Geoscience Reference
In-Depth Information
clouds are particularly effective since they act as black
bodies. For this reason, cloudiness and cloud-top tem-
perature can be mapped from satellites by day and by
night using infra-red sensors (see Plates 2, 3 and 15,
where high clouds appear cold). Radiative cooling of
cloud layers averages about 1.5°C per day.
For the globe as a whole, satellite measurements
show that in cloud-free conditions the mean absorbed
solar radiation is approximately 285W m
-2
, whereas the
emitted terrestrial radiation is 265W m
-2
. Including
cloud-covered areas, the corresponding global values
are 235 W m
-2
for both terms. Clouds reduce the
absorbed solar radiation by 50 W m
-2
, but reduce the
emitted radiation by only 30 W m
-2
. Hence global cloud
cover causes a net radiative loss of about 20 W m
-2
, due
to the dominance of cloud albedo reducing short-wave
radiation absorption. In lower latitudes this effect is
much larger (up to -50 to -100 W m
-2
), whereas in
high latitudes the two factors are close to balance, or
the increased infra-red absorption by clouds may lead
to a small positive value. These results are important
in terms of changing concentrations of greenhouse
gases, since the net radiative forcing by cloud cover
is four times that expected from CO
2
doubling (see
Chapter 13).
D HEAT BUDGET OF THE EARTH
We can now summarize the net effect of the transfers of
energy in the earth-atmosphere system averaged over
the globe and over an annual period.
The incident solar radiation averaged over the
globe is
Solar constant
π
r
2
/ 4
π
r
2
where
r
= radius of the earth and 4
r
2
is the surface area
of a sphere. This figure is approximately 342 W m
-2
,
or 11
π
10
9
J m
-2
yr
-1
(10
9
J = 1GJ); for convenience we
will regard it as 100 units. Referring to Figure 3.21,
incoming radiation is absorbed in the stratosphere
(3 units), by ozone mainly, and 20 units are absorbed in
the troposphere by carbon dioxide (1), water vapour
(13), dust (3) and water droplets in clouds (3). Twenty
units are reflected back to space from clouds, which
Figure 3.21
The balance of the atmospheric energy budget. The transfers are explained in the text. Solid lines indicate energy gains
by the atmosphere and surface in the left-hand diagram and the troposphere in the right-hand diagram. The exchanges are referred
to 100 units of incoming solar radiation at the top of the atmosphere (equal to 342 W m
-2
).
Source
: After Kiehl and Trenberth (1997) From
Bulletin of the American Meteorological Society
, by permission of the American Meteorological
Society.
