Massacand, Alexia C. (Institute for Atmospheric Science, ETH Zurich, Switzerland)
Wernli, Heini (same affiliation)
Davies, Huw C. (same affiliation)
In recent years there has been a spate of heavy precipitation events in the Alpine region, and in particular on the southern side. These events formed a major spur for, and significantly influenced the design of, the Mesoscale Alpine Programme (MAP).
The present study is directed toward eliciting the nature of the large-scale flow setting that parturates these precipitation events. It is shown that for the four MAP-designated major flooding events - i.e.
Vaison-la-Romaine (22.09.92); Brig (23-24.09.93); Piedmont (5-6.11.94); South Ticino (13.09.95) - the upper-level flow was characterized by a coherent and distinctive potential vorticity (PV) pattern. It took the form of a deep, quasi north-south aligned, elliptical-shaped, stratospheric intrusion that elongated slowly as it was advected toward the Alpine chain.
Diagnostic analysis of the ECMWF data, involving both computation of the vertically integrated PV distribution and backward trajectories of air-parcels emanating from the diabatically-induced PV tower associated with the heavy precipitation, helps establish the link between the large-scale setting and the meso-scale rain events.
Analysis of other severe precipitation events (e.g. Lake Maggiore, Italy (06.10.93); Friuli, Italy (19.09.95); Herault, France (11-12.11.96)) lends further support for the hypothesis that the endemic precursor signal is the advection of the forementioned distinctive PV anomaly toward the western Mediterranean.