperature observations from Chicago,
Illinois (ORD), St. Louis, Missouri (STL), Des Moines, Iowa (DSM), and
McCoy, Wisconsin (CMY) to calculate the temperature gradient.)
(b) What is the geostrophic wind at 850 hPa based on the temperature gradient
calculated in part (a)? (Hint: you will need to calculate the geostrophic wind
at the surface based on the sea level pressure observations at ORD, STL,
DSM, and CMY.)
6.11 (a) Use the upper air weather maps for the storm of 2003 on the CD-ROM to
estimate the geostrophic wind shear between 850 and 700 hPa at Oklahoma
City, Oklahoma at 00 UTC 16 Feb 2003. You may assume that the winds
reported on these weather maps are geostrophic winds.
(b) Calculate the horizontal temperature gradient required to produce the
geostrophic wind shear calculated in part (a).
(c) Is the horizontal temperature gradient calculated in part (b) consistent with
the temperatures shown on the 850 and 700 hPa upper air weather maps on
the CD-ROM?
116
SIMPLE STEADY MOTION
6.12 Use the weather observations at the surface and 850, 700, and 500 hPa on the
CD-ROM for 00 UTC 15 Feb 2003 to verify that the wind backs with height at
Dodge City, Kansas and veers with height at Nashville, Tennessee, consistent
with the discussion for the example in Section 6.4.1.
6.13 A hodograph displays the change in wind speed and direction with height,
giving a graphic depiction of vertical wind shear in a simple diagram.
(a) Using the data in Table 6.4 construct a hodograph using the blank hodograph
diagram provided on the CD-ROM.
Remember that the wind direction given by meteorologists indicates the
direction that the wind is coming from and that wind directions given
by numerical values have the following geographic directions: 90 = east,
180 = south, 270 = west, and 360 = north, with intermediate numerical
values corresponding to intermediate directions.
To make the hodograph, label the wind speed rings on the blank diagram
with values appropriate for the data you are using. Then, for each height
level, place a dot on the graph at the appropriate wind speed (given by
the circles) and wind direction (given by the radial lines). Note that the
wind directions plotted on the hodograph are the opposite to that which is
normally used: a north wind is plotted at the bottom of the hodograph, and
you proceed counterclockwise to east, south, and west. This convention is
used so that the final plot on the hodograph indicates the direction the wind
is going toward.
When you have placed all the dots, draw a line joining the dots from the
surface point to the uppermost point.
(b) Assume the sounding was taken at a Southern Hemisphere location with
latitude 44 S. Assume geostrophic and hydrostatic balance apply. Is the
horizontal advection warm or cold in (i) the layer from the surface to 3 km
Table 6.4 Vertical profile of wind speed and direction
observed at latitude 44 S
Altitude (km)
0.0
0.5
1.0
1.5
2.5
3.0
3.5
4.0
4.5
5.0
Wind speed (m s−1 )
Wind direction (deg)
00
92
92
92
83
92
13
14
14
14
0
233
245
254
290
296
276
263
254
233
REVIEW QUESTIONS
117
and (ii) the layer from 3 km to 5 km? Clearly explain the reasoning behind
your answer.
(c) Calculate the amount of warm or cold air advection at 3 km in Kelvin per
day. For simplicity, assume the average temperature is 273 K.
6.14 The mean temperature in a layer between 750 and 500 hPa decreases going
toward the east by 3 C per 100 km. If the 750 hPa geostrophic wind is from
the south-east at 20 m s−1 , what is the geostrophic wind speed and direction at
500 hPa? Let f = 10−4 s−1 .
6.15 Rewrite Equation (6.15) as separate equations for both horizontal components
of the wind, and expand the material derivative in this equation into local
derivatives.
6.16 (a) What is the direction of the ageostrophic wind due to advective acceleration
between Midland, Texas and Oklahoma City, Oklahoma at 300 hPa at 00
UTC 15 Feb 2003 based on the weather maps for the storm of 2003 provided
on the CD-ROM? You may assume that the winds reported on this map
are geostrophic winds.
(b) What is the direction of the ageostrophic wind due to advective acceleration
between Oklahoma City, Oklahoma and Springfield, Missouri at this time?
6.17 What is the direction of the isallobaric component of the ageostrophic wind at
the surface at Davenport, Iowa between 06 and 12 UTC 15 Feb 2003? Use the
surface weather maps for 06 and 12 UTC provided for the storm of 2003 on
the CD-ROM to answer this question.
7
Circulation and vorticity
7.1 Circulation
One way to describe an atmospheric flow that involves rotation or curvature is the
circulation, which is defined as the line integral of the wind around a closed curve
anywhere in the atmosphere (e.g. Figure 7.1):
⇀
⇀
C = u • ds
(7.1)
Unlike other measures of rotation in a flow, such as angular velocity, this definition
does not require the center of rotation to be defined, and hence it is a convenient
measure to use in situations where a single axis of rotation is difficult to identify. It
is conventional to perform the integration in a counterclockwise direction around the
curve and as a result C is positive for a counterclockwise flow.
7.1.1 Kelvin’s circulation theorem
In general, the circulation around a particular closed curve will be a function of both
time and space, since the velocity field is a function of time and space. So, if we
wish to determine the rate of change of circulation, we can write
⇀
DC Du ⇀
=
• ds
Dt
Dt
1 p
= −
ds +
ds + friction
s
s
⇀
where we have neglected the rotation of the frame of reference (that is, u in this
case is an absolute velocity relative to a fixed frame outside the Earth), and we have
expressed the gravity force in the form of the geopotential (see Section 4.3.1). There
is an additional term in this equation which arises from the advection of the closed
curve itself, but it turns out to be zero anyway, so it has also been neglected.
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
120
CIRCULATION AND VORTICITY
Vc = 0
Vg
Vg
Vc
Vc
Vg
Figure 7.1 Components Vc of a gradient flow Vg that contribute to the circulation around the
closed curve
Consider first the gravity term:
ds = d
s
= 2 − 1
= 0 around a closed circuit
It is intuitive that the gravity force would not be involved in the generation of rotating
motion, since gravity acts through the center of mass of any body.
Similarly, if the flow is barotropic (see Section 1.3.3), then density is a function
only of pressure, and we can write
1
1 p
dp
ds = −
−
p s
p
1
1
−
=−
p2
p1
= 0 around a closed circuit
For the moment, let us also assume that the fluid is inviscid: that is, frictionless.
From this, Kelvin’s theorem that circulation is constant in a barotropic, inviscid fluid
follows.
7.1.2 Bjerknes’ circulation theorem
In the atmosphere, the conditions required for Kelvin’s circulation theorem are rarely
met, and the more important conclusion is that changes in circulation can arise from
both viscous forces and baroclinicity (Section 1.3.3), as published by V. Bjerknes
in 1937. We will consider the generation of circulation by friction in Chapter 10.
The generation of a circulation by baroclinicity can be illustrated by considering the
development of a sea breeze (illustrated in Figure 7.2 and shown as an animation
121
CIRCULATION
925
H
L
L
H
950
975
Figure 7.2 The configuration of isobars (shown in hPa) and circulation (arrows) when a sea
breeze has reached a mature stage
on the accompanying CD-ROM). During a typical day at the beach, you may notice
a steady wind blowing off the water in the late afternoon. This wind is generally
noticeably cooler than the ambient air temperature. It is known as a sea breeze, and
it occurs in response to differences in temperature between a body of water and
neighboring land. That is, a sea breeze occurs in the presence of baroclinicity.
Overnight, the land and the ocean cool down from the previous day, and can
approximate a common equilibrium temperature. If there is no significant weather
passing through the region and the land surface near the coast is relatively flat, then
there will be at most a very small pressure gradient between the land and the water.
As the day progresses, the lower heat capacity of the land surface will cause it to
increase in temperature more rapidly than the ocean in response to the same amount
of solar energy impinging on the surface. Heat radiated back into the atmosphere
from the land surface will cause warming of the overlying air, and expansion of the
air column. This causes an increase in the thickness of the air column above the
land relative to the air above the water, resulting in a horizontal pressure gradient
aloft. This gradient can then initiate offshore flow at upper levels. Since the pressure
at any location is determined by the weight of the air above it, the removal of air
from higher levels causes the pressure at levels below to decrease. Hence, a pressure
gradient in the reverse sense is created at lower levels, creating an onshore flow that
we experience as a sea breeze. Rising and sinking air complete the circulation in
response to continuity requirements.
Since a rising parcel of air cools and moistens, the inland arm of the circulation
experiences conditions favorable for the formation of clouds. Hence, it is often
possible to observe this branch of the sea breeze in the form of cumulus clouds, and
sometimes even thunderstorms.
Example We can calculate an expected sea breeze based on a given temperature
difference as follows. First, we apply the circulation theorem around a vertical circuit
122
CIRCULATION AND VORTICITY
along the 975 mb and 925 mb isobars (Figure 7.2), using the ideal gas law and
neglecting friction:
1
DC
=−
dp
Dt
RT
=−
dp
p
= − RTd ln p
There is a contribution to the circulation only by the vertical segments because the
horizontal segments are taken at constant pressure. The resulting rate of increase in
the circulation is
pb
DC
T2 −T1
= R ln
Dt
pa
Taking T 2 − T 1 to be around 20 C, the rate of increase of circulation around this
circuit is
92 500
DC
× 20
= 287 × ln
Dt
97 500
= −302 m2 s−2
We expect the result to be negative since the temperature difference is inducing a
clockwise circulation.
The mean acceleration can be computed if we know the distance around the circuit
and the mean temperature in the atmospheric column. We will assume that the mean
temperature is about 15 C, and the horizontal distance 20 km. Sea breeze circulations
can typically penetrate inland a maximum of around 40 km from shore. This is due
to the increased surface friction, relative to the ocean surface, resulting from the
topography of the land. To calculate the vertical distance, recall the equation for
thickness (Equation (4.12))
RT
pa
97 500
287 × 288
Z=
= 434 m
ln
=
× ln
g
pb
10
92 500
Then
⇀
DC Du ⇀
=
• ds
Dt
Dt
⇀
Du ⇀
• ds
≈
Dt
Du DC
⇒
≈
/2 × Z + 2L ≈ −7 39 × 10−3 m s−2
Dt
Dt
123
CIRCULATION
In the absence of retarding forces this would produce a wind speed of 25 m s−1 (just
under 50 kts) in about an hour. In reality, as wind speed increases, the frictional
force near the surface increases, retarding the acceleration. In addition, temperature
advection (see Section 2.8) reduces the land–sea temperature contrast so that a balance
is obtained.
7.1.3 Observing sea breezes
Sea breezes are most common during the sunny days of spring and early summer,
since they require effective cooling at night followed by consistent heating during the
day. The leading edge of a sea breeze is characterized by decreased temperature and
rising motion, and hence has much in common with cold fronts (Section 9.1). The
sea breeze front can be identified in observations of clouds and in satellite imagery
by a line of fair weather cumulus clouds landward of clear air (see the CD-ROM
for an example). The sinking motion and lower temperatures over the ocean prevent
clouds from forming. Sea breezes can also occur in the vicinity of large lakes.
7.1.4 Relative circulation
So far, we have neglected the fact that we are in a rotating frame of reference. On the
Earth, a component of the circulation around any circuit will be due to the rotation
of the frame, that is
Cabsolute = Cearth + Crelative
The circulation due to the rotation of the Earth is then simply
Cearth =
⇀
⇀
uearth • d s
≈ R × 2 R sin
Cearth ≈ 2
R2 sin
where R is the radius of the Earth and
is the angular velocity of the Earth. The
primary importance of this component is the fact that it is dependent upon latitude.
Hence, a meridional flow of air under conditions of conserved absolute circulation
will experience an induced relative circulation to compensate for the change in the
circulation due to the rotation of the Earth. For example, consider the trade winds
over the Atlantic Ocean in northern summer (Figure 7.3). Trade winds flowing north
and south converge at the latitude of maximum heating which is around 10 N in
this figure. The southerly trades, originating in the Southern Hemisphere, acquire a
clockwise (that is, negative) rotation as they cross the equator and the circulation due
to the Earth’s rotation increases.
124
CIRCULATION AND VORTICITY
6
2
4
6
8
6
4
2
4
4
6
4
2
2
4
2
6
8
8
6
2
4
2
2
8
6
4
2
4 4 2
Figure 7.3 Average 1000 hPa vector wind (m s−1 ) for August 2003 for the tropical Atlantic. The
contours show the magnitude of the wind and the vectors show the wind direction. NCEP Reanalysis
data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from its Web site
at http://www.cdc.noaa.gov/
7.2 Vorticity
Another way to describe the curved motion of fluid parcels without reference to a
center of rotation is a quantity known as the vorticity, which is simply the circulation
per unit area:
=
C
A
(7.2)
This definition simplifies the analysis since, if we can calculate , we no longer
need to define a specific closed circuit in the flow of interest. Since the circulation
represents the flux of vorticity through a specified area closed circuit, the circulation
is often termed the vortex strength.
The calculation of vorticity is particularly simple in the case of solid body rotation.
Consider an infinitesimally small disk of radius r rotating with angular velocity
corresponding to a speed V at the rim:
=
2V
2 rV
C
=
=
= 2
2
A
r
r
If we expand this disk to a finite area, we find that components of the circulation on
opposite sides of the disk cancel out, and hence the relationship = 2 is generally
true for all solid body rotation. It follows that the vorticity of any fluid parcel on the
125
VORTICITY
Earth has a component due to the solid body rotation of the Earth, at any point of
which = sin and hence
earth = 2 sin = f
Therefore earth is positive in the Northern Hemisphere and negative in the Southern
Hemisphere.
We can derive the relative vorticity of a fluid in motion by calculating the circulation around a small rectangular fluid element, as shown in Figure 7.4, where we
have neglected motion in the vertical:
C=
⇀
⇀
u • ds =
udx + vdy
and hence the circulation around the infinitesimal fluid element ABCD:
C = CAB + CBC + CCD + CDA
u
v
= ux + v + x y − u + y x − vy
x
y
v
u
= xy − yx
x
y
v u
A
−
=
x y
where A is the area of ABCD. Then the relative vorticity is simply
C
A
v u
=
−
x y
=
(7.3)
δx
D
v
C
u
δy
y
A
B
x
Figure 7.4 Calculation of circulation around a rectangular fluid element
126
CIRCULATION AND VORTICITY
The relative vorticity will be positive for a counterclockwise rotation and will be
negative for a clockwise rotation. Therefore, for flow around a low-pressure center
in the Northern Hemisphere, the relative vorticity will be positive. In the Southern
Hemisphere the relative vorticity for flow around a low-pressure center is negative.
Example In August 1992, a category 5 hurricane, named Andrew, hit southern Florida
and the Louisiana coast. Hurricane Andrew caused 61 deaths and was one of the costliest
hurricanes in US history up to that date (a satellite image is available on the CD-ROM).
At the time the satellite image was taken, wind gusts up to 150 kts were observed at a
distance from the center of 20 km. The vortex strength at this time was then
⇀
⇀
C = u • ds
⇀
⇀
U = u • d s = 150 kts = 77 m s−1
⇒ C = 2 RU = 2 × 20 × 103 × 77
∴ C = 9 7 × 106 m2 s−1
The vorticity within the hurricane is roughly
9 7 × 106
C
=
A
R2
−3 −1
≈ 7 7 × 10 s
≈
Note that this value is two orders of magnitude larger than the background vorticity
due to the rotation of the Earth, which at latitude 25 N is 2 sin = 3 1 × 10−5 s−1 .
This is typically true of all moderate to strong circulations on the Earth, although it
does not imply that the background rotation is not important, as we will see in the
next section.
In fact, although the above calculation is straightforward and illuminating as far as
relative magnitudes are concerned, the most important aspect of relative vorticity is
not its value at a particular time, but its change in space or time. Temporal changes in
relative vorticity tell us about cyclone development – vorticity increases as cyclones
spin up, and decreases as they die. Spatial changes in relative vorticity can indicate
the influence of mountains or temperature gradients – the factors that can lead to the
generation of circulation according to Bjerknes’ theorem. One way to address the
spatial variation of vorticity in isolation is to develop a quantity that is conserved in
time – that quantity is called potential vorticity.
7.3 Conservation of potential vorticity
Recall Kelvin’s circulation theorem: for an inviscid, barotropic flow, the circulation
is conserved. We also saw that in most real cases, circulation (or vortex strength)
127
CONSERVATION OF POTENTIAL VORTICITY
changes due to the presence of baroclinicity or friction. However, we can simplify
this situation by making two assumptions: that we are far from the surface and hence
frictional effects are minimal, and that motion is adiabatic (that is, the potential
temperature is constant). Such a situation is often called potentiotropic, because on a
constant potential temperature surface, the density is a function of pressure alone:
p
p
=
=
RT
R
p0
p
Rd /cp
R /c
=p
cv /cp
p0 d p
R
This is analogous to a barotropic fluid, in which the density is a function only of
pressure everywhere in the fluid. However, adiabatic flow is more closely approximated in the real atmosphere than barotropic flow. Thus, on a constant potential
temperature surface, the pressure gradient term does vanish and the fluid satisfies
Kelvin’s circulation theorem. Mathematically, this is written
DCabsolute
Dt
=0
(7.4)
indicating that the circulation must be evaluated on a closed loop that lies entirely on
the constant potential temperature surface.
Using Equation (7.2) with the implied assumption that the constant potential temperature surface is approximately horizontal, we can write
+ f dA
Cabsolute =
A
Cabsolute
A
A + f = constant
+ f = lim
A→0
(7.5)
Suppose that a parcel is confined between potential temperature surfaces and +,
which are separated by a pressure interval p. Because the parcel is confined to a
given potential temperature (± 21 , the motion is by definition adiabatic. The mass
of the parcel, given by M = zA = − p/g A, must be conserved following
the motion. Hence we can write
Mg
Mg
≡−
×
p
p
Mg
A = −
p
A = −
Since M and are constant, this can be simplified to
A = −constant × g
p
(7.6)
128
CIRCULATION AND VORTICITY
where we have retained g by convention. Eliminating A between Equations (7.5)
and (7.6) yields
+ f = constant
constant × −g
p
∴ P = −g
+ f = constant
p
where we have defined a new quantity P, conserved in adiabatic frictionless flow,
which is known as the Rossby–Ertel potential vorticity. The term ‘potential vorticity’
is used in connection with several mathematical expressions. The physical meaning,
however, is that any quantity termed potential vorticity is always in some sense a
measure of the ratio of the absolute vorticity to the effective depth of the vortex.
The concept was introduced by C. G. Rossby in 1936 and shown in the form similar
to the one above by H. Ertel in 1939. The potential vorticity is a powerful concept
because it is an integration of small-scale fluid element properties and large-scale
flow properties.
Example A typical weather pattern when the prevailing flow impinges on a long
mountain chain that is normal to the flow is shown in Figure 7.5, which took place
as our case study storm ravaged the east coast. The Rocky Mountains are arrayed
Figure 7.5 Mean sea level pressure map, showing low- and high-pressure centers, fronts, and
troughs, at 00 UTC 17 Feb 2003
129
CONSERVATION OF POTENTIAL VORTICITY
north–south through the western third of the United States, and the prevailing flow
is westerly – recall from Chapter 6 that the thermal wind balance in the presence
of the decreasing equator-to-pole temperature gradient requires that the balanced air
flow be westerly. From the map, it is clear that high-pressure areas dominate to the
west of the Rocky Mountains, and a trough of low pressure sits on the lee side,
along with two closed low-pressure circulations. Why would such a configuration
be typical?
Let us assume that we have conditions that approximate adiabatic, frictionless flow.
Consider what happens when that flow impinges on a mountain barrier from the west
(Table 7.1).
As the column of air approaches the mountain, the upper potential temperature
surface rises, causing the column to increase in depth. This causes a decrease in
/p, and for conservation, this requires + f to increase. The only way in
Table 7.1 Modifications to vortex depth and vorticity as a westerly flow impinges on an
isolated mountain barrier in the Northern Hemisphere
z
x
Air column depth
Increase p ↑
Decrease p ↓
Increase p ↑
Decrease (return to
original value)
Decrease
Increase
Decrease
Change in + f
Sign of
Resulting motion
Increase
Positive
Northward
Decrease
Negative
Southward
Change in f
Increase
Decrease
Increase
Positive
Lee side
trough
Increase
Increase (return to
original value)
Decrease
Negative
Southward
p
Change in
y
x
Decrease
130
CIRCULATION AND VORTICITY
which the flow can respond to effect such an increase is to introduce a rotation that
causes to increase; that is, a cyclonic or counterclockwise rotation. This causes the
flow to turn slightly northward, which, in turn, causes an increase in the planetary
vorticity, f . By this time, the flow is impinging on the rising slopes of the mountain,
causing the air column to decrease in depth. This causes a rapid increase in /p
requiring a corresponding decrease in + f . This generates a strong anticyclonic
rotation – the source of the typical high pressure on the windward side of a long mountain range. The resulting southward flow causes the planetary vorticity to decrease
significantly.
After cresting the mountain, the vortex depth starts to increase rapidly again,
causing the generation of cyclonic flow and the acquisition of increasing planetary
vorticity. This is evident at the surface as a lee side trough, as we see in Figure 7.5. At
this point, the air column is likely to have reached its original value, and if no further
mountain ranges lie in the path of the air flow, one might expect no further changes
in vorticity. However, the air column now lies to the north of its original position,
and so with a larger value of f than initially, conservation requires < 0. This
causes clockwise rotation and equatorward motion. This interplay between relative
and planetary vorticity can continue downstream of the mountain, generating a lee
side wave.
The interplay between the relative vorticity of the fluid elements and the planetary vorticity imparted to the large-scale flow results in circulation patterns such
as that illustrated by Figure 7.5. As we will see in Section 8.2, this ‘trade-off’
between planetary and relative vorticity results in motions that are fundamental to
the dynamics of the atmosphere. It is also clear that the depth of the flow has an
influence – since variations in atmospheric depth are often associated with variations
in temperature (Section 4.3.4), this links the important concepts of baroclinicity with
vorticity generation. In general, it is useful to remember that the stretching of vortex
tubes causes acquisition of cyclonic vorticity, and shrinking produces anticyclonic
vorticity.
7.4 An introduction to the vorticity equation
From the example above, it is clear that even in situations where potential vorticity is conserved, the relative vorticity itself is not. In fact, it is this aspect that
gives rise to the interesting weather patterns we see in the vicinity of mountains.
We also saw in that example that the interplay between relative and planetary vorticity can be an important aspect of the generation of atmospheric circulations.
Because of this, we would like to have an equation that we can use to predict the
change in both time and space of the vorticity – this is called the vorticity equation. A complete form of this equation can be derived from first principles using
the Navier–Stokes equations (4.19). Here, the vorticity equation for a synoptic scale
mid-latitude weather system will be derived, based on the momentum equation for
AN INTRODUCTION TO THE VORTICITY EQUATION
131
such a system (Equation (5.9)) and using the definition of vertical relative vorticity
(Equation (7.3)):
⇀
⇀
1 pd ⇀ pd ⇀
Dh uh
⇀
i+
j − f k × uh
=−
Dt
x
y
Dh u
1 pd
Dh v
1 pd
=−
+ fv 1
=−
− fu 2
⇒
Dt
x
Dt
y
1 2 pd
Dh u
v
f
=−
1 ⇒
+f +v
1 ′
y
Dt y
xy
y
y
1 2 pd
Dh v
u
f
=−
2 ⇒
−f −u
2 ′
x
Dt x
xy
x
x
f
u v
f
Dh v u
′
′
= −f
−u −v
−
+
2 − 1 ⇒
Dt x y
x y
x
y
D
f
f
u v
∴ h +u +v
= −f
+
Dt
x
y
x y
where we have assumed that the density does not vary appreciably in the horizontal,
which is consistent with the scaling we are using (Section 5.4.2). Now, since we
also know that the Coriolis parameter does not change with time, we can write this
equation more efficiently as
u v
Dh
+ f = −f
(7.7)
+
Dt
x y
Thus, the synoptically scaled vorticity equation tells us that changes in time of relative
vorticity are generated by the advection of relative and planetary vorticity, and by
the convergence of planetary vorticity carrying air.
This conclusion suggests that if we want to understand the temporal evolution of
a mid-latitude cyclone, we should use a theoretical model that allows convergence
and divergence. Consider geostrophic flow: in this approximation the flow is nondivergent and hence the expression above for the change in total vorticity is identically
zero – in this case total vorticity (but not relative vorticity alone) is conserved
following the motion. More interesting is the case of quasi-geostrophic flow. In this
case, the total flow is non-divergent, but the model allows divergence in the horizontal
to be balanced by convergence in the vertical, and vice versa. By adopting the scaling
leading to Equation (6.15), we can write Equation (7.7) as
Dg
ua va
(7.8)
+ f = −f
+
Dt g
x
y
which can also be written, using Equation (6.14), as
Dg
w
g + f = f
Dt
z
132
CIRCULATION AND VORTICITY
Hence, this equation shows us that relative vorticity in a quasi-geostrophic situation
will be generated by the advection of relative vorticity, computed geostrophically,
and planetary vorticity, and by the convergence of planetary vorticity carrying air.
By our use of Equation (6.14), we can understand this last term equivalently as the
vertical stretching of planetary vorticity tubes.
Review questions
7.1 Consider a rectangular region of the atmosphere, with an east/west extent of
500 km and a north/south extent of 200 km. From the southern to the northern
edge of this region the zonal wind increases from 20 m s−1 to 50 m s−1 , and there
is no meridional wind. Assume that the zonal wind only varies in the meridional
direction.
(a) Calculate the circulation around this rectangular region.
(b) Does the sign of the circulation calculated in part (a) match the expected
sense of rotation induced by this flow?
(c) Repeat parts (a) and (b) by assuming that the wind decreases from
50 m s−1 to 20 m s−1 from the southern to northern edge of this rectangular
region.
(d) Calculate the relative vorticity for the flow in parts (a) and (c) using (i) the
circulation calculated above, and (ii) Equation (7.3).
7.2 Heavy morning snow has just fallen on a plain east of Bismarck, North Dakota.
Meanwhile, news radio from Pierre, South Dakota reports no snow in the state
that day.
(a) Write at least one paragraph describing why and how a local thermally driven
circulation is likely to develop during the afternoon. Support your description
with a diagram. You can assume: (i) the sky clears; (ii) no further snow falls;
and (iii) no large-scale pressure gradient.
(b) Calculate the rate of change of the circulation between the 1000 mb
and 850 mb levels, and between a location 20 km north of the snow
edge to a location 20 km south of the snow edge. Assume a temperature difference of 10 K between the two regions and an average temperature of 278 K. Estimate the wind speed 1 hour after the circulation
commences.
7.3 In the late autumn, with the onset of longer nights and shorter days, Lake
Superior in the northern United States remains relatively warm while the surrounding land begins to cool. The temperature difference between the land and
adjacent lake is largest at night, and is confined to the lowest 50 hPa of the
atmosphere.
REVIEW QUESTIONS
133
(a) Calculate the wind speed between the land and the lake 2 hours after the horizontal temperature difference described above has been established, assuming
that the temperature difference between 10 km inland to 10 km offshore is
5 K and that the average temperature is 278 K. You may assume that at sunset
there is initially no temperature difference or horizontal pressure gradient
between the land and the lake.
(b) Is this wind directed from the land to the lake or from the lake to the land?
Why?
7.4 Calculate the relative vorticity at 500 mb over Amarillo, Texas, at 00 UTC 15 Feb
2003 (Figure 1.8) using the wind observations at this level from Albuquerque,
New Mexico; Midland, Texas; Oklahoma City, Oklahoma; and Dodge City,
Kansas.
7.5 Hurricane Katrina became the third strongest hurricane in the Atlantic Basin as
it moved over the Gulf of Mexico in August 2005. Between 15 UTC 26 Aug and
15 UTC 28 Aug 2005 the hurricane intensified rapidly as the central pressure
decreased from 981 hPa to 907 hPa.
(a) Calculate the relative vorticity of this hurricane at 15 UTC 26 Aug when the
maximum winds were observed to be 36 m s−1 at a distance of 14 km from
the center of the hurricane.
(b) Calculate the relative vorticity of this hurricane at 15 UTC 28 Aug when the
maximum winds had increased to 77 m s−1 at a distance of 18.5 km from the
center of the hurricane.
7.6 Construct a table similar to Table 7.1 for westerly flow over a mountain range
in the Southern Hemisphere.
7.7 Construct a table similar to Table 7.1 for easterly flow over a mountain range in
the Northern Hemisphere.
7.8 A westerly flow at 50 N latitude is approaching the Canadian Rocky Mountains,
and will rise adiabatically over the mountains. As the flow approaches the mountains, it is bounded by two potential temperature surfaces: 296 K at a pressure
of 850 hPa and 320 K at a pressure of 300 hPa. You may assume that frictional
effects are negligible.
(a) If there is no meridional flow initially, and no horizontal shear in the zonal
wind, what is the initial relative and absolute vorticity of the flow before it
reaches the mountains?
(b) Calculate the relative and absolute vorticity of the flow at the crest of the
mountains, assuming that the upper potential temperature surface is still at
a pressure of 300 hPa, while the lower potential temperature surface has
risen to 700 hPa, and the flow has been deflected 5 of latitude toward the
south.
134
CIRCULATION AND VORTICITY
7.9 (a) What terms in the Navier–Stokes equations (4.19) have been neglected in
the derivation of Equation (7.7)?
(b) Is it reasonable to neglect these terms for a synoptic scale mid-latitude
weather system? Why or why not?
7.10 (a) Rewrite Equation (7.7) by expanding the material derivative in this equation.
(b) Describe what each term in this equation represents physically.
(c) Which terms in this expanded equation are equal to zero, and why?
7.11 (a) Calculate the relative vorticity at Oklahoma City, Oklahoma at 500 hPa at
00 UTC and 12 UTC 16 Feb 2003 based on the adjacent wind observations
(use the storm of 2003 weather maps on the CD-ROM).
(b) Using the wind observations at 00 UTC calculate the local rate of change
of the relative vorticity due to the convergence term in Equation (7.7).
(c) Based only on the change in relative vorticity due to convergence, what
value of relative vorticity would you expect at 500 hPa over Oklahoma City
at 12 UTC 16 Feb 2003?
(d) How does this compare to the value calculated from the 500 mb map at
this time? Why would the value predicted based on the 00 UTC data differ
from that calculated from the observations at 12 UTC?
7.12 (a) Rewrite Equation (7.8) by expanding the material derivative in this equation.
(b) Describe what each term in this equation represents physically.
(c) Which terms in this expanded equation are equal to zero, and why?
8
Simple wave motions
The dual concepts of vorticity and waves will provide us with the basis for understanding how mid-latitude cyclones work. Vorticity is important because, as we have
seen, we can use it to create quantities that are conserved in some situations, and the
interplay between changes in planetary vorticity and changes in relative vorticity can
generate interesting motion. Waves are important because all weather systems can
be understood physically as waves with particular wavelengths, excited by various
forces acting on the atmosphere. This is why particular weather systems have characteristic scales – the scales are associated with the wavelengths and frequencies of
the excited waves.
8.1 Properties of waves
In this section, we review some basic definitions in the description of waves. Such
a disturbance can take the form of a traveling or propagating wave, or a standing
wave. The simplest of such motions is the linear harmonic oscillator – for example,
a pendulum (Figure 8.1).
In such a system the limits of oscillation are equally spaced around the equilibrium
position. It satisfies the equation
= 0 cost −
so that the motion can be completely described by knowing the amplitude 0 , the
frequency , and the phase = t − of the pendulum swing. The constant is
called the phase constant.
The amplitude denotes the maximum deflection of the pendulum. The frequency
tells us how many complete oscillations are performed by the pendulum in a given
period of time. The unit of frequency is cycles per second, or hertz (Hz). The phase
of a wave is determined from the location of the pendulum mass at an identified time
t = 0. From a mathematical point of view, this is the value of as the cosine curve
crosses the t axis. The amplitude and the phase of a particular oscillation depend on
the initial position and initial speed, but the frequency depends only on the length of
the string (a convenient attribute for early clock makers.)
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
136
SIMPLE WAVE MOTIONS
θ
l
Mg sinθ
Mg
Figure 8.1 A pendulum of mass M is suspended on a frictionless, massless string of length l
In the atmosphere, wave motions are oscillations in field variables such as velocity
and pressure. In this context, traveling or propagating waves are most important.
Such waves transport energy, but not matter. Because the wave is traveling through
the medium, we need to include position as well as time in our description of the
disturbance. Most generally, this can be written
= 0 coskx + ly + mz − t −
Now the phase = kx + ly + mz − t − becomes a more complex concept,
because we need to fix ourselves in time and in space to determine what part of the
⇀
oscillation we are observing. The vector represented by k = k l m is known as the
wavenumber, which is related to the wavelength by
⇀ 2
k =
The phase speed of the wave, a scalar variable, is the rate at which the phase of the
wave propagates in each of the three spatial dimensions, and is given by
cx =
k
cz =
l
m
c = ⇀
k
cy =
(8.1)
Since the phase speed depends on the wavenumber, waves of different wavelengths,
starting from the same place, will propagate at different rates and hence will spread
out, or disperse. Because of this property, the relationship between frequency and
wavenumber
⇀for any given wave is called the dispersion relation. In the situation
where ∝ k, the phase speed is not a function of wavenumber, and the wave is
called non-dispersive.
The rate at which the envelope of observable disturbance, and, with it, the energy
of the waves, travels is a vector quantity called the group velocity. This means that
137
PROPERTIES OF WAVES
when a disturbance is made up of several waves of different phases and amplitudes,
we can determine a rate and direction of travel that are representative of the entire
group of waves. The components of the group velocity are determined by
cgx =
k
cgy =
l
cgz =
m
(8.2)
Example Consider an atmospheric wave in which buoyancy is the restoring force.
To simplify, assume the wave is in only two dimensions (x and z). The frequency of
such a wave is given by
Nk
Nk
= ⇀ = √
2
k + m2
k
where N is the Brunt–Väisälä frequency (Section 3.4). The phase speeds of this wave
in the zonal and vertical directions are
k
N
= ⇀
k
cx =
m
Nk
= ⇀
k m
cz =
and the total phase speed in the direction of travel is
Nk
c = ⇀ = ⇀2
k k
Note that the zonal and vertical phase speeds are not components of a vector whose
magnitude is the total phase speed. However, the group velocity is a vector, whose
components are given by
k
Nm2
= ⇀3
k
cgx =
m
−Nkm
= ⇀3
k
cgz =
An example of just such a wave, known in atmospheric dynamics as a ‘gravity wave’,
is shown on the CD-ROM and described in more detail in Chapter 13.
138
SIMPLE WAVE MOTIONS
8.2 Perturbation analysis
Perturbation analysis is a mathematical tool that allows us to study waves in the
atmosphere by making some rather strict assumptions to remove the nonlinearities
in the chosen equations of motion (see Section 5.4.2). These assumptions are very
different from, for example, the scale assumptions of geostrophic flow, since they are
designed to reflect specifically the behavior of waves. In perturbation analysis, we
consider that all variables are a sum of some (yet to be defined) basic state and some
small departure from that state. The basic state and the departure can be functions
of space and time, but an important requirement is that the basic state variables
themselves satisfy the governing equations. A second important requirement is that
the departures from the basic state, known as the perturbations, are sufficiently small
that any products of perturbation terms can be neglected.
Example Consider the x component of the quasi-geostrophic horizontal momentum
equation
Dg ug
+ fua = 0
Dt
We define a basic state and a perturbation for each variable in the equation:
ug x y t = ūg x t + u′g x y t
ua x y t = ūa x t + u′a x y t
vg x y t = v̄g x t + vg′ x y t
where we have neglected variations in height since they do not appear in our equation
of interest. Substituting into the equation gives
Dg
ū + u′g + fūa + u′a = 0
Dt g
u′g
u′g
u′g
u′g
u′g
ūg
ūg
ūg
+
+ ūg
+ ūg
+ u′g
+ u′g
+ v̄g
+ vg′
+ f ūa + fu′a = 0
t
t
x
x
x
x
y
y
Omitting quantities in which products of perturbation terms appear yields
u′g
u′g
u′g
ūg
ūg
ūg
+
+ ūg
+ ūg
+ u′g
+ v̄g
+ f ūa + fu′a = 0
t
t
x
x
x
y
In addition, we have defined that the basic state satisfies the governing equation alone,
that is
ūg
ūg
+ ūg
+ f ūa = 0
t
x
139
PERTURBATION ANALYSIS
so we can derive the governing equation for the time evolution of the perturbation field:
u′g
t
+ ūg
u′g
x
+ u′g
u′g
ūg
+ v̄g
+ fu′a = 0
x
y
Note that this equation is linear.
Using this approach, we can create a linear version of the quasi-geostrophic vorticity equation (Equation (7.8)). We begin by assuming that the vertical velocity
⇀
is identically zero, and the basic state is a westerly zonal flow u = u 0 which
is constant, or at most a function of t:
Dg
w
g + f = f
Dt
z
Dg g
f
+ vg
=0
Dt
y
Such a system is non-divergent. Let us also assume that the Coriolis parameter varies
linearly with latitude. This is known as the beta plane approximation, since we
write f as
f = f0 + y
In general,
=
2 cos 0
a
(8.3)
where 0 is the reference latitude, that is the latitude at which the plane is tangent to
the Earth. When we center f0 on the middle latitudes, has a magnitude of around
10−11 , and the approximation is quite accurate for small departures from the reference
latitude. Using the beta plane approximation, we can derive the perturbation form of
the equation:
Dg g
+ vg = 0
Dt
′
′
′
′
¯g
¯g
¯g
¯g
g
g
g
g
+
+ ūg
+ ūg
+ u′g
+ u′g
+ vg′
+ vg′
+ vg′ = 0
⇒
t
t
x
x
x
x
y
y
′
′
¯g
¯g
¯g
¯g
g
g
+
+ ūg
+ ūg
+ u′g
+ vg′
+ vg′ = 0 omit perturbation products
t
t
x
x
x
y
′
g
t
+ ūg
′
g
x
zonal basic flow: ¯ g = 0
+ vg′ = 0
We can find a wave-like solution to this equation by assuming a solution of the form
u′g = −lA coskx + ly − t
vg′ = kA coskx + ly − t
140
SIMPLE WAVE MOTIONS
We use this form for the perturbation wind field to ensure that the solution is
non-divergent. By deriving an expression for g′ and substituting the assumed solution
into the simplified equation we can determine the dispersion relation for this wave:
′
g
=
′
g
vg′
x
u′g
−
y
= −Ak2 + l2 sinkx + ly − t
′
g
+ vg′ = 0
t
x
−−Ak2 + l2 coskx + ly − t + kūg −Ak2 + l2 coskx + ly − t
+ ūg
+Ak coskx + ly − t = 0
Ak2 + l2 − kūg Ak2 + l2 + Ak = 0
⇒=
−k
+ kūg
k2 + l2
(8.4)
As we can see, this wave is certainly dispersive (that is, different wavelengths travel
at different phase speeds). In fact, from the dispersion relation, we can see that the
phase speed in the zonal direction is
cx =
k
⇒ cx − ūg = − ⇀2
k
(8.5)
Hence, we see that longer waves (with a smaller wavenumber) travel faster than
shorter waves. Ignoring the contribution of the zonal wind, propagation is toward the
west (that is, in the negative x direction). Equation (8.5) is known as Rossby’s formula,
named after Carl-Gustav Rossby, who substantially (though not solely) developed
this analysis in a series of important papers in the late 1930s and early 1940s.
8.3 Planetary waves
The formulation we derived in Section 8.2 made some very strong assumptions in
order to simplify the solution of the equations. Our assumption of non-divergence
(equivalent to assuming barotropic flow) can only be justified if we are considering
the scales of motion in the atmosphere that are much larger than mid-latitude synoptic
systems. At such large scales, disturbances in the atmosphere are two to three orders
of magnitude broader than they are deep, and the assumption of barotropy is not
so problematic. Hence, it is appropriate to apply this equation to the largest scales
of flow away from the surface, and in fact Rossby’s formula describes very well
the behavior of planetary waves, shown in Figure 8.2. These waves circle the globe
in the middle latitudes, and can be seen in this map as peaks above 1550 m and
PLANETARY WAVES
141
Figure 8.2 The 850 hPa geopotential height (m) over the Southern Hemisphere during
January 2003. Peaks in the wavenumber 3 pattern are labeled with an ‘X’. NCEP Reanalysis data
provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from its Web site at
http://www.cdc.noaa.gov/
troughs below 1525 m in the geopotential height field. The figure shows roughly
three peaks (indicated by an ‘X’), and hence this example is called a ‘wavenumber 3’
planetary wave.
We see from Equation (8.5) that the planetary waves, also known as Rossby waves,
are advected with the basic zonal flow. Because of this, we may consider three
possible situations:
1. Shorter waves, which have large wavenumber, will propagate more slowly. If
⇀2 < ūg
k
the waves will appear to move slowly eastward.
142
SIMPLE WAVE MOTIONS
2. Medium waves in which
⇀2 = ūg
k
will appear to be stationary on the Earth.
3. Longer waves, which have a small wavenumber, will propagate more quickly.
Hence, when
⇀2 > ūg
k
the waves will appear to move slowly westward; that is, in a retrograde sense
(that is, against the prevailing westerly flow.)
The critical wavenumber at which stationary waves will be observed is, of course,
⇀
/ūg . This is known as ks , the stationary wavenumber.
It is straightforward to use this relationship to derive the other aspects of interest
regarding this wave. We already know the form of the wind field in the zonal and
meridional direction, and the vorticity field (see the example in Section 8.2). The
wavelength can be determined simply by counting the number of peaks and troughs
around a given latitude circle, which has length 2 a cos , where a is the radius of
the Earth. For n waves at latitude 45 ,
2 a cos
n
√
2 a
=
n
=
⇒k=
2
√
2n
=
a
cx = ūg − ⇀2
k
= ūg −
a2
2n2
√
Example Since for 0 = 45 = 2/a, we can determine the period T for n
waves around a latitude circle. Neglecting the zonal wind,
T =
cx
PLANETARY WAVES
=
√
√
143
2 a 2n2
× 2
n
a
2n2 a
2 a
×√
n
2a2
2 n
=
=
Since = 2 /day, the period is simply n days. Hence, for example, the wavenumber
3 pattern that we see in Figure 8.2 has a period of 3 days, a wavelength of
√
√
2 a
2 × 637 × 106
=
= 943 × 106 m
=
n
3
and a phase speed, excluding the effects of a prevailing zonal flow, of
a2
2n2
a
= −√
2n2
7292 × 10−5 × 637 × 106
=−
√
2×9
cx = −
= −365 m s−1
This calculation also tells us that for a prevailing westerly zonal flow of 365 m s−1 ,
or 709 kts, 3 is the stationary wavenumber.
The vorticity equation we used to derive this information about planetary scale
waves was rendered geostrophic by our assumption of non-divergent flow. As a
result, the restoring force for the wave can be understood by considering the force
balance between Coriolis and pressure gradient forces expressed in the geostrophic
wind relationship. Because the Coriolis force increases with latitude, so too does
the pressure gradient, providing the restoring force for the wave motion. However,
the conservation of potential vorticity (Section 7.3) gives a clearer demonstration of
how planetary waves work. In fact, the example discussed in that section described
a topographically forced Rossby wave.
8.3.1 Forcing of planetary waves
Although the planetary wave patterns take on a great variety of configurations, there is
a tendency for the longer waves to occur in preferred geographic locations. Figure 8.3
shows the 500 hPa geopotential height anomaly (difference from the long-term mean)
for the northern winters from 1968 to 1996. Using the anomaly pattern we can discern
144
SIMPLE WAVE MOTIONS
Figure 8.3 The 500 hPa geopotential height (m) anomaly for the winter season (DJF) over the
Northern Hemisphere for the period 1968 to 1996. NCEP Reanalysis data provided by the NOAA-CIRES
Climate Diagnostics Center, Boulder, Colorado, from its Web site at http://www.cdc.noaa.gov/
the preferred locations of peaks and troughs of the geopotential height and see that
they are associated with meridionally oriented mountain ranges.
In the Northern Hemisphere winter, for example, a four-wave pattern is apparent,
with troughs located in the North Atlantic, in northern Scandinavia southward into
the Mediterranean Sea, the Aleutian region of the north Pacific, and in the vicinity
of the Rocky Mountains in North America. The persistence of particular patterns
seems to suggest that the forcing mechanisms for these waves also exhibit some
kind of persistence and that the waves are quasi-stationary. There must also be a
mechanism that allows variability. Of course, at any given moment the atmosphere
will not look like this pattern, not just because of this variability, but because transient
disturbances (weather) can be as large as the more slowly varying Rossby wave
disturbance.
In Figure 8.3, we see that troughs are located preferentially on the lee side (to
the east) of mountain ranges, with relatively higher pressure on the windward or
PLANETARY WAVES
145
upstream side. This can be interpreted as the atmosphere exerting an eastward force
on the solid Earth, with the Earth exerting an equal and opposite force on the
atmosphere.
In fact, forced waves are generated by three principal mechanisms. In addition to
the topographic forcing that can be inferred from the basic patterns, Rossby waves are
generated by thermal forcing due to longitudinal heating differences associated with
the distribution of oceans and continents, and by forcing due to nonlinear interactions
with smaller scale disturbances such as extra-tropical cyclones.
Consider now the vertical variation of geopotential height anomaly (Figure 8.4).
The patterns vary little with height, although the wind does become stronger as one
reaches the tropopause (usually round 150 hPa at middle latitudes.) This confirms that
our use of the barotropic model is quite appropriate. The more interesting feature is
that the pattern tilts westward with height. This is a reflection of the upward transport
of energy and westward momentum, as successive layers of air exert a westward
pressure force on the layer above.
The simplest approach to solve for a topographically forced Rossby wave is to
assume a sinusoidal mountain and search for a stationary solution (that is, omit any
Figure 8.4 Geopotential height anomaly for the winter season (DJF) at 45 N for the period 1975
to 2005. NCEP Reanalysis data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder,
Colorado, from its Web site at http://www.cdc.noaa.gov/
146
SIMPLE WAVE MOTIONS
variation with time). Drawing from the pioneering work of Charney and Eliassen
(1949), we can return to Equation (7.8):
Dg
w
g + f = f
Dt
z
In a barotropic fluid, the thermal wind vanishes, and hence the wind and the vorticity
are not functions of z. This allows us to integrate the above equation from a lower
boundary h x y to an upper boundary at the tropopause, which is assumed to be at
constant height H. Then,
Dg
Dz
Dz
H − hx y g + f = fwH − wh = f
−
Dt
Dt H Dt h
⇒H
Dg
Dh
h
+ f = −f
= −f ūg
Dt g
Dt
x
if we assume that h x y ≪ H. Defining the lower boundary as a sinusoidal mountain,
hx y = h0 sinkx + ly
we then assume a stationary wave solution:
u′g = −lA coskx + ly
vg′ = kA coskx + ly
Substituting this assumed solution into the governing equation yields an expression
for the amplitude of the wave, A, rather than the dispersion relation:
2
fh
⇀
A k −
= 0
ūg
H
A=
fh0
1
2
H ⇀
2
k − ks
(8.6)
⇀
We have in this solution an unrealistic case when k = ks and the amplitude goes
to infinity. Since this occurs at the zonal wind speed for which a free Rossby wave
becomes stationary, it can be thought of as a resonant mode, which is an expected
property of a linear system with no damping. Charney and Eliassen (1949) addressed
this problem by introducing friction to their model. This removed the singularity and
shifted the phase of the response so that it was more in line with the observations.
As a result, they developed a model which they rightly recognized as having the
potential to explain the winter mean circulation in the middle latitudes, and enable
the evolution of the polar front (Section 1.4) and the cyclone tracks associated with it.
REVIEW QUESTIONS
147
Review questions
8.1 Sound waves that are audible to the human ear have frequencies ranging from
20 to 20 000 s−1 . In air at a temperature of 20 C the phase speed of sound waves
is 343 m s−1 . This phase speed does not vary with wavelength. Consider a sound
wave that is propagating in time and in the x direction only.
(a) Calculate the wavenumber and wavelength for this range of frequencies.
(b) Calculate the group velocity of sound waves for the smallest and largest
frequency audible waves. How does the group velocity compare to the phase
speed of these waves?
(c) How long does it take sound waves with frequencies of 20 s−1 and 20 000 s−1
to travel 1 km?
(d) Are sound waves dispersive or non-dispersive?
8.2 An explosion generates sound waves with frequencies of 1000 and 2000 s−1 .
These sound waves have k = 292 m−1 and 583 m−1 respectively. Based on this
information, calculate the phase speed for these sound waves. Does this match
the value given for the phase speed of sound waves in question 8.1?
8.3 The waves discussed in the example in Section 8.1 are known as internal gravity
waves. As shown in this example, the frequency of these waves depends on
the Brunt–Väisälä frequency and the horizontal and vertical wavenumber of
the wave.
(a) For an atmosphere with N = 0023 s−1 find the frequency of waves with:
(i) k = 314 × 10−3 m−1 and m = 458 × 10−3 m−1 ; (ii) k = 628 × 10−3 m−1
and m = 574 × 10−3 m−1 .
(b) What is the horizontal and vertical wavelength of these waves?
(c) Calculate the horizontal and vertical phase speed of these waves.
(d) Calculate the horizontal and vertical group velocity of these waves.
(e) Are these waves dispersive? Why or why not?
8.4 (a) The value of the Coriolis parameter can be estimated using the beta plane
approximation (Equation (8.3)). Using a reference latitude of 45 , calculate
values of the Coriolis parameter using the beta plane approximation at
latitudes = 45 , 44 , 40 , 30 , 25 , and 20 .
(b) Use Equation (5.5) to calculate the value of the Coriolis parameter at these
latitudes.
(c) At which latitudes can the beta plane approximation be used with errors of
less than (i) 5% and (ii) 10%?
148
SIMPLE WAVE MOTIONS
8.5 Verify that the assumed solution for planetary waves, used to derive Rossby’s
formula, is indeed non-divergent.
8.6 Sketch the zonal and meridional wind fields and the vorticity field for a Rossby
wave over one wavelength in the x direction.
8.7 (a) Calculate the zonal wavelength and wavenumber for planetary waves at a
latitude of 40 N if there are a total of two, three, four, and five waves
around the latitude circle.
(b) Calculate the zonal phase speed of these waves assuming that ūg = 0 m s−1 .
(c) Which wave has the largest zonal phase speed? Is this consistent with the
discussion in Section 8.3?
(d) What is the period of these waves?
(e) What mean zonal wind speed is required to cause each of these waves to
be stationary?
8.8 (a) Using the weather maps for the storm of 2003 on the CD-ROM, estimate
the wavelength of the wave at 500 hPa at 00 UTC 16 Feb 2003.
(b) Calculate cx − ūg for this wave.
(c) Estimate the phase speed of this wave using the 500 hPa weather maps at
00 UTC and 12 UTC 16 Feb 2003.
(d) What value of ūg is required for the wave to have this phase speed? Is your
estimated value of ūg consistent with the observed zonal winds at this time?
8.9 (a) At a latitude of 40 S what is the wavenumber of a stationary planetary
wave if ūg = 5, 10, and 25 m s−1 ?
(b) What is the wavelength of each of the waves in part (a)?
(c) What is the direction of propagation for waves with k < ks ? What is the
direction of propagation for waves with k > ks ?
8.10 (a) Estimate the horizontal (zonal and meridional) scale of the Colorado Rocky
Mountains, Himalayas, Andes Mountains, and Southern Alps of New
Zealand.
(b) Calculate the amplitude and perturbation zonal and meridional geostrophic
winds for planetary waves forced by each of these mountain ranges for
ūg = 20 m s−1 . (You will need to estimate the height of each mountain range
and can assume that the tropopause is at an altitude of 11 km.)
9
Extra-tropical weather
systems
An important feature of the mid-latitude flow in the middle to upper troposphere is
the wave-like structure of westerly flow (Figure 9.1). Some of these waves, as we
have seen, are Rossby waves. These may be close to stationary, moving very slowly
eastward with the westerly flow, or long enough to retrogress (move westward). In
addition, short waves are embedded in the Rossby waves. There can be anywhere
between 6 and 18 short waves traversing a hemisphere at any one time, with wavelengths ranging from 4500 km down to 1500 km. As the short waves move through
the long-wave pattern, the various waves interfere with one another, resulting in the
complex flows that we observe in weather maps.
Extra-tropical synoptic scale weather systems are observed as closed circulations
at the surface but these are simply the surface expression of short waves in the upper
westerlies, first introduced in Section 1.3.3. Recall from Section 1.4 that typical
extra-tropical cyclones are frontal cyclones; that is, they originate at the polar front,
a highly baroclinic zone. Hence, the behavior of the short waves cannot assume the
barotropic model as was appropriate for long waves. While this results in a more
complex mathematical treatment, there are aspects of these short waves that we can
consider using highly simplified models.
9.1 Fronts
9.1.1 Margules’ model
The simplest model representing a front as a discontinuity in air properties is Margules’ model, first published in 1906 by the Austrian meteorologist Max Margules.
In this model, a cold front is idealized as a tilted plane separating two homogeneous,
geostrophic flows with different, but uniform, temperatures and densities. The effects
of friction near the surface are explicitly excluded. To develop this model, we choose
a coordinate system in which the x axis is normal to the front and the y axis is
parallel to the front (Figure 9.2). The front is stationary and infinitely thin, and we
assume that the across-front changes in temperature and density are relatively small
(the Boussinesq approximation).
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
150
EXTRA-TROPICAL WEATHER SYSTEMS
Figure 9.1 The 850 hPa geopotential height on 15 February 2003. NCEP Reanalysis data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from its Web site at
http://www.cdc.noaa.gov/
frontal surface
y
vg2
p2, ρ2
z
ε
vg1
p1 , ρ 1
x
Figure 9.2 Idealized depiction of a frontal discontinuity
In such a model, the only flow is along the front, in the y direction, since an
across-front wind would imply that the front was moving. Following Smith (1990),
we define the pressure, density, and along-front wind in the warm air mass at p1 1 ,
and vg1 , and similarly for the cold air mass p2 2 , and vg2 . Because we are making
the Boussinesq approximation, density differences across the front are important
only inasmuch as they create buoyancy forces, and hence we can use the vertical
momentum equation (6.10), where we have defined the reference density to be 2 :
1 p 2 − 1
+
g=0
2 z
2
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FRONTS
Using this equation we can integrate from the ground to the height of the frontal
surface h x, and determine how this varies by differentiating with respect to x,
yielding
1
2
p1 p2
−
x
x
=−
2 − 1 h
g
2
x
(9.1)
Geometry tells us that h x = −x tan and hence we can write
1
2
p1 p2
−
x
x
=−
2 − 1
g tan
2
To determine the along-front wind, we simply apply the geostrophic relationship
fvi =
1 pi
i = 1 2
i x
to get
− 1
g tan
f v2 − 1 v1 = − 2
2
2
(9.2)
which is Margules’ formula for relating frontal slope to the along-front wind differential. While this is a highly simplified model, several practical conclusions may be
drawn from it. For example, Equation (9.2) represents a balance between buoyancy
forces and net Coriolis forces important for maintaining the stationary front. This
balance is indicative of the dynamical processes in a real front. Also, the equation
requires a cyclonic change in wind across the front. For example, in the northern
hemisphere where f > 0, the along-front wind differential will be positive. Rarely,
horizontal acceleration in a real front may be sufficiently large that the cyclonic wind
shift at the front is suppressed, but more generally, this observation applies.
Example Figure 9.3 shows our case study cyclone as it reached Missouri on 15
February 2003. Do the weather reports from Oklahoma City and Little Rock, situated
roughly the same distance from the front, on either side of it, indicate a cyclonic
change in flow?
The station model for Oklahoma City tells us that the pressure was 1010.4 hPa,
the temperature was 42 F 6 C, and the dew point was 36 F 2 C. The wind was
15 kts, from the NNW. This is clearly in the cold dry air mass, as would be expected.
Little Rock reported a pressure of 1007.0 hPa, a temperature of 63 F 17 C, and
a dew point of 61 F 16 C and fog – a warm moist air mass in comparison. The
wind was 10 kts, from the south. The report from Dallas, just ahead of the cold front,
shows saturated air that has turned SW. Hence, we have a cyclonic (counterclockwise
in the Northern Hemisphere) change in the wind reports.
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EXTRA-TROPICAL WEATHER SYSTEMS
1016
37 120
12
10
34
63
59
L
1012
50
61
55
63 070
10
42 104
36
10
08
55 051
08
16
10
34
32
61
62 048
62
62 070
61
62 097
61
12
10
1008
Figure 9.3 Sea level pressure and station reports over Arkansas and surrounding states on 15
February 2003 at 12 UTC. The warm front is omitted for clarity
Nevertheless, the shortcomings of this model are severe. The model does not take
account of the vertical variations in temperature and density observed in the real
atmosphere. More importantly, this model cannot represent a front which is moving
normal to the frontal plane. This is because a simple coordinate transformation to
an inertial frame moving with the speed of the front would introduce an additional
Coriolis force in the along-front direction. The geostrophic approximation demands
that this be balanced by a pressure gradient force, but Smith and Reeder (1988) show
that this leads to an inconsistency, as follows.
Margules’model requires that we satisfy the requirement that the across-front wind
component is continuous, that is
u1 = u2
(9.3)
and also the requirement that the pressure across the front be continuous. This second
requirement has a corollary that the mass flux across the front be continuous (since
pressure is an expression of the movement of mass), that is
1 u1 = 2 u2
(9.4)
Since 1 = 2 we cannot simultaneously satisfy Equations (9.3) and (9.4) unless there
is vertical motion (which violates the assumption of geostrophy) or u1 = u2 = 0.
Hence, like the geostrophic model itself, Margules’ model should be considered to
be a useful conceptual ‘snapshot’ of a front rather than a truly descriptive model.
9.2 Frontal cyclones
The description of the common structure and dynamics of frontal cyclones
(Section 1.4) is drawn from the remarkable synthesis of Bjerknes and Solberg, who
FRONTAL CYCLONES
153
drew together the many disparate observational and theoretical studies into a single conceptual model, shown in Figure 1.10. The basic principle developed in this
work was that every cyclone is characterized by two fronts – the warm front in
advance of the cyclone center (known earlier as the ‘steering line’) and the cold front
behind the cyclone center (known earlier as the ‘squall line’). The model was further
developed by Bjerknes and Solberg to explain the formation and development of
cyclones, and how the frontal structure is created (Figure 1.9). This process indicates
that the nascent cyclone originates as a small wave whose amplitude then gradually
increases through some instability mechanism. It is this process of cyclogenesis and
the associated decay of cyclones, or cyclolysis, that will be discussed now.
9.2.1 Cyclogenesis
If a surface cyclone is to form or deepen, there must be net divergence of the mass of
air in the region above the cyclone, for only then will the surface pressure decrease.
As we saw in Section 1.4, a cyclone develops from a small wave in the polar front,
which generates a region of relatively lower pressure at the surface. Due to the
effects of friction near the ground disrupting the pure geostrophic balance, the air
will spiral in toward the center of low pressure. In a deepening low, the isallobaric
wind (Section 6.5.1) will also contribute significantly to the lower level convergence.
This convergence adds air to the column and hence causes the surface pressure to
increase. So, for surface pressure to continue to decrease, air must be removed from
the column above the center of the cyclone faster than it is spiraling in at the surface.
That is, the upper level divergence must outpace the lower level convergence.
We can explain why Rossby waves in the middle and upper troposphere generate
convergence and divergence by representing the wind field associated with the wave
to be the gradient wind, ignoring changes with time and frictional effects. By using
the gradient wind model, we are allowing an ageostrophic component of the model,
and hence we are allowing convergence and vertical motion (Equation (6.14)). Recall
from Section 6.2.4 that in cyclones, the gradient wind is less than the geostrophic wind
and in anticyclones the gradient wind is greater than the geostrophic wind. Hence,
as air moves through such a wave, parallel to the pressure contours (Figure 9.4),
it attains its highest speed in the ridges and lowest speed in the troughs, since the
true wind is most accurately represented by the gradient wind. Thus, the wind speed
must decelerate as the air moves from a ridge to a trough, causing a convergence
of mass. Similarly, as we move from the low-speed trough to the high-speed ridge,
acceleration and divergence must occur. And so these are the ideal conditions for
what we might expect to see in a surface development – a region of upper level
divergence occurring to the east of an upper level trough, allowing surface pressures
to decrease, resulting in lower level convergence.
However, in order for cyclogenesis and cyclone intensification to occur, the upper
level divergence must exceed the lower level convergence. How do we determine
this? First, recall that our model suggests that cyclones form on the polar front.
Due to thermal wind requirements, the strong horizontal temperature gradients at
the front will lead to strong geostrophic winds aloft. These persistent strong winds
154
EXTRA-TROPICAL WEATHER SYSTEMS
supergeostrophic
LO
convergence
y
divergence
HI
elevation (km)
subgeostrophic
10 convergence
convergence
x
divergence
convergence
convergence
divergence
5
z
divergence
0
x
Figure 9.4 Lines of geopotential height are related to the gradient wind and the divergence field
in the horizontal (top) and the vertical (bottom)
are called the polar jet stream. The polar jet stream circles the globe in the middle
latitudes, experiencing considerable variations in latitude in response to the long
waves and short waves propagating along the polar frontal zone. In addition, the jet
stream experiences considerable variations in magnitude along its path – an example
is shown on the CD-ROM. In fact, the jet speed can vary by a factor of 2 or even
3 – the zones of highest polar jet stream speeds are called jet maxima or jet streaks.
Jet maxima are associated with short waves in the westerlies and can be thought
of conceptually using the same gradient wind argument made above. Both the short
waves and the accompanying jet maxima generally propagate eastward through the
relatively slow moving Rossby waves.
What is generally observed is that cyclones form in specific regions of jet maxima,
either on the left side of the jet exit or on the right side of the jet entrance. For example,
the development of the offshore low on 16 and 17 February 2003 (Section 1.4.3)
takes place in a location corresponding to the right side of the jet entrance, and the
maximum is located over the North Atlantic (Figure 9.5).
So what is it about the jet maximum that causes divergence in the first place, and
how does that process continue to intensify? Recall the example in Section 6.5.1 in
which we found that, when a changing pressure field in space or in time creates an
imbalance of forces, a cross-isobaric flow is generated. This cross-isobaric flow in turn
creates regions of convergence and divergence and, as predicted by Equation (6.14),
vertical motion. Let us apply this concept to the idealized Northern Hemisphere jet
maximum depicted in Figure 9.6.
If we neglect changes in time and assume the geostrophic flow is approximately
zonal, we can apply a simplified form of Equation (6.17):
va =
1 ug
u
f g x
155
FRONTAL CYCLONES
36
1034
1032
44
36
4
12
20
1028
1012
1016
1020
1024
28
20
12
20
12
Figure 9.5 Sea level pressure (hPa, left) and 500 hPa wind shown as contours of isotachs (lines
of constant wind speed) with some indicative direction vectors (m s−1 , right) on 17 February 2003
(both fields slightly smoothed), showing the location of the jet maximum in the north Atlantic,
and the split jet over North America
Left Entrance
Convergence
Left Exit
Divergence
LOW
va
va
Divergence
Right Entrance
HIGH
Convergence
Right Exit
Figure 9.6 Horizontal view of the isobars (solid lines) and isotachs (dashed lines) with a typical
parcel trajectory (dotted line) in an idealized jet maximum. Jet flow is in the x direction. The four
quadrants of the jet are labeled
As in the earlier example from Section 6.5.1, the region of confluence at the jet
entrance (as the isobars come together) results in ug increasing as the flow enters the
jet streak. Since ug /x > 0 in the entrance region of the jet and all other terms in
the equation above are positive in the Northern Hemisphere, this results in va > 0
(an ageostrophic flow in the positive y direction from south to north). Since this
wind is toward low pressure, the flow extracts work from the pressure gradient and
accelerates. In the jet exit, a region of diffluence (where isobars are spreading apart),
ug /x < 0, and a southward ageostrophic flow is generated – this flow is toward high
pressure and decelerates. The cross-isobaric flow creates regions of convergence and
divergence on either side of the jet exit and entrance, as shown, which in turn give
rise to vertical motion. In the regions of upper level divergence, and thus ascending
air, high-level cirrus cloud is often observed. In the regions of convergence, and
descending air, any existing cloud may be expected to clear. In satellite pictures, the
position of the jet is often indicated by a streak of high-level cloud, frequently with
a sharp boundary along the jet axis.
156
EXTRA-TROPICAL WEATHER SYSTEMS
Thus, the additional divergence provided on the left jet exit and the right jet
entrance (in the Northern Hemisphere) provides the mechanism by which the upper
level divergence can outpace the lower level convergence, and allow cyclones to
intensify.
Example Figure 9.5 shows the newly developed cyclone on the east coast of North
America on 17 February 2003. It is just about to occlude, and at 12 UTC it was already
weakening. However, we can use this example to determine the typical magnitudes
of ageostrophic flow and vertical motion that may occur in such a cyclone. In this jet
maximum, air is accelerating from around 24 m s−1 to around 44 m s−1 over a distance
of about 20 of longitude, or about 1500 km at this latitude. This would generate an
ageostrophic wind of
44 − 24
1
24 + 44
1 ug
≈ −4
≈ 4 5 m s−1
va = ug
f x
10
2
1500 × 103
Assuming that the air 500 km on either side of the jet is undisturbed, and the velocity
maximum is around 2 km below the tropopause, the expected vertical motion would be
v
45
w
≈ 2 cm s−1
= − a ⇒ w ≈ 2 × 103 ×
z
y
500 × 103
Such a vertical displacement would be sufficient to cause cloud formation in the
ascending air. In this example, the development was aided by the warmth of the
Gulf Stream flowing northward along the Atlantic coast driving vertical motion
from below.
9.2.2 Sutcliffe Development Theory
Reginald C. Sutcliffe showed in 1938 that one can create a prognostic equation
for cyclogenesis by using the vorticity equation. This approach is more accurate
than trying to predict the net column divergence, because the integrated column
divergence is a small residual of the often large contributions at various levels. This
latter approach was used by Lewis F. Richardson in his famous numerical weather
prediction experiment which he published in 1922 – in a 6 hour hand-calculated
forecast still regarded as ‘heroic’, he predicted a pressure increase of 145 hPa for the
area around Munich, Germany on 10 May 1910. Richardson himself identified the
large apparent convergence of wind to be the source of the error. Sutcliffe’s method
using vorticity is a significant improvement on this approach.
Figure 9.7 shows an idealization of the vertical distribution of horizontal divergence
and vertical motion for a mid-latitude cyclone. There is not an exact correspondence,
level by level, between convergence and ascent. Ascent continues, for example,
even through the lower levels of divergent flow. It is also clear that the curvature
of the vertical velocity profile is consistently negative throughout the depth of the
157
FRONTAL CYCLONES
200 hPa
400 hPa
∆
.ua
600 hPa
w
800 hPa
1000 hPa
convergence
descent
divergence
ascent
Figure 9.7 Typical patterns of divergence (dashed line) and ascent (solid line) in a mid-latitude
cyclone
troposphere – our initial aim in developing this theory is to determine a diagnostic
formula for this curvature. We can do so using the quasi-geostrophic vorticity equation
(Equation (7.8)):
Dg
w
g +f = f
Dt
z
g
g
g
f
w
+ ug
+ vg
+ vg
=f
t
x
y
y
z
f
g
ug g
vg g
vg
2 g
2 g
2 w
f
+ fug
⇒
+ fvg
+f
+f
+f
= f2 2
z
t
z
xz
yz
z x
z y
z
z
We must eliminate the time derivative from the left hand side of the equation to turn
this from a prognostic into a diagnostic relation. To do this, we relate the hydrostatic
equation in Boussinesq form (Equation (6.10)) to vorticity using Equation (5.11):
vg ug
1
−
=
g =
x
y
f00
2 p 2 p
+
x2 y2
Then by applying the operator
2
2
+
x2 y2
t
to
− 0
1 pd
g
=−
00 z
00
158
EXTRA-TROPICAL WEATHER SYSTEMS
we get
2
1 2 p 2 p
2
+
+
=
t z 00 x2 y2
x2 y2 t
2
g
2
f
=
+
t
z
x2 y2 t
This may not seem like much of an improvement, but we can then eliminate time
from the buoyancy term using the continuity equation as follows:
D
=0
Dt
D
g
0 + − 0 = 0 × −
Dt
00
D
g D0
−
=0
Dt 00 Dt
D
g d0
−
w
=0
Dt 00 dz
We then define a buoyancy frequency Nd2 = −gd0 /gd00 z and employ quasigeostrophic scaling to omit the advection of the buoyancy term by vertical motion
and by the ageostrophic wind:
+ ug
+ vg
+ wNd2 = 0
t
x
y
Using this, we can write a (rather complex) diagnostic equation for the curvature of
the vertical velocity:
2
2
+ 2
2
x
y
−ug
2 g
2 g
ug g
− vg
− wNd2 + fug
+ fvg
+f
x
y
xz
yz
z x
(1)
(2)
(3)
(4)
2
vg g
vg
w
+f
+f
= f2 2
z y
z
z
(5)
(6)
In Sutcliffe’s derivation the adiabatic buoyancy term (2) was neglected, and he also
determined that terms (1) and (3) cancel in part, leaving a quantity known as the
deformation, which is small, and a term identical to term (4). By making these
simplifications, the complex equation above reduces to a much simpler one:
2f
ug g
vg g
vg
2 w
+ 2f
+f
= f2 2
z x
z y
z
z
159
FRONTAL CYCLONES
Now, it is possible to integrate this equation from the surface to some height H. By
choosing H to be a height at which the divergence is negligible, this can be written
w
=−
f
z 0
H
2
0
ug g
vg g
vg
dz
+2
+
z x
z y
z
(9.5)
Thus, we have derived a technique for diagnosing the vertical velocity based on the
vorticity. This relationship can also help to elucidate the mechanisms by which vorticity changes with time, in the quasi-geostrophic framework. If we can predict zones of
increasing vorticity, these will correspond to likely cyclogenesis and intensification.
Substituting Equation (9.5) back into Equation (7.8),
Dg
Dt
g
H
vg g
vg
ug g
dz
+2
+
+f = −
2
z x
z y
z
0
(9.6)
H
g
g
g
vg g
vg
ug g
dz
= −ug
− vg
− vg −
+2
+
2
t
x
y
z x
z y
z
0
To simplify this further, let us assume that all quantities vary linearly with height.
This constant vertical gradient is associated with the thermal wind variation and
hence we will use the subscript T . Then,
ug
u
= T
z
H
vg
v
= T
z
H
g
=
0+ T
z
H
We can substitute these into Equation (9.6) to create an approximate prognostic
equation for the surface vorticity:
g0
g
g0
= − ug0 + 2uT
− vg0 + 2vT
− uT T − vT T − vg0 + vT
(9.7)
t
x
y
x
y
1
2
3
This is Sutcliffe’s development equation.
Despite the considerable simplifications and approximations that have been made,
Sutcliffe’s formulation, like Margules’ model, yields important physical insights. Let
us consider each of the terms on the right hand side of this equation.
At the center of a surface low-pressure system, the geostrophic wind speed will be
much smaller than the wind shear, which is represented by uT . Hence at this location,
the advection term (1) can be written
−2uT
g0
g0
− 2vT
x
y
This suggests that the low-pressure center will propagate in the direction of the wind
shear, with a speed proportional to that wind shear. This is known as the thermal
160
EXTRA-TROPICAL WEATHER SYSTEMS
steering principle. Since we have assumed that ug0 ≈ 0, then uT ≈ ugH . This is why
the lines of constant thickness at upper levels are often broadly similar to lines of
geopotential height. It is often assumed that the level of zero divergence, H, is around
the 500 hPa level. If this is approximately true, then the low-pressure system will
tend to move in the direction of the 500 hPa wind. It is important to realize that the
system moves through a process of development, not simply translation.
The ‘thermal vorticity advection’ term (2) is the primary contribution to the development of mid-latitude cyclones. Consider the short-wave pattern shown in Figure 9.8,
represented by a wave in the thickness field. In such a diagram, the wind flowing
along the wave is the wind shear. If we assume the wave is sinusoidal in form, the
maximum gradient in T is one-quarter wavelength east of the thermal trough, and
hence we can expect that term (2) is likely to be a maximum here also. This location is referred to as a positive vorticity advection maximum. Similarly, one-quarter
wavelength east of the thermal ridge, we are likely to find a region of preferential
anticyclonic vorticity generation.
The final term in the equation is simply the contribution of the planetary vorticity.
Term (3) shows that cyclonic (that is, positive) vorticity is generated if the meridional
wind is equatorward. The term is a small contributor to the overall development
formula.
Used with care, Sutcliffe’s development equation can be, and has proved to be, a
very useful guide in weather forecasting. As with any simplified conceptual model,
we must take account of its limitations when applying it. First, it neglects the effects
of both adiabatic and diabatic heating. These often make important contributions
to the overall evolution of the system. Indeed, the omission of the adiabatic term
causes the speed of propagation to be overestimated by a factor of 2. Second, the
formulation gives an estimate of tendencies at a given moment, or in practical terms,
for a few hours. Thus, we would not expect Sutcliffe’s equation to be a substitute
for a good numerical weather prediction model. Finally, because of the assumptions
made, the model fails badly when the shear is small. This problem is of particular
uT
ζTmax
ζTmin
fmin
fmax
LO
uT
cold
divergence
uT
warm
convergence
HI
Figure 9.8 Lines of atmospheric thickness are related to the wind shear, the ‘thermal vorticity’,
and the divergence field
BAROCLINIC INSTABILITY
161
concern when a system undergoes occlusion and is then isolated from the polar front
and the associated strong shear.
9.2.3 Cyclolysis
The formation of an occluded front is often the first indication that a cyclone is about
to dissipate. However, the process of occlusion is still a subject of active research.
Even in the initial development of the theory, revisions occurred: originally Bjerknes
believed that the occlusion, where the cold front catches up to the warm front, would
initially occur away from the low center, but this view was probably influenced by
observations made in Scandinavia at the time which were influenced by the local
orography. Later work done in the United States by F. W. Reichelderfer suggested
that the warm sector closes from the low center outward. Further, some scientists
suggested that the ‘neutral occlusion’ suggested in the Norwegian model, in which
thermal gradients disappeared at all levels in the atmosphere, would be unusual.
This idea was supplanted with the ideas of warm occlusions, in which the polar air
behind the cold front is warmer than the polar air ahead of the warm front, and cold
occlusions, in which the reverse was true. It was suggested that cold occlusions would
be more commonly observed, since air behind the advancing cold front was less
likely to have been modified from cold stable polar conditions than air ahead of the
advancing warm front. The concept was further developed by Tor Bergeron in 1937 to
include the idea of a bent back occlusion, which occurs in the presence of secondary
developments. In 1969, R. K. Anderson introduced the idea of instant occlusions, in
which an occluded structure is created without the classic frontal ‘catch-up’ process.
This was followed by a period in which observational studies consistently failed to
verify the occlusion process, and in particular, the surface signals of occlusion.
So, the theory of occlusion and the Norwegian model in general was challenged for
a considerable period of time. Key researchers suggested in the 1980s and early 1990s
that the process did not occur. New conceptual models were developed. More recently,
though, a combination of intense observational studies and model simulations have
confirmed that the classical occlusion process does occur, though not in all cases
(some examples are shown on the CD-ROM). Even now, it is frequently observed
that a cyclone that appears to follow the classic life cycle will continue to intensify
after occlusion appears to have taken place. So, refinement of this conceptual model
continues.
9.3 Baroclinic instability
So far, we have been able to describe the basic ideas of Rossby waves, the polar front
and the polar jet, short waves and jet maxima, and how all of these factors combine to
form, intensify, and steer surface low-pressure systems. The missing piece from this
puzzle is: what causes the development of short waves in the vicinity of jet maxima?
162
EXTRA-TROPICAL WEATHER SYSTEMS
It has been observed that flows in which strong velocity shears occur are unstable
with respect to small perturbations. That is, a small perturbation will grow rapidly.
The role of a growing disturbance in any system is to bring the system back to a
stable state. It follows that unstable conditions cannot persist in the atmosphere for
any appreciable length of time. The transition from unstable to stable states involves
a reduction of the potential energy, and all systems left to themselves will try to
avoid instability and obtain a minimum of potential energy.
In this context, most synoptic scale systems in middle latitudes appear to develop
as a result of this instability of the jet stream flow, which is closely associated
through the thermal wind relationship with strong horizontal temperature gradients.
A horizontal temperature gradient at the surface is reflected in the atmosphere above
as slanting potential temperature surfaces whose height increases as one moves
poleward. Then, the source of kinetic energy for the growing disturbances is extracted
from the potential energy of the potential temperature field. This can occur when
there is a small transverse displacement of air along a path that is less steep than
the slope of the potential temperature surfaces. In this case, effectively colder air is
lowered, or warmer air is raised: this is an energy releasing process and so allows
the growth of the disturbance and reduces the potential energy of the atmospheric
state. Because of the relationship to the temperature structure, this process is called
baroclinic instability. A mathematical treatment of this process is beyond the scope of
this book, and so we refer the reader to, for example, J. R. Holton’s An Introduction
to Dynamic Meteorology for more details.
Review questions
9.1 Using the surface weather observations in Oklahoma City, Oklahoma and Dodge
City, Kansas estimate the slope of the cold front for the storm of 2003 shown
on the surface weather map in Figure 1.7. What is the height of the cold front
above the ground at Dodge City?
9.2 Consider a Northern Hemisphere cold front, oriented in the meridional direction,
moving from west to east. The geostrophic wind in the warm air mass is from the
south-west with a speed of 7 1 m s−1 , while the geostrophic wind in the cold air
mass is from the north-west. If there is no confluence or diffluence, what is the
magnitude of the geostrophic wind speed in the cold air mass? Find the distance
from the surface front at which the frontal plane is 1 km high, if the temperature
difference is 10 C, the average temperature is 15 C, and the latitude is 45 N.
You may assume a pressure of 1000 hPa to calculate the density on the warm
and cold sides of the front.
9.3 Consider a wave at the 500 hPa level that consists of a series of troughs and
ridges, with radius of curvature of 500 km and a horizontal pressure gradient
with a constant magnitude of 5 × 10−4 Pa m−1 . If the regions of cyclonic and
anticyclonic curvature are separated by a region that is 200 km long where the
height contours are straight and parallel, calculate the divergence across this
REVIEW QUESTIONS
163
region as the flow moves from a trough to a ridge and from a ridge to a trough.
You may assume that the gradient wind approximation is valid.
9.4 (a) Use the wind observations at 500 hPa at 12 UTC 16 Feb 2003 at Blacksburg,
Virginia and Wallops Island, Virginia to estimate the ageostrophic component
of the flow due to the change in geostrophic wind speed between these two
locations. You may assume that the reported winds at both radiosonde sites
are equal to the geostrophic wind.
(b) Calculate the divergence in the left and right entrance regions of this jet
streak by assuming that the ageostrophic flow at 500 km on either side of the
jet is 0 m s−1 .
(c) Are the regions of convergence and divergence calculated in part (b) consistent with the discussion in Section 9.2.1?
9.5 (a) Identify the location of the troughs, ridges, and jet streaks at the 300 hPa
level for 00 UTC on (i) 14 February 2003 and (ii) 16 February 2003. Use
the ETA model analyses on the CD-ROM.
(b) Is the change in sea level pressure between 00 UTC and 12 UTC 14 Feb 2003
in south-eastern Colorado consistent with the divergence of the ageostrophic
winds due to the troughs, ridges, and jet streaks identified in part (a)? If
not, what additional factors could cause this surface low-pressure system to
change as observed?
(c) Is the change in sea level pressure between 00 UTC and 12 UTC 16 Feb
2003 in Mississippi and Alabama consistent with the divergence of the
ageostrophic winds due to the troughs, ridges, and jet streaks identified in
part (a)? If not, what additional factors could cause this surface low-pressure
system to change as observed?
9.6 In a deepening mid-latitude cyclone, a horizontal divergence of −3 3 × 10−5 s−1
is observed at the surface. Assuming that this divergence is constant with height
in the lowest 1000 m of the atmosphere, calculate the vertical velocity at a height
of 1000 m. You may assume that the vertical velocity is 0 m s−1 at the surface.
9.7 Calculate the vertical velocity in the right and left entrance regions of the jet
streak described in question 9.4. Assume that the tropopause is located 1 km
above the jet streak and that there is no vertical motion at the tropopause.
9.8 Consider the surface low-pressure system at 00 UTC 16 Feb 2003 (see the
weather maps on the CD-ROM). Use the thermal steering principle to determine
the likely direction of travel of the cyclone.
Part II Atmospheric phenomena
10
Boundary layers
In Part I, we explored states of balance and adjustments to those states in what
we call the free atmosphere. By this we mean that we have implicitly or explicitly
excluded the effects of the viscous forces introduced in Section 4.3.3. However, it
was problems with the assumption of no viscosity that slowed the acceptance of the
mathematical theories of fluid flow amongst the experimenters and engineers who
worked with real fluids and saw first-hand the effects of friction. This prompted
Nobel Laureate Sir Cyril Hinshelwood, the British chemist, to observe at the start
of the twentieth century that ‘fluid dynamicists are divided into hydraulic engineers
who observe things that cannot be explained, and mathematicians who explain things
that cannot be observed’.
A serious problem in the study of these inviscid flows is that the theory predicts
that in a flow without vorticity there can be no drag (that is, a pressure differential
between one side of a moving body and the other). However, engineers attempting
to build flying machines in the nineteenth century knew that real fluids such as
air do produce drag. Otherwise, for example, parachutes would be useless. This
conclusion was known as d’Alembert’s paradox, after Jean Le Rond d’Alembert,
the French mathematician who demonstrated this paradoxical aspect of the theory
in 1768. D’Alembert himself saw that the conclusion was at odds with observation,
writing ‘I do not see then, I admit, how one can explain the resistance of fluids by the
theory in a satisfactory manner.’ The problem at the time was that without computers,
there was no way to solve the Navier–Stokes equations without simplifying them as
we did in Part I of the book. A crucial element of this simplification is the omission
of the friction term. D’Alembert’s paradox was finally resolved in 1904 by Ludwig
Prandtl who found a way to connect the purely empirical engineering tradition and
the purely theoretical mathematical tradition with this hypothesis:
Under a broad range of conditions, viscosity effects are significant, and comparable
in magnitude with advection and other inertial forces, in layers adjoining solid
boundaries and interfaces between fluids of different composition, and are small
outside these layers.
This hypothesis suggests that the effects of friction can be considered to be confined
to a layer adjoining the boundary between atmosphere and ocean, or atmosphere
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
168
BOUNDARY LAYERS
and land. For researchers at the beginning of the twentieth century, this hypothesis
meant that they could continue using simplifications that omitted friction, so long
as there were no boundaries in their problem. This led to the concept of the free
atmosphere, which we may now define formally as that part of the atmosphere above
this boundary layer. This is why the approaches described in Part I led to some quite
realistic models of the behavior of the real atmosphere.
It is important to note that this is an empirical hypothesis – that is, it does not arise
from theoretical consideration of the equations of motion. However, the hypothesis
has proved to be a useful perspective and yields credible results.
The existence of the boundary layer depends on the force of viscosity, but viscosity,
a molecular process, acts on an extremely short spatial scale. In general this force,
an expression of molecular diffusion, is several orders of magnitude smaller than
the other terms in the momentum equation. We saw this in the scale analysis in
Section 5.4.2 and quite reasonably concluded that we could omit this term from the
equations of motion. However, this force does become comparable to the other terms
in a very shallow layer directly adjoining the boundary – this is known as the viscous
sublayer, and in the atmosphere it is generally on the order of millimeters deep.
However, the boundary layer hypothesized by Prandtl is not just a few millimeters
deep – in the atmosphere it can be as deep as 1 kilometer. In fact, the role of viscosity
is an indirect one: it causes a no-slip boundary condition (that is, the wind is moving
at the same velocity as the boundary) at the surface or interface. This causes a large
wind shear, which is one method of generating turbulent eddies. The eddies transfer
heat and momentum much more efficiently than molecular diffusion. It is turbulence
that defines the extent of Prandtl’s boundary layer, and it is turbulence we must
include in our equations of motion for the boundary layer.
10.1 Turbulence
Turbulence is difficult to describe, but easy to recognize. Properties on which most
scientists agree are that it is three-dimensional and rotational in form, dissipative (that
is, it converts kinetic energy to heat energy), and nonlinear. In practical terms, as we
will see, the action of turbulence cannot be described by a differential equation.
In contrast with the motions we have studied so far, turbulent eddies have horizontal
and vertical length scales of the same order of magnitude. These length scales can
range from 10−3 m to 103 m. In the boundary layer, all of these scales are of critical
importance at all times. Hence, simplification through scale analysis is not possible.
Because of this, the observational study of and theoretical explanations for turbulence
are challenging. British meteorologist Lewis Fry Richardson suggested in 1920 that
turbulence is a process where the main energy containing eddies receive kinetic
energy directly, and then pass it down in a ‘cascade’ of eddy sizes to small eddies
which dissipate it into heat. He summarized this process in a poem:
Big whorls have little whorls,
that feed on their velocity,
REYNOLDS DECOMPOSITION
169
And little whorls have lesser whorls,
And so on to viscosity.
This cascade is caused by the stretching and twisting of the eddies. Because of it, if
the motion is not to fade away there must be continuous production of turbulence to
balance the dissipation. Making progress in simplifying this picture, Soviet scientist
Andrei Nikolaevich Kolmogorov showed in 1941 that the only parameter needed to
describe a turbulent flow is the average energy dissipation. A necessary condition
for this was to consider only length scales far from both the scale at which energy is
pumped into the flow and the scale at which the energy dissipates from the flow as
heat. Importantly, he showed that it did not matter where you were in the fluid or in
what direction you were traveling (a characteristic termed isotropy).
It turns out that isotropy is not always precisely maintained (see Section 10.3).
However, it is approximated enough of the time that his important insight allows us
to simplify our approach tremendously. This is because it makes clear that we do
not need to worry too much about the precise details of the turbulent flow, just its
overall effect on heat, momentum, and energy.
Despite the progress that has been made since Prandtl’s revolutionary hypothesis,
the problem of turbulence remains an extraordinarily difficult one. This was expressed
in 1932 by the British physicist Horace Lamb, who is reported to have said,
I am an old man now, and when I die and go to Heaven there are two matters on
which I hope for enlightenment. One is quantum electrodynamics, and the other is
the turbulent motion of fluids. And about the former I am really rather optimistic.
A very similar quote, however, is attributed to German physicist Werner Heisenberg,
and so this may be apocryphal. Nevertheless, it reflects the fundamental challenge
that turbulence represents, even today. In this chapter, we will consider some basic
conclusions arising from the study of turbulent boundary layers.
10.2 Reynolds decomposition
We have seen that it is not feasible to study the precise flow details in turbulent
motion, and in fact that it is not necessary. Hence, the complexity can be dealt with
through a process of averaging known as Reynolds decomposition. This procedure
was developed by British scientist Osborne Reynolds in the 1890s and has remained
a powerful tool in the study of turbulent flows. In Reynolds decomposition, we
represent any variable in a flow field (such as the wind velocity or temperature) by
a slowly varying ‘basic’ component and a rapidly varying turbulent component. For
example,
w = w + w′
where w is the basic wind speed in the z direction and w′ is the turbulent component.
To properly represent the basic state, we must average the flow over a period of
170
BOUNDARY LAYERS
time which is long enough to average out all of the turbulent fluctuations but short
enough to preserve any long-term trends and cycles. Generally, turbulent fluctuations
occur over seconds and minutes, while synoptic development occurs on a time scale
of hours. This suggests an averaging time somewhere between, such as 30 minutes.
By definition then, the time average of the turbulent terms is zero. So in the case of
the product of two variables, for example,
wT = w + w′ T + T ′ = wT + wT ′ + w′ T + w′ T ′
= wT + w
T ′ +
w′ T + w′ T ′
0
= wT
0
+ w′ T ′
Note that the time average of the product of two turbulent terms (the second term
above) is not necessarily zero. This term is most commonly called the covariance
term and we can interpret this mathematically or physically. In the mathematical
sense, if on average the turbulent vertical velocity is upward where the temperature
is larger than average and downward where it is smaller, the product is positive and
the variables are said to be positively correlated. From a more physical perspective,
where w′ T ′ is positive, vertical turbulent motions are transporting relatively warmer
air upward and relatively cooler air downward – these both result in a net flux of
warmer air upward. This net transport also applies to any water vapor that is present,
so long as the air does not reach saturation. Because these covariance terms are
a direct expression of the turbulent component of the flow, they are by Prandtl’s
hypothesis small in the free atmosphere, but significant in the boundary layer.
To apply Reynolds decomposition to the Navier–Stokes equations, first consider
the total derivative in the x direction:
u
u
u
Du u
=
+u +v +w
Dt
t
x
y
z
If we make an assumption, for simplicity, that the flow is non-divergent (and as we
have seen, this is a rather strict assumption), the continuity equation is
u v w
+ +
=0
x y z
Since this expression is zero, we can add it, multiplied by u, to the total derivative
with no impact:
Du u
u
u
u
u v w
=
+u +v +w +u
+ +
Dt
t
x
y
z
x y z
=
u u2 uv uw
+
+
+
t
x
y
z
REYNOLDS DECOMPOSITION
171
In order to Reynolds-decompose this expression, we separate each term into the mean
and fluctuating parts, and take the average of all terms (the details of this are shown
on the CD-ROM). The Reynolds-decomposed expression is then
′ ′ ′ ′ ′ ′
Du Du
=
+
uu +
uv +
uw
Dt
Dt x
y
z
where D/Dt is the rate of change following the mean motion. Using this term in
the zonal momentum equation, where for simplicity we denote the friction term,
representing molecular diffusion, from Equation (4.6) by Frx ,
1 p
Du
=−
+ fv + Frx
Dt
00 x
′ ′ ′ ′ ′ ′
Du
1
p + p′ + f v + v′ + F rx + Frx′
+
uu +
uv +
uw =−
Dt x
y
z
00 x
′ ′ ′ ′ ′ ′
1 p̄
Du
=−
+ f v + F rx −
uu +
uv +
uw
Dt
00 x
x
y
z
(10.1)
Thus, it is clear that the friction term is distinct from the turbulence term. This term,
because it is the sum of the gradients (that is, the divergence) of a product (that is, a
flux, Section 3.7), is usually known as the turbulent flux divergence. In the boundary
layer, the turbulent flux divergence is on the same order of magnitude as the other
terms in this equation. Hence, it cannot be neglected even when only the basic flow
is of direct interest.
Using the same process, we can write down the meridional momentum equation:
Dv
′ ′ ′ ′ ′ ′
1 p̄
vu +
vv +
vw
=−
− f u + F ry −
Dt
00 y
x
y
z
(10.2)
It is also useful, in the context of the boundary layer, to include some kind of
prediction for the temperature structure. For simplicity, we will assume that all motion
is adiabatic, and hence potential temperature is conserved. That is, the material
derivative of potential temperature is zero. Assuming a potential temperature structure
0 z associated with the hydrostatic profile, this can be written
D
+ 0 z = 0
Dt
D
+w 0 = 0
Dt
z
which in Reynolds-decomposed form becomes
D
= −w 0 −
u′ ′ +
v′ ′ +
w′ ′
Dt
z
x
y
z
(10.3)
172
BOUNDARY LAYERS
Equations (10.1)–(10.3) are equations to calculate the wind and temperature in the
boundary layer. However, with the tools we have available, we cannot solve these
equations, even numerically. This is because we have introduced three additional independent
For example, in Equation (10.1), these variables
variables
for each
equation.
are u′ u′ , u′ v′ , and u′w′ . In Equation (10.2), we have only two additional variables, since u′ v′ = v′ u′ , but in Equation (10.3) we again have three new variables.
Hence, to solve this system of equations we must develop eight additional equations
which either do not include any new variables, or include additional equations for any
introduced variables! Such a process is known as closure. Closure schemes are named
by the highest order equations that are retained. For example, a first-order scheme
is one in which Equation (10.1), the basic flow equation, is used and the turbulent
fluxes are expressed only in terms of the basic state. This necessitates assumptions
regarding the relationship between the basic state and the structure of the turbulent
flow. These are known as closure assumptions, to be discussed in Section 10.4. A
second-order closure derives additional prognostic equations for the turbulent flux
terms, but approximates any higher order perturbations. And so forth.
10.3 Generation of turbulence
For a turbulent flow to be maintained, continuous production of turbulence is required
to balance the dissipation. This means that, somehow, kinetic energy must be imparted
to the largest eddies in the flow. This turbulence generation can take place in two
ways. The first is through convection: when the temperature profile is unstable, air
must start to circulate vertically to redistribute heat upward until a stable profile is
achieved. This convection gives rise to large eddies that can then feed the turbulence
through the energy cascade. Of course, if the temperature profile is very stable,
this will act to suppress turbulence. One may think of this process as a conversion
from basic flow potential energy, embodied by the potential temperature profile, to
turbulent kinetic energy, through the action of buoyancy forces. Mathematically, this
can be written
g
w′ ′
0
where w′ is the turbulent vertical wind, ′ is the turbulent component of the potential
temperature variation, g is the acceleration due to gravity, and 0 is the basic flow
potential temperature profile. This term may be positive (for turbulence generation)
or negative (when a stable profile suppresses turbulence). The second is through
mechanical processes, due to the presence of wind shear. Because of the no-slip
boundary condition imposed by the viscous force, large gradients of wind are present
at the boundary. This is an unstable situation, and so large turbulent eddies are
generated to decrease the wind shear. This can be understood as a conversion from
basic flow kinetic energy to turbulent kinetic energy, and can be written
u
v
− u′ w′
− v′ w ′
z
z
CLOSURE ASSUMPTIONS
173
where u v, and w have their usual meaning as wind components. This term is always
positive. Finally, like all aspects of fluid flow, turbulence can also be modified by
transport – that is, it can be advected like temperature or basic flow momentum.
The source of turbulence in any particular situation will affect the structure of the
turbulence. In general, turbulence caused through mechanical generation has smaller
scales than turbulence caused by buoyant production. Thus, when both convective
turbulence and mechanical turbulence are present, relatively slow and large-scale
fluctuations are superimposed on relatively fast and small-scale variations, resulting
in very complex patterns. The large eddies tend to collect the smaller turbulent
circulations into their upward branches, with the result that these regions of the
convective boundary layer contain more turbulent kinetic energy than the downward
branches of the large eddies. Further, fluctuations in wind direction are amplified
by convective turbulence. It is also known that, close to the ground, all turbulent
eddies tend to be small. However, mechanically generated eddies near the ground are
elongated in the direction of the wind, whereas convective eddies are more isotropic.
The relative importance of buoyant production (or loss through stability) and
mechanical production in a particular turbulent flow field can be expressed as a
mathematical ratio of the terms representing these processes. The ratio is usually
written
⎤
⎡
g
w′ ′
0
⎥
⎢
−⎣
ū
v̄ ⎦
− u′ w ′
− v′ w ′
z
z
This ratio is called the flux Richardson number Rf (after Lewis Fry Richardson of
course). Experiments have shown that if this number is great than about 1/4, the air is
so highly stable that turbulence is suppressed even in the presence of wind shear. If
Rf is less than 0, then the turbulence is maintained by convection. Between these two
values, the mechanical production appears to be strong enough to sustain turbulent
flow even in a stable profile.
10.4 Closure assumptions
Consider a simplified case in which we assume that the structure of the boundary
layer is horizontally homogeneous (that is, /x /y = 0. Equations (10.1)–(10.2)
become
1 p̄
Du
′ ′
=−
+fv−
u w + Frx
Dt
00 x
z
1 p̄
Dv
′ ′
=−
−fu−
v w + Fry
Dt
00 y
z
Synoptic scaling is still applicable for synoptic scale motions in the boundary layer,
except that according to Prandtl’s hypothesis the turbulent component must scale as
174
BOUNDARY LAYERS
large as the other terms in the equations. If we also assume we are outside the viscous
sublayer, then synoptic scaling allows us to neglect the friction terms Frx and Fry .
These scale considerations result in
1 p
′ ′
+fv−
uw
00 x
z
′ ′
1 p
0=−
vw
−fu−
00 y
z
0=−
and the equations can be expressed in terms of the geostrophic wind:
0 = f v − vg −
u′ w′
z
v′ w ′
0 = −f u − ug −
z
(10.4)
To close this set of equations, we need expressions to determine the two remaining
turbulent flux terms. The assumptions made to devise these expressions determine
the ‘order’ of the closure scheme. Hence, a scheme which assumes the turbulent
flux terms are constant is called a ‘zero-order’ scheme. One that approximates the
turbulent flux in terms only of the mean flow is called a ‘first-order’ scheme. If the
turbulent flux terms are represented by their own additional equations, with higher
order fluxes then approximated, the scheme becomes ‘second-order’, and so on.
The higher the order of closure scheme, the more degrees of freedom and hence
the potentially more accurate (if the assumptions made are appropriate) the scheme
will be.
Consider the situation illustrated in Figure 10.1 in which a convective layer, heated
from below on a warm spring afternoon, is topped by a highly stable layer. In such a
situation, turbulent mixing can lead to the formation of a well-mixed layer in which
the wind speed and potential temperature are nearly independent of height.
Observations indicate that in such a situation, we can represent the surface turbulent
momentum flux by
⇀
⇀
u′ w′ s = −Cd u¯ u
v′ w′ s = −Cd u¯ v
⇀
where u¯ = u2 + v2 . That is, the turbulent flux is proportional to the square of the
basic wind speed, and the constant of proportionality Cd is obtained empirically. We
assume that the turbulent momentum flux at the entrainment layer is zero because
this is the point at which, moving into the free atmosphere, there should no longer be
significant energy in the turbulent component. This is known as a ‘half-order’ closure
scheme since we have not only assumed the turbulent flux terms are a function of the
basic flow, but also assumed the shape of the basic flow profile in advance. Hence,
we have reduced the degrees of freedom from a basic first-order scheme, but not so
greatly as to create a zero-order scheme.
175
CLOSURE ASSUMPTIONS
–20
700
–15
Wind Speed (knots)
–5
0
5
–10
10
15
20
entrainment zone
750
800
mixed layer
850
surface layer
900
305
310
315
320
Potential Temperature (K)
Figure 10.1 Atmospheric sounding for Amarillo, Texas at 6 p.m. local time on 26 May 2005. The
profiles of wind (dashed line) and potential temperature (solid line) from the surface (at 1099 m)
to 700 hPa are shown. The surface layer acts as the transition from the no-slip boundary condition
to the boundary layer. The entrainment zone is the region where the boundary layer air mixes
with the free atmosphere. Between lies the mixed layer, where the wind and temperature are
approximately uniform
We can then integrate Equations (10.4) from the surface to the height h of the
entrainment layer to find
⇀
⇀
Cd u¯ v
Cd u¯ u
u = ug −
v = vg +
fh
fh
Thus, this model produces a cross-isobaric component to the wind (that is, an
ageostrophic wind) in the well-mixed layer which does work to balance the frictional
dissipation at the surface. This is known as the mixed layer theory.
Consider as an illustration a case in which the geostrophic flow is purely zonal and
in the positive x direction (Figure 10.2). In this case, vg = 0, and the basic meridional
wind v is a small component in the northward direction. Since the geostrophic wind
is westerly, this meridional wind must be blowing toward low pressure. The zonal
basic flow u is modified also, through a small decrease arising from the contribution
of the cross-isobaric flow. Thus, this model produces the type of low-level slowing
and turning toward low pressure that we have come to expect from our examination
of weather charts.
Since generally the boundary layer turbulence tends to reduce the wind speed, the
turbulent momentum flux terms are often called boundary layer friction or turbulent
drag, but this ‘friction’ must not be confused with processes involving molecular
viscosity.
176
BOUNDARY LAYERS
isobar
low pressure
Cd u u
fh
u
ug
–
Cd u v
fh
y
x
isobar
high pressure
Figure 10.2 Resulting cross-isobaric flow in a well-mixed layer in the Northern Hemisphere (f > 0)
in the case of pure zonal flow
Example In June 1893 the Norwegian explorer and scientist Fridtjof Nansen loaded
6 years’ supply of food and 8 years’ supply of fuel onto the ice-strengthened ship
Fram and left Oslo port, heading east to Siberia. His plan was to sail northward from
Siberia, and let his ship be frozen into the ice when the Arctic Sea froze, and drift with
the ice, letting it carry him to the North Pole and then to Greenland. This plan was
based on his observations that surface ocean currents were not in the same direction
as the prevailing winds, but between 20 and 40 to the right of the wind’s direction.
Although he did not make it all the way to the North Pole, his expectations proved
correct, and three years later the Fram reached Spitsbergen, finally leaving the ice
and reaching Norway in August 1896 (see the map of his voyage in the CD-ROM).
How can boundary layer theory explain the discrepancy between the winds and
the surface currents that Nansen observed?
After hearing of Nansen’s observations, V. Bjerknes invited his student V. Walfrid
Ekman to use the new boundary layer theory to understand the discrepancy. In doing
so, Ekman had to account for the possibility of the presence of strong wind shear,
which occurs frequently in the Arctic. Hence, rather than using mixed layer theory, he
assumed that a turbulent flow transports heat and momentum in a manner analogous
to molecular diffusion:
u
z
v
v′ w′ = −Km
z
u′ w′ = −Km
(10.5)
where Km is the eddy viscosity coefficient, analogous to , the molecular viscosity
coefficient (Section 4.3.3). Since turbulence is more effective at mixing than viscosity,
one would expect that in such a formulation Km ≫ . This is a first-order closure
scheme since the flux terms are approximated using only the basic flow quantities, but
no assumptions are made about the form of the basic flow. In actuality, we have made
CLOSURE ASSUMPTIONS
177
some assumptions, since of course we have assumed synoptically scaling, horizontally
homogeneous flow. However, this is typical enough that it is not considered to reduce
the order of the closure.
Ekman further assumed no significant pressure gradients, which is better approximated in the ocean than in the atmosphere. Assuming some as yet unknown formulation for the eddy viscosity coefficients, Equations (10.4) become
2 u
=0
z2
2 v
− f u + Km 2 = 0
z
f v + Km
(10.6)
Equations (10.6) have come to be known as the Ekman boundary layer equations.
These equations can be solved to determine the currents in the ocean being driven
by a surface wind.
To find the solutions to these equations, we must define some boundary conditions
and postulate a form of the solution. We start by aligning our horizontal axes with
⇀
the surface current so that u z = 0 = u0 0. The other boundary condition is
u → 0 and v → 0 as z → −
We can postulate a form for the solution that conforms with this boundary condition.
To do so, we define a constant (and hence make the implicit assumption that Km is
constant) which will make our equations simpler to read:
2Km
(10.7)
h=
f
and then write
z
h
z
z
v z = Be h sin
h
z
u z = Ae h cos
The surface z = 0 boundary condition gives us A = u0 but does not allow us to
define B. We do this by substituting the assumed solution back into one component
of Equations (10.6). We choose the second component, but the first would give the
same result:
h2 2 v
2 z2
z
z
h2 2B z
z
u0 e h cos
=
e h cos
2
h
2 h
h
⇒ B = u0
u=
178
BOUNDARY LAYERS
Hence, we have derived the current profile
z
h
z
z
v z = u0 e h sin
h
z
u z = u0 e h cos
(10.8)
This profile, of course, does not tell us the relationship between the current at the
surface and the wind speed above it. To determine this, we must apply a further
boundary condition at the surface. Consider the following.
How does a fluid deform? By definition, the distinguishing property of a fluid
is that it cannot support a shearing stress (Section 3.1). Thus, at the surface, the
stress must be continuous across the air–water interface. Hence, the stress in the air,
the wind stress, must be equal to the stress in the water. This replaces the ‘no-slip’
boundary condition introduced at the start of this chapter that is appropriate when the
boundary is a solid surface. So, if we calculate the water stress, this will give us the
wind stress, and hence the direction of the wind.
We have assumed a form already for the stress in the water in Equations (10.5).
To write these in terms of stress directly, at the surface we have
u
Km u0
=
x = Km
z 0
h
Km u0
v
(10.9)
=
y = Km
z 0
h
Hence, assuming the surface current is purely zonal and toward the east, the surface
wind must be directed toward the north-east, or 45 to the left of the current.
This explains what Nansen observed on his ship.
At a greater depth, although the speed of the current will decrease, the direction
of the current will deviate even further from the wind’s direction. Eventually, at a
certain depth, the sea water may even flow upwind. The form of the current profile
is called the Ekman spiral (Figure 10.3).
While American scientist Kenneth Hunkins was probably the first researcher to
observe an Ekman spiral in the real ocean, from Ice Station Alpha during the International Geophysical Year 1957–1958 (see Figure 10.3), routine measurements capable
of discerning both current speed and direction from a stable platform were not possible until the 1980s. Observed spirals tend to turn less with depth than the Ekman
model predicts, but nevertheless the agreement between this simple theory and the
observations is substantial.
The height of the atmospheric mixed layer, or depth of the oceanic mixed layer, is
easily defined by the extent of uniform properties, particularly potential temperature,
close to the boundary. In the case of the Ekman spiral, a depth of the layer can also
be defined. For example, at the depth
z=− h
179
CLOSURE ASSUMPTIONS
N
Wind
4.1 ms–1
~
~
ice
2
16 8 4
0
10 cms–1
5
Figure 10.3 A wind-driven current velocity spiral as measured using an instrument, lowered from
an ice floe, that measures current as averages over around 15 minutes. The numbers show the depth
(m). From Hunkins (1966), perhaps the first field observation of an Ekman spiral in the ocean.
Reprinted from Deep Sea Research, 13, K. Hunkins, ‘Ekman drift currents in the Arctic Ocean’,
607–620, © 1966, with permission from Elsevier
the currents are
u = −0 04u0 and v = 0
Similarly,
u− 3 2 h = 0 and v̄− 3 2 h = −0 008u0
u−2
h
= 0 001u0 and v−2
h
=0
and so on, as the solution oscillates closer and closer to zero. Typically, the value
z = − h is taken to be a measure of the depth of the boundary layer. As expected
by the force balance between wind stress and Coriolis force, the depth depends on
the latitude and the eddy viscosity coefficient.
Turning again to the theoretical form of the spiral (Figure 10.4), it is apparent
that while the direction of the current changes with depth, integrated over the entire
water column the amount of water flowing upwind and downwind cancel, so that on
average there is no current either upwind or downwind. The average direction of the
current is at 90 to the right of the wind’s direction. This net current is called the
Ekman transport.
This net current has important implications for the observed oceanic circulation.
Because, at the middle latitudes, the oceans are often dominated by large, persistent,
180
BOUNDARY LAYERS
Ocean Surface
45°
d
in
W
ent
curr
Surface
Spiraling
currents
Net water
transport
Figure 10.4 Schematic showing the Ekman wind-driven current velocity spiral
high-pressure systems driving an anticyclonic flow, the Ekman transport directed to
the right of the wind stress predicts a transport of water into the center of the vortex,
or gyre. This convergence at the surface causes elevated sea level heights at the center
of the gyre, as is actually observed. The surface convergence also drives downward
motion and divergence deep below the center of the gyre, creating a circulation. This
secondary circulation is often referred to as Ekman pumping.
10.4.1 The analogy between molecular and turbulent diffusion
For the Ekman solution obtained above, an important assumption was made: that the
form of turbulent momentum transfer is analogous to molecular diffusion as described
in classical kinetic theory. The earliest and most popular form of this analogy was the
‘mixing length theory’, not to be confused with the ‘mixed layer theory’ described
above. The simplest form of this theory assumed that turbulent transfer was caused
by the motion of discrete parcels of fluid, like molecules, moving relative to one
another. Hence, a parcel originates from one location with a set of properties (for
example, momentum, temperature) and moves a certain distance at constant velocity
before mixing with its surroundings. From the resulting mixture, a new parcel is
identified which repeats the process anew. This distance is called the mixing length,
an analog of the ‘mean free path’ in kinetic theory, and in its simplest incarnations
yields a constant eddy diffusivity coefficient Km .
REVIEW QUESTIONS
181
Fundamental differences between the diffusion of momentum by molecular viscosity and by turbulence were identified as early as 1895 by Osborne Reynolds.
Molecular velocities are very large, on the same order of magnitude as the speed
of sound, but molecular diffusion is very small because collisions act constantly to
restore thermodynamic equilibrium, and it is only over the very small mean free
path before the collision that the molecule carries identifiable properties. In contrast,
fluid parcel velocities are relatively slow, but kinetic energy is extracted from the
mean flow and transferred to the turbulent flow rather rapidly. This is reflected in
empirically determined values of Km , which are flow dependent but usually of the
order of 101 m2 s−1 , in sharp contrast to values of , which are generally of the order
of 10−5 to 410−6 m2 s−1 .
Such contrasts demonstrate the important scale separation between the statistics of
molecular velocities in kinetic theory and the statistics of fluid velocities in turbulence.
They also illustrate why the concept of a mean free path in macroscopic flows is
unlikely. Nevertheless, refinements to this approach have continued over the years,
particularly with regard to approaches known as similarity theory, which involve
an empirical determination of dimensionless constants or functions. More recent
approaches have returned to the statistical description of turbulent fluxes, sometimes
combined with the explicit treatment of very large convective eddies that can have
length scales as large as the depth of the boundary layer.
Review questions
10.1 The following observations of w and were made at 1 second intervals during
a research flight over Kansas during a summer afternoon:
w = 1 1 1 5 1 4 0 9 1 0 1 2 0 8 0 7 1 3, and 1 1 m s−1
= 290 5, 291.0, 290.8, 290.3, 290.5, 291.0, 290.3, 289.9, 291.1, and 290 7 K
(a) Using Reynolds decomposition calculate the basic and turbulent component
for each time series.
(b) Calculate the covariance term, w′ ′ , based on the time series.
(c) Discuss the physical implications of the sign of the covariance term calculated in part (b).
10.2 Following the method used to derive Equation (10.1) show the derivations for
Equations (10.2) and (10.3).
10.3 Discuss how Figure 10.2 would change for
(a) CD = 0
(b) CD smaller than used in Figure 10.2
(c) CD larger than used in Figure 10.2.
182
BOUNDARY LAYERS
⇀
10.4 Plot the Ekman profiles of u v, and u¯ in the ocean, from the surface to a
depth of −2 h, for a surface current of 10 cm s−1 and h = 100 m.
10.5 Solve the Ekman equations for the basic flow in the atmospheric boundary
layer. Choose as your boundary conditions a ‘no-slip condition’ at the surface,
and
u → ug and v → vg as z →
11
Clouds and severe weather
Water exists in the atmosphere in three forms – as water vapor (a gas), as liquid
water, and as ice. Whereas water vapor is transparent to visible light and hence cannot
be seen in the air, liquid water and ice are readily seen in the atmosphere, as clouds,
rain, hail, and snow. It is the presence of water in the atmosphere that gives our
weather its character. Many of the most severe weather events, such as hurricanes and
thunderstorms, and also many of the beautiful atmospheric phenomena, are associated
with water. Rainbows are of course associated with refraction through raindrops. Ice
crystals in the clear air can also produce interesting effects such as sun dogs, halos,
and pillars. In this chapter, the role of moist processes in generating some examples
of severe weather will be explored.
11.1 Moist processes in the atmosphere
In Section 3.3.2, parameters were introduced that describe the amount of water vapor
in the air, including the water vapor pressure e and the mixing ratio r. These concepts
will be expanded here to help describe the formation of clouds in the atmosphere.
If the masses of water vapor and dry air in the mixture are known, the vapor
pressure can be calculated using Dalton’s law (Section 3.3.1), by
moles of water vapor
p
moles of water vapor and dry air
mv /Mv
p
=
mv /Mv + md /Md
e=
Since the mixing ratio is simply r = mv /md , this can be written
e=
r
p
r + Mv /Md
The ratio Mv /Md is a constant, usually denoted , having a value of 0.622.
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
(11.1)
184
CLOUDS AND SEVERE WEATHER
Equation (11.1) can be rearranged, so that
r=
e
p−e
(11.2)
can be used to calculate the mixing ratio from a known value of vapor pressure.
Clouds form in the atmosphere when the water vapor pressure exceeds the saturation vapor pressure, es , at that temperature. Saturation is, in fact, a dynamic equilibrium in which there are as many molecules returning to the liquid water (condensing)
as there are escaping (evaporating). At this point the vapor is said to be saturated,
and the vapor pressure is called the saturated vapor pressure. Equation (11.2) can be
used with es to calculate the saturation mixing ratio, rs . Since the molecular kinetic
energy is greater at higher temperature, more molecules can escape the surface and
the saturated vapor pressure and saturation mixing ratio are correspondingly higher.
There are a large number of semi-empirical equations that have been developed
to calculate the saturation vapor pressure as a function of temperature. The Goff–
Gratch equation (see Figure 11.1) is generally considered to be the reference, but
many other equations are in use in the scientific community. The differences between
these formulations at typical tropospheric temperatures and pressures are too small to
concern us here, although they can be important in the upper troposphere and above.
One of the simpler semi-empirical equations, which is used to calculate the saturation
vapor pressure over a liquid water surface, is known as Teten’s formula:
b T − T1
es = e0 exp
(11.3a)
T − T2
where e0 = 6112 hPa, b = 1727 K−1 , T1 = 27316 K, T2 = 3586 K, and T is the air
temperature, also with units of K. Similarly the vapor pressure, e, can be calculated with
b Td − T1
e = e0 exp
(11.3b)
Td − T2
where we have replaced T with the dew point temperature Td (Section 1.2.3). Equations (11.3a) and (11.3b) indicate that when T = Td , es = e and the air is saturated,
as expected from the discussion of dew point temperature in Section 1.2.3.
When the air is in dynamic equilibrium with a surface of ice, rather than liquid water, a lower saturation vapor pressure results (Figure 11.1). This is because
molecules escape from the surface of ice less readily than from liquid water. Hence,
it is important to always specify whether the saturation vapor pressure is with respect
to water or to ice. The ratio of the saturation vapor pressure over a liquid water
surface to the saturation vapor pressure oven an ice surface, esi , can be calculated using
es
=
esi
where T is given with units of K.
273 K
T
266
(11.4)
185
MOIST PROCESSES IN THE ATMOSPHERE
Saturation vapor pressure (hPa)
100
10
1
0.1
0.01
220
240
260
280
Temperature (K)
300
320
Figure 11.1 The saturation vapor pressure with respect to both water (solid line) and ice (dashed
line), and the difference between them (dotted line), calculated using the Goff–Gratch equation.
The vertical line indicates the freezing temperature
Finally, relative humidity is one measure of the amount of moisture in the air that
is frequently reported in weather forecasts and discussions. This quantity is defined as
RH =
r
e
× 100% ≈ × 100%
es
rs
(11.5)
When RH = 100%, e = es and the air is saturated with respect to a liquid water
surface. We can also determine the relative humidity with respect to an ice surface,
which is given by
RHi =
e
× 100%
esi
(11.6)
Example Use a skew T –log P diagram to determine the mixing ratio, saturation
mixing ratio, and relative humidity at the surface at Fort Worth, Texas at 00 UTC 5
Jun 2005.
Figure 11.2 shows the skew T –log P diagram for Fort Worth on this date, which was
a day that saw considerable thunderstorm activity across the central United States
(see the projects section of the CD-ROM.) Skew T –log P diagrams are commonly
used by meteorologists to diagnose the structure of the atmosphere at a particular
location and to determine how air parcels will be modified due to ascent or descent.
186
CLOUDS AND SEVERE WEATHER
100
0.1
200
0.4
0.8
1
2
90
50
pressure (hPa)
70
300
30
400
10
500
600
–1
0
700
–24
–8
8
16
32
800
900
1000
–40
0.4
–30
0.8
–20
1
–10
2
5
0
7 9
10
12 16 20 28 36
20
40
30
temperature (°C)
saturation Mixing ratio (g/kg)
Figure 11.2 ‘Skew T –log P’ diagram for a sounding taken at Fort Worth, Texas on 5 June 2005 at
00 UTC, showing the temperature and dew point temperature profiles
The logarithm of pressure is used as the vertical axis of the plot, and temperature
is plotted using an axis that is skewed at a 45 angle to the horizontal (hence the
name of the diagram). The saturation mixing ratio is also accorded a skewed axis,
as are lines that represent dry and moist adiabatic ascent. The lines that represent
dry adiabatic ascent, known as dry adiabats, are also lines of constant potential
temperature (Section 3.4). The temperature and dew point temperature profiles as
measured by a radiosonde (Section 1.3.1) are plotted on the diagram. The figure is
also shown on the CD-ROM with color coding to help identify the axes.
The saturation mixing ratio can be determined by locating the saturation mixing
ratio line that intersects a point defined by the measured temperature and pressure.
MOIST PROCESSES IN THE ATMOSPHERE
187
For this example, we are interested in the saturation mixing ratio at the surface. At the
surface, the temperature at this time was 31 C and the pressure was approximately
990 hPa. The point given by this temperature and pressure does not lie on any of the
saturation mixing ratio lines plotted on the diagram, and so we will need to estimate
the saturation mixing ratio based on the saturation mixing ratio lines that lie on either
side of our point of interest. Based on Figure 11.2 we estimate the saturation mixing
ratio for this point to be around 30 g kg−1 .
We will use the point defined by the surface dew point temperature (22 C) and
surface pressure (990 hPa) to determine the mixing ratio. To do this we first need to
note that the saturation mixing ratio lines on a skew T diagram also represent lines
of constant mixing ratio. The mixing ratio of the air parcel is given by the mixing
ratio line that intersects the point defined by the dew point temperature and pressure
of an air parcel. The point defined by the surface dew point temperature and pressure
does not lie on any of the mixing ratio lines and so, as before, we need to estimate
the mixing ratio based on the value of the mixing ratio lines that lie on either side
of our point of interest. Doing this, we estimate the mixing ratio at the surface to be
approximately 17 g kg−1 .
From the values of mixing ratio and saturation mixing ratio that we have estimated
from the skew T –log P diagram we can calculate the relative humidity of the air at
the surface using Equation (11.5):
r
× 100%
rs
17
≈
× 100%
30
≈ 57%
RH ≈
There are two ways in which a parcel of air may become saturated. One is to add
more water vapor to the air. More commonly, though, air becomes saturated because
its temperature decreases. This may occur through in situ processes such as radiative
cooling, or through ascent, as a parcel rises and cools adiabatically (Section 3.4).
This ascent may be due to a flow over a mountain, turbulent mixing arising from
convection, or low-level convergence in the vicinity of a front, for example. The
level at which saturation occurs, in this case, is called the lifting condensation level
(LCL). The LCL is a good estimate of the cloud base height for clouds that form
through this process.
If ascent and cooling continue, condensation of the excess water will start to
occur. Because of the heat released through this process, the cooling associated with
this ascent is less than that observed with dry adiabatic ascent, and the potential
temperature is no longer conserved. In observations, the liquid water content of a
cloud is generally less than expected from the process described here, because water
is lost from the cloud through mixing with surrounding unsaturated air, and through
rainfall.
188
CLOUDS AND SEVERE WEATHER
Example Find the lifting condensation level at Fort Worth, Texas at 00 UTC 5 Jun
2005.
To determine the lifting condensation level we will again use the skew T –log P
diagram shown in Figure 11.2, and will consider an air parcel that is lifted from the
surface. We will need to determine how the temperature and dew point temperature
of this air parcel change as the parcel is lifted through the atmosphere, and identify
the location at which the temperature and dew point temperature of the air parcel
first become equal.
Since the air parcel is not initially saturated (T = Td ) we know that this air parcel
will undergo dry adiabatic ascent (Section 3.4) when lifted, and will have a constant
potential temperature. To represent the change in air temperature of the air parcel
for this dry adiabatic process on the skew T –log P diagram we start from the surface
atmospheric temperature and draw a line from the surface temperature parallel to the
dry adiabats (see the color version of Figure 11.2 on the CD-ROM). Physically this
represents a conservation of potential temperature during the dry adiabatic process,
and each point on this line defines the temperature and pressure of the air parcel
required to maintain a constant potential temperature during ascent.
We also need to consider how the dew point temperature of this air parcel will
change during ascent. First, we note that during dry adiabatic ascent the total amount
of water vapor in the air parcel remains constant, and thus the mixing ratio will also
remain constant. In the previous example we found that the mixing ratio of the air
parcel is given by the intersection of the mixing ratio lines with the parcel dew point
temperature and pressure, and was equal to 17 g kg−1 at the surface. To represent the
conservation of mixing ratio we draw a line that starts at the value of the surface
mixing ratio and is parallel to the mixing ratio lines. Each point along this line has a
constant mixing ratio of 17 g kg−1 and defines the dew point temperature and pressure
of the air parcel as it rises through the atmosphere.
Where the two lines we have drawn intersect, the air parcel temperature and dew
point temperature are equal. It is at this point that the rising air parcel first becomes
saturated and a cloud will form, at the lifting condensation level (LCL). Reading this
level from the figure, it is around 860 hPa. Based on the pressure level heights shown
on the CD-ROM figure, the actual height of the LCL is roughly
969 + 04 × 1992 − 969 ≈ 1378 m
A check of the National Weather Service records for this date and location gives
an LCL of 851 hPa or 1456 m, and hence our graphical calculation is reasonably
accurate.
If ascent and adiabatic cooling continue, the temperature may drop below 0 C. This
is known as the freezing level, and above this level, ice crystals can start to form in
a process known as glaciation. In Figure 11.2, the freezing level is at 580 hPa, which
corresponds to 4680 m. The process of freezing leads to a release of latent heat, so
that continued cooling with ascent will be reduced further. Above the freezing level,
MOIST PROCESSES IN THE ATMOSPHERE
189
even though the cloud is saturated with respect to water, it is supersaturated with
respect to ice. Deposition of water vapor occurs on the surface of the ice crystals,
until humidity is reduced sufficiently that saturation with respect to ice is achieved.
This process also releases latent heat.
11.1.1 Stability in the moist atmosphere
Probably the most important driver of uplift for severe weather is moist convection.
The stability of a moist atmosphere to a vertical displacement can be understood as
follows. Recall that a dry atmosphere is neutrally stable if the potential temperature
is uniform with height. Such a dry adiabatic profile corresponds to a particular lapse
rate (that is, a value of −dT /dz that is usually written as d and has a constant value
of g/cp , where g is the acceleration due to gravity and cp is the specific heat of dry
air at constant pressure. It is this rate of temperature decrease which is represented
by the dry adiabat lines on the skew T –log P diagram in Figure 11.2. If the potential
temperature decreases with height, a situation which corresponds to a lapse rate
greater than the dry adiabatic lapse rate (that is, the atmosphere cools more quickly
than an unsaturated rising air parcel that has a constant potential temperature), the
air is unstable to a vertical displacement. An air parcel that is displaced upward any
distance vertically will be warmer than the surrounding environmental air, and will
experience an upward-directed buoyancy force. Thus, this air parcel will accelerate
upward, in the direction of the initial displacement. In such an environment, any
vertical disturbance of the atmosphere will tend to grow, with the air accelerating in
the direction of the initial displacement.
In a moist atmosphere, stability depends on the vertical profile of the wet-bulb
potential temperature (w . Details on the calculation of the wet-bulb potential temperature can be found in Rogers and Yau (1989). A uniform profile of the wet-bulb
potential temperature corresponds to the moist adiabatic lapse rate s , in which the
temperature decreases less rapidly than the dry case because of the latent heat release
from condensation. The moist adiabatic lapse rate is indicated on skew T –log P diagrams by the moist adiabats. If the actual lapse rate in the atmosphere falls between
the dry and the moist adiabatic lapse rates, the air will be stable in the dry case,
but unstable in the moist case, provided that the vertical displacement is sufficient
to saturate the air parcel and reduce its rate of cooling to the moist adiabatic lapse
rate. The atmosphere in this case is referred to as conditionally unstable, with the
condition for instability being whether or not the ascending air becomes saturated.
Hence, the presence of moisture in the air adds an additional potential for positive
buoyancy, and hence convection, to be generated.
Example Graphically determine the temperature and dew point temperature of an
air parcel lifted from the surface to 200 hPa at Fort Worth, Texas at 00 UTC 5 Jun
2005 using the skew T –log P diagram.
In the previous example, we determined the change in air parcel temperature and
dew point temperature between the surface and the LCL at 860 hPa. We found
190
CLOUDS AND SEVERE WEATHER
the temperature of the rising air parcel by drawing a line starting at the surface
temperature that was parallel to the dry adiabats. The dew point temperature of the
air parcel was found by drawing a line starting at the surface dew point temperature
that was parallel to the mixing ratio lines.
Once the air parcel is lifted above the lifting condensation level the potential
temperature and mixing ratio will no longer be conserved. The potential temperature
will increase due to latent heat release as water vapor condenses, and the change in
air parcel temperature will now be given by the moist adiabats on the skew T –log P
diagram. The mixing ratio will decrease due to the condensation of water vapor,
and the dew point temperature will remain equal to the air temperature (that is, the
rising air parcel will remain saturated). Thus the dew point temperature of the rising
air parcel will also be given by the moist adiabats on the diagram. Graphically this
is determined by drawing a line, starting at the LCL, that is parallel to the moist
adiabats. This is illustrated on the CD-ROM.
At 200 hPa the temperature and dew point temperature of the air parcel (not the
environmental air, which is indicated by the profiles) are both equal to −50 C.
A convenient measure of the energy available to accelerate a parcel vertically is the
convective available potential energy, or CAPE, which is the parcel buoyancy vertically integrated between the level of free convection (when the potential temperature
of the parcel is in excess of the environment) and the level of neutral buoyancy
(where this ceases to be the case). An example of this calculation performed graphically for the previous example is shown on the CD-ROM. A useful rule of thumb
in evaluating CAPE is that the addition of 1 g kg−1 of water vapor causes the same
increase in CAPE as a warming of 25 C. Values of CAPE over 2500 J kg−1 are very
unstable.
CAPE represents the potential for convective motion, but it can only be released
if a parcel is initially lifted to the level of free convection. Hence, CAPE may be
considered formally to be
CAPE = g
zEL T
zLFC
parcel − Tenv
Tenv
dz
(11.7)
where zEL is the height of the equilibrium level; that is, the level at which the
temperature of a buoyantly rising air parcel is equal to the environmental temperature.
In fact, there is usually an energy barrier to be surmounted before this can occur,
corresponding to the work needed to raise a parcel to its level of free convection. In
fact, large values of this so-called convective inhibition often correspond with large
values of CAPE, since it is the convective inhibition that allows CAPE above to
persist and develop.
Given that CAPE is a measure of the energy available to accelerate a parcel
vertically we can estimate the maximum vertical velocity of an air parcel as
w = 2 · CAPE05
(11.8)
191
AIR MASS THUNDERSTORMS
This relationship between CAPE and w indicates that for highly unstable environments, with CAPE > 2500 J kg−1 , the vertical velocity can reach values in excess of
70 m s−1 . In reality, vertical velocities in thunderstorms are often less than this, due to
mixing of environmental air with the thunderstorm updraft which reduces the CAPE
from the value given by Equation (11.7).
11.2 Air mass thunderstorms
As we have seen, the lifting of air causes adiabatic cooling and can result in the
rising air becoming saturated, leading to condensation and latent heat release. If the
atmosphere is conditionally unstable, the rising saturated air parcel will be warmer
than the environment and the result is a convective cloud referred to as a cumulus
cloud. As discussed in the previous section, the updraft velocities in convective
clouds can be quite large, resulting in the presence of supercooled (that is, below
0 C) liquid water well above the freezing level. There can be substantial horizontal
entrainment of environmental air into the cloud, altering the temperature and mixing
ratio of the rising air from the expected moist adiabatic values indicated on a skew
T –log P diagram. For this cloud to grow into a thunderstorm, continued ascent must
occur, allowing the cloud to grow vertically until a highly stable layer (often the
tropopause) prevents further lifting and creates an anvil (Figure 11.3). Sometimes
updraft velocities are sufficiently vigorous that the cloud tops may overshoot the
stable layer. Since the strength of the updraft increases with increasing CAPE, as
overshoot
anvil
pressure (hPa)
250
500
0 °C
700
sfc
5 km
cumulus
9 km
mature
12 km
dissipating
Figure 11.3 Life cycle of a typical air mass thunderstorm cell, showing cumulus, mature, and dissipating stages. Vertical lines between cloud base and the surface (sfc) indicate precipitation (rain
or hail). Arrows indicate downdrafts and updrafts within the cloud. The top of the cloud is at the
tropopause
192
CLOUDS AND SEVERE WEATHER
shown by Equation (11.8), the presence of an overshooting top is a visual indication
of an environment with large CAPE and the potential for severe weather.
At the mature stage of an air mass thunderstorm’s life cycle, precipitation will
be occurring. A vigorous downdraft is initiated from the downward frictional force
exerted on the air by the falling raindrops, and the downdraft region is associated
with the heaviest precipitation. At this stage the cloud is called a cumulonimbus, or
precipitating cumulus, cloud. Severe weather can be associated with these clouds,
including rain and strong winds, thunder and lightning, hail and even tornadoes. The
maximum updraft velocities in the mature thunderstorm cell are in the center of the
cloud. As precipitation continues to develop throughout the cloud and the downdrafts
become more extensive, the thunderstorm enters the dissipating stage of its life cycle
as the supply of supersaturated air from the lower atmosphere is depleted and the
thunderstorm starts to dissipate.
The most common and least severe type of thunderstorm is the air mass thunderstorm. These are single-cell thunderstorms (one cloud with one primary vertical
circulation) that develop in maritime tropical air masses, usually in the late afternoons
during summer, when surface heating and the consequent convection is at its maximum. Because there is no wind shear that would prevent the destruction of updrafts
by precipitation, air mass thunderstorms generally complete their life cycle over the
course of an hour.
Example A cumulus cloud has formed in an environment which has a temperature
of −10 C and a specific humidity of 07 g kg−1 at 500 hPa. During a process of lateral
entrainment, cloud droplets evaporate into the environmental air. What is the lowest
temperature to which the entrained air can be cooled by this process, if there are no
updrafts or downdrafts?
As unsaturated environmental air is entrained into the cumulus cloud, the water
droplets will evaporate. This evaporation will cool the air until saturation is reached,
and the temperature attained by this process is known as the wet-bulb temperature. In
this process, the amount of latent heat required to evaporate the water is provided by
the air, and as a result the air temperature will decrease. Energy must be conserved;
that is, the same amount of energy used to evaporate the water must be extracted
from the air. An equation that will allow us to calculate the wet-bulb temperature
can be derived (not shown). This equation is
−B
L
A exp
−r
Tw = T −
cp p
Tw
where Tw is the wet-bulb temperature, T is the environmental air temperature, L is
the latent heat of vaporization, cp is the specific heat of air at constant pressure,
= 0622 as defined in Section 11.1, p is the air pressure, r is the mixing ratio,
and A = 253 × 1011 Pa and B = 542 × 103 K are constants used to calculate
the saturation vapor pressure of the air. For this example all of the variables in the
equation are known, except for Tw . Tw appears on both the left and right hand sides
MULTI-CELL THUNDERSTORMS
193
of this equation, so to find Tw we must solve this equation iteratively. We start with
an assumed value for Tw = T = 263 K on the right hand side of the equation, and
solve for a new value of Tw :
25 × 106
Tw = 263 −
1004
= 256 K
0622
−542 × 103
11
− 00007
· 253 × 10 · exp
50 000
263
We then use this new value of Tw on the right hand side of the equation to get a
further refined estimate of Tw :
25 × 106
Tw = 263 −
1004
= 260 K
0622
−542 × 103
11
− 00007
· 253 × 10 · exp
50 000
256
We repeat (iterate) this process until the values of Tw do not change significantly.
For this example, this requires 12 iterations and the final value of Tw is 259 K. This
is the temperature to which the entrained air will be cooled and the air will now be
saturated.
If this saturated parcel is carried to the 850 hPa level in a downdraft, what will its
temperature be?
If the saturated air parcel is forced to sink it will warm adiabatically, and become
unsaturated immediately, thus warming at the dry adiabatic lapse rate. The air parcel
will conserve potential temperature (Equation (3.9)) and the actual temperature at
850 hPa can be determined from the potential temperature:
= T500
p0
p500
Rd /cp
1000 × 102
= 259
500 × 102
= 316 K
p0 −Rd /cp
T850 =
p850
287/1004
= 302 K
11.3 Multi-cell thunderstorms
In any air mass thunderstorm, the leading edge of the cold air associated with the
downdraft is called the gust front, which is characterized by a zone of gusty winds
194
CLOUDS AND SEVERE WEATHER
downdraft
updraft
gust front
Figure 11.4 A gust front (bold line) between the warm, moist updraft and cool, dry downdraft
allows the generation of new cells
and separates the warm moist updraft from the cooler downdraft and associated
precipitation (Figure 11.4). Hence, the gust front is able to force the warm, moist air
upward, generating a new cell. When a storm continuously regenerates in this manner,
a series of single-cell thunderstorms result that are in different stages of development.
Such a phenomenon is called a multi-cell thunderstorm. Multi-cell thunderstorms can
sometimes be severe.
11.4 Supercell thunderstorms and tornadoes
While air mass thunderstorms are ubiquitous, more interesting weather arises from the
dangerous supercell thunderstorm. Nearly all supercell thunderstorms produce severe
weather such as large hail, but only around 30% lead to the development of tornadoes.
The supercell is a single-cell thunderstorm, with the distinguishing characteristic that
the updraft of the storm is rotating, forming a mesocyclone. The term mesoscale
refers to systems that range in size from tens to hundreds of kilometers. Often
mesoscale systems are separated into three classes: meso-alpha (200–2000 km); mesobeta (20–200 km), and meso-gamma (2–20 km.) Supercells, however, are not defined
by their size, depth, or violence, but rather by this rotating updraft.
Supercell thunderstorms generally form in an environment that has moderate to
high values of CAPE (over 1500 J kg−1 ) and large vertical wind shear (more than
20 m s−1 in the layer below 6 km). The vertical shear plays a crucial role in preventing
the inbuilt ‘self-destruct mechanism’ of less severe single-cell thunderstorms, in
which precipitation destroys the updrafts, by inducing a circulation that separates the
zones of precipitation and of uplift (Figure 11.5). Because supercell thunderstorms
are coherent dynamical systems, they propagate in a continuous manner. Hence,
supercells can persist for up to 12 hours and may travel great distances.
These storms are still not well understood, due to the obvious difficulties in
observing them, but some aspects can be described here.
SUPERCELL THUNDERSTORMS AND TORNADOES
overshoot
anvil
pressure (hPa)
250
195
outflow
precipitation
500
0 °C
700
wall cloud
inflow
sfc
40 km
Figure 11.5 Structure of a supercell thunderstorm
A visible manifestation of the mesocyclone is the wall cloud (see photograph on
the CD-ROM). The development of the mesocyclone arises due to the presence of
vertical wind shear, calculated as the vector difference of the wind at two heights.
This is because vorticity can be produced wherever there is a gradient in the velocity
either normal or parallel to the direction of the existing vorticity vector. When the
velocity gradient is normal to the vorticity vector, the lines of vorticity are tilted and
new vorticity is generated that is proportional to the steepness of the wind gradient.
When the velocity gradient is parallel to the vorticity vector, the lines of vorticity are
stretched, generating new vorticity, or compressed, causing a reduction in vorticity.
The vertical wind shear can be enhanced by the presence of a low-level jet (a
distinct wind maximum in or just above the boundary layer). The shear results in the
rotation of air about an axis parallel to the ground. The rotating air is tilted into the
vertical by the thunderstorm updraft. The resulting rotation results in lowered pressure
in the center of the rotation. This pressure perturbation leads to an even stronger
updraft, which in turn enhances rotation through vortex stretching. This process
creates a feedback between the strength of the updraft and the cyclonic rotation. The
deeper the layer of wind shear, the more efficient this dynamical process. In this
context, buoyancy forces are of secondary importance, unlike the weaker air mass
thunderstorms. Because of this, supercell thunderstorms can continue after sunset,
surviving the loss of surface heating and the resulting buoyancy forces.
Tornado formation is strongly dependent on the dynamical structure of the thunderstorm, with the development of the mesocyclone and associated wall cloud likely
to be an essential precursor to tornado formation. The relatively slower (of order
10 m s−1 ) cyclonic circulation of the mesocyclone is generally observed up to an hour
before a tornado (spinning with velocities an order of magnitude larger) is evident. It
appears that this larger scale circulation leads to the observation that most tornadoes
196
CLOUDS AND SEVERE WEATHER
rotate cyclonically, despite the scale argument that tornadoes should have no preferred
rotation.
Within this favorable environment, then, tornado formation appears to be related
to a smaller, storm-scale process in which warm environmental air rises on the
warm side of the outflow boundary while cold air sinks and undercuts on the cold
side, which generates horizontal vorticity along the boundary. This vorticity then is
tilted and rapidly accelerated vertically into the dynamically generated mesocyclone
updraft, resulting in likely tornado formation. The horizontal vorticity associated with
this low-level process is in general not evident in the environment. It is generated
through the thunderstorm’s interaction with the environment.
Because of their violence, the wind speeds in tornadoes are impossible to measure directly. Hence, the strength of a tornado is estimated based on the resulting
damage, using the Fujita–Pearson scale (Section 6.2.2). Another useful tool is the
weather radar, which cannot observe individual tornadoes, but can discern the associated mesocyclone, which is an order of magnitude larger. Doppler radars can also
determine aspects of the wind velocity (see the projects section of the CD-ROM).
11.5 Mesoscale convective systems
In addition to supercell thunderstorms, the atmosphere can generate severe weather in
other forms, collectively known as mesoscale convective systems. Mesoscale convective systems include everything from individual squall lines to the larger mesoscale
convective complexes, and range widely in their degree of organization. There is
no strict size definition for these systems, although most often they fall into the
meso-beta scale range.
A squall line, or multi-cell line, is a line of thunderstorms which may be simple air
mass thunderstorms or supercells that share a common lifting mechanism. Examples
of possible lifting mechanisms include a cold front or a dryline boundary, which
may trigger a rather continuous line of thunderstorms, or a gravity wave, which
can generate a more scattered distribution. An example of an active squall line that
generated tornadoes in southern Canada and the northern United States along a cold
front on 11 July 2005 is shown on the CD-ROM. A dryline is simply the boundary
between a moist and a dry air mass across which the temperature does not vary
significantly. As with individual thunderstorms, a sufficient environmental CAPE
is favorable to squall line development. Severe squall line development is almost
always associated with higher values of CAPE (over 2000 J kg−1 ). In addition, a squall
line can be self-sustaining in an environment of wind shear, since this allows the
individual convective cells to be more severe and longer lived, and perhaps develop
into supercells.
Squall lines, like individual thunderstorms, generally undergo a characteristic life
cycle. Typically, the squall line develops between 100 and 300 km ahead of and
parallel to a cold front, at a location of optimum lift, CAPE, and shear. This location
is in the warm, moist sector of the associated mid-latitude cyclone, and the low-level
REVIEW QUESTIONS
197
warm advection in this sector contributes strongly to the instability of the environment. Squall line motion is a result of both advection and propagation. Individual
thunderstorm cells that make up the squall line tend to move with the low-level wind
field, but new cells continue to develop in the direction of the low-level wind shear
vector. During the early stages of a severe system, supercells may be present along
the entire squall line. However, the interacting circulations of the associated mesocyclones quickly disrupt the line so that only certain locations, such as the ends or at
breaks in a squall line, remain favorable for supercell maintenance. In the interior of
the squall line, more linear features such as bow echoes remain. Whether the narrow
band of convective cells develops into a severe system or not, the natural evolution
leads to an eventual spreading and weakening of the system.
Bow echoes are, as may be surmised from their name, a type of mesoscale convective system that was identified with the advent of weather radar. Bow echoes
are meso-beta scale boomerang-shaped systems of convective cells that occur either
as isolated phenomena or as part of a larger system such as a squall line. During
the development of a bow echo, cyclonic and anticyclonic vortices develop at either
end of the bow. As the cyclonic vortex becomes dominant, the bow evolves into
a comma-shaped system. The ‘notch’ behind the bow signifies the location of the
inflow jet, which can be a source of strong, damaging winds. Supercells are sometimes observed in bow echo systems, or may evolve directly into a bow echo as they
decay. Severe bow echoes require very high levels of CAPE – observations have
suggested that values larger than 2500 J kg−1 are necessary.
At the largest, meso-alpha, end of the spectrum are mesoscale convective complexes
(MCCs). Like other deep convective systems, MCCs are usually initiated in regions
of high low-level wind shear and CAPE, often in the vicinity of a stationary front.
MCCs are defined according to a set of criteria associated with size (greater than
100 000 km2 ), shape (elliptical rather than linear), and duration (greater than 6 h) of
the thunderstorm complex. Typically, MCCs are around 350 000 km2 (as defined by
the area of cloud cover) and last for around 11 h, with marine systems tending to
be larger and longer lived than terrestrial systems. Peak rainfall tends to occur early
in the development of the system. Unlike the sharp weather boundaries associated
with squall lines, MCCs are characterized by more persistent disturbances – long,
heavy rainfall events leading to flash floods, for example. More than 20% of extreme
summer rainfall in the United States has been attributed to MCCs. Similarly, areas
of southern China and the Sahel region in Africa rely on deep convective systems
such as these for their growing season rainfall.
Review questions
11.1 (a) Calculate the relative humidity with respect to an ice surface for temperatures from 0 C to −40 C, at 5 C intervals, for air that is saturated with
respect to a liquid water surface.
(b) At what temperature is the relative humidity with respect to ice greatest?
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CLOUDS AND SEVERE WEATHER
11.2 Use the surface weather map shown in Figure 1.7 to determine the vapor
pressure, saturation vapor pressure, mixing ratio, saturation mixing ratio, and
relative humidity at (a) Denver, Colorado, (b) Fort Worth, Texas, and (c)
Nashville, Tennessee. The surface pressure is 828.0 hPa at Denver, 984.1 hPa
at Fort Worth, and 989.5 hPa at Nashville.
11.3 (a) What is the pressure at the lifting condensation level for an air parcel that
has an initial pressure of 1000 hPa, an initial temperature of 20 C, and an
initial dew point temperature of 15 C?
(b) Recalculate the lifting condensation level for an initial dew point temperature of 10 C.
(c) Recalculate the lifting condensation level for an initial dew point temperature of 0 C.
(d) Recalculate the lifting condensation level for an initial dew point temperature of −10 C.
11.4 (a) Using the temperature, dew point temperature, and pressure at the surface
at Fort Worth, Texas at 00 UTC 5 Jun 2005 from the first example in
Section 11.1, calculate the relative humidity using the equations in this
chapter.
(b) How does the calculated value of relative humidity compare to that determined based on the values of mixing ratio and saturation mixing ratio
estimated from the skew T –log P diagram in the example from Section 11.1?
11.5 Use the storm of 2003 case study data on the CD-ROM from Fort Worth, Texas
at 00 UTC 15 Feb 2003 to:
(a) Plot a sounding on a blank skew T –log P diagram using data from the
surface and 850, 700, 500, 300, and 250 hPa levels.
(b) Indicate the temperature and dew point temperature of an air parcel lifted
from the surface to 250 hPa on the skew T –log P diagram.
(c) Mark the lifting condensation level, level of free convection, and equilibrium level on this sounding.
(d) Shade the portion of the sounding that represents CAPE.
12
Tropical weather
The tropical regions are unique in many ways, and yet the basic dynamics that govern
the weather in the tropics is the same as that in the middle latitudes. Nevertheless,
phenomena such as tropical cyclones are confined to these low latitudes. Many of the
differences observed in tropical dynamics can be attributed to the different scaling
that is appropriate there.
12.1 Scales of motion
In the middle latitudes, the strong horizontal temperature gradients and large values
of the Coriolis parameter give important guidance as to the appropriate approximations to be made in understanding the dynamics of the weather in these regions.
These attributes give rise to the very useful quasi-geostrophic equation, and the
conclusion that the available potential energy associated with the strong temperature gradients provides the energy for mid-latitude and polar atmospheric motion. In
the tropics, however, both the temperature gradients (Figure 12.1) and the Coriolis
parameter
f45 = 103 × 10−4
f10 = 253 × 10−5
are small.
Hence, a scale analysis for synoptic scale systems in the tropics may yield rather
different results compared to an analysis for the middle latitudes (Section 5.4.2). Consider the basic Navier–Stokes equations with synoptic scales as defined in Table 5.1,
so that
L ∼ 106 m
T ∼ several days ∼ 105 s
⇒ U ∼ 10 m s−1
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
200
TROPICAL WEATHER
Figure 12.1 The 1000–500 hPa thickness averaged over 1970 to 1999. NCEP Reanalysis data
provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from its Web site at
http://www.cdc.noaa.gov/
In this case, the Rossby number is still sufficiently small to assume a strong role for
the Coriolis force in synoptic scale disturbances:
Ro ∼
U
fL
10
25 × 10−5 × 106
Ro ∼ 04
∼
but clearly not as strong as for the mid-latitude case. Assuming that vertical motions
are not significantly larger than in the mid-latitude case (this assumption can be
justified based on an analysis of the thermodynamic equation), the following scaling
can be developed using the dynamic pressure (pd ) form of the equations:
2
u
u 2 u
u
u
1 pd
2 u
u
+w
=−
+v
+ v 2 + 2v sin −2w cos
+
+ u +v
2
2
t
x
y
z
x
y
z
x2
v 2 v
v
v
1 pd
2 v
v
v
+w
=−
+v
+
v
− 2u sin
y-eqn
+u +v
+
t
x
y
z
y
x2 y2
z2
2
2
U
U
UW
p
vU
vU
scale
fo U
≈ f0 W
L
L
H
L
L2
H2
−4
−4
−5
−16
−12
−4
magnitude 10
10
10
?
10
10
25 × 10
10−6
x-eqn
The Coriolis term is larger than the time rate of change and advection terms, just as
in the middle latitudes, but by a factor of 2, rather than by a factor of 10. In order for
SCALES OF MOTION
201
these equations to balance, therefore, the pressure gradient term must also be smaller
than in the mid-latitude scaling. Hence, the pressure perturbations associated with
synoptic scale disturbances must be smaller than for systems of a similar scale in
the middle latitudes, and increasingly so as we move toward the equator. There are
a range of waves in the tropical atmosphere that conform to this scaling. Here, one
important example will be considered.
Example Find the wave equation for a synoptic scale equatorial wave with no
meridional velocity perturbation component.
To solve for a wave, the perturbation method is appropriate. For simplicity, we will
assume that density is constant. From the scaling developed above
u
u
1 pd
u
+u +v = −
+ fv
t
x
y
x
v
v
v
1 pd
+u +v = −
− fu
t
x
y
y
the equations can be linearized assuming a zero basic state (u v = 0):
u′
1 pd′
=−
+ fv′
t
x
v′
1 pd′
=−
− fu′
t
y
For the equatorial zone, the beta approximation for the Coriolis parameter is simply
f ≈ y, since f0 = 0. With no meridional velocity perturbation, the equation set
simplifies to
u′
1 pd′
=−
t
x
1 pd′
0=−
− yu′
y
(12.1)
Hence, this wave will conform to a geostrophic force balance in the meridional
direction between the pressure gradient and the zonal velocity perturbation. Assume
a solution in the form of zonally propagating waves:
u′ = û y cos kx − t
pd′ = p̂d y cos kx − t
and substitute into Equation (12.1), resulting in
û sin kx − t =
k
p̂ sin kx − t
d
0=−
1 p̂d
cos kx − t − yû cos kx − t
y
202
TROPICAL WEATHER
These equations can be combined to eliminate the pressure term and yield a single
differential equation for û:
yû = −
û
k y
which can be integrated to give
−
ky2
= ln û + A
2
where A is some constant of integration. If the perturbation zonal velocity at the
equator is defined as û0 cos kx − t , this constant can be eliminated and the resulting
solution is
ky2
û = û0 exp −
2
ky2
′
cos kx − t
u = û0 exp −
2
Since c = /k, this can also be written
y2
u = û0 exp −
cos kx − t
2c
′
(12.2)
and the corresponding pressure perturbation is obtained from substituting this solution
into either component of Equation (12.1):
y2
pd′ = cû0 exp −
cos kx − t
2c
In order to yield a solution that decays away from the equator, c > 0, and hence
these waves are eastward traveling, synoptic, or planetary scale waves trapped near
the equator (Figure 12.2). Such waves are known as Kelvin waves.
Kelvin waves were first described in 1879 as water waves traveling along a vertical side boundary, by Sir William Thomson, Lord Kelvin (who was introduced in
Chapter 1). In their atmospheric, equatorially trapped form, the change of sign of the
Coriolis parameter at the equator acts in a manner analogous to a vertical boundary.
These waves contribute substantially to the temperature, pressure, and wind variations
observed in tropical regions, and are important both for influencing the dehydration
of air entering the stratosphere and for the formation of tropical cirrus clouds. Kelvin
waves are forced by variations in deep convection.
203
ATMOSPHERIC OSCILLATIONS
(a)
L
equator
H
z
x
(b)
height s
e
equator
z y
x
Figure 12.2 Equatorially trapped Kelvin wave shown as (a) a plan view and (b) a threedimensional view. Vectors in (a) indicate zonal wind associated with the wave. Note that the
pressure height surface and zonal wind magnitude vary sinusoidally in x (longitude) and decay
exponentially in y (latitude)
12.2 Atmospheric oscillations
Because the atmosphere contains waves of many different wavelengths and frequencies, propagating in many different directions, at all times, it should come as no
surprise that these waves sometimes interact to produce oscillating behavior on longer
time scales. There are many such atmospheric oscillations known in the atmosphere,
including the Southern Oscillation (which is associated with El Niño), the Pacific
Decadal Oscillation, and the North Atlantic Oscillation. All of these phenomena are
characterized by an alternation of two extreme states in a particular region of the
globe (characterized by pressure, temperature, and winds), usually with a period on
the order of years. For example, a ‘warm’ El Niño event occurs every 2 to 7 years,
and alternates with the ‘cold’ La Niña state and in between, or ‘neutral’, states, much
as one would expect if taking snapshots of an extremely slow pendulum.
The mechanism driving these oscillations is, in many cases, an open question, but
for some an interaction between different waves has been identified as a likely cause.
For example, the quasi-biennial oscillation or QBO, which has a period of around
26 months, appears to arise from just such a mechanism. This oscillation occurs
in the mean zonal winds of the equatorial stratosphere whereby zonally symmetric
easterlies and westerlies alternate regularly (Figure 12.3). It is currently thought that
the equatorially trapped Kelvin wave provides the westerly momentum and a type
of wave known as a combined Rossby–gravity wave provides easterly momentum to
204
TROPICAL WEATHER
20
15
(1)
10
5
0
–5
–10
–15
–20
(2)
–25
1965
1975
1985
10
15
(2)
10
30
30
300
10
15
3
30 5
20
25
20
25
10
0
30
0
0
10
0
5
5
50
50
100
40
5
10
10
15
20
50
5
0
20
10
20
5
2005
10
(1)
1995
60
–30
1955
10
0
20
0
10
10
500
0
5
90 °S
EQ
5
1000
90 °N 90 °S
EQ
90 °N
Figure 12.3 The top panel shows the zonal mean zonal wind at the equator at an altitude corresponding to the 30 hPa level, from 1955 to 2005, showing the quasibiennial oscillation between easterlies (negative) and westerlies (positive). An example of the westerly phase
is shown in lower panel (1) during March 2002, and of the easterly phase in lower panel
(2) during July 2003: these panels show the zonal mean zonal wind by latitude and height.
Westerlies are indicated by solid contours and easterlies by dotted contours. Reanalysis data
provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from its Web site at
http://www.cdc.noaa.gov/
produce the QBO. Recent studies suggest that stratospheric ozone variations are also
important to the generation and maintenance of the oscillation.
Although the period of the QBO is nominally defined as a little over 2 years,
because it arises from a complex interaction, the period varies substantially over time.
The fastest oscillation observed between 1948 and 1993 had a period close to 20
months (1959–1961) and the slowest was 36 months (1984–1987). Each new disturbance appears initially at around 30 km altitude, or around 10 hPa. The disturbance
then propagates downward at a rate of around 1 km per month, until it reaches around
23 km (35 hPa). At this point, the westerlies continue downward at the same rate,
but the propagation of easterly perturbations is slowed. This asymmetrical descent
TROPICAL CYCLONES
205
rate of the easterly and westerly zonal wind perturbations is one of the most notable
properties of the QBO.
The QBO is important for many aspects of tropical weather. It is thought to affect
rainfall patterns in the Sahel region (the semi-arid transition between the Sahara
Desert and the equatorial tropics of Africa). Predictions of both El Niño events
and monsoon strength and timing make use of information regarding the phase and
strength of the QBO. Finally, the phase of the QBO affects the frequency of tropical
cyclones in the Pacific, Atlantic, and Indian Oceans.
12.3 Tropical cyclones
Tropical cyclones are one of the most destructive of meteorological events. They
develop over the tropical and subtropical waters of all the world’s oceans and are
characterized by highly organized convection, a distinct ‘eye’, and strong cyclonic
winds near the surface. In order to be classified as a tropical cyclone, sustained
surface winds greater that 33 m s−1 or 64 kts must be observed. About 80 tropical
cyclones occur each year around the world.
Tropical cyclones go by different names depending on their region of formation. In the north Atlantic Ocean and the eastern side of the Pacific Ocean the
term ‘hurricane’ is used. In the north-west Pacific Ocean the term is ‘typhoon’,
and in other oceanic regions some variant of the more generic ‘tropical cyclone’
is used.
Tropical cyclones take form in a continuous process, usually spanning several days,
from the mesoscale convective complexes (Section 11.5) that develop frequently in
the tropical regions. Transition from MCC to tropical cyclone cannot occur unless
certain additional preconditions are met in the environment. These requirements are:
1. a conditionally unstable atmosphere (Section 11.1.1);
2. warm ocean temperatures (greater than 265 C);
3. a moist middle troposphere;
4. low vertical wind shear; and
5. at least 500 km from the equator.
The conditionally unstable atmosphere combines with heating from below to allow
the development of a robust MCC. Ascent can be initiated from the remains of a
higher latitude frontal system, from convergence associated with westward moving
disturbances known as African easterly waves, or at the Intertropical Convergence
Zone (ITCZ) which occurs where the north-easterly and south-easterly trade winds
converge. Warm waters at the surface are the ultimate source of energy for the
cyclone, and will be described in more detail below. Once convection commences,
the moist atmospheric layers in the middle of the troposphere reduce the amount of
206
TROPICAL WEATHER
evaporation within the cloud, and so enhance the rate of precipitation and associated
latent heat release. The small wind shear prevents the disruption of the gradually
organizing axisymmetric convective rain bands, in contrast to asymmetric mid-latitude
systems, which benefit from strong shear.
If the first four preconditions are met, a vigorous convective system will then
generate a meso-beta scale vortex, as mid-tropospheric expansion due to heating
leads to a lowering of surface pressure and associated surface convergence. This
mesoscale circulation, which entails a balance between pressure gradient, friction,
and Coriolis force, requires that the development occur some distance from the
equator. The thermodynamic disequilibrium between atmosphere and ocean then offsets through evaporation the energy lost to the system due to frictional dissipation.
This process is more efficient at higher wind speeds, since strong winds result in
very rough ocean conditions with large waves – these evaporate more efficiently
than a flat surface or water. Because of this, a feedback can develop, in which
escalating winds increase the rate of evaporation, intensifying convection, lowering surface pressure, and further increasing wind speeds. This conceptual model of
tropical cyclone intensification is known as WISHE (Wind-Induced Surface Heat
Exchange).
When the system reaches wind speeds of between 20 and 34 kts and presents a
closed circulation, it is classed as a ‘tropical depression’. If winds continue to increase
above 34 kts through the feedback process, the system is classed as a ‘tropical storm’,
and then, after 64 kts is reached, a ‘tropical cyclone’ (Figure 12.4a and b).
Tropical cyclones have an average central pressure of around 960 hPa, much
deeper than mid-latitude cyclones. Other differences include the location of strongest
winds, which are at the jet stream for mid-latitude cyclones but at the surface
for tropical cyclones. Mid-latitude cyclones have cold cores and are asymmetrical
in structure, with strong frontal systems. Tropical cyclones have warm cores, no
frontal structures, and are relatively axisymmetric. In addition, tropical cyclones
are meso-alpha scale in space but of longer duration than mid-latitude mesoscale
systems.
tropopause
eye
l rain
inversion
spira
band
l rain
spira
warm core
band
outflow
inflow
Figure 12.4(a) Cross-section through a mature tropical cyclone, showing the main features and
the vertical circulation
TROPICAL CYCLONES
207
eye
spiral rainbands
eyewall
Figure 12.4(b) Key features of a mature topical cyclone, as shown on an enhanced infrared
satellite image of Hurricane Ivan, taken at 0315 UTC 16 Sep 2004. Image courtesy of the NOAA
National Severe Storms Laboratory, Norman, Oklahoma, USA
Example Use the thermal wind relationship to explain why tropical cyclones are
typically characterized by anticyclonic circulation above around 300 hPa.
First, it is useful to write the thermal wind relationship in natural coordinates (see
Section 6.4):
Vg
g T
=−
z
fT00 n
⇀
Since the axis is normal to and to the left of the wind, this axis will point to the
center of a tropical cyclone in the Northern Hemisphere. Tropical cyclones are warm
cored, and hence
T
>0
n
and the thermal wind relationship requires that
Vg
<0
z
Hence, the geostrophic wind decreases with height and, if it reverses, will allow an
anticyclonic circulation aloft.
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TROPICAL WEATHER
A distinctive feature unique to tropical cyclones is the formation of an eye. The eye
is an area of relatively light winds (increasing as the eye wall is approached), low
pressures, and high temperatures found at the center of most severe tropical cyclones.
There is generally little or no precipitation. The eye of a tropical cyclone is usually
in the range of 30 to 60 km in diameter.
This warm, clear zone at the center of the cyclone is due to the fact that subsidence
within the eye results in adiabatic warming, particularly at middle levels, which
also creates an inversion. An inversion profile, as we have seen, is highly stable
and hence suppresses convection. The precise mechanisms by which the eye forms
are not well understood. One possible constraint that promotes eye formation is the
conservation of angular momentum. The winds converging around the center of the
cyclone would quickly rise to supersonic speeds to satisfy this requirement, and hence
air must ascend before reaching the center. Further, convection in tropical cyclones is
organized into long, narrow rain bands that converge toward the center (Figure 12.4),
and subsidence on both sides of the bands must contribute to initiating as well as
maintaining clearing within the eye.
The eye wall is the area of strong convection, highest surface winds and net
uplift. This net upward flow is a result of many updrafts and downdrafts within the
individual thunderstorm-like circulations. The formation of the eye wall is related to
the convergence of air in a shallow layer above the sea surface. The convergence
is due to the disruption of gradient wind balance due to the presence of turbulent
stresses in the boundary layer (see Section 6.2.4). The converging air is, of course,
very moist and as it rises out of the boundary layer the water vapor condenses to
form the eye wall clouds. The air flows outward above the boundary layer and, as a
result, clouds in the eye wall tilt outward with height.
Review questions
12.1 Derive Equation (12.1) from the Navier–Stokes equations, listing all assumptions and showing all of the steps in the derivation.
12.2 At 00 UTC 18 Aug (17 Local Standard Time, LST), a Kelvin wave convective
envelope at approximately 3 N and 132 W was observed by researchers on a
ship. This Kelvin wave had a phase speed of 15 m s−1 and a period of 6 days.
Plot u′ and pd′ for this wave using û0 = 5 m s−1 at t = 0 s and y = 0 , 1 , and
5 latitude. Describe the phase relationship between u′ and pd′ and how these
variables vary with latitude.
12.3 Calculate the gradient wind for a landfalling hurricane at Miami, Florida assuming a horizontal pressure gradient of 2 mb km−1 at a distance of 50 km from the
center of the storm.
12.4 Using the answer from question 12.3 calculate the altitude at which the winds
shift from cyclonic to anticyclonic assuming that the eye is 5 C warmer than a
location 10 km from the center of the storm.
13
Mountain weather
Many interesting features of mountain weather are related to the generation of internal
gravity waves by flow over orography (mountains). These waves are commonplace
in a stable atmosphere and can be generated by many mechanisms. In this chapter
we will derive the governing equations for internal gravity waves, explore how these
waves are generated by flow over orography, and how the waves affect the winds,
pressure, and temperature to result in some unique aspects of mountain weather.
We will conclude the chapter by considering the dynamics associated with severe
downslope windstorms that occur in the lee of major mountain barriers around
the world.
13.1 Internal gravity waves
13.1.1 Derivation of the internal gravity wave equation
In a stable atmosphere, an air parcel that is displaced vertically will experience a
buoyancy force that accelerates the air parcel back toward its original position. This
buoyancy force is the restoring force for internal gravity waves, first introduced in
Section 8.1. In that example, the dispersion relationship, which relates the frequency
of the wave, , to the horizontal, k, and vertical,
⇀ m, wavenumbers, and the Brunt–
Väisälä frequency, N , was given as = Nk/k. We will now derive the governing
equations for this type of wave motion and show that the dispersion relationship
assumed in Section 8.1 is indeed correct.
For our analysis of internal gravity waves we will start with the Navier–Stokes
equations (4.19). These waves are very much smaller than synoptic scale systems,
and hence we cannot use the synoptic scaling to simplify the equations. However, we
can make some simplifications. These include making the Boussinesq approximation
(Section 6.3), and ignoring the effects of friction. We will also assume that the flow is
two-dimensional in the xz plane, such that we can neglect the meridional component,
v, of the wind. Finally we note that the gravity waves are characterized by sufficiently
short time and space scales such that the Rossby number is large and the Coriolis
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
210
MOUNTAIN WEATHER
force terms can be neglected. With these assumptions the Navier–Stokes equations
(4.19) simplify to
u
u 1 p
u
+u +w +
=0
t
x
z x
w
w
w 1 p
+u
+w
+
+g = 0
t
x
z z
u w
+
=0
x z
(13.1a)
(13.1b)
(13.1c)
Finally, we can assume that the flow is adiabatic, which results in an equation for
the potential temperature:
+u +w = 0
t
x
z
(13.1d)
To derive the governing equations for internal gravity waves we will use the perturbation method introduced in Section 8.2. The variables in Equations (13.1) will
be assumed to consist of a basic state and a small departure from that basic state as
follows:
u = u + u′
w = w′
p = pz + p′
(13.2)
= z + ′
= + ′
The basic state is constant in both the x and z directions and with time except for
pz and z, which vary in the vertical direction, and we have also assumed that
the basic state vertical component of the wind, w, is zero. Further, we will require
that the basic state pressure and density satisfy the hydrostatic equation
dp
= −g
dz
(13.3)
Combining the definition of the basic and perturbation states given by Equations (13.2)
and the horizontal momentum equation given by Equation (13.1a) we get
u
u′
u
u′
u
u′
1
u u′
+
+u +u
+ u′ + u′
+ w′ + w′
+
t
t
x
x
x
x
z
z + ′
p p′
+
x x
=0
211
INTERNAL GRAVITY WAVES
Derivatives of the basic state variables are zero (except for vertical derivatives of p
and and terms that involve products of perturbations are sufficiently small that
they can be neglected. This then gives
1 p′
u′
u′
+u
+
=0
t
x + ′ x
By noting that
1
1
=
+ ′
′
1+
−1
1
≈
′
1−
then for ′ / ≪ 1, this reduces to
u′ 1 p′
u′
+u
+
=0
t
x
x
(13.4a)
It can be shown that −′ / ≈ ′ /. Using this approximation, the other equations
(13.1) can be simplified in a similar manner:
w′ ′
1 p′
w′
+u
− g+
=0
t
t
z
u′ w′
+
=0
x
z
′
′
′
+u
+ w′
=0
t
x
z
(13.4b)
(13.4c)
(13.4d)
In Equations (13.4) we have four equations and four unknowns (u′ w′ p′ , and ′ ),
if we assume that the basic state is known. To solve this, we can reduce this set
to a single equation with a single unknown (the details of this are shown on the
CD-ROM) to give
+u
t
x
2
2 w′ 2 w′
2 w′
+
+N2 2 = 0
2
2
x
z
x
(13.5)
This is a complex equation, but can be solved rather easily if we assume a form for
the solution.
13.1.2 Dispersion relation for internal gravity waves
Like Rossby waves and Kelvin waves, the dispersion relation (Section 8.1) for gravity
waves can be derived by assuming a wave-like solution of Equation (13.5):
w′ = w0 coskx + mz − t
(13.6)
212
MOUNTAIN WEATHER
where w0 is the amplitude of the wave, k and m are the horizontal and vertical
wavenumbers, and is the wave frequency. Substitution of this assumed solution
into Equation (13.5) results in
− uk2 k2 + m2 − N 2 k2 = 0
(13.7)
which is the dispersion relationship for internal gravity waves. For the case of u = 0
we find that
Nk
= ±√
2
k + m2
Nk
= ± ⇀
k
(13.8)
which is the equation used in the example in Section 8.1. The dispersion relation tells
us that in order for our assumed solution to be valid, this specific relationship between
k m, and N must be satisfied, and arbitrary values of these variables are not
allowed. Also, note that, unlike the simplified example in Section 8.1, Equation (13.8)
has two roots. The correct root of this equation is selected by noting that frequency
is positive (by convention), so for k > 0 the positive root is used and for k < 0 the
negative root is used.
The components of the phase speed, as we saw in Section 8.1, are
cx =
N
= ± ⇀
k
k
Nk
cz = = ± ⇀
m
m k
(13.9)
and the components of the group velocity are
cgx =
cgz =
Nm2
= ± ⇀3
k
k
−Nkm
= ± ⇀3
m
k
(13.10)
and we choose the appropriate root according to the sign of k, just as for Equation (13.8).
Table 13.1 shows the phase speeds and group velocities resulting from all possible
combinations of signs of k and m when u = 0 The signs of cx and k are always
the same and the signs of cz and m are always the same, so the wave propagates
⇀
in the direction of the wavenumber vector, k = k m, as expected. One interesting
213
INTERNAL GRAVITY WAVES
Table 13.1 All possible combinations of phase speed
(cx and cz ) and group velocity (cgx and cgz ) are shown
here for the case of zero basic wind (u = 0) according to
the sign of k (which determines the sign of root chosen)
and the sign of m
Sign of k
Sign of m
Root
cx
cz
cgx
cgz
+
+
−
−
+
−
+
−
+
+
−
−
+
+
−
−
+
−
+
−
+
+
−
−
−
+
−
+
feature of gravity waves is that cgz is always of opposite sign to cz ; that is, the
direction of vertical energy propagation in an internal gravity wave is opposite the
direction of vertical wave propagation. Thus, a downward propagating internal gravity
wave will direct energy upward and vice versa. This feature will be essential in our
understanding of the orographically forced internal gravity waves discussed below.
Example Show that for an internal gravity wave the direction of energy propagation,
as given by the group velocity, is perpendicular to the wavenumber vector.
First recall that when the scalar product of two vectors is equal to zero the two vectors
⇀
are perpendicular (Section 2.2.3). So it is simply necessary to verify that k • c g is
identically zero:
⇀
⇀
⇀
⇀
⇀
⇀
k • c g = kx + m z • cgx x + cgz z
⎛
⎞
2
Nkm ⎟
⇀
⇀
⎜ Nm
= kx + m z • ⎝ ⇀3 x − ⇀3 z⎠
k
k
Nkm2 Nkm2
= ⇀3 − ⇀3
k
k
=0
And hence the wavenumber and group velocity vectors are perpendicular.
For the case of u = 0, Equation (13.7) gives a more complex relationship:
Nk
= ± + uk
k
214
MOUNTAIN WEATHER
We can also define an intrinsic frequency, which is characteristic of a particular
stability profile (or stratification) of the atmosphere, as given by N :
Nk
ˆ ≡ − uk = ±
k
(13.11)
This is the frequency that would be measured by an observer who is moving with the
basic wind at a speed of u. The positive root of this equation applies to cases when
> uk and the waves are propagating toward the east relative to the mean wind,
while the negative root applies to cases when < uk and the waves are propagating
toward the west relative to the mean wind.
The intrinsic phase speed and group velocity of the wave are identical to Equations (13.9) and (13.10), while the actual phase speed is
cx =
N
= ± +u
k
k
Nk
cz = = ±
m
m k
(13.12)
and the components of the group velocity are
cgx =
Nm2
= ± ⇀3 + u
k
k
cgz =
−Nkm
= ± ⇀3
m
k
(13.13)
As for the dispersion relation, the positive and negative roots of Equations (13.12)
and (13.13) apply to the cases of > uk and < uk respectively. Hence, the only
difference between the phase speed and group velocity of an internal gravity wave
in the presence of zero or nonzero mean wind is that the mean horizontal wind acts
to modify the phase speed and group velocity in the x direction, while leaving the
vertical phase speed and group velocity unchanged.
13.1.3 Structure of internal gravity waves
We can now use the equations we have developed to create a diagram of the structure
of an internal gravity wave. First, we will use our assumed solution of Equation (13.5)
to derive the corresponding solutions for u′ p′ , and ′ .
Example Derive an equation for u′ that is not a function of any other perturbation
variables.
215
INTERNAL GRAVITY WAVES
Using the assumed solution for w, Equation (13.6), in Equation (13.4c) gives
u′
w coskx + mz − t = 0
+
x z 0
u′
= −mw0 sinkx + mz − t
x
m
u′ = −w0 cos
k
(13.14a)
So, for m and k of the same sign u′ is exactly out of phase with w′ ; that is, when w′
is a maximum u′ is a minimum.
Derivation of the equations for p′ and ′ are left as review questions. When we
assume zero basic flow, the equations result in
p′ = −w0
′ =
m
cos
k2
w0 d
sin
dz
(13.14b)
(13.14c)
We can now use these equations to take a ‘snapshot’ of the wind, pressure, and
temperature distribution of an internal gravity wave. We will consider the case
of k < 0 and m < 0, an internal gravity wave that is propagating toward the
west and downward. The lines of constant phase
at a particular time can be
determined by
= kx + mz − tconstant = constant
⇒z=−
k
x + constant
m
(13.15)
where the constant in Equation (13.15) depends on both the phase and the time.
The lines of constant phase for a wave at t = 0 s are shown in Figure 13.1. The
⇀
wavenumber vector, k, is directed down and toward the left in this figure since m < 0
and k < 0, and this is the direction of phase propagation. Since the wave is propagating
toward the left in this figure the phase decreases toward the right and increases
toward the left. Both Figure 13.1 and Table 13.2 indicate the location of maximum,
minimum, and zero values of u′ w′ p′ , and ′ calculated from Equations (13.6) and
(13.14).
Figure 13.1 shows that the air parcel oscillations in the wave are parallel to the
wave fronts (lines of constant phase) and perpendicular to the direction of wave
216
MOUNTAIN WEATHER
z
λz
cg
α
3 λz
4
λz
2
k
u′ min,
w ′ max
u′ max,
w ′ min
λz
4
θ′ min
u′, w ′, p′ = 0
θ′ = 0
p ′ min
θ′ max
u′, w ′, p′ = 0
λx
4
φ=0
θ′ = 0
p ′ max
λx
2
θ′ min
u′, w ′, p′ = 0
3 λx
4
φ = −π/2
λx
φ = −π
φ = −3π/2
x
φ = −2π
Figure 13.1 Schematic illustration of the phase relationships between u′ w ′ p′ , and ′ for an
internal gravity wave. Locations of maximum, minimum, and zero values of u′ w ′ p′ , and ′ are
labeled. The thin diagonal lines show lines of constant phase (labeled with values of ). The angle
between the lines of constant phase and the vertical direction is (top left corner). Distances
along the x and z axes are labeled relative to the horizontal, x , and vertical, z , wavelength of
the wave
Table 13.2 Phase location of the maximum, minimum, and zero values of
u′ w ′ p′ , and ′ for an internal gravity wave with k < 0, m < 0, and u = 0
0
−/2
−
−3/2
−2
x
w′
u′
p′
′
0
x /4
x /2
3 x /4
x
Maximum
Zero
Minimum
Zero
Maximum
Minimum
Zero
Maximum
Zero
Minimum
Maximum
Zero
Minimum
Zero
Maximum
Zero
Minimum
Zero
Maximum
Zero
propagation. The angle between the air parcel oscillations and the vertical direction,
, is given by
cos =
z
2x + 2z
=
=
0 5
2/m
2/k2 + 2/m2
2/m
√
2/km k2 + m2
0 5
217
FLOW OVER MOUNTAINS
=√
k
k2 + m2
k
cos = ⇀
k
Combining this result with the dispersion relationship, Equation (13.8), gives
= N cos
(13.16)
So, the frequency of the wave depends only upon the Brunt–Väisälä frequency and
the angle of the air parcel oscillations in the wave. For air parcel oscillations that are
vertical, = 0, the wave propagates in the horizontal direction, and the frequency of
the wave is equal to the Brunt–Väisälä frequency ( = N ). As the angle of air parcel
oscillation becomes increasingly horizontal, the frequency of the wave decreases,
eventually to a value of 0 for air parcel oscillations that are purely horizontal. More
rapid air parcel oscillations, with a frequency greater than N , are not possible in
internal gravity waves.
13.2 Flow over mountains
13.2.1 Idealized case: an infinite series of ridges
Armed with the knowledge of internal gravity wave dynamics we now turn our
attention to the generation of such waves in response to flow over mountains. We
will first consider the simplest, most idealized case: a constant mean wind flowing
over an infinite series of sinusoidal ridges. This orography can be written
hx = h + h′
= h′
= h0 coskx
where we have assumed that the orography consists of a mean and perturbation part and
that the mean orography (h has zero elevation. The perturbation orography consists of
an infinite series of ridges with a horizontal wavenumber of k and a height of h0 .
Since the wind cannot flow into or out of the ground, the flow at the surface must
be parallel to the orography. Hence, the perturbation vertical velocity at the ground
will be given by
dh
dt
h′
=u
x
′
w x 0 = −ukh0 sinkx
w′ x 0 =
(13.17)
218
MOUNTAIN WEATHER
Consider the case of a stationary wave: /t = 0 and = 0. The governing equation
for internal gravity waves (Equation (13.5)) with these assumptions simplifies to
u2
2
x2
2 w′
2 w′ 2 w′
+
+N2 2 = 0
2
2
x
z
x
Integrating this equation twice with respect to x gives
2 w′ 2 w′ N 2 ′
+ 2 + 2 w =0
x2
z
u
(13.18)
Substituting into Equation (13.18) an assumed wave-like solution of the form w′ =
w0 sin kx + mz (where we have used sin to be congruent with the boundary condition
in Equation (13.17)) gives the dispersion relationship
N2
− k2
u2
N2
− k2
m=±
u2
m2 =
(13.19)
Applying the lower boundary condition to the assumed solution gives
w′ x z = −ukh0 sinkx + mz
(13.20a)
Since the stationary wave assumption requires that = 0, Equation (13.11) gives
ˆ = −uk. This gives an intrinsic phase speed of the wave of
ĉx =
−uk
ˆ
=
= −u
k
k
That is, the wave has a phase speed exactly opposite to the mean wind. Because
the wave propagation and the advection by the mean wind exactly cancel, the wave
remains stationary relative to the ground, and hence this is consistent with our
assumption.
We can determine the sign of m in Equation (13.19) by noting that the vertical
group velocity (Equation (13.13)), and thus vertical energy propagation, is in the
opposite direction to the vertical wave propagation (Equation (13.12)). For the case
of flow over orography, the source of wave energy is at the lower boundary, and so
energy must be propagating upward, and therefore m < 0.
The structure of the wave is shown in Figure 13.2(a) for the case of u > 0. The
lines of constant phase tilt westward with height. Relative to the mean wind, the
wave propagates down and to the west and energy propagates up and to the west as
shown by the group velocity vectors on this figure.
219
FLOW OVER MOUNTAINS
cg
k
Figure 13.2(a) Vertical cross-section through a vertically propagating internal gravity wave
generated by flow over an infinite series of ridges. Streamlines are shown as thin lines, and the
mean flow is from left to right in the figure. Streamlines are plotted at intervals of an eighth of a
vertical wavelength, and the top streamline is plotted at a height of one vertical wavelength. The
⇀
heavy black lines mark lines of constant phase. The group velocity vector ( c g ) and the wavenumber
⇀
vector ( k ) are also shown
cg
k
Figure 13.2(b) Vertical cross-section through an evanescent internal gravity wave generated by
flow over an infinite series of ridges. Streamlines are shown as thin lines, and the mean flow
is from left to right in the figure. Streamlines are plotted at intervals of an eighth of a vertical
wavelength, and the top streamline is plotted at a height of one vertical wavelength. The heavy
⇀
black line indicates a line of constant phase. The group velocity vector ( c g ) and the wavenumber
⇀
vector ( k ) are also shown
220
MOUNTAIN WEATHER
Example Calculate the mean wind speed that causes N 2 /u2 = k2 for flow over a
series of ridges with a horizontal wavelength of 10 km when the atmospheric static
stability is given by N = 0 02 s−1 .
First, we will calculate the horizontal wavenumber, k, that is forced by these ridges:
2
x
2
=
10 000
= 6 3 × 10−4
k=
Then, the magnitude of u required for N 2 /u2 = k2 is
u =
N
k
0 02
6 3 × 10−4
= 32 m s−1
=
This wind speed is often referred to as the critical wind speed. When the magnitude of the mean flow is less than this critical wind speed, Equation (13.19) gives
a real result. However, if the mean flow is faster than this critical wind speed,
Equation (13.19) is not valid and it is clear we have assumed the wrong form of the
solution.
Let us now consider the case of a more rapid mean wind. When u > N/k a
different form of the solution must be assumed because the forcing for the internal
gravity wave at the lower boundary occurs at a frequency greater than N , and as
such Equation (13.18) indicates that the atmosphere is unable to support a wave at
this frequency. However, the boundary condition Equation (13.17) must still apply,
and hence the solution will be wave-like in the x direction and this component of
the assumed solution will remain unchanged. Because the forcing originates at the
ground, it is reasonable to assume, in the absence of a wave-like solution in the
vertical, a bounded solution in the z direction. Hence, we assume a solution of
the form
w′ = w0 e−mz sinkx
(13.20b)
which satisfies Equation (13.17) if w0 = −ukh0 as before. A bounded solution requires
that m > 0. Verification that this solution also satisfies the governing Equation (13.18)
is left as a review question. The structure of the solution for this case is shown in
Figure 13.2(b).
FLOW OVER MOUNTAINS
221
The case of the vertically propagating internal gravity wave (Figure 13.2a) is known
as the wide ridge case since widely spaced ridges will have a small wavenumber k,
and hence are likely to satisfy the condition uk < N . This case is also favored for
situations with weak winds (small u) or strong static stability (large N ). Conversely,
the case of the vertically decaying internal gravity wave, also known as an evanescent
wave, is referred to as the narrow ridge case, where the ridges are closely spaced and
k is large. This case is also favored when the mean wind speed is large or the static
stability is weak.
13.2.2 Flow over an isolated ridge
Flow over an infinite series of sinusoidal ridges, while illustrative of the processes
acting to generate topographic internal gravity waves, is not encountered in the real
world. Instead, the atmospheric flow is more likely to encounter isolated topographic
obstacles. Fortunately, the methods used above can also be used to determine the
structure and characteristics of internal gravity waves generated by flow over an
isolated ridge.
The topography of an isolated ridge, h′ x, can be represented as a sum of sines
and cosines, with varying amplitudes, hn , and wavenumbers, kn , of the same form as
that used to represent an infinite series of ridges. Summing a large number of sines
and cosines over a range of appropriately chosen wavelengths can create a single
curve that approximates the shape of the isolated ridge.
The atmospheric response to flow over each individual sine or cosine term is
identical to that discussed in the previous section (Equation (13.20)). For ukn < N
Equation 13.20(a) applies and the wave is vertically propagating, while for ukn > N
Equation 13.20(b) applies and the wave is evanescent. The perturbation method yields
a linear equation, and hence we can sum the atmospheric response for the flow over
each individual sine or cosine term to reconstruct the resulting wave field. For wide
ridges the sum is dominated by sines and cosines with small kn and the resulting
wave is vertically propagating as illustrated in Figure 13.3(a). For narrow ridges the
sum is dominated by sines and cosines with large kn and the wave that is generated
by the flow over this topography is evanescent. An example of this type of situation
is shown in Figure 13.3(b).
Example Minna Bluff is a ridge 5 km wide and 900 m tall that extends onto the
Ross Ice Shelf of Antarctica. This ridge is oriented east/west while the prevailing
flow in this region is from the south. Observations during a recent winter found a
wind of 20 m s−1 from the south and N = 0 02 s−1 . Determine if the flow over Minna
Bluff for these conditions will result in a vertically propagating or evanescent internal
gravity wave.
222
MOUNTAIN WEATHER
10
4
(a)
9
0
0
–4
0
L
–6.2
8
0
height (km)
7
6
0
0
5
H
7.7
4
4
3 0
0
–4
0
2
L
–8.4
1
0
W
20
40
60
80 100 120
distance (km)
140
160
10
(b)
9 0
180
E
L
–1.2
0
8
Height (Km)
7
6
0
5
0
4
3
2
0
1
W
1
2
3
distance (km)
4
E
Figure 13.3 Vertical cross-section through (a) a vertically propagating internal gravity wave
and (b) an evenescent internal gravity wave generated by flow over an isolated ridge. Contours
of vertical velocity are plotted, with upward vertical velocity shown by solid contour lines and
downward vertical velocity shown by dashed lines. The ridge is shown at the bottom of the figure
by the solid shading, and is shown with a vertical exaggeration of two times
FLOW OVER MOUNTAINS
223
We calculate the horizontal wavenumber of this ridge as
2
x
2
=
5000
= 1 3 × 10−3 m−1
k=
and note that the representation of this ridge as a sum of sines and cosines will be
dominated by sines and cosines with a wavenumber greater than 1 3 × 10−3 m−1 (that
is, wavelengths less than 5 km).
We use this value of k to evaluate uk as
uk = 20 × 1 3 × 10−3
= 0 026 s−1
and note that this value of uk is greater than N . Therefore this flow will result in
an evanescent internal gravity wave of the type illustrated in Figure 13.3(b).
13.2.3 Flow with vertical variations in wind speed and stability
While Section 13.2.2 addressed the more realistic case of internal gravity waves
generated by flow over an isolated ridge, the solution for this type of flow is still
limited by the assumption of constant background wind speed and stability. In the
real atmosphere these conditions are rarely met, and if they are present are likely
to represent a rather uninteresting example. More typical is the case where the
background wind, u, and the static stability, N , vary in the vertical direction.
For this more general case the governing equation for the internal gravity waves,
Equation (13.18), contains an additional term proportional to the curvature of the
wind profile:
d2 u
2 w′ 2 w′ N 2 ′
+ 2 + 2 w − u−1 2 w′ = 0
2
x
z
dz
u
(13.21)
where the coefficients of w′ are grouped as a single term known as the Scorer
parameter:
l2 =
d2 u
N2
− u−1 2
2
dz
u
In fact, often the second term is small and can be neglected.
(13.22)
224
MOUNTAIN WEATHER
As in Section 13.2.1 we will assume a wave-like solution of the form w′ = w0 sinkx +
mz. Substituting this assumed solution into Equation (13.21) gives the dispersion
relationship
m2 = l 2 − k 2
√
m = ± l 2 − k2
Our assumed solution is valid for l2 > k2 and results in a vertically propagating
wave. If l2 < k2 our assumed solution is not valid, and instead we must assume a
solution of the form w′ = w0 e−mz sinkx, which results in an evanescent wave. We
can define a critical horizontal wavenumber as kc = l, where internal gravity waves
will be vertically propagating for k < kc and evanescent for k > kc .
Let us now consider an atmosphere with two layers, each with constant l, such that
in the lower layer lL > kc and in the upper layer lU < kc . This suggests an atmosphere
which is highly stable with comparatively weak winds in the lower layer and lower
stability, perhaps combined with strong winds, in the upper layer.
Flow over an isolated ridge in this situation will generate a vertically propagating
internal gravity wave in the lower layer. This vertically propagating wave will be
reflected at the boundary between the two layers, since vertical wave propagation is
l<k
l>k
k
Figure 13.4 Vertical cross-section through a trapped lee wave generated by flow over an isolated
ridge. Streamlines are shown as thin lines, and the mean flow is from left to right in the figure.
Tick marks along the horizontal axis indicate the horizontal wavelength of the trapped lee wave
and of the topography. The tick mark along the right side of the figure indicates the boundary
between layers of the atmosphere with differing Scorer parameter (l). The relationship between l
and k in each layer is also indicated
225
FLOW OVER MOUNTAINS
not possible in the upper layer. The wave will then be repeatedly reflected between
the interface between the two atmospheric layers and the ground surface, resulting
in a trapped lee wave downstream of the isolated ridge. This situation is illustrated
schematically in Figure 13.4. If sufficient moisture is present in the atmosphere clouds
will form at the crest of each wave shown in Figure13.4. Clouds that form in this way
are known as lee wave clouds and are a common sight downwind of mountainous
regions, particularly during the winter months.
Example Lee wave clouds were observed over Boulder, Colorado on 20 December
2004. Use the rawinsonde observations listed in Table 13.3 from Grand Junction,
Colorado (upstream of the Rocky Mountains) at 12 UTC 20 Dec 2004 to determine
the horizontal scale of topography required to generate trapped lee waves.
Trapped lee waves require that lL > kc and lU < kc . We will calculate l for the lower
layer (surface to 700 mb) and upper layer (700 mb to 300 mb) using Equation (13.22)
and neglecting the curvature term. We first calculate u , and N for each
layer as
15 9 + −1 5
2
= 7 2 m s−1
24 1 + 15 9
2
= 20 0 m s−1
uL =
uU =
300 8 + 281 7
2
= 291 3 K
320 1 + 300 8
2
= 310 5 K
L =
U =
N2 =
NL2 =
300 8 − 281 7
9 81
×
291 3
3053 − 1453
= 4 02 × 10−4 s−2
g d
dz
NU2 =
9 81
320 1 − 300 8
×
310 5
9270 − 3053
= 9 81 × 10−5 s−2
Table 13.3 Rawinsonde observations from Grand Junction, Colorado at 12 UTC
20 Dec 2004
Pressure (mb)
Elevation (m)
850
700
300
1453
3053
9270
Potential
temperature (K)
281 7
300 8
320 1
Zonal component
of the wind (m s−1
−1 5
15 9
24 1
226
MOUNTAIN WEATHER
Using these values we calculate the Scorer parameter:
l2 =
4 02 × 10−4
7 22
= 7 7 × 10−6 m−2
N2
u2
9 81 × 10−5
202
= 2 5 × 10−7 m−2
lL2 =
lU2 =
⇒ lL = 2 8 × 10−3 m−1
⇒ lU = 4 9 × 10−4 m−1
The wavenumber of the orography that is forcing the wave must be less than lL , but
greater than lU . The horizontal wavelength of the topography is given by
2
k
and hence the bounds of orographic wavelength are derived from the critical wavelength in each layer so that
x =
2
2 8 × 10−3
= 2243 m
cL =
= 2 2 km
2
4 9 × 10−4
= 12 823 m
cU =
= 12 8 km
Hence, for a trapped lee wave, the orography must have a wavelength longer than
2.2 km and less than 12.8 km.
The width of the Colorado Rocky Mountains is approximately 200 km, too large
to satisfy the conditions identified above for trapped lee waves. Within the Colorado
Rocky Mountains are numerous smaller mountain ranges, with the Front Range being
located just to the west of Boulder, Colorado. The horizontal scale of the high peaks
in the Front Range is roughly 6 km, and could force a trapped lee wave for the
atmospheric conditions observed on 20 December 2004.
As has been shown in this section, internal gravity waves generated by flow over
topography can take many forms depending on the scale of the topographic forcing
(k) and the state of the atmosphere (u N l). We now turn our attention to the case of
strong downslope windstorms which can result in property damage and loss of life
downwind of major mountain ranges in extreme situations.
13.3 Downslope windstorms
The lee slopes of mountain ranges around the world occasionally experience strong,
damaging downslope winds. These winds are often a defining characteristic of the
DOWNSLOPE WINDSTORMS
227
local climate and are given unique regional names. In the United States strong
downslope winds in the lee of the Rocky Mountains are known as chinooks and can
have speeds in excess of 60 m s−1 . In the Alps the same type of downslope windstorm
is known as a foehn, while downwind of the Southern Alps of New Zealand these
windstorms are referred to as norwesters, for the north-west wind direction common
to these events in this part of the world.
To understand the dynamics of downslope windstorms, we will consider the flow
of a constant density fluid over a mountain ridge, similar to the flow of water over a
rock in a stream. To simplify the analysis, we will try to define a height of the fluid
surface, given by hx. If the height of the barrier is given by hm x, then the depth
of the fluid layer is simply h − hm .
Starting with the x component of the Navier–Stokes equations (4.19), simplifying
assumptions can be made. We will consider flow in the xz plane such that we
can neglect the meridional component, v, of the wind and spatial derivatives in the
y direction (/y = 0. We will assume that the zonal component, u, of the flow
does not vary in the vertical direction (u/z = 0. We will ignore the effects of
friction and will only consider flows with sufficiently short time and space scales
such that the Coriolis force terms can be neglected. Finally, we will only consider
steady state flows such that /t = 0. With these assumptions the x component of
the Navier–Stokes equations (4.19) simplifies to
u
u
1 p
=−
x
x
where the density is constant but u and p may vary. Note that this equation, unlike
the equations considered in previous sections of this chapter, is a nonlinear equation
(see Section 5.4.2), due to the advection term on the left hand side.
We will also require that the flow satisfies the hydrostatic equation
p
= −g
z
We can then use this to rewrite the pressure gradient force term, by assuming that
the pressure at the top of the fluid layer is equal to zero. In this case, the pressure at
any depth z in the fluid is given by pz = gh − z, and the pressure gradient force
term can be written as
h
1 p
= −g
−
x z
x
This equation states that the horizontal pressure gradient depends only upon the slope
of the fluid surface, and further that the pressure gradient will not vary in the vertical
direction in the fluid. Using this result the u momentum equation can be written
u
h
u
= −g
x
x
(13.23)
228
MOUNTAIN WEATHER
If the height of the fluid surface slopes downward in the positive x direction the zonal
component of the flow must increase in this direction. If the fluid surface slopes
upward in the positive x direction the zonal component of the flow must decrease in
this direction.
Equation (13.23) still contains two unknown variables, u and h. To create a closed
set of equations, the principle of continuity can be invoked. For a fluid of constant
density,
u w
+
=0
x z
Integrating this equation over the depth of the fluid, from hm to h, yields
h u
h w
z = −
z
z
x
z=hm
z=hm
wh − whm = −h − hm
u
x
but the vertical velocities at the boundaries of the fluid layer are simply
wh = u
h
x
whm = u
hm
x
and the continuity equation then reduces to
h
h
u
− u m = −h − hm
x
x
x
uh − hm
=0
x
u
(13.24)
This equation requires that changes in the zonal wind speed must be compensated for
by changes in the depth of the fluid, h − hm , such that as the wind speed increases
the total depth of the fluid must decrease and vice versa.
Equations (13.23) and (13.24) now make a closed set of equations in the two
unknown variables u and h. These equations can be combined by multiplying Equation (13.23) by u and using Equation (13.24) to replace h/x to give
2
u
h
= −ug m
u − g h − hm
x
x
We simplify this equation by defining the Froude number, Fr, as
Fr =
u2
g h − hm
229
DOWNSLOPE WINDSTORMS
and hence
1 − Fr 2
u
x
=
u hm
h − hm x
(13.25)
The changes in the zonal flow in response to the topography depend on the value of
the Froude number. For Fr < 1 the flow is termed subcritical and u/x ∝ hm /x.
When Fr > 1, u/x ∝ −hm /x and the flow is termed supercritical. The Froude
number can be understood as the ratio of the magnitude of nonlinear advection to
the magnitude of the pressure gradient. Thus, in supercritical flow, the nonlinear
advection term dominates and balance is satisfied only when the flow is accelerated.
The flow for both of these cases is illustrated in Figure 13.5.
Fr < 1 everywhere
z
x
ground
Fr > 1 everywhere
z
x
ground
Fr < 1
Fr > 1
Fr < 1
z
x
ground
Figure 13.5 Vertical cross-sections of a two-layer fluid flowing over a ridge for (a) Fr < 1 everywhere (subcritical flow), (b) Fr > 1 everywhere (supercritical flow), and (c) Fr < 1 transitioning to
Fr > 1 over the ridge (hydraulic jump)
230
MOUNTAIN WEATHER
Consider a case where a subcritical flow approaches the crest of a mountain. The
wind speed increases and the total depth of the fluid layer decreases, both of which
cause Fr to increase. If the value of Fr upstream of the barrier is sufficiently large
that Fr increases to a value greater than 1, the flow will transition from subcritical
to supercritical. As the supercritical flow descends the lee of the barrier the wind
speed will increase further and the depth of the fluid will continue to decrease. This
flow eventually becomes unstable and a turbulent hydraulic jump occurs. The jump
allows the flow to adjust back to subcritical conditions: the wind speed decreases and
the depth of the fluid increases. This situation can result in very large wind speeds
in the lee of the barrier, the downslope windstorm. Conditions in the hydraulic jump
are extremely turbulent and pose a serious hazard to aircraft.
Example Measurements were taken on 24 January 1992 at experimental towers near
the town of Bettles, located in the lee of the Brooks Range in Alaska, near the top of
the Brooks Range, and on the northern, windward side. During the early hours of the
morning, a rapid and turbulent acceleration of the wind from relatively calm to 90 kts
was observed at the Bettles site. Around the same time, the prevailing northerlies
were impinging on the Brooks Range at 10 kts. Soundings indicated a very shallow
stable layer extending to around 1700 m was topped by a strong inversion. Does the
data indicate the presence of a hydraulic jump?
The elevation of the Brooks Range is around 1500 m. Assuming that the altitude of
the inversion does not change substantially as the air flows over the Brooks Range,
the Froude number can be calculated:
Frupstream =
u2upstream
g h − hm
Frcrest =
2
5 14
9 81 × 1700
= 1 6 × 10−3
=
u2upstream
g h − hm
46 32
9 81 × 200
=11
=
This calculation indicates that the formation of the strong inversion created a shallow
stable layer, which enabled the development of a hydraulic jump as the air flowed
over the Brooks Range. This was experienced as a turbulent, strong, but short-lived
windstorm in Bettles.
Review questions
13.1 Determine the wind speed required for our assumption of negligible Coriolis terms
in the Navier–Stokes equations for flow over the following mountain ranges:
(a) Rocky Mountains, Colorado, width = 200 km
(b) Southern Alps, New Zealand, width = 100 km
(c) Snowy Range, Wyoming, width = 20 km.
231
REVIEW QUESTIONS
13.2 Starting with the Navier–Stokes equations, as simplified by the assumptions
at the start of this chapter (Equations (13.1)), use the definition of the basic
state and perturbation variables in Equations (13.2) to derive the perturbation
governing equations (13.4). (Hint: use the fact that the basic state is assumed
to satisfy the hydrostatic assumption and that −′ / ≈ ′ /.)
13.3 Verify that the dispersion relationship for internal gravity waves given in
Section 13.1.2 is correct.
13.4 Derive equations for p′ and ′ for an internal gravity wave that are not functions
of any other perturbation variables, such that the phase relationship between
u′ w′ p′ and ′ can be determined.
13.5 Sketch a figure similar to Figure 13.1 for k > 0 and m > 0, showing lines of
⇀
constant phase, direction of phase propagation (k), direction of energy propa⇀
gation (c g , and locations of maximum, minimum, and zero u′ w′ p′ , and ′ ,
and wind vectors.
13.6 Verify that the dispersion relationship given by Equation (13.19), for internal
gravity waves generated by flow over an infinite series of ridges, is correct.
13.7 Verify that the assumed solution given by Equation (13.20b) for an evanescent
internal gravity wave is a solution of Equation (13.18).
13.8 Consider the case of a stably stratified flow (N = 0 02 s−1 ) over an infinite
series of ridges with h0 = 50 m and a horizontal wavelength of 1 km.
(a) Calculate ucritical for this situation.
(b) Plot h as a function of x for −x ≤ x ≤ x .
(c) Plot w′ as a function of x for −x ≤ x ≤ x at z = 0 and a height equal to
one vertical wavelength for u = 1 and 9 m s−1 .
13.9 It is winter in Wyoming, and the flow approaching the Snowy Range
(x = 20 km) has the following properties:
P (mb)
Z (m)
(K)
850
700
500
1500
3000
5500
270
287
289
The mean wind speed in the layer between 850 and 700 mb is 5 m s−1 and the
mean wind speed in the layer between 700 mb and 500 mb is 30 m s−1 .
232
MOUNTAIN WEATHER
(a) Calculate the Brunt–Väisälä frequency for each layer.
(b) Calculate the Scorer parameter for each layer, neglecting the curvature term.
(c) What types of internal gravity waves would be expected in each layer for
this flow over the Snowy Range?
(d) Sketch the flow over and downwind of the Snowy Range for this case.
14
Polar weather
While the dynamics that govern the weather of the polar regions are the same as
those that govern the weather of the middle latitudes and tropics, there are some
unique aspects of polar weather. One such feature is the juxtaposition of strong lowlevel static stability with rugged orography. The large ice sheets of Greenland and
Antarctica reach elevations in excess of 3000 m and are marked by steep ice slopes
along their margins. The weather over these ice sheets is strongly constrained by this
confluence of orography and stability, resulting in the characteristic katabatic winds.
Over the polar oceanic regions intense low-pressure systems, smaller than typical
mid-latitude cyclones, occur. These polar lows can be as intense as tropical cyclones,
and in fact share some common characteristics with their tropical relatives.
14.1 Katabatic winds
A defining characteristic of the atmospheric circulation over the Antarctic and Greenland ice sheets is the persistent low-level wind. This katabatic wind is defined as a
wind that flows downslope under the influence of gravity. The flow is negatively
buoyant; that is, the air in the katabatic flow is colder and thus more dense than the
ambient air outside of the flow. Katabatic winds can occur over any sloped surface
when the air in contact with that surface is cooled (due to turbulent heat transfer
to the underlying surface and/or by direct radiative cooling). Short-lived katabatic
winds can occur in mountainous terrain anywhere on Earth on a nightly basis as
the air near the surface cools after sunset. Over the large polar ice sheets, persistent
inversion conditions (temperature increasing with altitude away from the ice sheet
surface, see Section 4.3.4) result in katabatic winds that can last for days, weeks, and
even months at a time.
Early explorers of the Antarctic continent were the first to report the existence
of strong katabatic winds along the edge of the ice sheet. An expedition led by
Sir Douglas Mawson from 1911 to 1914 reported wind speeds at Cape Denison
671 S 1427 E of 25 to 35 m s−1 49 to 68 kts for months at a time, with gusts in
excess of 45 m s−1 . More recent meteorological observations at Cape Denison, made
by automatic weather stations, have reported an annual mean wind speed of 209 m s−1
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
234
POLAR WEATHER
(41 kts) for the year 1995. Annual mean wind speeds from other years are less reliable
as the anemometer at this site is often destroyed by the strong katabatic flow long
before the harsh Antarctic winter ends and the weather station can be repaired.
Another unique characteristic of the katabatic wind regime is the nearly constant
wind direction. The directional constancy (DC), defined as the ratio of the vector
mean wind speed to the scalar mean wind speed, was an amazing 0.96 at Cape
Denison during 1995. A directional constancy of 1.0 would indicate that the wind
was always from the same direction. The Cape Denison resultant wind direction (that
is, the vector-averaged wind direction), measured counterclockwise from due north,
was 162 (SSE). This wind direction is oriented approximately 20 to the left of the
local downslope direction of the ice topography, often referred to as the fall line. We
will see later that over the Antarctic ice sheet we should expect katabatic winds to
be directed to the left of the ice sheet fall line, while over the Greenland ice sheet we
would expect the katabatic winds to be directed to the right of the ice sheet fall line.
Example Calculate the scalar mean wind speed, resultant wind speed and direction, and the directional constancy for March 1997 at Terra Nova Bay, Antarctica
7495 S 16369 E using the automatic weather station observations provided on
the CD-ROM.
The scalar wind speed is simply the average of the magnitude of the wind speed
observations for the month, but the resultant wind speed takes into account both
positive and negative values of the wind components u and v. Consider the simple
case of two wind observations: one is a wind of 10 m s−1 from the south and the other
is a wind of 10 m s−1 from the north. The scalar average wind speed would simply
be 10 m s−1 . For the resultant wind speed we would average the u and v components
of the wind speed and then calculate the total wind speed from the averages of the
two components. For the south wind we have u = 0 m s−1 and v = 10 m s−1 . For the
north wind we have u = 0 m s−1 and v = −10 m s−1 . The average u component for
these two wind observations would be 0 m s−1 and the average v component would
also be 0 m s−1 . The resultant wind speed in this case would be 0 m s−1 . The scalar
wind speed will always be greater than or equal to the resultant wind speed.
The scalar mean wind speed can be obtained from the individual wind speed
observations as follows:
⇀
V =
N
1
WSi
N i=1
= 152 m s−1
where N = 248 is the number of three-hourly weather observations for the month of
March 1997 at this location.
Each of the individual wind speed and direction observations can be converted
into zonal and meridional components:
u = −WS × sinWD
v = −WS × cosWD
KATABATIC WINDS
235
where WS is the observed wind speed and WD is the observed wind direction. Then,
the mean zonal and meridional components of the wind are
u=
N
1
u
N i=1 i
= 144 m s−1
v=
N
1
v
N i=1 i
= −33 m s−1
Using these results we can calculate the resultant wind speed as
resultant WS = u2 + v2
= 148 m s−1
The resultant wind direction can be calculated from the horizontal wind components
using the formula
180 −1 v
WD = 90 −
+ WD0
tan
u
where WD0 = 180 for u > 0 and WD0 = 0 for u < 0. This equation is not valid for
ū = 0: in this case WD = 180 when v > 0, WD = 360 when v < 0, and WD = 0
when v = 0. The resultant wind direction is then
180 −1 v̄
+ WD0
tan
resultant WD = 90 −
u
180 −1 −33
+ 180
tan
= 90 −
144
= 283
The fall line at Terra Nova Bay is directed toward the east, and so the resultant wind
direction of 283 is oriented 13 to the left of the fall line.
The directional constancy at Terra Nova Bay is
u2 + v2
DC =
N ⇀
1
V
N i=1 i
1442 + −332
=
152
= 097
236
POLAR WEATHER
Strong winds, large directional constancy, and a wind direction that is 10 to 50
to the left of the local fall line are all characteristics of Antarctic katabatic winds.
We will now explore the dynamics responsible for driving this persistent wind
regime.
Consider the near surface atmospheric state over the Antarctic ice sheet during
the long polar night. At a location in the interior of the Antarctic ice sheet, at
75 S 135 E for example, the ice surface slopes gently to the north. The Sun has
set in early May and will not rise again until August. Without sunlight the surface
cools by emitting long-wave radiation and the air immediately in contact with the
surface cools due to the turbulent transfer of heat to the ice surface. This results in
the development of a strong temperature inversion in the lowest 100 to 1000 m of the
atmosphere. We will approximate this situation by an atmosphere that consists of two
layers, each of constant density, with the lower layer having a greater density than
the upper layer. The lower layer is of constant depth, h, and the interface between the
lower and upper layers is parallel to the underlying ice topography, as illustrated in
Figure 14.1.
For an analysis of katabatic winds the horizontal wind components of the Navier–
Stokes equations (4.19) can be approximated as follows. Observations of katabatic
winds in the Antarctic indicate that the winds are nearly constant in time, so we
will assume that the flow is steady state (/t = 0). We will also neglect nonlinear advection terms in the equations of motion, an assumption which is evaluated
in review question 14.4 at the end of this chapter. We will orient our coordinate
system such that the positive x direction points in the downslope direction and
the positive y direction points to the left of the downslope direction. With these
B
C
∆x
h
ρtop, θtop
A
D
z
x
ρbot, θbot
Figure 14.1 Vertical cross-section through the two-layer atmosphere used to represent katabatic
winds. h is the depth of the inversion layer, h/x is the slope of the ice sheet surface, is the
density, and is the potential temperature. The subscripts bot and top refer to the bottom and
top layers respectively. The points labeled A, B, C, and D are discussed in the text
237
KATABATIC WINDS
assumptions and using Equations (10.4) to simplify the friction term, Equations (4.19)
simplify to
1 p
+ fv − CD Vu
x
1 p
− fu − CD Vv
0=−
y
0=−
(14.1)
where V is the scalar wind speed. Using the hydrostatic equation, the pressure at
point A (pA ) in Figure 14.1 is given by
p
=− g
z
pB
zB
⇒ p = −
pA
zA
pB − pA = −
bot g zB
p A = pB +
bot gz
− zA
bot g zB
− zA
Similarly, the pressure at point D (pD ) in Figure 14.1 is given by
pD = pC +
top g zC
− zD
The downslope pressure gradient force acting between points A and D can then be
estimated as
1 pD − pA
1 p
≈−
−
x
x
≈−
≈−
1 pC +
top gh − pB
+
bot gh
x
1
pC − pB
+
x
top −
bot
gh
x
where is the mean density in the two layers and x is the horizontal distance
between points A and D. The first term on the right hand side of this equation, the
pressure gradient force in the upper layer, can be written as
1 pC − pB
1 p
−
≈−
x
x top
This term is referred to as the ambient or background pressure gradient force. Because
h/ x is simply the terrain slope −z/x, the equation for the downslope pressure
gradient force is
1 p
1 p
top − bot z
−
+g
=−
x
x top
x
238
POLAR WEATHER
Using the fact that
top −
bot
≈−
top − bot
=−
where ¯ is the mean potential temperature in the two layers, this equation can be
rewritten in terms of the inversion strength
1 p
z
1 p
−
−g
=−
x
x top
x
The second term on the right hand side of this equation is referred to as the katabatic
force, which arises solely due to the presence of a potential temperature inversion
over sloping terrain. If the terrain slope is zero or the potential temperature profile is
uniform, this term is zero. For a stably stratified atmosphere, where top > bot , and
for terrain sloping down in the positive x direction, this term is positive and air will
be accelerated in the downslope direction.
Since our coordinate system is oriented with the positive x direction aligned with
the ice fall line there is no terrain slope in the y direction and the pressure gradient
force in the y direction in the lower layer is due only to the ambient pressure gradient
force so that
1 p
1 p
−
=−
y
y top
Now, Equations (14.1) can be rewritten as
z
1 p
−g
+ fv − CD Vu
0= −
x top
x
1 p
− fu − CD Vv
0= −
y top
(14.2a)
(14.2b)
If the ambient pressure gradient is neglected, the downslope wind is given by a
balance between the katabatic force, the Coriolis force, and the frictional force. We
will consider a situation where the katabatic force term is positive: an inversion over
terrain that slopes down in the positive x direction. For the simple case of no frictional
force, CD = 0, the Coriolis force and the katabatic force create a geostrophic balance.
In the Southern Hemisphere this results in a wind that is perpendicular to the fall line
of the ice sheet and directed such that higher elevations of the ice sheet are to the left
of the wind vector (Figure 14.2a). For the case of no Coriolis force, an unrealistic
situation for high latitudes but one which may pertain on a tropical glacier such as
the Quelccaya Ice Cap in the Andes Mountains of South America, the downslope
momentum equation would be given by a balance between the katabatic force and
the frictional force. In this case the wind would be directed down the fall line with
239
KATABATIC WINDS
(a) – No Fr; KF and CoF balance
2500 m
2500 m
2000 m
y
2000 m
CoF
y
KF
Wind
x
x
(b) – No CoF; KF and Fr balance
2500 m
y
2500 m
2000 m
y
Wind
x
2000 m
Fr
KF
x
(c) – KF, CoF, and Fr balance
2500 m
2500 m
2000 m
Wind
y
2000 m
CoF
y
x
x
KF
Fr
Figure 14.2 Katabatic wind vector (left column) and force vectors for flows when the katabatic
and Coriolis forces balance (a), the katabatic and friction forces balance (b), and the katabatic,
Coriolis, and friction forces balance (c). Force vectors are shown with filled arrowheads and KF is
the katabatic force, CoF is the Coriolis force, and Fr is the frictional force. Wind vectors are shown
with small arrowheads. Ice sheet elevation contours are labeled as 2000 m and 2500 m, and in all
panels the higher elevation is to the left
no cross-slope component (Figure 14.2b). The more realistic case of all three forces
acting results in a wind that is directed downslope and to the left of the fall line in
the Southern Hemisphere (Figure 14.2c).
We will now define as the deviation angle of the wind from the terrain fall line
(and the x direction), with positive values taken for winds that are directed to the left
of the fall line. Using this definition we can write
u = V cos
v = V sin
Neglecting the ambient pressure gradient force terms in Equations (14.2), V and
are given by
V=
g
f
= −
z
sin
x
g
CD
z
cos
x
05
(14.3a)
240
POLAR WEATHER
cos
05
z 2
g
f2
f
+4
+
CD
f
x
CD2
=
g
z
2
f
x
(14.3b)
Example Calculate the downslope and cross-slope components of the wind speed for
a katabatic flow over the Antarctic ice sheet at 75 S 135 E if the average potential
temperature near the surface is 230 K and 250 K in the middle troposphere, and the
terrain slope is −0002. Assume that the background pressure gradient is negligible
and that the drag coefficient is 167 × 10−5 m−1 .
From the information given, we find
=
top − bot
= 250 K − 230 K
= 20 K
=
top + bot
2
250 K + 230 K
=
2
= 240 K
f = 2 sin
= 2 × 729 × 10−5 s−1 × sin−75
= −14 × 10−4 s−1
Using z/x = −0002 and g = 981 m s−1 in Equation (14.3b) gives
cos
=
−14 × 10−4
+
167 × 10−5
= 070
2
2 05
−14 × 10−4
981
20
−0002
+4
167 × 10−5
14 × 10−4 240
981
20
2
−0002
−14 × 10−4 240
and hence = 45 , so the wind is directed at an angle of 45 to the left of the ice slope
fall line. We can now use this value in Equation (14.3a) to find the scalar wind speed:
05
981
20
V= −
×
× −0002 × 070
167 × 10−5 240
= 83 m s−1
241
KATABATIC WINDS
The downslope component of the wind is
u = 83 × sin45
= 59 m s−1
and the cross-slope component of the wind is
v = 83 × cos45
= 59 m s−1
Thomas Parish and David Bromwich used this approach to estimate the average
katabatic wind flow over the whole of the Antarctic continent (Figure 14.3). We see
that the katabatic winds diverge from the high interior of the continent and flow
downslope towards the coast as expected.
0°
15°
45°
15°
30°
30°
45°
60°
65°
60°
65°
75°
75°
90°
90°
70°
85°
105°
105°
120°
80°
135°
75°
150°
65°
165° 180°
75°
70°
165°
120°
135°
150°
Figure 14.3 Mean wintertime streamlines over the Antarctic continent calculated using the twolayer model of katabatic flow. Ice sheet elevation contours are shown as thin gray lines. (From
Parish and Bromwich, 1987. Reprinted with permission of Nature Publishing Group)
242
POLAR WEATHER
The persistent katabatic winds over the Antarctic continent, with flow directed
downslope away from the high elevations of the ice sheet, are an important component
of the meridional circulation of high southern latitudes. This divergence of katabatic
winds from the high interior of the Antarctic continent acts to remove mass from
over the continent, and results in sinking motion over the continent. As a result of the
sinking motion cloud and precipitation formation is inhibited and Antarctica receives
very little precipitation. Offshore, the katabatic winds converge with the winds of
the middle latitudes, resulting in rising motion, and extensive cloudiness over the
Southern Ocean. The final branch of this circulation is a poleward-directed flow in
the middle and upper troposphere of the high southern latitudes, forming the polar
cell discussed in Chapter 15.
14.2 Barrier winds
The combination of strong static stability and steep topography can result in other
types of low-level flows. For example, when a low-level wind in a stable atmosphere
is directed toward a topographic barrier, the flow may either pass over the barrier, as
discussed in Chapter 13, or be blocked by the barrier. Whether the flow passes over
the barrier or is blocked is determined by a Froude number given by
Fr =
V
gH
/ 05
(14.4)
where V is the wind speed directed toward the barrier, H is the height of the barrier,
is the potential temperature difference between the surface and the top of the
barrier, and is the potential temperature of the flow approaching the barrier.
Note that the Froude number defined here differs from that defined in Section 13.3
for the discussion of downslope windstorms. This unfortunate situation of referring
to these two different quantities both as the Froude number has a long history that is
discussed in detail by Baines (1995). For the case of barrier winds the Froude number
is proportional to the ratio of the kinetic energy of the flow to the potential energy
required for the flow to pass over the barrier. When Fr > 1 the flow has sufficient
kinetic energy to pass over the barrier. When Fr < 1 the flow lacks sufficient kinetic
energy to allow the airstream to pass over the barrier and the flow is blocked. This
situation is favored when the wind speed is weak or when the atmosphere is very
stable, so that the potential temperature difference between the top and bottom of the
barrier is large.
Example The Ross Ice Shelf is a large, flat, floating ice mass centered roughly on
the International Date Line at the edge of the Antarctic continent. This ice shelf is
surrounded by elevated terrain to the east, south, and west, with the steepest terrain
being the Transantarctic Mountains that form the western boundary of the ice shelf.
Calculate the Froude number for an observed easterly wind of 10 m s−1 flowing across
the Ross Ice Shelf. The Transantarctic Mountains have an elevation around 2000 m
243
BARRIER WINDS
and climatological conditions on the ice shelf during the austral winter give a near
surface potential temperature of 260 K and a potential temperature difference between
the ice shelf and the top of the Transantarctic Mountains of 10 K.
Fr =
10
981 × 2000 × 10/26005
= 036
Since Fr < 1, we would expect this easterly wind across the Ross Ice Shelf to be
blocked by the Transantarctic Mountains. What is the atmospheric response to a
blocked flow?
Consider a two layer model of the atmosphere as shown in Figure 14.4. The top
panel of this figure illustrates the initial conditions, with a stably stratified atmosphere ( bot > top and bot < top ) and a lower layer of undisturbed depth h which is
experiencing an easterly flow. As this stable air impinges on the barrier it is unable
to flow over the barrier (since Fr < 1) and accumulates at the base of the barrier.
In response to this situation the lower layer bulges upward adjacent to the barrier,
resulting in a modification of the surface pressure at point A in the bottom panel of
Figure 14.4. Using the hydrostatic equation, the pressure at point A in the figure is
given by
p A = pB +
bot
h + h g
(a)
ρtop,
θtop
H
z
ρbot,
h
θbot
x
(b)
B
H
z
ρtop, θtop
C
∆h
h + ∆h
h
ρbot, θbot
x
A
D
Figure 14.4 Vertical cross-sections of a two-layer atmosphere with the flow in the lower layer
impinging upon a barrier (a) and the response of the two-layer fluid to this flow (b). h is the
undisturbed depth of the lower layer, h is the increased depth of the lower layer adjacent to the
barrier, H is the height of the barrier, is the density, and is the potential temperature. The
subscripts bot and top refer to the bottom and top layers respectively. The points labeled A, B, C,
and D are discussed in the text
244
POLAR WEATHER
and the pressure at point D in the figure is given by
pD = pC +
bot hg +
top
hg
The difference in pressure between points A and D is thus
p A − pD = p B +
bot
h + h g − pC −
= pB − pC +
= pB − pC +
bot hg −
bot
−
bot hg +
bot hg −
bot
top
hg −
hg
top
hg
hg
top
If there is no ambient horizontal pressure gradient (that is, pB = pC ), then the pressure
difference between points A and D reduces to
pA − pD =
Since bot >
point D.
top
and
bot
−
top
hg
(14.5)
h > 0, the pressure at point A is greater than the pressure at
Example Calculate the pressure difference between a point adjacent to the
Transantarctic Mountains and a point located near the center of the Ross Ice Shelf for
the previous example. Assume that the undisturbed depth of the lower layer flow is
500 m and the depth of this layer adjacent to the Transantarctic Mountains is 1000 m.
Consistent with the stable stratification, we can take the density of the bottom layer
to be 13 kg m−3 and of the upper layer to be 12 kg m−3 .
Using Equation (14.5),
pA − pD = 13 − 12 × 500 × 981
= 491 Pa
= 49 mb
The easterly wind in the lower layer will respond to this new pressure distribution,
and come into geostrophic balance. The resulting geostrophic wind will be parallel to
the barrier and is referred to as a barrier wind. In order to calculate the geostrophic
wind that results from the blocked flow we need to estimate the distance over which
the pressure difference occurs. The Rossby radius of deformation (rR ) provides an
estimate of this distance and is given by
1
rR =
gh
f
bot
05
(14.6)
Physically, the Rossby radius of deformation is the horizontal distance over which the
atmosphere will adjust to geostrophic balance for a given disturbance, and depends
on the intensity of the disturbance and its time scale, here 1/f .
BARRIER WINDS
245
Using the pressure difference given by Equation (14.5) and the horizontal scale of
the disturbance suggested by Equation (14.6) we can estimate the horizontal pressure
gradient:
p
p
≈
x
rR
for flow against a north–south-oriented barrier. The geostrophic wind is then given by
1 p
f x
1 p
vg ≈
f rR
vg =
(14.7)
Example Calculate the geostrophic barrier wind that results from the blocked flow
in the previous example. The latitude at the center of the Ross Ice Shelf is 80 S.
At 80 S f is
f = 2 × 729 × 10−5 · sin−80
= −14 × 10−4 s−1
The Rossby radius of deformation can be calculated using Equation (14.6) using
= 10 K, bot = 260 K, and h = 500 m (the undisturbed depth of the lower layer)
from the previous example, and f = 14 × 10−4 s−1 :
10 05
1
981 × 500 ×
rR =
260
−14 × 10−4
= 98 000 m
= 98 km
The resulting geostrophic wind can now be calculated with Equation (14.7) using
−3
and p = 491 Pa from the previous example:
bot = 13 kg m
vg =
1
−491
−4
13 × −14 × 10 98 000
= 28 m s−1
where the sign of the pressure gradient is negative since the pressure decreases toward
the east, and the resulting geostrophic wind is from the south (positive vg ).
While this example is a rather extreme case, the blocking of a stable low-level
flow can result in strong barrier parallel flows that are known as barrier winds.
246
POLAR WEATHER
Barrier winds often develop on the Ross Ice Shelf when a cyclone passes north of
the ice shelf. The clockwise flow around the cyclone results in easterly winds across
the Ross Ice Shelf. If this easterly flow is sufficiently weak and/or the stability is
sufficiently large, Fr will be less than unity and the low-level flow will be blocked.
An example of a barrier wind on the Ross Ice Shelf that developed in this manner on
11 April 2005 is shown in Figure 14.5. Similar situations give rise to barrier winds
along the east side of the Antarctic Peninsula and along the steep margins of the
Greenland ice sheet. These strong winds can create difficult conditions for travel and
may influence the distribution of sea ice if they occur over the ocean.
180
170 E
170 W
160 W
160 E
76
4
98
968
972
97
6
988
78
98
0
992
996
80
98
4
988
82
992
84
996
86
Figure 14.5 Surface weather map over Ross Ice Shelf, Antartica, for 12 UTC 11 Mar 2005. Sea level
pressure contours are shown for locations with elevations less than 500 m and wind observations
from automatic weather stations are plotted using the standard convention. Map provided by
M. Seefeldt, CIRES/ATOC, University of Colorado
POLAR LOWS
247
14.3 Polar lows
The category of mesoscale cyclones spans a broad range of weather systems, and one
of the most intense types is the polar lows. These systems can be extremely intense,
with winds comparable to those in a tropical cyclone and heavy snowfall. Like tropical
cyclones, polar lows are relatively axisymmetric and warm cored, at least in the
lower troposphere. Early interest in polar lows developed in Scandinavia, where small
violent storms were known to affect both ships and coastal communities over very
limited areas. Such storms were well known but impossible to predict. With the advent
of the satellite era, these small, intense, damaging storms could finally be identified,
tracked, and, to an extent, predicted. They appear to be a phenomenon of the eastern
Arctic, around Greenland and the north Atlantic, being exceedingly rare on the Pacific
side. Even more recently, such cyclones have been identified in the Southern Ocean
around Antarctica, although they tend to be weaker due to the strong statically stable
conditions.
An accepted definition of a polar low is that it is a maritime mesoscale cyclone
with near surface winds in excess of 15 m s−1 that forms poleward of the polar front.
Many methods have been used to classify the broad variety of systems that satisfy
this definition, which includes systems that form in highly baroclinic regions along
the polar front and near ice edges, as well as systems that form convectively in highlatitude, nearly barotropic environments. It is most accurate, therefore, to consider
polar lows to be a spectrum of mesoscale systems, ranging from purely baroclinic
to purely convectively driven, with a range of hybrid systems in between. Further,
a particular polar low may owe its development to baroclinic instabilities or thermal
instabilities at different stages of its life cycle. Additional details on polar lows can
be found in Rasmussen and Turner (2003).
Example In February 1990, a meso-beta scale polar low passed close to the meteorological station located in Tromso, Norway (69 40′ N, 18 56′ E), which registered
winds of 158 m s−1 at a time when atmospheric pressure was 1001 hPa. Estimate the
central pressure of the cyclone.
Since the cyclone is assumed to be mesoscale and relatively axisymmetric, the
gradient wind model is appropriate. We can assume an order of magnitude for the
diameter of the cyclone to be 100 km. Then the model (Equation (6.1)) predicts
V2
+ fV
R
1582
−5
= −12
+ 2 × 7292 × 10 × sin 6967 × 158
50 × 103
pd
=−
n
= −12 499 × 10−3 + 216 × 10−3
= −858 × 10−3
248
POLAR WEATHER
pd = −858 × 10−3 × 50 × 103 + 100 100
= 997 hPa
14.3.1 Baroclinic instability
It has been widely accepted for some time that mid-latitude synoptic scale systems
form through the process of baroclinic instability as described in Chapter 9. Detailed
observations have made it possible to identify the various stages in the life cycle of a
baroclinic development as expected by the theoretical model (Figure 1.9). Although
in this model the lower level cyclone and upper level trough develop simultaneously
as a continuous process, in fact initiation will happen at one level in the presence of a
favorable environment in the other level. So, for example, a well-defined upper level
short-wave trough may move over a pre-existing lower level, rather shallow, frontal
zone. Alternatively, a fairly zonal upper level flow may encounter a particularly
strong region of lower level baroclinicity, which supplies the energy to perturb the
upper level flow. In some cases, polar lows develop in regions of reverse shear,
where the horizontal wind decreases with height.
Consider the shallow baroclinic type. In this case, a shallow frontal zone may
persist between a region of snow-covered land or sea ice and the relatively warmer
open ocean. As an upper level trough passes over this frontal zone toward the warmer
surface, the strong static stability of the polar atmosphere is lowered, perhaps even
becoming neutral. In such conditions, cyclogenesis may take place. The lower the
static stability, the smaller the horizontal scale of the system. In addition, the planetary
vorticity is large at high latitudes, and this can contribute to rapid cyclogenesis.
In deep baroclinic zones, the development is more typical of mid-latitude cyclogenesis. In this process, a small initial perturbation in an otherwise uniform zonal
flow will engender a wave-like structure in the zonal oriented isotherms. As we saw
in Chapter 9, this wave is displaced a quarter wavelength to the west of the wave in
pressure. Horizontal temperature advection due to the pressure perturbation and associated geostrophic wind then amplifies the temperature perturbation. This allows the
development of a thermally direct circulation, in which cold air is sinking and warm
air is rising, which converts available potential energy in the horizontal temperature
gradient to kinetic energy of cyclogenesis.
14.3.2 Convection
It has been recognized since the earliest studies of polar lows that convection often
plays a significant role, although the exact nature of this role has been disputed and
remains an area of active research. Nevertheless, a fairly consistent picture has now
emerged. The polar atmosphere, like the tropical one, is nearly neutral to deep moist
convection. Hence, CAPE must be generated in order for a development to take
place. This can be achieved by strong surface fluxes of latent and sensible heat that
REVIEW QUESTIONS
249
destabilize lower layers. Alternatively, or in addition to surface forcing, CAPE can be
generated through the advection of cold air aloft. Hence, polar lows do not develop
from existing reservoirs of CAPE, but rather co-develop with the CAPE reservoir as
the air flows over a warm sea surface. The maximum observed CAPE value in a polar
low has been observed to be around 1100 J kg−1 , and values around 400–600 J kg−1
are more typical. Such moderate values compared to mid-latitude thunderstorms are
nevertheless a significant source of energy in most polar low developments. If CAPE
is not consumed as rapidly as it is generated, the reservoir may build up and continue
to feed the cyclogenesis over time.
As with a tropical cyclone, heating in the middle troposphere is the most effective
way to enhance cyclone development, since it leads directly to a decrease in surface
pressure. The formation of a warm core may arise from the surface turbulent heat
transport noted above (the WISHE model, Section 12.3), a mechanism which requires
no CAPE in the atmospheric profile. Alternatively, it has been suggested that polar
low intensification may occur more explosively through the heat release associated
with mid-tropospheric cumulus convection, which does require a reservoir of CAPE in
the atmospheric profile. This conceptual model is called CISK (Conditional Instability
of the Second Kind), and was once thought to be of primary importance in the
development of tropical cyclones. It is likely that both instability processes contribute
to polar low development in different cases and even at different stages of a single low.
It has been found that disturbances must be of a finite amplitude in order to
intensify a polar low through the thermal instability mechanisms described above.
Hence, it appears that the development of polar lows is a two-stage process. Initial
development, then, occurs through a process of baroclinic instability, or arises from
another disturbance such as a topographically generated cyclone. Then, thermal instability can play a role in the intensification of the disturbance into an intense polar
low. This two-stage process is supported by satellite observations, particularly in the
Arctic, and has led some authors to characterize thermal instabilities as a cyclone
‘afterburner’. However, because upper level troughs are generally associated with
cold air advection, which enables the generation of CAPE, the two processes may be
thought of as cooperating to bring about the polar low.
Review questions
14.1 Using Equations (14.2) derive Equations (14.3). (Hint: to derive the two expressions for V use v times Equation (14.2a) minus u times Equation (14.2b) to
derive one of the equations for V and u times Equation (14.2a) plus v times
Equation (14.2b) to derive the other equation for V . The equation for cos
can be derived using the two expressions for V and by noting that sin2 +
cos2 = 1.)
14.2 An idealized representation of the Antarctic ice sheet topography can be given by
1000 km − x 05
z = 4000 m × 1 −
1000 km
250
POLAR WEATHER
for 0 km < x ≤ 1000 km.
(a) Write an equation for the terrain slope between x = 0 and 1000 km.
(b) Plot the wind speed and deviation angle as a function of x for inversion
strengths of 5, 10, and 20 K. Assume that CD = 167 × 10−5 m−1 f = 14 ×
10−4 s−1 , and ¯ = 250 K.
14.3 Based on your answers to question 14.2(b) plot the ratio of the katabatic force
to (a) the Coriolis force and (b) the frictional force as a function of x.
14.4 Based on your answers to question 14.2(b) plot the ratio of the downslope
advection term (= u u/x) to the katabatic force term as a function of x. When
deriving the governing equations for katabatic flow, is the assumption that the
advection term is negligible reasonable?
14.5 Using the idealized two-layer model of katabatic winds, and assuming that there
is no background pressure gradient force, indicate the expected wind direction,
relative to an ice sheet topography, in the Northern Hemisphere for
(a) a balance between the katabatic and Coriolis forces;
(b) a balance between the katabatic and frictional forces;
(c) a three-way balance between the katabatic, Coriolis, and frictional forces.
14.6 Minna Bluff is an east/west-oriented ridge that extends from the Transantarctic
Mountains onto the Ross Ice Shelf at a latitude of 78.5 S, with a height of 900 m.
This ridge is often in the path of the climatological southerly winds that blow
across the western side of the Ross Ice Shelf. Seefeldt et al. (2003) analyzed
a case of southerly flow impinging on Minna Bluff on 22 April 1994. For this
case, there were south winds at 5 m s−1 , the potential temperature difference
between the surface of the Ross Ice Shelf and the top of Minna Bluff was 10 K,
and the surface temperature over the Ross Ice Shelf was 248 K.
(a) Calculate the Froude number for this flow impinging on Minna Bluff.
(b) Assume that the depth of the undisturbed flow upstream from Minna Bluff
is 500 m, and that the flow deepens to a depth of 750 m adjacent to Minna
Bluff. What is the horizontal pressure difference between the undisturbed
flow and the flow at the base of Minna Bluff?
(c) What is the horizontal distance over which the pressure change in part (b)
occurs?
(d) Estimate the geostrophic wind speed and direction upstream from Minna
Bluff for this case.
15
Epilogue: the general
circulation
15.1 Fueled by the Sun
In this book we have been studying many aspects of the motion of the atmosphere,
but we have not considered the ultimate source of energy for that motion. In fact,
the atmosphere and ocean circulation that we observe on the Earth is a system that is
fueled by the Sun – more specifically, by the imbalance of energy input to the Earth
system at the equator and the poles (Figure 15.1).
Energy from the Sun is received as short-wave radiation, light of wavelengths
between 0.2 and 4 m. This is equivalent to a blackbody radiating at 5776 K, but with
rather less ultraviolet than would be expected. About 40% of the incoming energy is
in the visible part of the spectrum 04–067 m and about 10% is in the ultraviolet
< 04 m. The energy flux emitted by the Sun is not perfectly constant and varies
at a rate and over a range that is important for studies of climate, though not for
meteorology and numerical weather prediction.
Even considering only time scales where the energy flux emitted by the Sun may
be considered constant, the energy received at the top of the atmosphere varies with
season and with latitude. The elliptical orbit of the Earth causes a variation of ±35%,
and the tilt of the earth’s axis, at an angle of 235 from the orbital plane results in
seasonal variations which far exceed the variation arising from the elliptical orbit.
When account is taken of these variations, the average flux in 1 year varies with
latitude as shown in Figure 15.1.
All of the energy impinging on the Earth is not absorbed: a percentage is reflected
or scattered. This percentage has an annual average value of about 30%, which
consists of 6% back-scattered by the air, 20% reflected by clouds, and 4% reflected by
the surface (land and oceans) itself. This fraction of reflected and scattered incoming
solar radiation is known as the albedo .The albedo can vary enormously with space
and time, because it depends on such factors as surface type and cloud distribution.
Hence, the total short-wave energy absorbed by the system is not as symmetrical
about the equator as the total incoming short-wave energy (Figure 15.1).
In the long term, the Earth system should neither gain nor lose heat. Hence all of
the absorbed solar radiation is re-emitted back to space in the long-wave portion of
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
252
EPILOGUE: THE GENERAL CIRCULATION
500
shortwave in
Wm–2
400
shortwave absorbed
300
surplus
longwave outgoing
200
deficit
deficit
100
–90° –60°
–30°
EQ
30°
60° 90°
Latitude
Figure 15.1 Average radiation balance for the Earth, showing incoming short-wave radiation
(solid line), absorbed short-wave radiation (dashed line), and outgoing long-wave radiation (dot
dash line). The horizontal axis shows the sine of the latitude, and is hence proportional to surface
area. Stippled zones indicate latitudes of mismmatch between annual mean energy in and out.
For long-term balance areas of surplus must exactly balance areas of deficit. NCEP Reanalysis data
provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado, from its Web site at
http://WWW.cdc.noaa.gov/
the spectrum (greater than around 4 m). Of the total outgoing long-wave radiation,
37% is emitted by clouds, 54% is emitted by gases in the atmosphere such as water
vapor and carbon dioxide (and much of this is long-wave energy that was absorbed in
the atmosphere from the surface below), and 9% is emitted directly from the surface
of the Earth, including the oceans. The important role of the atmosphere in the energy
balance of the Earth system is apparent.
As for the absorbed solar energy, the emitted long-wave energy varies dramatically
in space and time, creating a surplus of energy at some locations and times and a deficit
at others. While this varies according to season and surface and with transient effects
of features such as clouds and snow cover, the most systematic pattern of surplus
and deficit is the variation with latitude. It is this energy imbalance which drives
atmospheric and oceanic movement to transport energy; in Sections 15.2 and 15.3
we will look at some ways in which this imbalance can be corrected.
15.1.1 The greenhouse effect
Because of the absorption and re-emission of long-wave energy by the atmosphere,
the average temperature at the surface of the Earth is around 33 K warmer than
would be expected if the Earth had no atmosphere. This is because, while about
74% of short-wave energy entering the system passes through the atmosphere to the
surface of the Earth, around 62% of long-wave energy emitted from the surface is
reabsorbed in the atmosphere before it leaves the system. This energy is re-emitted of
RADIATIVE–CONVECTIVE EQUILIBRIUM
253
course, since balance must be maintained, but in so doing some energy is re-emitted
downward to the surface of the Earth, and hence increases the temperature near the
surface. This so-called ‘greenhouse effect’ creates the benign conditions for life on
Earth that otherwise would require an orbit closer to the Sun.
The enhanced greenhouse effect is the process whereby long-wave radiation absorbers
such as carbon dioxide and methane are added to the atmosphere in the course of
human activities. In such a situation, the energy balance of the Earth system is modified,
and with this comes changes in the circulation of the atmosphere and the ocean.
15.2 Radiative–convective equilibrium
The atmosphere is, in large part, heated from below due to the emitted long-wave
radiation from the surface. Because of this source of heat from below, the resulting
atmospheric profile must tend to be statically unstable and hence convective motions
are generated. This aspect of the energy transformations in the Earth system led to the
idea in the seventeenth century of a radiative–convective equilibrium, whereby air
heated at the surface in the tropics rises convectively and flows to the poles, where it
sinks and returns at surface level to the tropics. This results in a net transport of heat
from equator to pole. The surface flow was identified as the trade winds, so called due
to their consistency, which was good for rapid, reliable transport of goods by sailing
ship. This conceptual model was first postulated in 1686 by English astronomer
Edmund Halley. Halley suggested that the trade winds north of the equator blow
from east to west due to a build-up of flowing air that follows the heat of the Sun as
it travels from east to west due to the rotation of the Earth.
In 1735, professional lawyer and amateur meteorologist George Hadley made an
important refinement to Halley’s theory. Hadley correctly surmised that at the Earth’s
surface, the planet and the atmosphere rotate together. Because every latitude circle on
the Earth completes one revolution in one day, this means that the air and the surface
at higher latitudes rotate more slowly than air and the surface at the equator. So, wind
moving toward the equator would come from a region of lower eastward velocity
and hence result in a westward turning of the wind. Hadley’s conclusions were
published 100 years before Coriolis systematically described motions in a rotating
coordinate system (Section 4.4.2). While neither Halley nor Hadley had all of the
conceptual tools necessary to understand the dynamics of the trade winds, the role
of the atmosphere in carrying excess heat to the poles was an important first step in
understanding the global circulation.
Such a process, then, causes a simple direct circulation carrying heat from equator
to pole (Figure 15.2), correcting the energy imbalance caused by the differential
in short-wave absorption and long-wave emission. A direct circulation is one in
which hot air rises and cool air sinks. Hadley’s single-cell theory goes some way to
explaining the behavior of the trade winds and tropical convection. In his conceptual
model, the upward motion of the Hadley cell is driven by latent heat release as
water vapor is converted into precipitation. The rising motion, then, is embedded
within thunderstorms, which in fact occupy only around 0.5% of the tropical surface
254
EPILOGUE: THE GENERAL CIRCULATION
sinking and cooling
60 °N
Hadley
cell
40 °N
20 °N
trade winds
trade winds
ITCZ
convection
EQ
Figure 15.2 Hadley’s postulated single-cell circulation which accounts for the trade winds and
convection in the tropics. The zone of convection, called the Intertropical Convergence Zone (ITCZ),
follows the latitude of maximum solar heating throughout the year. The cell illustrated is not to
scale and only the Northern Hemisphere complete circulation is shown
area. However, models representing the single-cell theory predict an equator-to-pole
temperature gradient of around 120 K, instead of the observed value closer to 45 K.
A further problem with the Halley/Hadley conceptual model is that it is not
dynamically realistic on a rotating Earth. Conservation of angular momentum of the
Earth–atmosphere system requires that winds from the same direction cannot exist
at all locations on the Earth. Easterly winds in one location must be balanced by
westerly winds somewhere else. This conservation requirement is discussed in further
detail in Section 15.4.
15.3 The zonal mean circulation
The baroclinically driven mid-latitude cyclones explored in Part I cannot be accounted
for in the Hadley single-cell model. Yet these systems derive their energy from the
potential energy available in the mean meridional temperature gradient, and hence
act to reduce this gradient. This implies that the observed mean temperature field is
due to a balance between the competing effects of a differential radiation distribution
and a broader range of instabilities.
255
THE ZONAL MEAN CIRCULATION
An early attempt to address this problem was made by American meteorologist
and oceanographer William Ferrel, who is best known for his work on tides. In
1856, he proposed a mid-latitude cell characterized by an indirect circulation, in
which air at lower latitudes sinks and flows poleward and eastward, while cooler
air at higher latitudes rises and flows equatorward and westward. This model does
not match up precisely with the observations, and he supplied no mathematical
justification. Nevertheless, like Halley before him, it was an important attempt to
explain aspects of the global circulation (in this case, the mid-latitude westerlies)
that laid the groundwork for future research. On this important foundation, new
insights continued to be generated. For example, in 1921, German meteorologist
and oceanographer Albert Defant suggested that traveling mid-latitude cyclones and
anticyclones could be viewed as turbulent elements in a roughly horizontal process
of heat exchange between air masses. This idea turned out to be pivotal in analyses
of the angular momentum and energy budgets of the Earth. Then, in the 1930s, Carl
Gustav Rossby relied on the work of Hadley, Ferrel, and Defant, among others, to
create the more complete conceptual model shown in Figure 15.3.
The strength of the Hadley circulation in this model varies with longitude, being
strongly affected by topography and land/sea contrast. The rising motion at the ITCZ
60 °N
polar front
Ferrel
cell
prevailing westerlies
40 °N
horse latitudes
Hadley
cell
20 °N
trade easterlies
EQ
ITCZ
Figure 15.3 A revised picture of the zonal mean circulation, based on Rossby’s work of the 1930s
and 1940s
256
EPILOGUE: THE GENERAL CIRCULATION
is a region also called the doldrums, because prevailing horizontal winds are light
and variable. After rising near the equator, the air in the Hadley cell moves poleward
in the upper region of the troposphere, sinking near 30 (N or S) and generating belts
of high pressure. The subtropical jet streams arise near the upper, poleward portions
of the circulation. The circulation is completed by the equatorward flow of the trade
winds. The downward branch of the Hadley cell is associated with the world’s great
deserts (e.g. Mojave Desert, the Gibson Desert, the Sahara Desert, the Gobi Desert,
etc.).This region of light and variable winds at the downward branch is known as the
horse latitudes, although there are various explanations as to the origin of this term.
In Rossby’s original work, he suggested a second direct circulation at the poles,
to create what was called the ‘three-cell model’. However, the polar cell, if it can
be defined, is rather weak (Figure 15.4a–c). The Ferrel cell should be understood
as a simple Eulerian mean that does not represent the actual motion of air parcels.
Nevertheless, without Rossby waves and the associated baroclinic developments, the
Ferrel cell would not be observed in the zonal mean.
Example Use the principle of thermal wind balance to determine the expected zonal
mean wind u at the Northern Hemisphere jet maximum in Figure 15.4(a). How does
this compare to the jet maximum in a single cell, or Hadley, regime?
Figure 15.4(a) Aspects of the observed zonal mean general circulation: the zonal mean zonal
wind (m s−1 ) with westerlies shown as positive and easterlies shown as negative. This is a time
mean of 20 vernal equinoxes (21 March). The jet maxima at around 200 hPa are apparent
257
THE ZONAL MEAN CIRCULATION
(b) (i)
3 ms–1
height (hPa)
100
300
500
1000
–90°
–45°
ITCZ
0°
latitude
45°
3 ms–1
90°
(b) (ii)
height (hPa)
100
300
500
1000
–90°
–45°
0°
latitude
ITCZ
45°
90°
Figure 15.4(b) Aspects of the observed zonal mean general circulation: the zonal mean
meridional circulation for (i) December–January–February and (ii) June–July–August averages.
NCEP Reanalysis data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado,
from its Web site at http://WWW.cdc.noaa.gov/
258
EPILOGUE: THE GENERAL CIRCULATION
(c)
–80
–70
–60
–50
–40
–30
100
300
height (hPa)
–20
–10
500
0
700
10
900
1000
–90°
20
0°
latitude
–45°
45°
90°
Figure 15.4(c) Aspects of the observed zonal mean general circulation: the zonal mean temperature ( C). This is a time mean of 20 vernal equinoxes (21 March)
The global circulation can be represented approximately by the requirement for
thermal wind balance:
f0
u
g T
=−
z
T0 y
where the overbar indicates a zonal mean quantity and T0 is a global average surface
temperature. The mean radius, a, of the Earth is 637 × 106 m, and hence the equator–
pole distance is approximately
y =
2 a
= 100 × 107 m
4
In the case of the observed jet (see Figure 15.4a), the temperature gradient that leads
to the zonal wind maximum is confined to around 20 of latitude (Figure 15.4d
on the CD-ROM). In this region, the temperature decreases around 20 C. From the
figure, we assume a surface zonal wind of 2 m s−1 , and the jet maximum at around
10 km,
−20
T
= −90 × 10−6 K m−1
=
y
0222 × 107
THE ANGULAR MOMENTUM BUDGET
259
g T
u
=−
z
f0 T0 y
981
=−
× −90 × 10−6
10 × 10−4 × 288
= 31 × 10−3
u = 31 m s−1
ujet = 33 m s−1
which accords quite well with the observed jet. In the case of the Hadley regime,
T
−120
= −12 × 10−5 K m−1
=
y
100 × 107
and hence the predicted jet will be around 30% stronger than observed. In fact, if
the jet is similarly confined to a smaller latitudinal band, this jet may be somewhat
stronger.
Rossby’s work has been refined since then by scientists such as Victor Starr, Norman
Phillips, and Edward Lorenz. Their important insights led to the current view that the
Hadley circulation is driven primarily in fact through cooling by transient baroclinic
waves in the mid-latitude storm tracks. It is for this reason that the Hadley cells
translate with season. Warm tropical oceans do determine where upward motion is
favored, but the primary source of moisture is transported by trade winds from the
subtropics. Nevertheless, the fundamental view that the Hadley circulation is the most
efficient means of poleward heat transport in the tropical regions remains.
The thermal wind balance requirement imposes a strong constraint on the zonal
mean circulation, in that any pressure gradient that results from a departure from
balance will drive a mean meridional circulation which adjusts the mean zonal wind
and temperature fields so that thermal wind balance is restored. Thus, the mean
meridional circulation plays the same role in the zonal mean circulation that the
ageostrophic flow plays in synoptic scale quasi-geostrophic systems.
15.4 The angular momentum budget
Because there are no external sources of torque, the total angular momentum of the
Earth–atmosphere system must be conserved. Since the average rate of rotation of
the Earth is approximately constant, the atmosphere must also on average conserve
its angular momentum. The role of the ocean in this angular momentum budget is
becoming better understood at present.
The mid-latitude prevailing westerlies are an important part of the global circulation. These winds circulate around the Earth in the same sense as the Earth’s rotation.
Conversely, the tropical easterlies rotate less rapidly than the rotation of the Earth.
260
EPILOGUE: THE GENERAL CIRCULATION
Tm
Tf
Tw
6
torque
3
0
–3
–6
–90°
–60°
–30°
0°
latitude
30°
60°
90°
Figure 15.5 Annual mean zonally integrated surface pressure or mountain torque (Tm), surface
friction torque (Tf), and gravity wave drag torque (Tw) in hadleys 1018 kg m2 s−2 . With perfect
data, these torques would be exactly balanced by the annual mean angular momentum transport.
NCEP Reanalysis data provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, Colorado,
from its Web site at http://www.cdc.noaa.gov/
To maintain balance, angular momentum that is gained in the tropics due to the interaction between surface and atmosphere must be transported poleward and transferred
back to the Earth at higher latitudes.
The transfer of angular momentum between the Earth and atmosphere is achieved
through the action of three torques (which is the force multiplied by the distance from
the axis of rotation). These torques are caused by (i) surface friction, (ii) the pressure differential between the leeward and windward sides of meridionally oriented
mountain ranges, and (iii) vertical momentum transfer due to internal gravity waves
(Figure 15.5). Until the late 1980s, the role of gravity wave stresses was not known,
and the difference between torques and angular momentum transport was attributed
to measurement error. In the middle latitudes of the Northern Hemisphere the surface
pressure differential or ‘mountain torque’ provides nearly half of the total atmosphere surface momentum exchange. In the tropics and the Southern Hemisphere the
exchange is dominated by surface friction and the turbulent eddy stresses it engenders.
This angular momentum budget is balanced by the poleward transport of angular
momentum, which is achieved when poleward moving air carries more angular
momentum than equatorward moving air. In the equatorial regions a large fraction of
this transport is done by the zonal mean circulation: the Hadley cell. In the Hadley
cell, parcels containing the angular momentum transferred from the surface are lifted
to the upper troposphere and poleward, while parcels with less angular momentum
sink and move equatorward, resulting in a net poleward transport.
Example Use the principle of conservation of angular momentum to determine
the expected zonal mean wind u at the Northern Hemisphere jet maximum in
Figure 15.4(a).
THE ENERGY CYCLE
261
Angular momentum is given by r 2 , where = u/r is the total rate of rotation of
the atmosphere relative to an observer in an inertial frame, and r is the distance to
the axis of rotation. Hence,
ueq 2
u30 N
2
+
r 30 N =
+
r eq
r30 N
req
2
req
ueq
−
u30 N = r30 N
+
2
req r30
N
a2
−4
= a cos 30 7292 × 10−5 +
a a2 cos2 30
−7292 × 10−5
As noted above, the radius of the Earth, a, is 637 × 106 m, and hence we calculate that u30 N = 129 m s−1 . This is much larger than the observed jet maximum,
and suggests that the zonal mean circulation alone cannot be responsible for the
angular momentum transport. This reflects the importance of what is termed the
eddy momentum flux: that is, the momentum transport by disturbances that may
be described as transient and small scale relative to the scale of the zonal mean
circulation.
Hence, weather systems, though transient and small scale compared to the global
circulation, are an important component of angular momentum transport. In middle
latitudes, they predominate. The mid-latitude eddy angular momentum transport is
a maximum at upper levels (around the jet stream), and is accomplished through
the pronounced eastward tilt with increasing latitude of the horizontal flow in
weather systems (Figure 15.6). This important asymmetry means that air flowing poleward will have a stronger westerly wind component than the equatorward
return flow.
15.5 The energy cycle
We have seen that the requirements for thermal wind balance and for angular momentum conservation give us just part of the story when we consider only the components
of the large-scale circulation. Now that we turn our attention to the energy cycle, we
will find once again the central role for synoptic weather systems in the functioning
of the Earth’s atmospheric dynamics.
Like angular momentum, energy is conserved and hence can be followed as it
transforms from one form to another. The total energy of a parcel in the atmosphere is
made up of the kinetic energy of its motion, arising predominantly from the horizontal
component, and the moist static energy h, which is given by
h = cp T +
+ Lr
(15.1)
262
EPILOGUE: THE GENERAL CIRCULATION
60 °N
40 °N
20 °N
Figure 15.6 The relationship between the tilt of mid-latitude waves at jet stream level and the
net poleward transport of positive angular momentum
where L is the latent heat of condensation, r is the mixing ratio, is the geopotential,
T is the temperature, and cp is the specific heat at constant pressure of dry air.
The first two terms on the right hand side of Equation (15.1) are generally grouped
together as the total potential energy, or dry static energy, of the parcel. Around
0.1% of the total potential energy, termed the available potential energy, is accessible
to be extracted and converted into kinetic energy. The available potential energy,
in an important insight by meteorologist Edward Lorenz, arises from the equatorto-pole temperature gradient, and is the source of energy for synoptic disturbances,
as first discussed in Section 9.3. Because the available potential energy resides in
the temperature gradient, diabatic heating or cooling will only increase the available
potential energy if it enhances that gradient. If the heating or cooling acts to decrease
the temperature gradient, then available potential energy is actually decreased.
In the long-term mean, the production of available potential energy must exactly
balance the dissipation through turbulent and thence frictional processes (via the
energy cascade). However, this balance is not maintained in regional or temporal
subsets, and can only be considered as a global, long-term budget. The derivation of
the equations that describe this budget are beyond the scope of this book, but they
may be summarized and illustrated in a ‘box diagram’, an approach pioneered by
Lorenz in 1955 and shown in Figure 15.7.
263
THE ENERGY CYCLE
baroclinic disturbances
transform mean APE
zonal mean
to eddy APE
available
potential
1.3
energy
11.1
33.3
1.12
net heating of tropics
relative to poles
large scale
overturning
(reverse energy
flow in tropics)
surface and
internal friction
0.18
0.18
zonal circulation
maintained primarily by
zonal mean conversion from eddy KE
kinetic
0.36
energy
4.5
eddy
available
potential
energy
0.7
diabatic heating by clouds
and surface fluxes
eddy APE transformed
to eddy KE by
vertical motions
2.0
eddy
kinetic
energy
surface and
internal friction
1.64
7.3
Figure 15.7 Lorenz energy cycle for the global atmosphere in the annual mean. Estimates of
energy density in 105 J m−2 are given in italics at each box and rates of energy transfer (that
is, generation, conversion, and dissipation) are given in W m−2 at each arrow. Quantities are
estimates only, and based on the work of Abraham Oort and José Peixoto using a 10 year mean of
meteorological station data
This diagram is based on station data, which limits the accuracy due to problems of
missing data, large gaps over the oceans, and an inability to fully account for vertical
motion. The direction of the flow of energy between reservoirs of available potential
and kinetic energy must be deduced rather than observed. Even in more recent
analyses using global gridded datasets, considerable differences between datasets
and methods arise. However, these analyses all reflect the fact that radiative heating
from the Sun and moist diabatic processes represent a net influx of energy into the
system, which ultimately is balanced by frictional dissipation. Some versions of this
analysis split the kinetic energy into stationary and transient components, since the
effects of forced orographic waves and stationary planetary waves also contribute
substantially to the poleward heat flux. In fact, it is apparent in recent studies that
there is compensation and mutual adjustments between the stationary and transient
components of energy transport, resulting in a total energy transport as shown in
Figure 15.7. While all attempts to characterize the global energy cycle are limited by
the many assumptions made in deriving them and, as always, by limited data, they
do lend considerable insight into the processes maintaining the general circulation.
In particular, the energy cycle highlights the primary role of weather disturbances.
Appendix A – symbols
a – average radius of the Earth (constant): 637 × 106 m
⇀
a – acceleration vector
A – area (also unit area, also wave amplitude, also coefficient or constant of integration
to be solved for per boundary conditions in Ekman layer theory)
b – constant in Teten’s formula: = 1727 K −1
B – coefficient (constant of integration) to be solved for per boundary conditions in,
for example, Ekman layer theory
c – phase speed
C – circulation
Cabsolute – absolute circulation, including contributions from planetary rotation and
from other sources of rotation relative to the surface of the planet
CAPE – Convective Available Potential Energy
CD – drag coefficient
Cearth – circulation resulting from the Earth’s rotation
CFtotal – total centrifugal ‘force’
cgx – zonal group velocity
cgy – meridional group velocity
cgz – vertical group velocity
cp – specific heat at constant pressure (constant): 1004 J kg−1 K−1 (for dry air);
1952 J kg−1 K −1 (for water vapor)
Cp – phase speed, or speed of propagation, of ocean waves
Crelative – circulation due to sources of rotary motion relative to the surface of a planet
cv – specific heat at constant volume (constant): 717 J kg−1 K−1 (for dry air);
1463 J kg−1 K −1 (for water vapor)
cx – zonal phase speed
ĉx – intrinsic zonal wave propagation speed
cy – meridional phase speed
cz – vertical phase speed
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
266
APPENDIX A
d – channel width
DC – wind directional constancy
d/d – derivative with respect to variable
D/D – Lagrangian, substantial, or material derivative with respect to variable
/ – local, Eulerian, or partial derivative with respect to variable
Dg /D – geostrophic Lagrangian derivative with respect to variable
Dh /D – horizontal Lagrangian derivative with respect to variable
e – vapor pressure
EL – Equilibrium Level
es – saturation vapor pressure
esi – saturation vapor pressure over ice
f – Coriolis parameter, defined as 2 sin
f0 – ‘mid-latitude’ reference Coriolis parameter, defined as 2 sin 45
⇀
F – force vector
FCoriolis – Coriolis force
Fr
– Froude number
⇀
F r – viscous force vector
g – effective gravity (gravitational acceleration plus centrifugal effect) (constant):
981 m s−2 at sea level at equator
⇀∗
g – gravitation vector
g0∗ – average gravitational acceleration at sea level (constant): 983 m s−2
−2
G – gravitation constant: 6673 × 10−11 N m2 kg
h – lower boundary height as a function of location xy; frontal height as a function
of distance x; constant in Ekman layer theory h = 2Km /f ; moist static energy
H – scale height H = Rd T /g ∗ ; high-pressure center; vertical length scale
hm – barrier height
⇀
i – unit x direction vector in a Cartesian coordinate system
⇀
i ′ – unit x direction in rotated Cartesian coordinate system
I⇀ – moment of inertia
j – unit y direction vector in a Cartesian coordinate system
⇀
j ′ – unit y direction in rotated Cartesian coordinate system
⇀
k – unit z direction vector in a Cartesian coordinate system; wavenumber vector
⇀
k′ – unit z direction in rotated Cartesian coordinate system
k – zonal wavenumber
Km – eddy viscosity
⇀
ks – stationary wavenumber
L – length scale; low-pressure center
l – meridional wavenumber; ocean wave length
l2 – Scorer parameter
LCL – Lifting Condensation Level
LFC – Level of Free Convection
m – mass; vertical wavenumber
M – mass of the Earth (constant): 59742 × 1024 kg; molecular mass
APPENDIX A
Mair – mass of a volume of air
Md – average molecular mass for dry air (constant): 28966 g mol−1
Mv – average molecular mass for water vapor (constant): 18016 g mol−1
n – normally oriented coordinate in a natural coordinate system
N – Brunt–Väisälä or buoyancy frequency
p – pressure
p0 – surface pressure; reference pressure
P – Ertel’s potential vorticity
pd – perturbation, or dynamic, pressure
⇀
P g – pressure gradient force vector
pn – pressure of the nth gas in a mixture
pz – pressure at height z
q – specific humidity
Q – heat energy
r – mixing ratio; radius of curvature
R – gas constant for a particular gas; radius of curvature
R∗ – universal gas constant: 8314 J mol−1 K−1
Rd – gas constant for dry air: R∗ /Md = 287 J kg−1 K−1
Re – Reynolds number
RH – Relative Humidity
Rn – gas constant for the nth gas in a mixture
Ro – Rossby number
rR – Rossby radius of deformation
rs – saturation mixing ratio
Rv – gas constant for water vapor: 461 J kg−1 K−1
R – radial distance from rotation axis to surface of the Earth at a given latitude
s – tangentially oriented coordinate in a natural coordinate system
⇀
S – vector describing a smooth curve
t – time
T – temperature; time scale; period of oscillation
T0 – reference temperature, most commonly a function of z
T00 – constant reference temperature
T1 – constant in Teten’s formula: 273.16 K
T2 – constant in Teten’s formula: 35.86 K
TA – temperature of parcel A
Td – dew point temperature
Tenv – temperature of the environment
Tparcel – temperature of a parcel
Tparcelz – temperature of a parcel at height z
Tv – virtual temperature
Tw – wet-bulb temperature
Tz – temperature at height z
u – zonal wind component
u0 – reference zonal velocity component
267
268
APPENDIX A
U – horizontal velocity scale
⇀
u – velocity vector
û – zonal velocity amplitude
û0 – equatorial zonal velocity amplitude
⇀
u′ – velocity vector in a rotated coordinate system
⇀
ua – ageostrophic wind vector
ua – zonal ageostrophic wind component
⇀
uearth – velocity vector attributable to motion due to rotation of the Earth
ui – velocity at a reference level i
⇀
ug – geostrophic velocity vector
⇀
uh – horizontal velocity vector
ux – x-directed component of the velocity vector
v – meridional wind component
V – wind speed in natural coordinates; volume
⇀
⇀
V – wind vector in a natural coordinate system = V
va – meridional ageostrophic wind component
Vg – geostrophic wind in a natural coordinate system
vi – velocity at a reference level i
w – vertical wind component
w0 – amplitude of vertical velocity perturbation
W – vertical velocity scale
WD – wind direction
WS – wind speed
⇀
x – i -directed Cartesian
coordinate value
⇀
x0 – reference i -directed Cartesian coordinate value
⇀
y – j-directed Cartesian coordinate value
⇀
y0 – reference j-directed Cartesian coordinate value
⇀
z – k-directed Cartesian coordinate value; height; vertical coordinate in a natural
coordinate system
Z – geopotential height
⇀
z0 – reference k-directed Cartesian coordinate value
– angle; specific volume; phase constant for simple harmonic motion
– meridional gradient of Coriolis parameter f/y; ocean wave phase speed constant
of proportionality
– perturbation or variation
– change or difference
– angle subtended through a curve s, the path taken in the tangential direction
in the natural coordinate system
– frontal slope in Margules formula; also ratio of molecular mass of dry air to
molecular mass of water vapor: 0.622
– vorticity
earth – vorticity due to the Earth’s rotation, f
g – geostrophic vorticity
APPENDIX A
269
g0 – geostrophic vorticity at the surface
– vorticity on a constant potential temperature surface, isentropic vorticity
⇀
– normal unit vector in a natural coordinate system
– potential temperature; angle of rotation of a pendulum
0 – amplitude of angular rotation of pendulum
parcelz – potential temperature of a parcel at height z
w – wet-bulb potential temperature
z – potential temperature of air at height z
– wavelength
– dynamic viscosity
– kinematic viscosity: 15 × 10−5 m2 s−1 (Earth atmosphere), 10 × 10−6 m2 s−1
(Earth ocean); frequency
ˆ – intrinsic frequency of oscillation
– pi (constant): 3.141 59
– density
0 – surface density; reference density, most commonly a function of z
00 – constant reference density
i – density at a reference level i
n – density of nth gas in a mixture
parcelz – density of a parcel at height z
v – density of water vapor contained in a volume of air
z – density at height z
– buoyancy force
– stress vector; tangentially directed axis in natural coordinate system
x – x-directed stress vector component
y – y-directed stress vector component
′ – perturbed , where is any variable or operator
– averaged , where is any variable or operator
L – variable or parameter in a defined lower layer in a two-layer model
U – variable or parameter in a defined upper layer in a two-layer model
– latitude; smooth differentiable scalar function; phase of a simple harmonic
oscillator
– geopotential
– deviation angle of the wind from the terrain fall line
– angular velocity
– angular velocity of the Earth’s rotation (constant): 7292 × 10−5 s−1
Appendix B – constants and
units
Useful constants
a – average radius of the Earth: 637 × 106 m
cp – specific heat at constant pressure: 1004 J kg−1 K−1 (for dry air); 1952 J kg−1 K−1
(for water vapor)
cv – specific heat at constant volume: 717 J kg−1 K −1 (for dry air); 1463 J kg−1 K−1
(for water vapor)
g – effective gravity (gravitational acceleration plus centrifugal effect): 981 m s−2 at
sea level at equator
g0∗ – average gravitational acceleration at sea level: 983 m s−2
G – gravitation constant: 6673 × 10−11 N m2 kg−2
M – mass of the Earth: 59742 × 1024 kg
Md – average molecular mass for dry air: 28966 g mol−1
Mv – average molecular mass for water vapor: 18016 g mol−1
R∗ – universal gas constant: 8314 J mol−1 K−1
Rd – gas constant for dry air: R∗ /Md = 287 J kg−1 K −1
Rv – gas constant for water vapor: R∗ /Mv = 461 J kg−1 K −1
– kinematic viscosity: 15 × 10−5 m2 s−1 (Earth atmosphere); 10 × 10−6 m2 s−1
(Earth ocean)
– pi: 3.141 59
– angular velocity of the Earth’s rotation: 7292 × 10−5 s−1
Units
Primary SI units (Système International d’Unités)
Length: meter (m)
Mass: kilogram (kg)
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
272
APPENDIX B
Temperature: kelvin (K)
Time: second (s)
Derived units
Force: newton (N): = kg m s−2
Pressure: pascal (Pa): = N m−2 = kg m−1 s−2
Energy: joule (J): = kg m2 s−2
Power: watt (W): = kg m2 s−3
Other commonly used units in atmospheric science
Pressure: millibar (mb): 1 mb = 100 Pa = 1 hPa
Pressure: hectopascal (hPa): 1 hPa = 100 Pa = 1 mb
Temperature interval: degree Celsius C 1 C = 1 K
Temperature: Celsius degrees C C = K − 27315
Temperature: Celsius degrees C C = F − 32 F/18 F C−1
Temperature: Fahrenheit degrees F F = C × 18 F C−1 + 32 F
Wind Speed: knots (kts): 1 kt = 051 m s−1
Wind Speed: miles per hour (mph): 1 mph = 048 m s−1
Wind speed: kilometers per hour: (kph): 1 kph = 028 m s−1
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Index
Note: The bold numbers are the primary definitions/references.
A
absolute temperature 5, 42
absolute zero 5
adiabatic 46, 47, 48, 127
adiabatic, dry 49, 186, 188
adiabatic, moist 49, 189
adiabatic cooling 47
adiabatic warming 47
adjustment 108, 113, 263
advection 35, 37, 85
advection, cold 36, 107
advection, thermal 106
advection, warm 36, 107, 197
ageostrophic wind 109, 110–112, 155
albedo 251
air mass 9–10
amplitude 135
anomaly 143
anticyclonic 89, 99, 107
atmospheric oscillations 203–205
available potential energy 262
B
balanced flow 95, 95–103
baroclinic 14, 247
baroclinic instability 161–162, 247–248
baroclinicity 120, 121, 127, 130
barotropic 14, 120, 127, 140, 146
beta plane approximation 139
bow echoes 197
boundary layer 168
Boussinesq approximation 104, 149
Brunt-Väisälä frequency 49
buoyancy force 48, 74, 105
C
centrifugal force 67, 68
centripetal force 67, 69, 95
chinook 80, 227
circulation 119
circulation, absolute 123
circulation, Bjerknes’ theorem 120–121
circulation, indirect 255
circulation, Kelvin’s theorem 119–120
circulation, relative 123
circulation, thermally direct 113
closure 172, 174
closure assumptions 172
cold core cyclone 16
cold occlusion 161
conditionally unstable 189, 191, 205
confluence 112, 155
continuity 51
continuity equation 52, 104
continuum hypothesis 50
convection 46, 172, 189, 205, 248
convective available potential energy 190
convective inhibition 190
Applied Atmospheric Dynamics Amanda H. Lynch, John J. Cassano
© 2006 John Wiley & Sons, Ltd
278
convergence 15, 51, 53
covariance term 170
Coriolis force 69–71
Coriolis parameter 84
critical wind speed 220
cyclonic 89, 99, 107
cyclogenesis 153
cyclolysis 153
cyclostrophic 97
D
Dalton’s Law 43
dew point depression 11–12
diffluence 112, 155
dimensional homogeneity 79
dispersion relation 136
divergence 15, 51, 52–53
doldrums 256
downslope windstorm 226
drag 167, 175
dryline 196
dry static energy 262
dynamics 4
dynamic similarity 83
d’Alembert’s paradox 167
E
eddy momentum flux 261
eddy viscosity coefficient 176
effective gravity 68
Ekman boundary layer equations 177
Ekman spiral 178, 179
Ekman transport 179
enhanced greenhouse effect 253
entrainment 174, 191
evanescent wave 221
exact differential 32
extra-tropical cyclone 3
eye 205, 208
F
fall line 234
First Law of Thermodynamics 46
flux 52
flux divergence 171
flux, momentum 261
flux, vorticity 124 (see also vortex
strength)
INDEX
flux Richardson number 173
foehn 83, 227
force 57
Fram 176
free atmosphere 167
frequency 135
front 9, 10–12, 149–152 (see also
Margules’ Model)
front, cold 11
front, occluded 11, 16
front, polar 15, 149
front, stationary 11, 151
front, warm 11
frontal cyclone 14, 152
frontal system 3
G
geopotential 59
geopotential height 59
geostrophic 87, 88, 112–113
geostrophic approximation 87, 98
geostrophic wind 87
glaciation 188
gradient wind 99, 100
gravity 58–59
greenhouse effect 252–253
Greenwich Mean Time (GMT) 9
group velocity 136
gust front 193
H
hodograph 96
horse latitudes 256
hydraulic jump 230
hydrostatic approximation 86
hydrostatic balance 63, 86
hydrostatic pressure 73
hypsometric equation 64
I
ideal gas 42
Ideal Gas Law 42
incompressible 72
inertial frame of reference 58
inertial oscillation 96
inertial term 81
instant occlusion 161
Intertropical Convergence Zone
205
279
INDEX
non-dispersive 136
non-inertial frame of reference 28
non-linear differential equation 84
normal stress 41
norwesters 227
intrinsic frequency 214
inversion 66
inviscid 120
isallobaric wind 111
isobaric 12
isotropy 169
isotherm 14
isothermal layer 66
O
occlusion 17, 161
occlusion, bent back 161
occlusion, cold 161
occlusion, instant 161
occlusion, warm 161
open wave cyclone 15
orography 209
J
jet 154, 155
jet, low level 195
jet, polar 154
jet entrance 154
jet exit 154
jet maximum 154, 256
K
katabatic force 238
katabatic wind 91, 233
Kelvin waves 202
kinematic viscosity coefficient
63
L
lapse rate 75, 189
lapse rate, adiabatic 189
lapse rate, moist adiabatic 189
lee wave 225
lifting condensation level 187
long waves 13, 17, 154 (see also planetary
waves and Rossby wave)
M
Margules’ formula 151
Margules’ Model 149–151
mesocyclone 194
mesoscale 194
mesoscale convective complexes 197
mesoscale convective systems 196
mid-latitude westerlies 14, 255
mixed layer theory 175
mixing length 180
mixing ratio 44
moist static energy 261
N
neutrally stable 49
no-slip boundary condition
168
P
parcel 35
phase 135
phase speed 136
planetary waves 13, 82, 140–143 (see also
Rossby wave and long waves)
polar low 247
polar vortex 90
positive vorticity advection maximum 160
potential function 33
potential vorticity 126, 128
potentiotropic 127 (see also adiabatic)
pressure 5
pressure gradient force 59
pressure, atmospheric 6
pressure, hydrostatic 73
pressure, mean sea level 6–7, 8
pressure, partial 43
pressure, perturbation 73
propagate 17
propagating wave 135
Q
quasi-biennial oscillation 203
quasi-geostrophic flow 108, 131
R
radiosonde 10
retrograde 142
retrogress 149
reverse shear 248
Reynolds decomposition
rheology 42
169
280
Richardson number 173
ridge 13
Rossby number 81
Rossby’s formula 140
Rossby wave 141 (see also planetary
waves and long waves)
S
saturation 184
saturation vapor pressure 184
sensitive volume 50
scale height 64
Scorer parameter 223
shear stress 41
short wave 13, 15, 149, 154, 248
similarity theory 181
specific humidity 44
specific volume 46
standing wave 135
state 42
station model 7–8
stratification 214
subcritical 229
supercritical 229
supersaturated 189
surface weather map 11
synoptic 82, 86
T
Taylor-Proudman theorem 104
temperature
temperature, advection 37
temperature, atmospheric 4–5
temperature, dew point 7
temperature, potential 46, 49, 127
INDEX
temperature, virtual 45
temperature, wet bulb 192
Teten’s formula 184
thermal steering principle 159–160
thermal wind relationship 106, 113, 259
thickness 65
trade wind 123, 205, 253
tropopause 66, 105, 145
troposphere 4
trough 13
turbulence 168
turbulent eddy 168
turbulent flux divergence 171
U
unit vector 23
Universal Time (UTC)
9
V
vapor pressure 44
variable, intensive 42
variable, state 42
viscous 41
viscous force 61–63
viscous sub-layer 168
vortex strength 124 (see also flux, vorticity)
vorticity 124
vorticity equation 131
W
wall cloud 195
warm sector 16, 18, 161
wavenumber 136, 141, 142
well-mixed layer 174
wind shear 104, 106, 113
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