Entrainment due to a thermal impinging on a stratified interface with and without buoyancy reversal

The aim of the present paper is to understand the interaction between a rising thermal and an inversion in the atmosphere and to quantify the turbulent entrainment rate due to a thermal impingement on a stratified interface. This problem was simulated in the laboratory using a water tank. The thermal is created by releasing a small volume of buoyant fluid into a stratified environment composed of two layers of different densities. A thin interface separates the lower layer from the lighter upper layer. The entrainment of upper layer fluid into the thermal is investigated using a passive dye flow visualization technique. The entrainment rate is found to obey a Ri power law, as predicted by Cotel and Breidenthal [1997]. The effect of simulated evaporative cooling on the entrainment of a thermal impinging on a stratified interface is also investigated experimentally. Evaporative cooling in atmospheric clouds is simulated in the laboratory using alcohol-water mixtures, so that the mixed fluid is denser than either parent parcel. This is realized in the laboratory by releasing a mixture of ethyl alcohol and ethylene glycol in an aqueous solution. It rises first through a relatively dense lower layer fluid and then impinges on a thin stratified interface, above which is a layer of relatively light fluid. The entrainment rate for values of the buoyancy reversal parameter D* between 0 and 0.5 was found to obey a Ri-(3/2) power law. The entrainment rate is independent of D* between 0 and 0.5 for a range of Richardson numbers Ri from 3 to 25. This is consistent with the behavior of the buoyancy-reversing thermal in an unstratified environment observed by Johari [1992].