REGIONAL CLIMATE MODEL STUDY OF THE SCALE DEPENDENCE OF CLOUD-RADIATION INTERACTIONS

The scale dependence of cloud-radiation interaction associated with th e parameterizations for fractional cloudiness and radiation used in a global climate model is studied by examining the averages, for differe nt spatial scales, of detailed structure of cloudiness and radiation s imulated from a regional climate model that incorporates these paramet erizations. The regional model simulation is conducted over an area ab out (360 km)(2) located on the southern Great Plains for the period 10 -17 April 1994 during which both satellite and surface measurements of radiation fluxes and clouds are available from the Intensive Observin g Period of the Atmospheric Radiation Measurement program. The area co rresponds approximately to one gridpoint size of a global climate mode l with horizontal resolution T31. The regional model simulates well th e overall cloud and radiation temporal features when averaged over the entire region. However, specific biases exist in the spatial patterns such as the high clouds, the TOA upwelling solar radiation under clou dy conditions, and the net longwave surface flux under clear condition s at night. The cloud and radiation parameterizations are found to be sensitive to the spatial scale of the computation. The diagnosed total cloudiness shows a strong horizontal resolution dependence that leads to large changes in the surface and TOA radiation budgets. An additio nal experiment, in which the diagnosed cloud at each level is held con stant while the radiation parameterization is recalculated, still prod uces a substantial sensitivity to spatial scale in the calculated radi ation quantities. This is because the nature of the cloud vertical ove rlapping assumption changes as the horizontal scale of the computation varies.