| Abstract: 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-troposophere. 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 micro-physics, 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. |