Role of convection triggers in the simulation of the diurnal cycle of precipitation over the United States Great Plains in a general circulation model

Abstract

Recent comparisons of a number of general circulation models (GCMs) have shown that most of them have deficiencies in the simulation of the diurnal cycle of warm season precipitation. The deficiencies are particularly pronounced over the United States Great Plains where the models generally fail to capture the nocturnal rainfall maximum found in the observations. By using the National Centers for Environmental Prediction's Global Forecasting System (NCEP GFS) GCM, which is unusual in that it produces a realistic nocturnal rainfall signal over the Great Plains, this study examines the nature and realism of the mechanisms responsible for the nocturnal rain in the GCM. A series of sensitivity experiments highlight the importance of triggers implemented in the convection scheme. Specifically, the convection trigger function that the cloud base (defined as the level of free convection) must be within 150 hPa depth from the convection starting level (which crudely represents an upper limit of convective inhibition) plays a key role on the realistic simulation of the diurnal phase of convection. On the basis of this trigger, the nighttime elevation of the convection starting level (defined as the maximum level of moist static energy from the surface) above the boundary layer inversion provides the condition favorable for the development of nocturnal precipitation over the Great Plains. The results are discussed in terms of their implications for improving our understanding and parameterizations of the physical processes that generate nocturnal rain in this and other regions with large diurnal cycles.