Colloquium, Thurs., 11 Dec., 3:00 pm, EN6085A (NOTE: THURSDAY)

Observations of the horizontal structure of cumulus clouds

Aircraft observations of shallow to moderately deep cumulus clouds over the tropical Atlantic, over a mountain in Arizona, and over the high plains of Wyoming are analyzed with the purpose to describe the typical horizontal structure of cloud, thermodynamic and kinematic parameters from the middle of the cloud across the cloud edge into the ambient clear air. The analysis of 1624 cumulus penetrations shows that the vertical mass flux, the buoyancy, the buoyancy flux, and the turbulent kinetic energy all tend to reach a minimum near the cloud edge. These variables, and also the liquid water content, the droplet concentration, and the mean droplet size generally decrease towards the cloud edge, starting at about halfway between cloud center and cloud edge, or, in physical space, at about 500 m from the cloud edge. The prevailing subsidence in the cloud margin is interpreted as the result of local negative buoyancy, which is interpreted as the result of evaporative cooling in entraining and detraining eddies. The tendency for a downward mass flux to occur in the cloud margin and in a thin shell of clear air around cumuli, and the apparent dynamical forcing of this flux are consistent with other recent observational and modeling studies, but inconsistent with the accepted view that the environment in a cumulus cloud field subsides uniformly. Two observations give evidence for the concept of a cloud top toroidal ring which contains the least diluted air.