Origin of the moisture feeding the Heavy Precipitating Systems over Southeastern France

Abstract. In the Northwestern Mediterranean region, large amounts of precipitation can accumulate over the coasts in less than a day. The present study aims at characterising the origin and the pathways of the moisture feeding such heavy precipitation. The ten Heavy Precipitating Events (HPEs) that occurred over the French Mediterranean region during the autumns of 2008 and 2009 are simulated with the non-hydrostatic research numerical model Meso-NH at 2.5 km, 10 km and 40 km horizontal resolution.

Using eulerian on-line passive tracers, high-resolution simulations (2.5 km horizontal resolution) show that the heavy precipitating systems are fed by a south-southwesterly to easterly low-level moist flow. It is typically 1000 m deep and remains almost unchanged all along an event. This low-level feeding flow crosses the most northwestern part of the Mediterranean in 5 to 10 h.

Larger-scale simulations (40 km and 10 km horizontal resolution) show that the moisture of the low-level feeding flow is provided by both evaporation of the Mediterranean Sea within the last 2 days before the HPE triggering and transport from remote sources in the lower half of the troposphere over more than 3 to 4 days. Local Mediterranean moisture is gained along the air mass low-level progress towards the Northwestern Mediterranean basin following two main branches along the Spanish coast and west of Sardinia. The Mediterranean Sea is the main moisture source when anticyclonic conditions prevail during the last 3 or 4 days before the HPE. When cyclonic conditions prevail before the HPE, the relative contribution of local and remote sources is more balanced. Remote moisture comes most of the time from the Atlantic Ocean. African tropical moisture is a less frequent but larger remote source.