QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 133: 897–916 (2007) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.70 Fohn¨ in the Rhine Valley during MAP: A review of its multiscale dynamics in complex valley geometry Philippe Drobinski,a* Reinhold Steinacker,b Hans Richner,c Kathrin Baumann-Stanzer,d Guillaume Beffrey,e Bruno Benech,f Heinz Berger,g Barbara Chimani,b Alain Dabas,e Manfred Dorninger,b Bruno Durr,¨ h Cyrille Flamant,a Max Frioud,i Markus Furger,j Inga Grohn,¨ b Stefan Gubser,c Thomas Gutermann,h Christian Haberli,¨ b,h Esther Haller-Scharnhost,¨ c,h Genevieve` Jaubert,e Marie Lothon,f Valentin Mitev,i Ulrike Pechinger,d Martin Piringer,d Matthias Ratheiser,b Dominique Ruffieux,g Gabriela Seiz,k Manfred Spatzierer,b Simon Tschannett,b Siegfried Vogt,l Richard Wernerm and Gunther¨ Zangl¨ n a Institut Pierre Simon Laplace, Service d’A´eronomie, Paris, France b Department of Meteorology and Geophysics, University of Vienna, Austria c Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland d Central Institute for Meteorology and Geodynamics, Vienna, Austria e Centre National de Recherches M´et´eorologiques, M´et´eo-France, Toulouse, France f Laboratoire d’A´erologie, Toulouse, France g MeteoSwiss, Payerne, Switzerland h MeteoSwiss, Zurich, Switzerland i Observatoire de Neuchˆatel, Switzerland j Paul Scherrer Institute, Villigen, Switzerland k Institute of Geodesy and Photogrammetry, ETH Zurich, Switzerland l Institut f¨ur Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany m Umweltinstitut des Landes Vorarlberg, Austria n Meteorologisches Institut der Universit¨at M¨unchen, Germany ABSTRACT: This paper summarizes the findings of seven years of research on fohn¨ conducted within the project ‘Fohn¨ in the Rhine Valley during MAP’ (FORM) of the Mesoscale Alpine Programme (MAP). It starts with a brief historical review of fohn¨ research in the Alps, reaching back to the middle of the 19th century. Afterwards, it provides an overview of the experimental and numerical challenges identified before the MAP field experiment and summarizes the key findings made during MAP in observation, simulation and theory. We specifically address the role of the upstream and cross-Alpine flow structure on fohn¨ at a local scale and the processes driving fohn¨ propagation in the Rhine Valley. The crucial importance of interactions between the fohn¨ and cold-air pools frequently filling the lower Rhine Valley is highlighted. In addition, the dynamics of a low-level flow splitting occurring at a valley bifurcation between the Rhine Valley and the Seez Valley are examined. The advances in numerical modelling and forecasting of fohn¨ events in the Rhine Valley are also underlined. Finally, we discuss the main differences between fohn¨ dynamics in the Rhine Valley area and in the Wipp/Inn Valley region and point out some open research questions needing further investigation. Copyright 2007 Royal Meteorological Society KEY WORDS orographic flow; valley flow; cold-air pools Received 14 February 2006; Revised 2 November 2006; Accepted 3 November 2006 1. Some highlights and controversies from Alpine or high pollution levels (Nkemdirim and Leggat, 1978; fohn¨ studies since 1850 Hoinka and Rosler,¨ 1987), so that it is of great practical importance to better understand and predict the structure Fohn¨ is a generic term for strong downslope winds expe- of the fohn¨ flow. In the Alps, fohn¨ occurs most frequently riencing warming at the lee of a mountain ridge. Fohn¨ in the presence of strong synoptic-scale flow perpendicu- is associated with a decrease in cloudiness in the lee, lar to the Alpine crest, i.e. either northerly (north fohn)¨ or is strong and gusty, and is often channelled along gaps and valleys cut into the main ridge (Brinkmann, 1971; southerly (south fohn)¨ flow (Hoinka, 1980). While both Seibert, 1990). Fohn¨ may cause damage due to severe flow directions are similarly frequent, the vast majority storms (Brinkmann, 1974), snow melting (Hoinka, 1985), of the scientific literature on the Alpine fohn¨ deals with the south fohn¨ (e.g. Frey, 1953; Seibert, 1985). This is probably because south fohn¨ usually has a much stronger * Correspondence to: Philippe Drobinski, Service d’Aeronomie,´ Uni- impact on the local temperature than north fohn¨ (or even versite´ Pierre et Marie Curie, Tour 15, Couloir 15-14, 4 Place Jussieu, 75252 Paris Cedex´ 05, France. west and east fohns¨ which are more infrequent, how- E-mail: [email protected] ever), which is in turn related to the different origin Copyright 2007 Royal Meteorological Society 898 P. DROBINSKI ET AL. of the air masses (moist subtropical air mass for south fohn¨ to originate from the Sahara desert (Hann, 1866, fohn,¨ polar air mass for north fohn).¨ Usually, fohn¨ flow reprinted in Kuhn, 1989). Hann also found that latent is restricted to the respective lee side of the Alpine crest, heat release related to orographic precipitation is one but exceptions can be found in a few regions. For exam- important factor contributing to the temperature differ- ple, the Inn Valley (located north of the Alpine crest, see ence between the windward side and the lee side of the Figure 1) can also experience north fohn¨ in situations Alps. However, in contrast to subsequent textbook ver- of strong northerly or northwesterly flow (Hann, 1891; sions of the so-called thermodynamic fohn¨ theory, he did Zangl,¨ 2006). not claim that latent heat release is the only relevant fac- In the scientific literature on Alpine fohn,¨ an impor- tor (Seibert, 1985). In fact, cold-air blocking over the tant distinction is made between deep fohn¨ and shallow Po basin may give rise to much larger cross-Alpine tem- fohn.¨ A fohn¨ is termed deep when the cross-Alpine syn- perature differences than could be explained by latent optic flow extends significantly above the height of the heat release (Seibert, 1985, 1990). While the thermody- Alpine crest (e.g. Seibert, 1985, 1990). The dynamics namic explanation for the warmth of the fohn¨ was readily of deep fohn¨ is strongly influenced by vertically prop- accepted in the scientific community, there was a lot of agating gravity waves. Due to their three-dimensional debate on dynamical aspects of the fohn,¨ particularly the (3D) dispersion characteristics, gravity waves excited question how the fohn¨ is able to penetrate into Alpine over mountain ranges encompassing a valley can also valleys and to remove the denser cold air lying there. affect the flow dynamics in the valley proper, leading One of the earliest theories was proposed by Wild (1868), to pronounced wind maxima under suitable topographic who hypothesized that the fohn¨ ‘sucks’ the cold air out conditions (Zangl,¨ 2003). On the other hand, shallow of the valleys in a way similar to a vacuum cleaner. fohn¨ flow is restricted to a relatively small number of This was questioned by Billwiller (1878) who ascribed deep valley transects in the Alpine crest. So far, shal- a more passive role to the fohn¨ flow, merely replacing low fohn¨ has been reported from southerly directions the cold air that had been driven away by some synoptic- only, occurring under approximately westerly synoptic- scale pressure gradient. These theories survived several scale flow conditions. Shallow fohn¨ frequently precedes decades and gave rise to a remarkably heavy dispute (e.g. deep fohn¨ when the synoptic-scale flow direction grad- von Ficker, 1913; Streiff-Becker, 1931). Entirely different ually backs from west to south-west or south (Seibert, hypotheses were brought up in the mid-twentieth century, 1990). However, there are also shallow fohn¨ cases that for example by Roßmann (1950) who ascribed the downs- do not develop into a deep fohn.¨ The mesoscale dynam- lope acceleration of the fohn¨ to evaporation of cloud ics of shallow fohn¨ have been investigated by Sprenger water and spilled-over precipitation. (Seibert, 1985, gave and Schar¨ (2001) and Zangl¨ (2002a). They found that a critical review of fohn¨ theories; article reprints appeared the synoptic-scale pressure gradient related to geostroph- in Kuhn, 1989.) ically balanced westerly flow, frictional flow deflection With the advance of the theories of orographic gravity towards the lower pressure, and cross-Alpine tempera- waves (Lyra, 1943; Queney, 1948) and shooting hydraulic ture contrasts with cold air lying in the south play an flows (Schweitzer, 1953; Long, 1953), a deeper under- important role in generating shallow fohn.¨ On the local standing and more complete picture than provided by the scale, shallow fohn¨ flow is mainly governed by hydraulic early fohn¨ theories became available. Nevertheless, the dynamics (Flamant et al., 2002; Gohm and Mayr, 2004). variety of different hypotheses reflects the important fact Fohn¨ research has a long history in the Alps, reaching that local flow patterns related to fohn¨ differ strongly back to the middle of the 19th century. Already in 1866, between various Alpine valleys. Wild (1868) and Streiff- Hann recognized that adiabatic warming in the lee of the Becker (1931) observed that the southerly fohn¨ starts Alpine crest is the main reason for the fohn¨ being warm close to the Alpine crest and then gradually penetrates and dry, rejecting earlier hypotheses that assumed the toward the north, whereas light opposing (northerly) flow 3.5 47.5 3 Rhine valley 2.5 target area N) ° 2 47 Wipp valley target area 1.5 Latitude ( 1 46.5 8.5 9 9.5 10 10.5 11 11.5 12 12.5 0.5 Height above sea level (km) Longitude (°E) 0 Figure 1. Topography of the Alps. The two boxes indicate the Rhine Valley target area and the Wipp/Inn Valley target area for fohn¨ studies during MAP. Copyright 2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj FOHN¨ IN THE RHINE VALLEY DURING MAP 899 is present in the cold-air pool.