Analysis and simulation of local and regional conditions for the rainfall over the Lago Maggiore Target Area during MAP IOP 2b

The Mesoscale Alpine Programme (MAP) operational and research network collected a large variety of measurements during the two days of Intensive Observing Period (IOP) 2b (19 and 20 September 1999), which is one of the most intense rainfall episodes during the Special Observing Period over the Lago Maggiore area. The synoptic situation is a typical one of a southerly flow and associated convection over Alpine orography. Simulations with the non-hydrostatic Meso-NH model are used to describe the rainfall conditions and are successfully validated at the different scales allowed by the MAP data. The two types of pre-frontal orographically triggered convective episodes are described: (1) Sporadic and shallow convection occurs over the coastal mountains and propagates over the Po Valley. The chronology and the intensity of these convective episodes are driven by two main features: the low-level humidity anomalies and the middle-troposphere dry anomaly present over the Mediterranean Sea. (2) The Lago Maggiore orographic convection has a very long duration and is mainly controlled by both the intensity of the easterly low-level wind in the north part of the Po Valley and the upper-level clouds of the cold front. The different origins of the air masses transported by this easterly barrier wind and the associated local circulations are detailed.