Collating Airborne and Surface Observations of the Microstructure of Precipitating Continental Convective Clouds

Abstract

The observational results from sailplane flights into the updrafts of developing cumulus clouds in north-eastern Colorado show some important variations in the microstructure of the cloud droplet and ice particle distributions. Some of these variations are apparently caused by the combined interactions of cloud droplets and precipitation particles with the horizontal and vertical components of the updraft and its horizontal and vertical structure. Data from these observations are introduced into a circulation framework in an attempt to understand how the microphysics and the circulation can interact to give the features observed. The results cast doubt on the validity of the often made assumption that the microphysical properties of a cloud are distributed randomly with respect to each other on the smaller scales, and that this condition exists uniformly throughout the cloud. The observed precipitation shafts with bimodal size distributions in the middle and lower parts of a cloud can be recreated in a two-dimensional simulation of the observed cloud air circulation with embedded microphysics. The observed and calculated frozen water content can increase by one to two orders of magnitude over the liquid water content when moving from cloudy air into a precipitation shaft. The observed change in concentration with height of the ice particles exceeds (by over two orders of magnitude) the expected ice nuclei concentration usually found in the atmosphere at comparable temperatures. The average concentrations of ice particles observed occasionally exceed 400 l?1.