The Use of Partial Cloudiness in a Warm-Rain Parameterization: A Subgrid-Scale Precipitation Scheme

Abstract

A method is proposed on how to handle the effects of partial cloudiness in a warm-rain microphysical scheme and how to generate subgrid-scale precipitation. The method is simple and concerns essentially two ideas: the use of the vertical distribution of the partial cloudiness and the use of environmental and cloud-scale values for the thermodynamic variables instead of their grid-mean values. It applies to any microphysical scheme. Here, the method has been applied to a warm-rain parameterization scheme that has been implemented in a mesoscale model using a statistical partial cloudiness scheme. Numerical tests have been done for two one-dimensional cases of boundary-layer cloudiness: a cumulus case and a case of a decoupled stratocumulus layer. The results show that the correct coupling of a partial cloudiness scheme and a microphysical scheme allows for a better description of the actual cloudiness and precipitation fields by ensuring a consistent computation of partial cloudiness, cloud water, and rainwater in partly cloudy regions.