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SUMMARYOn 8 and 9 September 2002, an extreme flash-flood event occurred over south-eastern France. This event was simulated with the non-hydrostatic mesoscale model Meso-NH. Several experiments were performed, differing in their initial conditions. A first experiment used the 4D-Var large-scale French operational analysis, as initial state. For the second experiment, the initial state was provided by a mesoscale initialization using surface observations, radar and satellite data. Finally, in the last experiment, only the mesoscale analysis of surface observations was used as initial state. The aim of this study is to assess the validity of the high-resolution simulations for hydrological purposes. Then simulated and observed mean areal rainfall depth, over nine watersheds with surface areas ranging from 200 to 2200 km 2 , were compared. This comparison highlighted the contribution of both initialization procedures to a better location of the mesoscale convective system and to larger amounts of rain, during the first ten hours of simulation. Also, a hydrological model was used to compare stream flows: the simulated and observed rainfall fields were used as input data to a single-event TOPMODEL version. The comparison of the hydrological responses shows that the mesoscale initialization leads to higher peak flows and more realistic flood timing than with the simulation starting from large-scale analysis. The full initialization allowed a simulation of peak flows higher than the ones obtained with the mesoscale surface-observation initialization.

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Abstract. Within the framework of the European Interreg IIIb Medocc program, the HYDROPTIMET project aims at the optimization of the hydrometeorological forecasting tools in the context of intense precipitation within complex topography. Therefore, some meteorological forecast models and hydrological models were tested on four Mediterranean flash-flood events. One of them occured in France where the South-eastern ridge of the French "Massif Central", the Gard region, experienced a devastating flood on 8 and 9 September 2002. 24 people were killed during this event and the economic damage was estimated at 1.2 billion euros.To built the next generation of the hydrometeorological forecasting chain that will be able to capture such localized and fast events and the resulting discharges, the forecasted rain fields might be improved to be relevant for hydrological purposes.In such context, this paper presents the results of the evaluation methodology proposed by Yates et al. (2005) that highlights the relevant hydrological scales of a simulated rain field. Simulated rain fields of 7 meteorological model runs concerning with the French event are therefore evaluated for different accumulation times. The dynamics of these models are either based on non-hydrostatic or hydrostatic equation systems. Moreover, these models were run under different configurations (resolution, initial conditions). The classical score analysis and the areal evaluation of the simulated rain fields are then performed in order to put forward the main simulation characteristics that improve the quantitative precipitation forecast.Correspondence to: S. Anquetin (sandrine.anquetin@hmg.inpg.fr)The conclusions draw some recommendations on the value of the quantitative precipitation forecasts and way to use it for quantitative discharge forecasts within mountainous areas.