Three-dimensional radiative transfer in tropical deep convective clouds

Abstract

This study focuses on the interaction between short-wave radiation and a field of tropical deep convective clouds generated using a cloud-resolving model to assess the significance of three-dimensional radiative transport. Comparisons with an independent pixel approximation approach and a plane-parallel radiative scheme are used to assess the importance of the subgrid-scale variability and organization of the clouds for their radiative properties. The horizontal transport of radiation is responsible for a significant energy redistribution inside the cloudy and clear regions. Local heating rates in both the cloudy and clear-sky regions can differ by 1 K d-1 or more, with mean heating rates altered by 20% and 15%, respectively. These figures are larger than previously reported for convective scenes, most probably because of the lower cloud fraction of the scene used here. This energy imbalance is mainly controlled by two opposing effects: side illumination and shadowing, whose relative importance is driven by the spatial arrangement of cloud elements in the domain, the Sun position and the aspect ratio of clouds. These effects partially cancel, giving lower mean biases. Nevertheless, comparison between independent pixel approximation and plane-parallel biases shows that for deep convective cloud, geometry-related effects can have a larger influence on radiative transfer calculation than internal optical inhomogeneities. © 2003 by the American Geophysical Union.