Convective Trigger Function for a Mass-Flux Cumulus Parameterization Scheme

A precipitation physics package for the National Centers for Environmental Prediction Regional Spectral Model designed to improve the skill of precipitation forecasts is proposed. The package incorporates a prognostic grid-resolvable precipitation scheme and a parameterized convection scheme with a convective trigger function that explicitly couples boundary layer and convective precipitation processes. Comprehensive sensitivity experiments were conducted with a grid spacing of approximately 25 km for a heavy rain case over the United States during 15–17 May 1995. In this paper, the trigger function setup in the convective parameterization scheme and its impact on the predicted precipitation are discussed. Special attention is given to the interaction of cloud properties in the parameterized convection with the evolution of grid-resolvable precipitation physics. The impact of convective forcing due to different convective triggers on the large-scale pattern downstream is also discussed. The implementation of the prognostic cloud scheme and performance are presented in a companion paper.