Potter T.D., Colman B.R. (co-chief editors). The handbook of weather, climate, and water: dynamics, climate physical meteorology, weather systems, and measurements
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Figure 20 Annual cycle of monthly mean net radiation in W=m
2
determined from the
ERBE observations. See ftp site for color image.
Figure 21 (see color insert) January mean OLR in W=m
2
determined from ERBE
observations. See ftp site for color image.
366
RADIATION IN THE ATMOSPHERE: OBSERVATIONS AND APPLICATIONS
tropical convection regions (e.g., Indonesia, Congo Basin, central South America).
These thick clouds also yield local minima in the January and July averaged plane-
tary albedos (Figs. 23 and 24). Regional albedo maps for January and July indicate
the presence of maritime stratus regions located off the west coast of continents.
Figure 22 (see color insert) July mean OLR in W=m
2
determined from ERBE observa-
tions. See ftp site for color image.
Figure 23 (see color insert) January mean albedo in percent determined from ERBE
observations. See ftp site for color image.
4 TOP-OF-ATMOSPHERE RADIATION BUDGETS 367
These features do not appear on the OLR maps because of the similarity between the
sea surface temperature and cloud effective temperature. The ITCZ appears in the
albedo maps as a regional enhancement, while in maps of OLR it is a relative
minimum.
Analysis of the regional distribution of the net radiative energy gains and losses
for January and July is shown in Figures 25 and 26, respectively. These figures
clearly indicate that the summer hemisphere gains radiative energy while the
winter hemisphere is a net radiative sink. In July, the large desert regions of northern
Africa and Saudi Arabia also exhibit a net radiative loss. This arises from the high
surface albedos and high surface temperatures and overall cloud-free conditions. The
largest energy gains are associated with cloud-free ocean regions in the summer
hemisphere, due to the relatively low albedo and high solar input.
The measured outgoing longwave radiation and albedo also indicate regional
forcing mechanisms. For example, in the tropics longitudinal variations can be as
large as the zonal averages and are associated with east–west circulations. While
tropical regions in general display a net radiative heating, the Sahara is radiatively
cooling. This is due to the high surface albedo, the warm surface temperatures,
and the dry and cloud-free atmosphere. The radiative cooling is maintained by
subsidence warming, which also has a drying effect and therefore helps maintain
the desert.
Figure 24 July mean albedo in percent deter mined from ERBE observations. See ftp site for
color image.
368
RADIATION IN THE ATMOSPHERE: OBSERVATIONS AND APPLICATIONS
Cloud-Radiative Forc ing
One of the major research problems in radiative trans fer applications is how clouds
affect the energy budget of the planet and thus impact climate . The net radiative
heating H within a column is
Figure 25 (see color insert) January mean net radiation in W=m
2
determined from ERBE
observations. See ftp site for color image.
Figure 26 (see color insert) July mean net radiation in W=m
2
determined from ERBE
observations. See ftp site for color image.
4 TOP-OF-ATMOSPHERE RADIATION BUDGETS 369
H ¼ S
0
ð1 aÞOLR ð13Þ
where S
0
ð1 aÞ is the absorbed solar energy and OLR is the outgoing longwave
energy. To describe the effects of clouds on H from observations we define the cloud
forcing,
C ¼ H H
clr
ð14Þ
where H
clr
is the clear-sky net heating. Thus, cloud-radiative forcing is the difference
between the all-sky (cloudy and clear regions) and cloud-sky fluxes. We can write
C ¼ C
SW
þ C
LW
ð15Þ
where
C
SW
¼ Sða
clr
aÞð16Þ
and
C
LW
¼ OLR
clr
OLR ð17Þ
In the longwave, clouds generally reduce the LW emission to space and thus result
in a heating. While in the SW, clouds reduce the absorbed solar radiation, due to a
generally higher albedo, and thus result in a cooling of the planet. Results from
ERBE indicate that in the global mean, clouds reduce the radiative heating of
the planet. This cooling is a function of season and ranges from approximately
13 to 21 W=m
2
, with an uncertainty of approximately 5 W=m
2
. On average,
clouds reduce the globally absorbed solar radiation by approximately 50 W=m
2
,
while reducing the OLR by about 30 W=m
2
. These values may be compared with
the 4 W=m
2
heating predicted by a doubling of CO
2
concentration.
Variations in net cloud forcing are associated with surface type, cloud type, and
season. These dependencies can be seen in maps of January and July cloud net
radiative forcing (Figs. 27 and 28). Maritime stratus tend toward a negative net cloud
forcing as the shortwave effects dominate the longwave. The deserts of North Africa
and Saudi Arabia have a positive cloud radiative forcing as the longwave dominates
the cloud impact on the albedo.
5 RADIATION AND THE GREENHOUSE EFFECT
The energy that fuels the atmospheric and oceanic circulations originates from the
sun. If the total energy content of the Earth–atmosphere syst em does not vary
significantly with time, there must be a close balance between the incoming
absorbed solar radiation and the outgoing terrestrial emitted thermal radiation. In
370 RADIATION IN THE ATMOSPHERE: OBSERVATIONS AND APPLICATIONS
Figure 27 (see color insert) January cloud radiative forcing in W=m
2
. See ftp site for color
image.
Figure 28 (see color insert) July cloud radiative forcing in W=m
2
. See ftp site for color
image.
5 RADIATION AND THE GREENHOUSE EFFECT 371
other words, when averaged over the entire globe for the year, the net energy gain
equals the net loss
Absorbed sunlight ¼ terrestrial emission ð18Þ
S
0
pR
2
e
ð1 a
e
Þ¼4pR
2
e
sT
4
e
ð19Þ
where a
e
is the albedo (broadband reflectance) of Earth and T
e
is the effective
temperature. For an albedo of a
e
¼ 0:3, T
e
255
, which is considerably cooler
than the average surface temperature of 288 K.
The average surface temperature is greater than the effective radia tive temperature
because of the radiative properties of the atmosphere. The atmosphere is relatively
transparent to solar radiation while absorbing and emitting longwave radiation
effectively.
The spectral selectivity of absorption by atmospheric gases is the fundamental
cause of the greenhouse effect. Much of the shortwave, or solar, energy passes
through the atmosphere and warms the surface. While the atmosphere is transparent
to shortwave radiation, it is efficient at absorbing terrestrial or longwave radiation
emitted upward by the surface. So, while carbon dioxide and water vapor comprise
only a very small percentage of the atmospheric gases, they are extremely important
because of their radiative properties, i.e., their abilities to absorb this longwave
radiation and emit it throu ghout the atmosphere.
Gases that are transparent to solar energy while absorbing terrestrial energy will
warm the atm osphere because they allow solar energy to reach the surface and
inhibit longwave radiation from reaching outer space. These radiatively active
gases are called greenhouse gases. In addition to carbon dioxide, other important
greenhouse gases are water vapor, methane (CH
4
), and CFCs. Methane and CFCs
are important because they absorb terrestrial radiation in the 10- to 12-mm infrared
(IR) atmospheric window. The concentration of these latter two gases has also been
increasing. Increases in the greenhouse gases with time can potentially result in a
climate change, as the atmosphere becomes more effective at absorbing longwave
energy emitted by the surface.
Greenhouse warming or the enhanced greenhouse effect are the terms used to
explain the relationship between the observed rise in global temperatures and an
increase in atmospheric carbon dioxide. In this chapter we have considered one
aspect of the enhanced greenhouse effect, absorption and emission of radiation by
certain atmospheric gases. How does this play a role in the enhanced greenhouse
effect? Let us use carbon dioxide as an example. Increasing the carbon dioxide
concentrations in the atmosphere does not appreciably affect the amount of solar
energy that reaches the surface. However, since carbon dioxide absorbs longwave
radiation, the amount of longwave energy emitted by the surface and absorb ed by
the atmosphere increases as the atmospheric concentration of carbon dioxide rises.
The increased absorption increases the temperature of the atmosphere. The warmer
atmospheric temperatures increase the amount of longwave energy emitted by the
atmosphere toward the surface, which increases the energy gain of the surface,
372 RADIATION IN THE ATMOSPHERE: OBSERVATIONS AND APPLICATIONS
warming it. Thus, increased concentration of carbon dioxide may result in a warming
of Earth’s atmosphere and surface.
Water vapor is the most effective greenhouse gas because of its strong absorption
of longwave energy emitted by the surface. A warmer atmosphere can mean more
water vapor in the atmosphere and possibly more clouds.
Clouds and the Greenhouse Effect
Increased concentration of greenhouse gases can lead to a warming of the atmo-
sphere. As the air temperature warms, the relative humidity initially decreases.
Evaporation depends on relative humidity; so, as the atmosphere warms, more
evaporation occurs, which adds more water to the atmosphere and enhances the
greenhouse warming. Earth and atmosphere keep heating up until the energy emitted
balances the amount of sunlight absorbed. But greenhouse gases are not the whole
story of climate change. Clouds have a large impact on the solar and terrestrial
energy gains of the atmosphere. Clouds reflect solar energy and reduce the
amount of solar radia tion reaching the surface, and thus cause a cooling of Earth.
The thicker the cloud, the more energy reflected back to space, and the less solar
energy available to warm the surface and atmosphere below the cloud. By reflecting
solar energy back to space, clouds tend to cool the planet. Clouds are also very good
emitters and absorbers of terrestrial radiation.
Clouds block the emission of longwave radiation to space. Thus, in the longwave,
clouds act to warm the planet, much as greenhouse gases do. To complicate matters,
the altitude of the cloud is important in determining how much they warm the planet.
Cirrus are cold clouds. Thick cirrus therefore emit very little to space because of
their cold temperature, while at the same time cirrus are effective at absorbing the
surface-emitted energy. Thus, with respect to longwave radiation losses to space,
cirrus tend to warm the planet. Stratus also warm the planet but not as much as
cirrus. This is because stratus are low in the atmosphere and have temperatures that
are more similar to the surface than cirrus clouds. Stratus absorb radia tion emitted by
the surface, but they emit similar amounts of terrestrial radiation to space as the
surface. To complicate matters still further, how effective a cloud is at reflecting
sunlight is a function of how large the cloud droplets or cloud ice crystals are.
Figure 29 depicts the dependence of cloud radiative forcing as a function of cloud
temperature and the change in the planetary albedo due to the cloud. Thin, cold
clouds tend to warm the planet while thick, warm clouds tend to cool. The zero line
indicates those clouds that have no net effect on the top of the atmosphere energy
budget. Cold, thick clouds, such as convective systems, have little impact on the
energy budget at the top of the atmosphere.
So clouds can either act to cool or warm the planet, depending on how much of
Earth they cover, how thick they are, how high they are, and how big the cloud
particles are. Measurements indicate that on average, clouds’ reflection of sunlight
dominates the clouds’ g reenhouse warming. Thus, today’s distribution of clouds
tends to cool the planet. But this may not always be the case. As the atmosphere
warms the distribution of cloud amount, cloud altitude, and cloud thickness may all
5 RADIATION AND THE GREENHOUSE EFFECT 373
change. We do not know what the effect of clouds will be on the surfa ce tempera-
tures if the global climate changes. Clouds could dampen any greenhouse warming
by increasing cloud cover or decreasing cloud altitude. On the other hand, clouds
could increase a warming if the cloud cover decreases or cloud altitude increases.
Climate is so sensitive to how clouds might change that an accurate prediction of
climate change hinges on correctly predicting cloud formation.
Radiative Equilibrium
A fundamental challenge of modern science is to predict climate. Recent concerns
about global warming and the effect of greenhouse gases added to the atmosphere by
humans have heightened the need to understand the processes that cause climate
variations. Models are used to gain a better understanding of the atmosphere. We can
use a simple model to derive the temperature distribution of an atmosphere in
radiative equil ibrium by assuming that the only source term is due to thermal
Figure 29 Change in net radiation at top of atmosphere as function of cloud effective
temperature and change in planetary albedo due to the cloud. Contours are in W=m
2
(after D.
Hartmann, (1994) Global Physical Climatology, Academic Press, NY).
374
RADIATION IN THE ATMOSPHERE: OBSERVATIONS AND APPLICATIONS
emission: B ¼ sT
4
. For demonstration, we also assum e that the atmosphere absorbs
at all wavelengths. The upwelling and downwelling fluxes are then
dðF"F#Þ
dt
¼ðF"þF#Þ þ 2B ð20Þ
dðF"þF#Þ
dt
¼ðF"F#Þ ð21Þ
We can directly solve this set of equations to yield the emission as a function of
optical depth:
BðtÞ¼
F"F#
2
t þ B
0
ð22Þ
where B
0
is the emission of the first layer of atmosphere ðt ¼ 0Þ.
For thermal equil ibrium, the atmosphere and surface outgoing longwave flux
must balance the incident solar flux, F
0
, so that
F"ð0Þ¼F
0
ð23Þ
and
ðF"F #Þ ¼ F
0
ð24Þ
yielding
BðtÞ¼
F
0
2
ðt þ 1Þð25Þ
At the surface, the upwelling flux emitted by the surface must balance the
absorbed solar flux and the downwelling flux emitted by the atmosphere ½F#ðt
s
Þ.
BðT
s
Þ¼F
0
þ F#ðt
s
Þð26Þ
which leads to
BðT
s
Þ¼Bðt
s
Þþ
F
0
2
ð27Þ
This simple model demonstrates that the atmospheric temperature profile is a
function of its optical depth. It also shows that the surface temperature is warmer
than the overlying air. The greater the optical depth the greater the difference
between the surface temperature and the effective temperature of the planet.
5 RADIATION AND THE GREENHOUSE EFFECT 375