Monte Carlo radiative transfer calculations for inhomogeneous mixed phase clouds

Albedo of three-dimensionally inhomogeneous mixed-phase clouds at visible w avelengths have been calculated by means of a Monte Carlo radiative transfe r model. The cloud physical properties, liquid water, ice, snow, and rain, have been obtained from the non-hydrostatic mesoscale atmospheric model GES IMA. Monte Carlo results are shown for a hierarchy of simplifications in cl oud geometry ranging from clouds with spatial variability of both optical t hickness and microphysical properties to plane-parallel homogeneous clouds with prescribed microphysical properties. The results clearly demonstrate t he albedo reduction due to cloud inhomogeneity compared to plane-parallel h omogeneous water clouds (albedo bias). It is shown that the albedo bias is caused in about equal parts by internal cloud inhomogeneities and by horizo ntally finiteness of clouds. A detailed consideration of spatial variation in single scattering properties does not appear to be critical for realisti c radiative transfer calculations. (C) 1999 Elsevier Science Ltd. All right s reserved.