The role of vertically varying cloud fraction in the parametrization of microphysical processes in the ECMWF model

General-circulation models (GCMs) have generally treated solely the radiative impacts of vertically varying cloud fraction by using a cloud-overlap assumption. In this study, the microphysical impacts of vertically varying cloud fraction are addressed by developing a subgrid-scale precipitation model which resolves the vertical variation of cloud fraction. This subgrid model subdivides the grid boxes into homogeneous columns which are either clear or completely cloudy. By comparing the column-averaged microphysical quantities from the subgrid-scale precipitation model with the parametrization in the European Centre for Medium-Range Forecasts (ECMWF) model, the ability of the ECMWF model to account for the subgrid nature of cloud and precipitation microphysics is assessed. It is found that the ECMWF model overestimates the evaporation of precipitation in the tropical mid-troposphere. This results from (a) an incorrect parametrization of the area of the grid box covered by precipitation, and (b) the inadequacy of assuming a single value for the precipitation rate in the grid box. In addition to assessing the ability of the ECMWF model to parametrize the subgrid nature of cloud microphysics, the subgrid precipitation model is used to show that the cloud-overlap assumption has a large impact on the evaporation of precipitation. In light of the current trend towards more sophisticated cloud and precipitation parametrizations in GCMs, more attention should be paid to the impact of vertical cloud-fraction variations on the parametrized microphysics.