A Warm Conveyor Belt Mechanism Accompanying Extreme Precipitation Events Over North-Eastern Italy

A Lagrangian methodology for the reconstruction and the analysis of the airstreams governing the transport of water vapour has been applied to analyse three recent extreme precipitation events over the Alps (3-5 November 1966, 16-18 November 2000 and 24-26 November 2002). The analysis outlines that the precipitation over the North-East of Italy is strongly conditioned by the evaporation rate and by properties of airmasses over the Central Mediterranean area. In particular most of the water vapour contributing to the precipitation ending over the Alps originates over Tunisia, the coastal regions in the western Lybia, the Channel of Sicilia, the Gulf of Gabes and the southerly Thyrrenian Sea. A simplified conceptual model is also proposed to visualize the traits of the moist airstreams inside the Mediterranean cyclones. Two patterns of airstreams can be identified, according to the scheme proposed by Browning and Roberts, 1994. The trajectory population of the two portions of WCB is a crucial factor controlling the precipitation production.