The Effect of Finite Geometry on the Three-Dimensional Transfer of Solar Irradiance in Clouds

Abstract

The transfer of solar irradiance in three-dimensional clouds has been modeled by both a Monte Carlo process and an analytical technique. At the expense of considerable computation time the Monte Carlo model provides an accurate test bed of results, while for a more modest computational effort the analytical model provides approximate results which are in generally good agreement with the Monte Carlo results. For the shortwave radiation budget of a cloud to be accurately determined, the results presented here indicate the need for a three-dimensional description of the cloud's geometry. Such a description helps to explain the low light levels observed beneath thunderstorms, and how the single-scatter albedo may remain close to unity despite the fact that the observed cloud albedo seldom exceeds 0.8. The commonly used plane-parallel approximation introduces error through neglect of the diffusion of radiation out through the cloud sides and neglect of possible illumination of the cloud side(s) by the direct solar beam. These errors typically exceed 10% if the cloud width to height ratio is less than 8:1 and increase noticeably with increasing solar zenith angle.