Geoscience Reference
In-Depth Information
4.7 RADIATIVE EFFECTS OF CLOUDS
Clouds interact with both shortwave and longwave radiation in the earth's
atmosphere. Their most important interaction with shortwave radiation is by
reflection, which changes the direction of the incident radiation but not its
wavelength. Cloud albedos range from 0.2 to 0.9 (see
Table 5.1
). Some of this
reflected radiation is redirected back into space, making the climate system
cooler. At the same time, clouds behave as nearly perfect blackbodies at ter-
restrial wavelengths. They absorb almost all the longwave radiation incident
on them, and reradiate the energy at different wavelengths according to their
temperatures and the Stefan-Boltzmann law (Eq. 4.1). If the longwave radia-
tion incident on a cloud was originally directed out of the atmosphere, for
example, as upward emission from the surface, then the cloud will reemit this
radiation in all directions and at lower temperatures, turing the energy back
into the climate system to warm it. This, of course, is just the greenhouse ef-
fect at work.
Which of these cloud effects on radiation dominates for the earth system?
Do clouds cool the earth system, or warm it? The two idealized cases that fol-
low make it clear that the answer to this question depends on factors such as
the cloud's albedo and its altitude.
CLOUD CASE 1
Consider a low cloud with a shortwave albedo of 0.3 and a longwave emis-
sivity (and absorptivity) of 1. To isolate the effects of this one cloud, assume
that the rest of the atmosphere above and below the cloud is empty of other
clouds, greenhouse gases, and particles, and model the cloud as a semi-ininite
slab. The cloud top is located near 2 km elevation, so a reasonable value for the
cloud- top temperature,
T
C
, is
(4.34)
TT
=+=
2 87.5
Γ
K km
−
(2
)(6
K/km
)
=
75.5
K
,
C
S
where the globally averaged surface air temperature and a typical tropospheric
lapse rate are used. Further assume that the solar radiation incident on the
cloud,
S
INC
, is also the globally averaged value of 342 W/m
2
(Eq. 4.8).
What is the net radiative effect of this cloud?
• Shortwave effect: The cloud relects 30% of the incoming solar radiation.
Because of this reflection—because of the presence of the cloud—the climate
system receives 102.6 W/m
2
less radiative heating.
• Longwave effect: The cloud absorbs all the
σ = longwave
radiation incident on it from the surface and emits thermal radiation of
.
T
3874W/
4 2
.
σ = Because of the presence of the cloud, the climate system
retains 60.8 W/m
2
more heat
T
326 6W/
4 2
.
Thus, the net effect of the presence of this low cloud is to cool the climate sys-
tem by 41.8 W/m
2
.
