Cumulus Convection and Larger Scale Circulations II. Cumulus and Mesoscale Interactions

Abstract

This paper (part II) is the second of two papers dealing with cumulus and broader scale flow interaction. The first (part I) is a companion paper that deals with this topic from broad-scale considerations. The second (part II) deals with this topic from the cumulus scale point of view. A numerical model of individual convective clouds has been used to investigate the effects of a typical population of cumulus clouds on the large-scale features of a tropical disturbance or cloud cluster. The typical cloud population has been determined from U.S. Navy aircraft reconnaissance radar data in the tropical Atlantic Ocean. This study shows that the detrainment of cloud mass from the population produces net cooling of the air around the clouds. This occurs because the cooling produced by the evaporation of the detrained cloud liquid water over-compensates for the detrainment of sensible heat excess. A heat and moisture budget for a steady-state tropical cloud cluster has been computed using the typical cloud population. A mass budget of the cluster reveals that the vertical circulation required to explain typically observed rainfall rates has a magnitude many times larger than the synoptic mass circulation. The synoptic scale vertical mass circulation is, in fact, a small residual between both the much larger upward mass transport by the clouds and the downward mass transport induced in the cloud-free region around the clouds.