Observations of the Effects of Entrainment and Mixing on the Droplet Size Spectra in a Small Cumulus

Abstract

Sulfur hexafluoride was released at the base of a small nonprecipitating warm cumulus to study cloud mixing and entrainment processes. The tracer gas traveled to the top of the cloud where, during a 2.5 min period, it had mixed to produce a dilute mixture containing 30%, 19% and 51% of air from the original tracer region, an adjacent region of the same cloud, and the environment surrounding the cloud, respectively. The droplet size distributions measured at the top of the cloud represented a mixture of larger droplets that had been growing from the base and smaller, recently activated droplets. The observations suggest that the source region for the small droplets was near cloud top. The large droplet concentration was conserved during the mixing process. These observations are compared with predictions from some recent models for cloud entrainment and droplet evolution.