Diurnal variability of the hydrologic cycle and radiative fluxes: Comparisons between observations and a GCM

The simulated diurnal cycle is in many ways an ideal test bed for new physi cal parameterizations. The purpose of this paper is to compare observations from the Tropical Rainfall Measurement Mission, the Earth Radiation Budget Experiment, the International Satellite Cloud Climatology Project, the Clo uds and the Earth's Radiant Energy System Experiment, and the Anglo-Brazili an Amazonian Climate Observation Study with the diurnal Variability of the Amazonian hydrologic cycle and radiative energy budget as simulated by the Colorado State University general circulation model, and to evaluate improv ements and deficiencies of the model physics. The model uses a prognostic c umulus kinetic energy (CKE) to relax the quasi-equilibrium closure of the A rakawa-Schubert cumulus parameterization. A parameter, alpha is used to rel ate the CKE to the cumulus mass flux. This parameter is expected to vary wi th cloud depth, mean shear, and the level of convective activity, but up to now a single constant value for all cloud types has been used. The results of the present study show,clearly that this approach cannot yield realisti c simulations of both the diurnal cycle and the monthly mean climate state. Improved results are obtained using a version of the model in which alpha is permitted to vary with cloud depth.