CUMULUS ENTRAINMENT, FINE-SCALE MIXING, AND BUOYANCY REVERSAL

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.