Laboratory experiments on the cloud-top entrainment instability

The stability of stratocumulus clouds with strong evaporative cooling effects is explored in laboratory simulations. Two fluids, initially separated by a thin horizontal plate, contain mixtures of water, alcohol and glycol which have a strongly nonlinear density as a function of mixture ratio. Initially, the fluid below the plate is more dense than that above the plate. When the plate is suddenly withdrawn, the turbulence in its wake mixes the two fluids together, producing mixtures with densities greater than that of either initial fluid. It is found that the system is unstable to strong perturbations only in cases of relatively large buoyancy reversal. The system is stable to strong perturbations if the buoyancy reversal is comparable to or less than the initial stratification. A simple model is presented to explain the results.