Dynamics of the Cumulus Cloud Margin: An Observational Study

Aircraft observations of shallow to moderately deep cumulus clouds are analyzed with the purpose of describing the typical horizontal structure of thermodynamic and kinematic parameters near the cumulus margin from the cloud center into the ambient clear air. The cumuli were sampled in a broad range of environments in three regions: the tropical Atlantic Ocean in winter, the Sonoran Desert during the monsoon, and the arid high plains of Wyoming in summer. The composite analysis of 1624 cumulus penetrations shows that the vertical mass flux, temperature, buoyancy, the buoyancy flux, and the turbulent kinetic energy all tend to reach a minimum near the cloud edge. Most of these variables, and also the liquid water content, the droplet concentration, and the mean droplet size, generally decrease in value from within the cumulus toward the cloud edge, slowly at first and rapidly close to the cloud edge. These findings corroborate recent observational and modeling studies and provide further evidence for significant evaporative cooling in laterally entraining and detraining eddies in the cloud margin, a transition zone within similar to 200 m (or similar to 10% of the cloud diameter) of the cloud edge. This cooling explains the tendency for downward accelerating, buoyantly driven subsidence in the cloud margin.