High resolution simulations of an intense convective precipitation event

Summary A newly developed non-hydrostatic model (MOLOCH), operating at a resolution of about 2 km, is run for a case of heavy precipitation over southeastern France. The event (8–9 September 2002) was characterized by intense convective activity leading to a severe flash flood in the region of the Gard river, south of the Massif Central. An almost stationary mesoscale convective system (MCS), developing well in advance of an approaching cold front, discharged a huge amount of rainfall over the same area, more than 600 mm in 24 hours. Several simulations are performed in order to test the model set-up, evaluate the sensitivity on different initial conditions, and analyse the case-study. The quantitative precipitation forecasts (QPF) appear to vary widely among the experiments, depending on the initialization time chosen (00, 06, and 12 UTC, September 8). Only the run starting at 06 UTC predicts, with a satisfactory degree of accuracy, the location where the MCS developed and its almost stationary behavior during the first stage (∼12 hours) of the event. In all the simulations, the convective system then propagated northward over the Massif Central. In addition to experiments starting from standard ECMWF analyses, an assimilation procedure, based on Optimal Interpolation, is applied to the initial conditions. Surface observations of temperature, wind and relative humidity have been assimilated. The assimilation produces an improvement in the forecasts of surface fields and leads to a better location of the initial triggering phase. Further experiments, performed by changing the orography in the model, allow the investigation of the role of the Massif Central in triggering the mesoscale convective system and in controlling its evolution.