A parametrization of the effects of cloud and precipitation overlap for use in general-circulation models

The necessity for treating the effects of vertically varying cloud fraction when parametrizing microphysical processes in general-circulation models (GCMs) was recently highlighted by Jakob and Klein. In this study a parametrization to include such effects in a GCM is developed, and the new scheme is applied in the ECMWF global model. The basic idea of the new scheme is to separate the model's rain and snow fluxes into a cloudy and a clear-sky part. The scheme is tested using the subgrid-scale precipitation model of Jakob and Klein as a benchmark. The impact of the new scheme on the model climate is also investigated. It is shown that the new parametrization leads to a better representation of the effects of cloud and precipitation overlap, and that it alleviates most of the problems connected with their treatment in the current scheme. Due to the better treatment of cloud and precipitation overlap the new parametrization leads to a reduction in precipitation evaporation and an increase in accretion rates. When tested in seasonal model integrations the new scheme produces a drier tropical mid-troposphere with consequences for the hydrological cycle.