The Effects of Cloud Detrainment on the Diagnosed Properties of Cumulus Populations

Richard H. Johnson National Hurricane and Experimental Meteorology Laboratory, NOAA, Coral Gables, Fla. 33124

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Abstract

Lateral detrainment from cumulus updrafts and its effect on the properties of cumulus cloud populations (as determined from large-scale observations) are examined. This detrainment, which can be related to the cumulus life cycle, has been specified for a spectrum of cloud sizes by considering initially the character of detrainment for the two extremes of convection: deep cumulonimbi and shallow cumuli. The main purpose is to determine for some reasonable assumption of individual cloud detrainment to what extent diagnosed cloud ensemble properties differ from those given by applying a steady-state plume model for convective updrafts and downdrafts.

Application of the model to tropical western Pacific data indicates, as earlier studies have shown, that in convectively disturbed situations a bimodal cloud population exists of predominantly deep and shallow cumuli. However, the contribution to the total cloud-base mass flux from deep cumuli is increased and from shallow cumuli decreased somewhat when the effects of lateral detrainment are taken into account. Convective downdrafts maintained by precipitation evaporation are found to contribute in an important way to the total convective mass flux regardless of whether side detrainment from updrafts is included or not.

Abstract

Lateral detrainment from cumulus updrafts and its effect on the properties of cumulus cloud populations (as determined from large-scale observations) are examined. This detrainment, which can be related to the cumulus life cycle, has been specified for a spectrum of cloud sizes by considering initially the character of detrainment for the two extremes of convection: deep cumulonimbi and shallow cumuli. The main purpose is to determine for some reasonable assumption of individual cloud detrainment to what extent diagnosed cloud ensemble properties differ from those given by applying a steady-state plume model for convective updrafts and downdrafts.

Application of the model to tropical western Pacific data indicates, as earlier studies have shown, that in convectively disturbed situations a bimodal cloud population exists of predominantly deep and shallow cumuli. However, the contribution to the total cloud-base mass flux from deep cumuli is increased and from shallow cumuli decreased somewhat when the effects of lateral detrainment are taken into account. Convective downdrafts maintained by precipitation evaporation are found to contribute in an important way to the total convective mass flux regardless of whether side detrainment from updrafts is included or not.

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