Radiative Properties of Finite Cloud Fields over a Reflecting Surface

Abstract

Monte Carlo calculations have been performed for finite cloud fields over a reflecting surface in order to parameterize the effect of underlying surface on the cloud albedo. The radiative transfer calculations show that the results with the plane-parallel approximation approach the Monte Carlo results with increasing surface albedo for oblique sun angles. The parameterization scheme allows for the contribution of the multiple reflections between cloud layer and the surface. The model yields a reasonable approximation to Monte Carlo results for finite cloud fields. The effective cloud fraction obtained with the parameterization shows that it should be a nonlinear function of the cloud cover viewed normally and the area enhancement ratio, indicating that the plane-parallel approximation is not satisfactory for finite cloud fields. For high surface reflectivity, however, the effective cloud fraction approaches unity. Therefore, the plane-parallel approximation may be adequate for finite cloud fields over a highly reflecting surface.