An experimental technique for testing the validity of cumulus cloud parameterizations for longwave radiation calculations

Cumulus cloud bulk geometry, size, and spatial distributions have long been recognized as important factors for longwave radiative transfer under brok en cloud conditions. Most current climate models, however, still ignore the se factors and estimate the effects of broken cumulus clouds as the cloud a mount-weighted average of clear and black-cloud overcast conditions, that i s, 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 definin g an effective cloud fraction, the validity of these parameterizations has long been assumed because of inadequate measurements of the instantaneous a tmospheric radiative properties. Now ground-based measurements at the Atmos pheric Radiation Measurement Program southern Great Plains Cloud and Radiat ion Test Bed site allow the derivation of the effective cloud fraction, abs olute cloud fraction, cloud aspect ratio, and many other variables characte rizing cumulus clouds. Using an empirically determined sampling period of 1 0 min, several different parameterizations for effective cumulus cloud frac tion were tested by comparing effective amounts derived from hemispheric fl ux observations with values predicted by the parameterizations. Within the range of data and among the models rested, the better results were obtained with the cuboidal model with exponential cloud size and spatial distributi ons, the random cylinder model, the regular cuboidal model. and the shifted -periodic array cuboidal model. However, there are few cases in the range o f greatest sensitivity where model comparisons demonstrate larger disparity .