The effect on absorption in clouds of having an inhomogeneous distribution
of droplets is shown to depend on whether one replaces a homogeneous cloud
by an inhomogeneous cloud that has the same mean optical thickness, or one
that has the same spherical albedo. For the purposes of general circulation
models (GCMs) the more appropriate comparison is between homogeneous and i
nhomogeneous clouds that have the same spherical albeo. so that the radiati
on balance of the planet with space is maintained. In this ease it is found
, using Monte Carlo and independent pixel approximation calculations, that
inhomogeneous clouds can absorb more than homogeneous clouds. It is also fo
und that because of the different effects of cloud inhomogeneity on absorpt
ion and on the transmission of the direct beam the absorption efficiency of
an inhomogeneous cloud may be either greater (for low and high optical dep
ths) or lesser (for intermediate optical depths) than that for a homogeneou
s cloud of the same mean optical depth. This effect is relevant both to in-
cloud absorption and to absorption below clouds. In order to include these
effects in GCMs a simple renormalization of the single-scattering parameter
s of radiative transfer theory is derived that allows the effects of cloud
inhomogeneities to be included in plane-parallel calculations. This renorma
lization method is shown to give reasonable results when compared with Mont
e Carlo calculations, has the appropiate limits for conservative and comple
tely absorbing cases, and provides a simple interpretation of the effects o
f cloud inhomogeneities that could readily be incorporated in a GCM.