Role of moisture patterns in the backbuilding formation of HyMeX IOP13 heavy precipitation systems

Mediterranean regions are regularly affected by heavy convective precipitation. During the Hydrological Cycle in the Mediterranean Experiment Intensive Observation Period 13 (HyMeX-IOP13), the multi-platform observation strategy allowed analysis of the backbuilding convective systems that developed on 14 October 2012, as well as the associated moisture structures in the environment upstream of convection. The numerical simulation at 2.5 km horizontal resolution succeeds in reproducing the location and time evolution of the observed heavy precipitation systems and the main characteristics of the marine air mass. Convection develops in Southeastern France over the foothills closest to the coast when a moist conditionally unstable marine boundary layer topped by particularly dry air masses is advected inland. Cold air formed by evaporative cooling under the precipitating cells flows down the valleys, slowly shifting the location of the backbuilding convective cells from the mountainsides to the coast and over the sea. Surface observations confirm that these simulated backbuilding mechanisms describe the processes involved in maintenance of the heavy precipitation event realistically. A Lagrangian analysis shows that the moisture supply to the convective system is provided by the moist conditionally unstable marine boundary layer, while the dry air masses above are involved in cold-pool formation. Four days before the event, both dry and moist air masses come from the Atlantic Ocean in the lower half of the troposphere. The dry air mass involved in cold-pool formation results from both the advection of mid-level air masses and the drying of low-level air masses lifted up over Spain. For the moist air mass feeding the backbuilding convective systems, most of the air parcels overpass France before travelling almost 48 h in the lowest 1000 m above the Mediterranean. About 50% of the moisture supply to the precipitating system originates from evaporation over the sea.