ENTRAINMENT AND MIXING IN BUOYANCY-REVERSING CONVECTION WITH APPLICATIONS TO CLOUD-TOP ENTRAINMENT INSTABILITY

A consensus seems to exist throughout the cloud-physics community that buoyancy reversal associated with evaporative cooling affects not onl y the global (cloud-scale) dynamics of a convective cloud, but also th e rate of mixing between the cloud and its environment. The latter eff ect is associated with the concept of the so-called 'cloud-top entrain ment instability' (CTEI), which assumes a positive feedback between bu oyancy reversal and the rate of entrainment. In this paper, effects of buoyancy reversal on cloud dynamics are discussed in the context of a n unstratified anelastic two-fluid system. Convection in this system m imics some essential features of cumulus convection. Two-dimensional n umerical experiments, with and without the effects of buoyancy reversa l, have been performed. It was found that buoyancy reversal has a dram atic impact on the overall flow evolution, but that its effect on the rate of mixing between the two fluids is small, i.e. flows which diffe red dramatically depending on whether there was buoyancy reversal or n ot still resulted in similar amounts of mass being mixed. This result casts doubt on the concept of CTEI in cumulus dynamics. The distinctio n between strong global effects of buoyancy reversal and minor effects on the rate of mixing in buoyancy-reversing convection is supported b y laboratory experiments with classical and buoyancy-reversing thermal s.