ON THE FORMATION AND STRUCTURE OF DOWNDRAFTS IN CUMULUS CLOUDS

Abstract

With use of the model of the steady-state entraining cumulus draft, it is possible to describe the formation and structure of the downdraft observed at the down-shear edge of an oceanic trade-cumulus cloud. The observations are airplane traverses made at several levels giving the temperature, humidity, and vertical draft structure in cloud and environment, and a nearby pilot-balloon run. It is found that this downdraft must be mixing primarily with the updraft; that it gains its negative buoyancy mainly from evaporation; and that it is permitted to go through a life cycle of formation, strengthening, and decay which is closely dependent upon that of the updraft.