Abstract
Measurements are described of the distributions of humidity in the clear air surrounding small to medium sized, isolated cumulus clouds. Wind shear is the most important factor in determining the radial distribution of enhanced humidity around cumulus clouds. Significant humidity enhancements occur in the clear air on the downshear and cross-shear sides of cumulus clouds; comparatively little humidity enhancement occurs on the upshear side. The horizontal extents of the humidity perturbations in the clear air increase with cloud age. Stable atmospheric layers can have a significant effect on the vertical distribution of the humidity field; enhancements in humidity are generally confined below any stable layer.
Four detailed case studies are used to evaluate the performance of the cumulus detrainment criteria suggested by Bretherton and Smolarkiewicz and Taylor and Baker. Although these criteria were formulated for clouds in an environment with no vertical wind shear, they predict the altitudes of maximum detrainment in four case studies where wind shear was present. However, wind shear had a significant effect on the locations and depth of the detrained air. A conceptual model is proposed for the processes that appear to be responsible for the patterns of humidity enhancement in the clear air surrounding cumulus clouds.
