Abstract : 1C.6
Mediterranean synoptic scale ingredients involved in heavy precipitating events triggering over southern France: a clustering approach

Bruno Joly, Olivier Nuissier, Véronique Ducrocq, Alain Joly
GAME/CNRM (Météo-France, CNRS)

In this study, we focus on the large scale environments propitious to Heavy Precipitating Events (HPE) over the southeastern mountainous region of France. Firstly, the method of automatic classification is applied to investigate the synoptic variability of the climatological period of such events, the fall season (September to December). The data comes from the ERA40 reanalysis database of the ECMWF for the whole period (1957-2001). A seven Weather regimes classification of the 500hPa geopotential height (Z500) has then been found out to be significant. A basic contingency analysis shows that none of the regimes can be singularly associated to the majority of the typical rainfall events of the region. Thus, a classification of the rainfall events themselves has been undertaken.

A set of a thousand precipitating events over southern France has then been sampled from the standard rainfall series of Meteo France. Their related Z500 are found out to parse in four classes for which the corresponding spatial rainfall distributions are markedly set apart. This is underlining that the Z500 and the rainfall structures are noticeably connected. The most frequent Z500 class includes 50% of the HPEs with a corresponding maximum rainfall composite over the Massif Central foothills. It consists in a trough just west of France and a ridge downstream, combined with a strong sourtherly low-level jet carrying warm and moist air over the region.

This study has allowed to draw up the features of the large scale conditions governing the main part, particularly the most intense one, of the high precipitating events over the Southern France.