Salby M.L. Fundamentals of Atmospheric Physics

20

I A Global View

.{ ~/'" ".:~,-.--~-." 7 ...... 7.11 ....................... "

:'z. .,"~"%~..-..,- . .., .. [...~.~.~. ....... ....j

.~--(b) 5OO rob: Jan-Mar, 1984

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...... ~'~176176

Figure 1.9

(Continued)

(b) Time-mean height of and velocity on the 500-mb isobaric surface

for January-March 1984. Isobaric heights shown in meters. From National Meteorological Center

(NMC) analyses.

time-mean circulation contains locally intensified jets that mark the "North

Atlantic" and "North Pacific storm tracks." Also marked by a steep merid-

ional gradient of height, these are preferred regions of cyclone development

and accompanying convective activity. By determining these and other longi-

tudinally dependent features of the time-mean circulation, planetary waves

control regional climates.

The circulation in the stratosphere is also unsteady. However, the synoptic

disturbances that prevail in the troposphere are not evident in the instanta-

neous circulation at 10 mb, which is shown in Fig. 1.10a for the same day

as in Fig. 1.9a. Instead, only planetary-scale disturbances appear at this al-

titude. The time-mean circulation at 10 mb (Fig. 1.10b) is characterized by

1.2 Composition and Structure 21

........

(a) Omb: Mar , 4

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...---~ .,, .:.".:::" " ".,.!..."'-~ ",,

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313875

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: ~176

[" ~176176

Figure 1.10 As in Fig. 1.9, but for the 10-mb isobaric surface.

(continues)

strong westerly flow that corresponds to the polar-night jet in Fig. 1.8 and to a

circumpolar vortex. Like motion at 500 mb, the time-mean circulation is dis-

turbed from zonal symmetry by steady planetary waves that are not eliminated

by averaging over time. The instantaneous circulation in Fig. 1.10a is much

more disturbed than the time-mean circulation. Counterclockwise motion as-

sociated with the polar-night vortex has been displaced well off the pole and

highly distorted by a clockwise circulation that has amplified and temporarily

invaded the polar cap. This anomalous feature deflects the air stream through

large excursions in latitude, which transport air from one radiative environ-

ment to another.

In addition to the large-scale features described, the circulation is also

disturbed on smaller dimensions that are not resolved in the global analyses

shown in Figs. 1.9 and 1.10. These small-scale disturbances, which are known

22

1 A Global View

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Figure 1.10

(Continued)

as

gravity waves,

owe their existence to buoyancy and the stratification of mass.

Gravity waves are manifested in wavy patterns that appear in layered clouds,

as are often observed from the ground and in satellite imagery like that shown

in Fig. 1.11. Like planetary waves, gravity waves contain both transient and

steady components, so they are present even in time-mean fields.

1.2.4 Trace Constituents

Beyond its primary constituents, air contains a variety of trace species. Al-

though they exist in relatively minor abundances, several of these play key

roles in radiative and chemical processes in the atmosphere. Perhaps the sim-

plest such trace species is CO2 because it is chemically inert away from the

surface and therefore well mixed throughout the homosphere. Like N2 and 02,

1.2 Composition and Structure

23

Figure 1.11 Satellite image of wavy cloud patterns found downwind of mountainous terrain.

From Scorer (1986). Reproduced with permission of Ellis Horwood Ltd.

carbon dioxide has a nearly uniform mixing ratio, rco 2 ~ 350 ppmv. However,

unlike the primary constituents of air, CO2 is tied to human activities.

CARBON DIOXIDE

Involved in chemical and biological processes, CO2 is produced naturally

near the surface. However, increasing levels of carbon dioxide in historical

records point to human activities as an important perturbation to the natural

budget of CO2. Since the dawn of the industrial age, global burning of fossil

fuels has steadily increased the amount of carbon dioxide that is introduced

into the atmosphere. Even though interactions with the oceans and the bio-

sphere make the budget of CO2 complex, the influence of human activities

is strongly suggested in recent measurements. Figure 1.12 shows the record

of rco 2 at Mauna Loa in the latter half of the twentieth century. A rather

modest annual variation of about 1%, which is associated with biological pro-

duction and the growing season of the Northern Hemisphere, is superposed

on a steady trend that reflects an increase of nearly 15% in just the last four

decades.

The rapid increase of CO2 in recent years has prompted concerns over

global warming because of the role carbon dioxide plays in trapping radiant

energy near the earth's surface. Such concerns are supported, in part, by large-

24

1 A Global View

355

~, 350-

>

E

c}.. 345-

(D..

0 340-

I--

< 335-

n"

330-

Z

X 325-

oj 320

0

0 315

310

Figure 1.12

(1989).

9 i | i | i | l ! i ! a | i | i | I ! i ! i ! i | i | i | i | l

I I l I l I I I I I I I . I . i . I . I . l . I . I . a . I . I

58 60 62 64 66 68 70 72 74 76 78 80 82 84 86 88

YEAR

Mixing ratio of carbon dioxide over Mauna Loa, Hawaii. After Keeling et aL

scale numerical integrations. General circulation models (GCMs) are used to

study climate by including a wide array of physical processes, many of which

can be only crudely parameterized. Nevertheless, most GCMs are in agree-

ment in predicting a 2 to 3 K increase of global-mean surface temperature in

response to a doubling of CO2, as appears imminent in the next century. In

the absence of mitigating factors, such an increase of global-mean tempera-

ture would introduce important changes to the earth's climate, for example,

by melting polar ice caps and increasing the level of the oceans, not to men-

tion changes in weather and regional climates. In the stratosphere, where CO2

plays a dominant role in the radiative energy balance through infrared (IR)

cooling to space, temperature decreases as large as 10 K have been suggested,

which could alter other radiatively active constituents such as ozone.

Concerns over increasing CO2 are also supported by historical records of

the earth's climate. Glacial ice cores, drilled from great depths, provide a

record of atmospheric composition dating far into the past. In combination

with geological information on atmospheric temperature, that record suggests

a link between atmospheric CO2 and global temperature. During previous cli-

mates of the earth, these quantities varied in a systematically related fashion.

Figure 1.13 shows geological records of CO2 and temperature that extend back

160,000 years. Sharp minima in CO2 coincide with anomalously cold temper-

atures that have been identified with ice ages 20,000 and 140,000 years ago.

Some caution should be exercised in interpreting these records due to uncer-

tainties in the stability of gases trapped inside ice and in fi~ng the times of

individual events in the records. Equally important is the issue of causality:

DEPTH (m)

AT

o C

2

0

-2

-4

-6

---8

-10

CO2

ppmv

300

280

260

240

220

200

180

500 1000 1500 2000

I I I I

1.2 Composition and Structure 25

(b)

0 40 80 120

AT

o C

2

0

-2

-4

-6

-8

-10

CO~

ppmv

30O

280

26O

240

220

J

200

-

180

160

AGE (kyr BP)

Figure 1.13 (a) Temperature change and (b) atmospheric carbon dioxide as functions of

time before present (BP), inferred from an ice core drilled at Vostok, Antarctica. Reprinted with

permission from

Nature

(Barnola

et aL,

Although they illustrate the interdependence of CO2 and temperature, these

geological records provide no information on which produced changes in the

other. Nevertheless, geological records like those in Fig. 1.13 provide impor-

tant evidence of previous climates of the earth and the roles atmospheric

constituents play in the current climate.

WATER VAPOR

The uniform composition of dry air in the homosphere is not to be confused

with the distributions of trace species such as water vapor and ozone. These

constituents are highly variable because they are not merely redistributed by

atmospheric motions, which would eventually homogenize them. Instead, wa-

ter vapor and ozone are continually produced in some regions and destroyed

in others. By transporting them from their source regions to their sink regions,

26

1 A Global

View

the circulation exerts an important influence on these species and makes their

distributions dynamic.

Owing to its involvement in radiative processes, cloud formation, and in

exchanges of energy with the oceans, water vapor is the single most important

trace species in the atmosphere. The zonal-mean distribution of water vapor

is shown in Fig. 1.14 as a function of latitude and altitude. Water vapor is

confined almost exclusively to the troposphere. Its zonal-mean mixing ratio

?I~2O decreases steadily with altitude, from a maximum of about 20 g kg -1

at the surface in the tropics to a minimum of a few parts per million at the

tropopause. The absolute concentration of water vapor, or

absolute humidity,

Piano (shaded area) decreases with altitude even more rapidly. From (1.14),

the density of the ith constituent is just its mixing ratio times the density of

dry air

Pi -- riPd.

(1.24)

Because

Pd

decreases exponentially with altitude, water vapor tends to be con-

centrated in the lowest 2 km of the atmosphere. The zonal-mean mixing ratio

also decreases with latitude, falling to under 5 g kg -1 poleward of 60 ~ These

characteristics of water vapor reflect its production at the earth's surface, re-

distribution by the atmospheric circulation, and destruction at altitude and at

middle and high latitudes through condensation and precipitation.

Owing to those production and destruction mechanisms and the rapid trans-

port of air between source and sink regions, tropospheric water vapor is short

Water Vapor (g kg "1)

' I ' ' I ' ' I ' ' I ' ' I '

200 -

s ,..~ .....- ~ J .....~ .~

uJ 400 - 1 -- .. -- .. ........ " -- -- .. .. " -- -- -- ...... _

co 600 1

i / / .4

-80 ~ ~ -60 ~ .50 ~ .40 ~ .30~ ~ -10 ~ 0 ~ 10 ~ 20 ~ 30 ~ 40 ~ 50 ~ 60 ~ 70 ~ 80 ~

LATITUDE

Figure 1.14 Zonal-mean mixing ratio of water vapor (contoured) and density of water vapor

or absolute humidity (shaded), as functions of latitude and pressure. The shaded levels correspond

to 20, 40, and 60% of the maximum value.

Source: Oort and Peixoto (1983).

1.2 Composition and Structure 27

lived. A characteristic lifetime, which may be defined as the time for rn2 o in-

side an individual parcel to change significantly, is of order days. Every few

days, an air parcel encounters a warm ocean surface, where it absorbs mois-

ture through evaporation, or a region of cloudiness, where it loses water vapor

through condensation and precipitation.

Most of the water vapor in Fig. 1.14 originates near the equator at warm

ocean surfaces. Consequently, transport by the circulation plays a key role

in determining the mean distribution ~H20- Vertical and horizontal transport,

which are referred to as

convection

and

advection,

respectively, each contributes

to the redistribution of rH2 o. Introduced at the surface of the tropical atmo-

sphere, water vapor is carried aloft by deep convective cells and horizontally

by large-scale eddies that disperse rH20 across the globe in complex fashion.

Some bodies of air escape production and destruction long enough for rH20

to be rearranged as a tracer.

Figure 1.15 presents for the day shown in Fig. 1.9 an image from the

6.3-/zm water vapor channel of the geostationary satellite Meteosat-2, which

observes the earth from above the Greenwich meridian (see Fig. 1.24 for

geographical landmarks). The gray scale in Fig. 1.15 represents cold emis-

sion temperatures (high altitudes) as bright, and warm emission temperatures

(low altitudes) as dark. Since the water vapor column is optically thick at

this wavelength (i.e., outgoing radiation is emitted by H20 at the highest

levels), behavior in Fig. 1.15 corresponds to the top of the moisture layer.

Bright regions indicate moisture at high altitudes and deep convective dis-

placements of surface air, whereas dark regions indicate moisture that remains

close to the earth's surface. Thus, Fig. 1.15 reflects the horizontal distribution

of rile O.

Unlike the mean distribution in Fig. 1.14, which is fairly smooth, the global

distribution of water vapor on an individual day is quite variable. The mois-

ture pattern is granular in the tropics, where water vapor has been displaced

vertically by deep convective cells that have dimensions of tens to a few hun-

dred kilometers. At middle and high latitudes, the pattern is smoother, but

still complex. Swirls of light and dark mark bodies of air that are rich and

lean in water vapor, respectively, for example, air that originated in tropical

and extratropical regions and has been rearranged by the circulation. The lo-

cal abundance reflects the history of the air parcel residing at that location,

namely, where that parcel has been and what processes influencing water vapor

have acted on it. A tongue of water vapor stretches northeastward from deep

convection over the Amazon Basin (see Fig. 1.24), across the Atlantic, and

into Africa, where it joins a tongue of drier air that is being drawn southward

behind a cyclone in the eastern Atlantic (compare Fig. 1.9a). In the Southern

Hemisphere, a band of high moisture is sharply delineated from neighboring

lower moisture along a front that trails behind a cyclone in the South Atlantic.

More relevant to radiative processes than the local concentration is the

total abundance of a species over a position on the earth's surface. The

28

1 A Global View

Figure 1.15 Water vapor image on March 4, 1984, from the 6.3-/xm channel of Meteosat-

2, which is in geostationary orbit over the Greenwich meridian. Gray scale displays equivalent

blackbody temperature from warmest (black) to coldest (white). (See Fig. 1.24 for geographical

landmarks.) Supplied by the European Space Agency.

column abundance,

fo ~176

Zi -- pidz

__

1 f ps

ridp~

Jo

g

(1.25)

describes the mass of the ith species contained by an atmospheric column of

unit cross-sectional area. Figure 1.16 displays the distribution of EH~O, which

is referred to as

total precipitable water vapor

and expressed in millimeters of

liquid water, for the day shown in Fig. 1.15. Sharply confined to the tropics,

EH2O resembles the distribution of temperature. Signatures of deep convec-

tion appear in enhanced column abundance (e.g., over tropical Africa, South

America, and the eastern Atlantic), but EH2O is distributed more uniformly

than rH~ o. In part, this feature of the water vapor distribution follows from

90

March 4, 1984

1.2

Composition and Structure

29

60

30

-30

-60

-90

0 60 120 180 240 300 360

0 Y.H20 (mm) 60

Figure

1.16 Global distribution of the column abundance of water vapor, or

total precipitable

water vapor,

on March 4, 1984, derived from the TIROS Operational Vertical Sounder (TOVS).

Data courtesy of I. Wittmeyer and T. Vonderharr (CSU).

the concentration of PH20 in the lowest 2 km. But it also indicates that, at

higher altitudes, even deep convective towers contain comparatively little wa-

ter in vapor phase.

Together, the zonal-mean and horizontal distributions illustrate the pre-

vailing mechanisms controlling atmospheric water vapor. Large values of FH20

at the earth's surface in the tropics reflect production of water vapor through

evaporation of warm tropical oceans. Only inside convective towers and in

analogous features of greater dimension are large mixing ratios found far

above the ground. Even there, vertical transport of H20 is limited by thermo-

dynamic constraints that prevent water vapor from reaching great altitudes,

where it would be photodissociated by energetic solar radiation.

OZONE

Another radiatively active trace gas, ozone plays a key role in supporting

life at the earth's surface. By intercepting harmful UV radiation, ozone allows

life as we know it to exist. In fact, the evolution of the earth's atmosphere

and the formation of the ozone layer are thought to be closely related to the

development of life on Earth.

Salby M.L. Fundamentals of Atmospheric Physics

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