Geoscience Reference
In-Depth Information
CLOUD CASE 2
Consider a high cloud with the exact same radiative characteristics as the low
cloud in Case 1, that is, a shortwave albedo of 0.3 and longwave emissivity of
1. Again, assume that the rest of the atmosphere above and below the cloud is
empty of other clouds, greenhouse gases, and particles. The cloud top is located
near 10 km elevation, so a reasonable value for its temperature is
(4.35)
TT
=+=
2 87.5
Γ
K
−
(10
km K/km
)(6
)
=
227.5
K
.
C
S
What is the net effect of this high cloud on the heat balance?
• Shortwave effect: The cloud interacts with the solar radiation in the same way
as the low cloud because the albedo is independent of the cloud height and
temperature, and the climate system receives 102.6 W/m
2
less solar heating.
• Longwave effect: The cloud again absorbs the entire
σ =
T
4
387.4
W/m
2
longwave radiation incident on it from the surface and emits thermal
radiation of
σ = The decrease in longwave radiation emitted
to space due to the presence of the high cloud is 235.5 W/m
2
.
T
W/
4 2
151.9
.
In the high-cloud case, the reduction in OLR due to the presence of the cloud
is greater than the reduction in solar heating by 132.9 W/m
2
. The net effect of
the presence of the high cloud is to
warm
the climate system by 132.9 W/m
2
.
These simple calculations demonstrate that, to first order, interactions between
clouds and solar radiation are independent of cloud height, but interactions be-
tween clouds and longwave radiation depend on cloud temperature and, there-
fore, cloud altitude. High clouds are much more effective at trapping longwave
radiation in the climate system than low clouds because the temperature differ-
ence between high clouds and the surface is greater. For this reason, high clouds
tend to warm the climate system and low clouds tend to cool the system.
For the earth system as a whole, we assumed previously that there is a bal-
ance between the incoming solar radiation and the outgoing longwave radi-
ation. Locally, however, this state of radiative equilibrium does not hold in
general. For the cloudless case, the net radiative heating at a particular location
is given by
(1
−
α
)
S
CS
0
4
H
=
−
σ
T
,
(4.36)
CS
4
CS
where
H
CS
is the “clear-sky” heating rate, a
CS
is the clear-sky albedo, and
T
CS
is the atmospheric temperature. Similarly, the net radiative heating in a cloudy
atmosphere,
H
CL
, is
(1
−
α
)
S
CL
0
4
H
=
−
σ
T
.
(4. 37)
CL
4
C
L
Cloud forcing
,
F
C
, is the net local radiative heating rate due to the presence
of clouds, so it is the difference in radiative heating between the cloudy and
clear- sky cases:
