Buoyancy reversal in the cloud-top mixing layer

Abstract

Results from theoretical and numerical studies of the cloud-top mixing layer with buoyancy reversal due to evaporative cooling in a small size system (of the order of 10 m) will be presented. An ideal two-layer configuration has been defined following previous literature and a linear stability analysis has been performed, showing the buoyancy reversal instability and providing a reinterpretation of the buoyancy reversal parameter D in terms of the ratio between the stable and the unsatble time-scales. A high-order numerical scheme has been employed to investigate the nonlinear regime, and results suggest that a turbulent state in the lower layer can develop from this instability for cases D << 1 typical of stratocumuli, without additional external forcing. Three-dimensional simulations confirm this result. It is also observed that mixing with the upper layer occurs mainly through laminar diffusion, without engulfing pure upper fluid, and the turbulent lower layer is capped by the inversion. Characterization of the resulting turbulent fields from three-dimensional simulations is currently in progress.