A climatological Analysis of the Link between breaking synoptic-scale Rossby Waves and heavy Precipitation Events in the Alps

Extreme precipitation events on the south-side of the European Alps can lead to floods and land slides and thereby cause severe damage. Case studies indicate that these events are frequently associated with meridionally elongated breaking synoptic-scale Rossby waves (so-called "streamers") located to the west of the Alpine ridge. In combination with the highly-structured orography they lead to localized severe rain events.

This study considers from a climatological stand point the role of potential vorticity (PV) streamers as upper-level precursors to heavy precipitation events. A climatology of streamers is derived base on the ERA-40 data set. Days of extreme precipitation along the Alpine south-side are determined from an observational Alpine precipitation climatology (1966-1999,Frei and Schaer,1998). For these days, the presence, the location and the orientation of the streamers is recorded.

A quantitative evaluation is undertaken of the co-occurrence of streamers and heavy precipitation, and shows that a significant fraction of the extreme precipitation is associated with a streamer situated over western Europe. An investigation of structural differences of the upper-level precursor and the ambient flow conditions for heavy and extreme precipitation events, underlines the important influence of moisture for the formation of the extreme precipitation. The occurrence and the development of PV intrusions is linked to Rossby wave propagation and breaking along the extra-tropical tropopause and hence the propagation serves as precursor to heavy precipitation events.