Monte Carlo radiative transfer calculations for inhomogeneous mixed phase clouds

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