156
R. Stuhlmann
Net - Radiation W/m2
-180 -140 -100
-60
-20
o 40 80 120
Figure 7.3: Monthly mean net radiation (QRADJ for April 1985.
-
Lalltude, deg
(-J~
i
: ., . ... -April
\, . , ......... .
----. . . -< . . ::- Oaooe<
\:--=--- Januuy
~~~~~~o~~~ ~~~~~~
utllud_, deg
N
Figure 7.4: Zonal average outgoing longwave and reflected shortwave radiation for the months
April, July, October 1985 and January 1986 derived from ERBE.
There is one problem in calculating the last term of Equation 7.2 directly from the satellite data.
The foot prints of the satellite instruments (PIXELs) on Earth, determined by the instruments'
instantaneous field of views (IFOVs), have a nonnegligible spatial extend compared to the
spatial scale of broken cloudiness. The ERBE instruments. for instance. have a pixel size of
about 60 km 2 at the sub satellite point. Thus, pixels partly filled by clouds and clear-sky is a
very common situation, which does not allow to unequivocally determine N and QR~Dca.s t from