Abstract : 3K.12
Weather developments leading to heavy snow in the South-Eastern Alpine region

Panziera Luca, Brian Hoskins
Department of Meteorology, University of Reading

Heavy precipitations events occurring in the Western and Central European Alps have been the focus of a number of studies. However, extreme Alpine precipitation (EAP) events observed in the South-Eastern Alps appear to have received little attention. Here we concentrate on the winter EAP events that led to heavy snowfall in the South-Eastern Alpine region in the last 25 years. Four types of synoptic pattern are identified as being responsible for the extreme precipitation, and for each type a detailed case study is presented. Their analysis has been performed by looking at the large-scale dynamics of the Atlantic-European region, with particular emphasis on the potential vorticity (PV) view of synoptic development. These synoptic patterns differ mainly in shape and position of the upper level positive PV anomaly. In one case, the latter appears as a narrow streamer elongated in the north-south direction, approaching the Alps from the West and associated with a broad trough extended over Western and Central Europe. In a second case, the same initial PV structure occurs but, in presence of a cold advection and north-westerly flow impinging upon the Alps at the lower levels, Alpine lee cyclogenesis is triggered. In the other two cases, isolated tropopause level positive PV anomalies lead to the heavy snow events over the South-Eastern Alps. The PV anomaly may penetrate into the Mediterranean Sea from the North approaching the Southern slopes of the Alps from the South-West, or it may move from the North-East towards the northern side of the Alpine ridge. A common feature of all the synoptic patterns is the presence of moist southerly winds over the Alps at the time of the most intense phase of the snowstorms.