Back-tracking water vapour contributing to a precipitation event over Trentino: a case study

The relation between individual precipitation patter-is over an Alpine target area and the origin of the water vapour which contributes to them is investigated. Back-trajectories are calculated, starting from selected ending points over the target area, by means of the Lagrangian model FLEXTRA, on the basis of meteorological fields provided by ECMWF 4d-Var analysis data. Suitable clustering procedure is devised to identify representative flow patterns from calculated trajectories. The procedure is applied to a recent Precipitation event (24-27 November 2002) adopted as a test case. The analysis allows to identify the contribution of various air streams to the precipitation, as well as the regions where most of the evaporation into the air streams occurred at various phases of the event.