INFLUENCES OF ISOLATED CUMULUS CLOUDS ON THE HUMIDITY OF THEIR SURROUNDINGS

Measurements are described of the distributions of humidity in the cle ar air surrounding small to medium sized, isolated cumulus clouds. Win d shear is the most important factor in determining the radial distrib ution 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 occ urs on the upshear side. The horizontal extents of the humidity pertur bations in the clear air increase with cloud age. Stable atmospheric l ayers can have a significant effect on the vertical distribution of th e humidity field; enhancements in humidity are generally confined belo w any stable layer. Four detailed case studies are used to evaluate th e performance of the cumulus detrainment criteria suggested by Brether ton and Smolarkiewicz and Taylor and Baker. Although these criteria we re formulated for clouds in an environment with no vertical wind shear , they predict the altitudes of maximum detrainment in four case studi es 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 cumu lus clouds.