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Orographic rainfall over the Cévennes-Vivarais region

Anquetin Sandrine

At LTHE, a working group focus its research on orographic rainfall in the Cévennes-Vivarais mountainous region (South-East of France). Rainfall is mainly estimated from raingauge and radar data provided by the hydro-meteorological observatory OHMCV (http://www.lthe.hmg.inpg.fr/OHM-CV). To depict atmospheric processes associated with observed rainfall, mesoscale simulations are performed [1]. During the 4 last years, efforts have been made to improve our capacity to objectively compare simulated and observed rainfall [6], and to qualify the simulated rainfall for hydrological use [5, 4].
A climatological study [3] has been based on both recent (2000-2004) and long term (1958-1991) rainfall series. The spatial distributions and the structure of rainfall are analysed at short time steps (1 to 24 hours). It is shown that rain mainly falls over the south-west north-east main mountain ridge and over localized spots on the foothill like around the town of Alès. Extreme rainfall climatology reveals that both the structure and distribution of rainfall differs depending on the time steps. At longest time steps, rainfall is stationary over the main ridge with rainfall rates from 10 to 20 mm.h. At the 1-hour time scale, high rainfall rates (80 to 100 mm.h) strike specific spots along the main ridge and over the foothill. Then, we distinguish between the shallow stationary convection leading to extreme rainfall at longest time steps and the deep one (extreme rainfall at shortest time steps).
Our objective is to study from a climatological point of view, those two kinds of convection and associated rainfall. This already starts by analysing the initial atmospheric conditions leading to shallow convective clouds [2]. It results that the main factors are:
- the wind velocity, direction and shear;
- the mid level humidity;
- the low level layer stability.
The developed methodology would be applied to deep convective events.

[1] Anquetin, S., Miniscloux, F., and Creutin, J. D. Numerical simulation of orographic rainbands. J. Geophys. Res. 108, D8, doi:10.1029/2002JD001593 (2003).
[2] Godart, A. Climatologie des pluies en zone de relief marque - recherche des paramètres atmosphériques associés à la convection peu profonde. Master's thesis, M2R Sciences de la Terre, de l'Univers et de l'Environnement Université Joseph Fourier de Grenoble, 2006.
[3] Molinié, G., Yates, E., Bois, P., Boudevillain, B., Anquetin, S., and Creutin, J. Rainfall regimes in a mountainous mediterranean region: Statistical analysis at short time steps. xx (In preparation).
[4] Yates, E. Convection en région Cévennes-Vivarais : Etude de données pluviométriques, simulations numériques et validation multi-échelles. PhD thesis, ENSHMG, INP, Grenoble, France, November 2006.
[5] Yates, E., Anquetin, S., Ducrocq, V., Creutin, J.-D., Ricard, D., and Chancibault, K. Point and areal validation of forecast precipitation fields. Met. Applications 13, 1 (march 2006), 1-20.
[6] Yates, E., Creutin, J.-D., Anquetin, S., and Rivoirard, J. A scale dependant quality index of areal rainfall prediction. submitted at J. Hydrometeor. (2006b).