Entrainment and Detrainment in Cumulus Clouds

Gregory R. Taylor Department of Geology and Physical Sciences, California State University at Chico, Chico, California

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Marcia B. Baker Geophysics Program, University of Washington, Seattle, Washington

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Abstract

Vertical redistribution of air and its properties inside convective clouds can be studied by standard thermodynamic analyses (Paluch and saturation point diagrams) if the clouds are nonprecipitating and ice free. It is shown from such analysis that the linear patterns often seen in these diagrams can result from continued “lateral” entrainment. The observations are consistent with the hypothesis that buoyancy sorting is an important factor in determining cloud composition. This yields a simple, graphical means for estimating the thermodynamic properties of in-cloud air at any pressure.

Field observations of detrained fluxes near cumulus clouds reveal the existence of rather thin detrainment layers which can occur anywhere over the cloud layer. These observations are shown to be consistent with the conclusion of Bretherton and Smolarkiewicz that detrainment is governed by vertical gradients of cloud buoyancy. This allows prediction of detrainment regions from cloud base temperature and pressure and the environmental sounding.

Abstract

Vertical redistribution of air and its properties inside convective clouds can be studied by standard thermodynamic analyses (Paluch and saturation point diagrams) if the clouds are nonprecipitating and ice free. It is shown from such analysis that the linear patterns often seen in these diagrams can result from continued “lateral” entrainment. The observations are consistent with the hypothesis that buoyancy sorting is an important factor in determining cloud composition. This yields a simple, graphical means for estimating the thermodynamic properties of in-cloud air at any pressure.

Field observations of detrained fluxes near cumulus clouds reveal the existence of rather thin detrainment layers which can occur anywhere over the cloud layer. These observations are shown to be consistent with the conclusion of Bretherton and Smolarkiewicz that detrainment is governed by vertical gradients of cloud buoyancy. This allows prediction of detrainment regions from cloud base temperature and pressure and the environmental sounding.

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