Intercomparison of Model Simulations of Cloudy Rayleigh-Bénard Convection in a Laboratory Chamber

The Pi Chamber at the Michigan Technological University is a cloud chamber (1 m high x 2 m x 2 m) that can support cloudy Rayleigh-Bénard convection. Supersaturation is produced by turbulent mixing of air saturated at the (cold) top and (warm) bottom wall temperatures. Droplets grow on continuously injected aerosol particles, and eventually fall out due to sedimentation. A statistically steady state is obtained for steady boundary conditions. By varying the aerosol injection rate, the injected aerosol sizes, and the temperature gradient in different experiments, a range of droplet size distributions (DSDs) is obtained. Theoretical analyses demonstrate that the DSD shape (i.e., the droplet size PDF) depends on both the mean and the variance of the supersaturation (SS). However, the PDFs for the extreme cases of no mean SS or no variance of SS do not differ much, which makes experimental discrimination between cases difficult based on the PDF alone. To help elucidate the relative roles of mean SS and variance of SS, a model intercomparison study has been performed as part of the 10th International Cloud Modelling Workshop. Seven models were involved: Three large-eddy simulation models, two direct numerical simulation models, and a 1D linear eddy model.