Orographic influence on deep convection: case study and sensitivity experiments

The non hydrostatic convection resolving model MOLOCH is employed in order to evaluate its capability to realistically simulate the evolution of the mesoscale convective system responsible for an episode of extremely heavy rainfall and flood over southeastern France (Gard event, 2002). Numerical experiments indicate large sensitivity of modelled precipitation amounts and distribution, due to different cell organization and propagation, to the specification of the initial conditions. Among different experimented initialization times (00, 06 and 12 UTC, September 8) the run starting at 06 UTC is able to predict the development and the almost stationary behaviour of the convective system, at least in the early stage of the event. Additional experiments, aimed at studying the role played by the orography in triggering the convection and controlling its evolution, have been performed in order to characterize sensitivity to ambient wind and orography. Simulations demon- strate that the presence of the orographic barrier is crucial for both triggering and maintaining the mesoscale convective system. Moreover, the location and intensity of precipitation turns out to be sensitive to variations of the mountain height and of the mean meridional wind component. A partial explanation of this behaviour in terms of the Froude number is suggested.