Synoptic-scale weather patterns during Alpine heavy rain events

Autumnal streamer-type elongated troughs are related to heavy precipitation observed at the southside of the European Alps. In order to study the development of this configuration, the typical structural evolution of the ambient flow in the event of heavy rain is investigated by lagged analyses. The precipitation amounts, both observations at the Alpine southside and from ERA40 forecasts by the ECMWF, are chosen as parameters to be correlated with atmospheric fields provided by the ERA40 data. Indeed, the resulting statistics indicate that the preferable position of elongated streamers is quasi-north-south aligned and extending southward over the Mediterranean Sea with its southern end located between the Pyrenees and northern Africa. A significant orographic impact on the streamer is revealed in the regression fields of the upper-level potential vorticity showing an indentation above the western Alps. The lower-tropospheric regression fields show a zonally elongated pressure signal due to the orographic influence. Up to two days in advance of heavy rain events a regressed meridional humidity flux occurs towards the south-western Alps, which at the event's peak time is found impinging upon the entire Alpine ridge. The regressed vertical velocity fields exhibit two maxima south of the Alps, a strong one at the western and a weaker one over the eastern Alps.