PNNL

Abstract

Cumulus clouds can become tilted or elongated in the presence of wind shear. Nevertheless, most studies of the interaction of cumulus clouds and radiation have assumed these clouds to be isotropic. This paper describes an investigation of the effect of fair-weather cumulus cloud ?eld anisotropy on domain-averaged solar ?uxes and atmospheric heating rate pro?les. A stochastic ?eld generation algorithm was used to produce twenty three-dimensional liquid water content ?elds based on the statistical properties of cloud scenes from a large eddy simulation. Progressively greater degrees of x-z plane tilting and horizontal stretching were imposed on each of these scenes, so that an ensemble of scenes was produced for each level of distortion. The resulting scenes were used as input to a three-dimensional Monte Carlo radiative transfer model. Domain-average transmission, re?ection, and absorption of broadband solar radiation were computed for each scene along with the average heating rate pro?le. Both tilt and horizontal stretching were found to signi?cantly affect calculated ?uxes, with the amount and sign of ?ux differences depending strongly on sun position relative to cloud distortion geometry. The mechanisms by which anisotropy interacts with solar ?uxes were investigated by comparisons to independent pixel approximation and tilted independent pixel approximation computations for the same scenes. Cumulus anisotropy was found to most strongly impact solar radiative transfer by changing the effective cloud fraction, i.e., the cloud fraction with respect to the solar beam direction.