A numerical study of three catastrophic precipitating events over southern France. II: Mesoscale triggering and stationarity factors

Abstract : In the western Mediterranean basin, large amounts of precipitation can accumulate in less than a day when a Mesoscale Convective System (MCS) stays over the same area for several hours. Heavy Precipitating Events (HPEs) in this region (especially southern France) are not only characterized by significant precipitation rates (typically more than 200 mm in less than 24 or 48 hours) but also by quasi-stationary behaviour. The aim of this present study is to use realistic simulations of past events to analyze and better understand the physical mechanisms which lead to the stationarity of HPEs over southern France using a high-resolution (2.5 km) non-hydrostatic mesoscale atmospheric model. We focused on three HPEs: 13-14 October 1995 (the Cévennes case), 12-13 November 1999 (the Aude case) and 8-9 September 2002 (the Gard case). The experimental design for simulating realistic structures and evolutions of the HPEs with the Meso-NH model has been presented in a companion paper. In that study, the evolution of the synoptic patterns in which the HPEs occurred were analysed. In this paper, the importance of mesoscale ingredients, such as the moist and conditionally unstable low-level jet (LLJ), is examined in terms of their ability to focus the deep convection over the same area for several hours. The Cévennes case is typical of a so-called Cévenole event in France for which orographic forcing is the primary mechanism which acts to continuously generate convective cells upwind of the 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 relief-like feature to force the conditionally unstable and moist LLJ to rise. This constitutes a quasi-stationary forcing as the cold pool is blocked in the Rhône Valley. For the third case, diabatic cooling and the Massif Central act to enhance the precipitation but have almost no impact on the location of the anchoring point of the precipitating system.