A Study of the Broadening of Droplet Size Distributions in Cumuli

Abstract

Cloud droplets may take a variety of paths to reach some location in a cloud and therefore may encounter varying growth conditions. This paper examines the effect of these variations on broadening of the droplet size distribution in cumuli. Diffusional growth of a collection of droplets is studied in detail using measurements obtained during the Cooperative Convective Precipitation Experiment (CCOPE) from the University of Wyoming's King Air research aircraft. A technique is described by which these measurements may be converted to the frame of reference of a moving droplet. Entrainment processes that are associated with variations in vertical velocity appear to have the effect of intensifying fluctuations in integral radius, which in turn produce variations in the supersaturation encountered by the droplets and thus opportunities for variations in growth. Contributions to spectral broadening arise from turbulent structures in the cloud, producing variations in integral radius as well as in the correlation between integral radius and vertical velocity. A relation among these variations and the resultant spectral broadening is developed and applied to measurements in growing cumuli. In unmixed updraft, the resulting spectral broadening is small. Calculations based upon measurements obtained in mixed parcels produce reasonable qualitative and quantitative agreement with the observed dispersions. The relation of recent studies on spectral broadening to this work is explored.