The Role of Convective-Scale Precipitation Downdrafts in Cumulus and Synoptic-Scale Interactions

Abstract

A diagnostic model has been developed to determine the contribution by convective-scale precipitation downdrafts to the total cumulus fluxes of mass, heat and moisture an the synoptic scale. Populations of cumulus clouds are assumed to consist of individual cloud elements of various sizes, each possessing an updraft and downdraft that are modeled as steady-state, entraining plumes. An integral equation for a function representing the cloud base mass flux for each type of cloud is derived based on an assumption regarding the relative intensities of the updraft and downdraft. The model is tested on the Reed and Recker (1971) western Pacific composite easterly wave data and on a heavy-rain-producing tropical depression that occurred over northern Florida in 1969. Results from both studies indicate that cumulus downdrafts contribute significantly to the total convective mass transport in the lower troposphere. It is shown that the neglect of cumulus downdrafts and their associated rainfall evaporation leads to the diagnosis of excessively large populations of shallow cumulus clouds in highly convective situations. Analysis of the subcloud-layer moisture budget shows downdraft water vapor transport to be important in the water vapor balance for this layer. The implications of the findings of this diagnostic study on theories for the parameterization of cumulus convection are discussed.