A numerical study of moist stratified flow regimes over isolated topography

Numerical simulations of moist flows over simply shaped, three-dimensional mountains were undertaken using a mesoscale meteorological model, with the purpose of examining the existence of different flow regimes near obstacles and the transitions among them. Moving the system in parameter space resulted in different solutions for a particular set of parameters. This method was adopted to explore numerically 'multiple regimes', stable for at least meteorologically interesting time periods. The evolution of the flow was studied in two different experimental set-ups. In the first case, the height of the obstacle was progressively changed over time. In the second case, the effect of an advective change of the humidity content inside the channel was analysed. In both experiments, a dependence on the history of the flow became apparent. A comparison among cases of obstacles with the same aspect ratio and non-dimensional height, but with different horizontal cross-sections, highlighted the importance of the obstacle geometry in perturbing the upstream flow, favouring the persistence of upstream blocked regions in the case of an arc-shaped mountain. The sensitivity of the reversal flow volume to several control parameters was analysed with additional experiments. In particular, the importance of the cooling associated with the evaporation of precipitation was emphasized.