Clouds and the Radiative Heating ...
161
Short-Wave Cloud Forcing W/m2
-90 -70 -60 -50 -40 -30 -20 -10
0
20
Figure 7.7: Monthly mean shortwave cloud forcing (C Fsw) for April 1985.
Similar as for the top of the atmosphere the cloud forcing at the surface, C F( S), is calculated as
the difference between the monthly averaged net radiation at the surface for all-sky conditions,
QRAD(S), and that for clear-skies, Q~e;;(S). The net cloud forcing at the surface again can
be separated into its two components, CFsw(S), for the shortwave and, CFLW(S), for the
longwave. A discussion on the estimate of the effect of clouds on solar surface absorption
derived from different satellite observations is given in the paper by Keevallik and Rannik
(1995). Most results published for the longwave cloud forcing at the surface are derived from
climate model runs (Ramanathan, 1986; 1987; Slingo and Slingo, 1988; Randall, 1990). The
few results based on satellite data strongly depend on the assumption of the cloud base altitude
distribution and thus need further validation in the future.
Rieland and Stuhlmann (1993), for instance, present the solar cloud forcing at the surface
derived from Meteosat data for a one year period (February 1985 to January 1986). Their annual
average is presented in Figure 7.10. Compared to calculations for the top of the atmosphere it
is found that both, C Fsw(S) and C Fsw are of the same order of magnitude. The maximum
cooling due to the albedo effect of clouds, CFsw(S), is about 80-100 Wm- 2 . The regions
of maximum cloud forcing are located above the north and south midlatitude oceans and
correspond very well to those regions with a maximum in cloudiness. Secondary maxima with
a cooling of about 60-70 Wm- 2 appear above the regions of tropical convective clouds in the
ITCZ and low stratus clouds off the western African coast. Minimum values of a cooling less
than 30 Wm- 2 are located above several near cloud-free subtropical regions. A very interesting
area is that of the Sahara Desert. Here, cloud forcing at the top of the atmosphere is found to
be a very small cooling of CFsw ~ 10 Wm- 2 , while at the surface the cooling, CFsw(S) >
30 Wm- 2 , is significantly larger. The different behavior of these components indicates that
161
Short-Wave Cloud Forcing W/m2
-90 -70 -60 -50 -40 -30 -20 -10
0
20
Figure 7.7: Monthly mean shortwave cloud forcing (C Fsw) for April 1985.
Similar as for the top of the atmosphere the cloud forcing at the surface, C F( S), is calculated as
the difference between the monthly averaged net radiation at the surface for all-sky conditions,
QRAD(S), and that for clear-skies, Q~e;;(S). The net cloud forcing at the surface again can
be separated into its two components, CFsw(S), for the shortwave and, CFLW(S), for the
longwave. A discussion on the estimate of the effect of clouds on solar surface absorption
derived from different satellite observations is given in the paper by Keevallik and Rannik
(1995). Most results published for the longwave cloud forcing at the surface are derived from
climate model runs (Ramanathan, 1986; 1987; Slingo and Slingo, 1988; Randall, 1990). The
few results based on satellite data strongly depend on the assumption of the cloud base altitude
distribution and thus need further validation in the future.
Rieland and Stuhlmann (1993), for instance, present the solar cloud forcing at the surface
derived from Meteosat data for a one year period (February 1985 to January 1986). Their annual
average is presented in Figure 7.10. Compared to calculations for the top of the atmosphere it
is found that both, C Fsw(S) and C Fsw are of the same order of magnitude. The maximum
cooling due to the albedo effect of clouds, CFsw(S), is about 80-100 Wm- 2 . The regions
of maximum cloud forcing are located above the north and south midlatitude oceans and
correspond very well to those regions with a maximum in cloudiness. Secondary maxima with
a cooling of about 60-70 Wm- 2 appear above the regions of tropical convective clouds in the
ITCZ and low stratus clouds off the western African coast. Minimum values of a cooling less
than 30 Wm- 2 are located above several near cloud-free subtropical regions. A very interesting
area is that of the Sahara Desert. Here, cloud forcing at the top of the atmosphere is found to
be a very small cooling of CFsw ~ 10 Wm- 2 , while at the surface the cooling, CFsw(S) >
30 Wm- 2 , is significantly larger. The different behavior of these components indicates that