Ww. Grabowski, CUMULUS ENTRAINMENT, FINE-SCALE MIXING, AND BUOYANCY REVERSAL, Quarterly Journal of the Royal Meteorological Society, 119(513), 1993, pp. 935-956
The formation of buoyancy reversal due to entrainment of dry environme
ntal air, and its implication for cumulus dynamics, are discussed. Con
cepts originating from laboratory experiments with reacting turbulent
flows, and from numerical simulations of homogeneous and isotropic tur
bulence, are applied to distinguish between large-scale entraining edd
ies developing at the interface. subsequent development of smaller-sca
le motions, and final homogenization by microscale processes. The phys
ics of the microscale homogenization by molecular diffusion and sedime
ntation of cloud droplets is discussed. It is shown that, as a result
of droplet sedimentation, much smaller negative buoyancy (buoyancy und
ershoots) may be generated on cloud microscale as compared with the va
lue predicted by a classical nonlinear mixing diagram of cloudy and cl
oud-free air. Highly idealized numerical experiments aimed at simulati
ng the temporal evolution of a small convective cloud were performed w
ith and without the effects of molecular mixing that results in buoyan
cy reversal. It is argued that these experiments provide two limiting
cases, and that the dynamics of a real cumulus cloud is located somewh
ere between them. The major effect of buoyancy reversal as suggested b
y these experiments is to increase the intermittency associated with c
loud evolution. The temporal variation of cloud-top height, maximum up
draughts, and minimum downdraughts increases significantly when buoyan
cy reversal is allowed. It is argued that results of numerical experim
ents, together with other laboratory and theoretical studies, cast ser
ious doubts on the concept of cumulus entrainment being driven by the
cloud-top entrainment instability.