Generalization of Albrecht's Cumulus Cloud Amount Parameterization

Abstract

A diagnostic parameterization of cumulus cloud amount based on the decay of passive cloud material is reinvestigated. It is shown that assumptions previously made in the derivation may be relaxed by taking into account the heat budget of the decay process. The resulting parametric expression for cloud amount is a function of the ratio between initial cloud liquid water content and ambient saturation deficit. Unlike the original formulation, it is valid also for negatively buoyant clouds, which frequently form as a result of penetrating boundary layer thermals. It is shown that within the framework of the parameterization the effect on cloud amount of a constant rate of longwave cooling is equivalent to a reduced ambient saturation deficit.