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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Idealized numerical study of Mediterranean heavy precipitating events : identification of favouring ingredients

Emilie Bresson (Météo-France/CNRM/GAME); Véronique Ducrocq, Olivier Nuissier, Didier Ricard, Céline De Saint-Aubin

During the fall, in the western Mediterranean region, Heavy Precipitating Events (HPE) frequently occur and can cause torrential rain and flash-floods. The meteorological ingredients favoring these events are quite well-known. A slow-evolving synoptic environment associated to conditional convective instability, low-level moist flows from the sea and an orographic barrier are favourable to HPE triggering. Understanding how these ingredients combine and interact to produce more or less precipitation with different locations of the quasi-stationary convective systems (over the mountain, upstream over the plains or the Sea) is still an open question. This study contribute to address this question, based on high-resolution idealized simulations of mesoscale convective systems over Northwestern Mediterranean for which the intensity of the flow, the environmental humidity and the orography vary alternatively. Diagnostic tools as Lagrangian backward trajectories and eulerian passive tracers help also to identify the prominent mechanisms that take part in the triggering and maintenance of quasi-stationary precipitating systems.

The location and intensity of the Mediterranean quasi-stationary mesoscale convective systems depend not only on a unique combination of characteristics of the mesoscale environment in which the systems form and evolve. The same location of the system can be obtained by varying either the flow speed or the moisture. Rapid or moist flows favour precipitation over the mountain range, whereas slower or dryer flows locate the quasi-stationary system upstream over the Sea. The backward trajectories show that the air parcels within the convective cells originate from the very lower levels (below 1000 m)over the Sea. The deflection of the flow by the neighboring mountain range, increased with a slower or a dryer impinging flow, is evidenced by removing the mountain range. This deflection increases the low-level convergence. The presence and intensity of a low-level cold pool beneath the precipitating systems are also favoured by a slower flow or a dryer environment. The MESO-NH sensitivity experiments evidence thus four main low-level mesoscale forcing which combine or compete to explain the intensity and the location of the system: cold pool forcing, orographic lifting, low-level wind convergence and flow deflection. This study helps to identify regions and mechanisms that are important to better document during the HyMeX SOP.