A case of convection development over the western Mediterranean Sea: A study through numerical simulations

A convective case producing heavy precipitation in the western Mediterranea n region, characterized by pronounced upper level forcing and main rainfall over the sea, is studied. On the day of the event (September 28(th) 1994), more than 140 mm of precipitation were recorded in coastal lands of easter n Spain, and 180 mm were estimated over the sea with radar data. Synoptical ly, the case appears to combine warm and moist easterly advection at low le vels, typically observed in torrential rainfall events of the region, with a less common strong upper level dynamical forcing. A set of mesoscale nume rical simulations using the Hirlam model is performed to investigate the me chanisms responsible for the convection development, and to assess the infl uence of the orography on the rainfall field. Model output diagnosis indica tes that in addition to the lower level forcing, a two-jets interaction is decisive for the triggering and driving of the convection during the event. Moreover, a non-topographic simulation reveals a relatively weak influence of the orography on this event when compared with other similar heavy prec ipitation cases in eastern Spain. Previous studies have shown an orographic influence of more than 90% on the rainfall whereas in this case about 50% of the precipitation over the area is attributed to the orographic forcing. The study is extended with an analysis of the individual effects of the At las and Iberian Peninsula, by means of a factor separation technique. It is shown that the Atlas range induces a redistribution of the precipitation o ver the Mediterranean, whereas local enhancements can be attributed to the Iberian topography.