Regimes in moist stratified flows over isolated topography: Numerical experiments

Moist flows over simply shaped 3D mountains have been studied in numerical simulations made with a mesoscale meteorological model. Our aim is to examine the possible existence of multiple solutions, searching for different solutions depending on the path followed by the system in the parameter space. Results from three different sets of experiments are discussed here. In the first set of simulations, the height of the mountain has been progressively changed in time. In the second group of experiments, the humidity of the air flowing over the obstacle has been increased in time by adding a source term to the equation of evolution of moisture. The case of advection of moist air towards an obstacle, initially embedded in dry air, has been studied as a third type of flow. A dependence on the past history of the flow seems to characterise some types of system evolution, leading to different flow regimes over the obstacle. The experiments indicate that this result is mainly a consequence of changes of state of water, associated with the presence of humidity in the atmosphere. These effects axe emphasised in the case of an elliptical mountain, with its longer axis perpendicular to the main flow. In the three different sets of experiments presented here, evident differences with simulations where flow parameters are kept constant from the beginning persist in the flow regimes, also for periods of time much longer than the characteristic time of evolution towards stationary solutions.