L’événement des 8-9 septembre 2002 : situation météorologique et simulation a mésoéchelle

The 8-9 September 2002  extreme flash-flood : Meteorological description and mesoscale simulations

Résumé

La situation extrême des pluies diluviennes du 8-9 septembre 2002 fait l’objet d’une étude a posteriori avec le modèle de recherche MESO-NH utilisé à une résolution de quelques kilomètres. Après une brève description du contexte et de l’événement météorologique qui ont conduit aux pluies exceptionnelles, le potentiel de la modélisation à haute résolution pour la prévision quantitative des précipitations est examiné à l’aide de différentes expériences de sensibilité aux conditions initiales et de surface.

Dans un second temps, la modélisation à haute résolution est utilisée comme outil de retour d’expérience pour mieux comprendre les phénomènes météorologiques à l’origine des fortes précipitations.

Abstract

The Southeastern France is prone to heavy precipitating events. A major event has occurred the 8-9th September 2002, with maxima of accumulated rainfall totals having reached locally 600-700 mm; there were more than 20 deaths. The high resolution non-hydrostatic model MESO-NH (2.4 km grid-mesh) has been run a posteriori on this case. It has been found that the high resolution simulation may significantly improves the quantitative precipitation forecast of the event. A sensitivity study to initial conditions has been conducted and shows clearly that mesoscale data assimilation is the most efficient to improve the short range forecast : assimilation of mesonet surface observations, radar and satellite data helps to improve significantly the location of the precipitation maximum.

The high resolution model has been also used to study the event, and in particular to understand the relative unusual location of the largest precipitation over the upwind lower mountainous area of the Gard department rather than over the Massif Central crests. The location of the most active convection is explained by a cold pool beneath the simulated storm that faced the low-level moist and unstable southeastern flow from Mediterranean sea. This density current, produced by the evaporation, sublimation and melting of precipitation, forced the unstable air to lift, generating therefore continuously new convective cells at the leading edge of the cold pool.