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Abstract:
Numerous studies based on observations and large-eddy simulation (LES) have shown that cumulus clouds are typically composed of one or more ascending large thermals (“large” meaning a thermal size similar to the width of the cloud as a whole). Here, mechanisms driving entrainment in cumulus thermals are examined and contrasted with entrainment in dry thermals. Dry thermals entrain mainly by a process of baroclinic vorticity generation which results from their initial buoyancy becoming concentrated near the thermals’ rotation centers, while they undergo little detrainment. As a result, net entrainment of environmental fluid is driven by an organized flow into the thermals, and they grow in volume as they rise. A similar picture holds for both laminar and turbulent dry thermals, suggesting that smaller-scale turbulent eddies are relatively unimportant in driving entrainment. The nature of entrainment for moist cumulus thermals is substantially different mainly because of condensation and latent heating in their cores. This limits the spreading induced by baroclinic generation of vorticity and thus entrainment efficiency is only about one-half that for dry thermals, all else being equal. This result is consistent with LES showing that cumulus thermals undergo little increase in size as they ascend. Thus, the dilution that does occur in cumulus thermals is mainly associated with entrainment (inflow) balanced by detrainment (outflow), presumably driven by smaller-scale turbulent eddies, in contrast to the situation for dry thermals. A conceptual model based on these ideas will be presented, as well as implications for representing entrainment in cumulus parameterizations.
