An Experimental Technique for Testing the Validity of Cumulus Cloud Parameterizations for Longwave Radiation Calculations

Abstract

Cumulus cloud bulk geometry, size, and spatial distributions have long been recognized as important factors for longwave radiative transfer under broken cloud conditions. Most current climate models, however, still ignore these factors and estimate the effects of broken cumulus clouds as the cloud amount?weighted average of clear and black-cloud overcast conditions, that is, the black plate approximation. Although several groups have adopted the simplicity of the black plate approximation and extended it to include the effects of cloud geometry, cloud size, and spatial distributions by defining an effective cloud fraction, the validity of these parameterizations has long been assumed because of inadequate measurements of the instantaneous atmospheric radiative properties. Now ground-based measurements at the Atmospheric Radiation Measurement Program southern Great Plains Cloud and Radiation Test Bed site allow the derivation of the effective cloud fraction, absolute cloud fraction, cloud aspect ratio, and many other variables characterizing cumulus clouds. Using an empirically determined sampling period of 10 min, several different parameterizations for effective cumulus cloud fraction were tested by comparing effective amounts derived from hemispheric flux observations with values predicted by the parameterizations. Within the range of data and among the models tested, the better results were obtained with the cuboidal model with exponential cloud size and spatial distributions, the random cylinder model, the regular cuboidal model, and the shifted-periodic array cuboidal model. However, there are few cases in the range of greatest sensitivity where model comparisons demonstrate larger disparity.