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 only the global (cloud‐scale) dynamics of a convective cloud, but also the rate of mixing between the cloud and its environment. The latter effect is associated with the concept of the so‐called ‘cloud‐top entrainment instability’ (CTEI), which assumes a positive feedback between buoyancy reversal and the rate of entrainment. In this paper, effects of buoyancy reversal on cloud dynamics are discussed in the context of an unstratified anelastic two‐fluid system. Convection in this system mimics some essential features of cumulus convection. Two‐dimensional numerical experiments, with and without the effects of buoyancy reversal, have been performed. It was found that buoyancy reversal has a dramatic 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 differed dramatically depending on whether there was buoyancy reversal or not still resulted in similar amounts of mass being mixed. This result casts doubt on the concept of CTEI in cumulus dynamics. The distinction between strong global effects of buoyancy reversal and minor effects on the rate of mixing in buoyancy‐reversing convection is supported by laboratory experiments with classical and buoyancy‐reversing thermals.