EVALUATION OF PARAMETRIC ASSUMPTIONS FOR SHALLOW CUMULUS CONVECTION

A large-eddy simulation (LES) model has been utilized to study nonprec ipitating shallow convective clouds such as observed during the undist urbed BOMEX period in the trade wind areas. By choosing a realistic la rge scale forcing the authors have been able to simulate shallow conve ctive clouds under quasi-steady-state conditions over a long period of 7 hours. This is a necessary condition to investigate diagnostic cumu lus parameterization schemes since such schemes usually assume steady- state conditions. The response of the model to the applied large-scale forcing compares well with budget study results of BOMEX. In addition , the LES model delivers detailed information concerning the dynamics of shallow convective clouds. This is used to verify basic parameteriz ations of turbulent fluxes and entrainment and detrainment rates used in large-scale models. The most important conclusions are (i) the frac tional entrainment and detrainment rates used in present large-scale a tmospheric models are one order of magnitude too small, confirming pre vious results obtained by Esbensen, and (ii) estimates of turbulent fl uxes by bulk cloud updrafts and environmental downdrafts give an under estimation of 20% to 50% depending on the variable that is transported . Implications of these results for cumulus parameterizations will be discussed.