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Triggering and stationarity factors for Heavy Precipitating Events over Southern France


Nuissier olivier

Based on high-resolution modelling with the Méso-NH model, we have studied two cases of quasi-stationary Mesoscale Convective Systems (MCSs) which generated extreme rainfall amount (> 500 mm in 24 hours) on September 8-9th 2002 over Gard and on November 12-13th 1999 over Aude, and an other case less paroxismic but more representative of "cévenol" event (precipitation total ~ 250 mm in 24 hours) on October 13-14th 1995.
The precipitating systems form in a slow-evolving synoptic environment favourable to the development of convective systems (diffluent upper-level southerly flow, PV anomalies,...). At low-levels, a moderate to intense southerly to easterly flow provides conditionally unstable and moist air as it moves over a warm Mediterranean Sea at this period of the year (late summer and fall season). Moreover, backward trajectories and sensitivity experiments show that the Cévennes case is a typical event in which mesoscale orographic forcing is the primary mechanism acting in continuously generating convective cells upwind of Massif Central. The unusual location of the Gard event is explained by a cold pool induced by the evaporation of precipitation which acts as a pseudo-orography to force the conditionally unstable and moist low-level jet to rise. This constitutes a quasi-stationary forcing as the cold pool is blocked in the Rhône valley.