Q. J. R. Meteorol. Soc. (2006), 132, pp. 2853–2860 doi: 10.1256/qj.05.239 Synoptic-scale weather patterns during Alpine heavy rain events By KLAUS P. HOINKA 1 ∗ , CORNELIA SCHWIERZ 2 and OLIVIA MARTIUS 2 1 Institut f¨ ur Physik der Atmosph¨ are, DLR, Oberpfaffenhofen, Germany 2 Institut f¨ ur Atmosph¨ are und Klima, ETH, Z¨ urich, Switzerland (Received 28 December 2005; revised 18 May 2006) SUMMARY 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. KEYWORDS: Alps flooding ERA40 PV-streamer 1. I NTRODUCTION Western Europe is a preferred region for the occurrence of narrow meridionally elongated troughs, which may extend equatorward from Britain towards the Iberian Peninsula (Holopainen and Rontu 1981). These elongated troughs are characterized by high values of potential vorticity (PV) and are therefore termed ‘PV-streamer’ (Appenzeller and Davies 1992). Among others, Massacand et al. (1998) pointed out that these upper-tropospheric troughs accompany events of heavy precipitation on the southern slopes of the Alps. In the last decades, various notable events occurred which exhibited the characteristic synoptic features of streamers combined with heavy precipitation at the Alpine southside, e.g. the ‘Brig-event’ on 23 September 1993 and the ‘Piedmont-event’ on 5 November 1994, both with a maximum rain rate of more than 220 mm per 24 h. The knowledge of the streamer’s impact on regional heavy precipitation is a necessary prerequesite for a successful numerical weather prediction (NWP). Numerical simulations were performed in order to understand this impact and its significance for NWP as outlined by various studies in special journal issues of international projects (Mesoscale Alpine Programme (MAP) (Bougeault et al. 2003); Heavy Precipitation in the Alpine Region (HERA) (Volkert 2000)). Coordinated experimental efforts, such as the MAP experiment, helped further to understand the streamer’s evolution and its impact on the generation of flooding. The climatological relationship between heavy precipitation along the Alpine southside and upper-tropospheric PV-streamers over western Europe was investigated in a recent study by Martius et al. (2006, hereafter MZSD). They determined climatological statistics of streamers based on the ERA40 data (Simmons and Gibson 2000) provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) and related them to observed precipitation data (Frei and Sch¨ ar 1998). The resulting structure of the averaged upper-tropospheric PV field shows the typical PV-streamer spatial pattern known from various prominant events (Massacand et al. 1998): quasi-north–south aligned; extending southward over the Mediterranean Sea with its southern end located between the Pyrenees and northern Africa. The study of MSZD showed also that the combination of a synoptic-scale PV-streamer associated with heavy rain above the Alps is not a rare event. For a 34-year (1966–99) period they determined about 4 days per year with heavy precipitation of more than 29 mm d -1 on the Alpine southside. The time series of rain rates used consists of area-mean values representative for southern Switzerland. On average about 3 days per year of the heavy precipitation subsample showed the presence of an upper-level streamer over western Central Europe, whereas the rest of this subsample contains days with other features, e.g. the ∗ Corresponding author: Institut f¨ ur Physik der Atmosph¨ are, DLR, Postfach 1116, D-82230 Wessling, Germany. e-mail: Klaus.Hoinka@dlr.de c  Royal Meteorological Society, 2006. 2853