Mixing in Thermals with and without Buoyancy Reversal

Abstract

The mixing characteristics of turbulent thermals were investigated in a water tank via laser-induced fluorescence techniques. The concentration of mixed fluid in the far field of a ?classical? thermal is approximately uniform, whereas the near field is dominated by a moderately diluted toroidal core. The effects of atmospheric evaporative cooling on thermals were simulated by using chemicals with nonlinear buoyancy-reversing behavior. Even though the trajectory of an ?evaporating? thermal depends strongly on a buoyancy-reversal parameter, the minimum mixing rate is not greatly modified. A simple derivation, based on the self-similar properties of thermals, provides the minimum molecular-scale mixing rate, which agrees with the measurements. These observations may enhance the understanding of cumulus clouds in weakly stratified environments.