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. On the other hand, Bill- (1935) chose all cases with southerly wind directions willer (1878) and von Ficker (1905, 1913) observed out- between 1 and 3 km above ground to detect typical flow of cold air into the Alpine foreland before the onset profile structures during fohn¨ conditions. Particularly of the fohn.¨ Recently, Zangl¨ (1999, 2003) demonstrated in spring, positive temperature deviations of 3 °C with that these different flow patterns are related to differ- respect to the climatological average were found at 1 ent orientations of the valleys. Valleys aligned with the to 2 km altitude. Near the ground, inversions were fre- impinging flow tend to exhibit the former flow pattern quently observed in the kite soundings. Studying the with converging flow at the leading edge of the fohn¨ zone spatial flow variability along the Rhine Valley, Guter- because the gravity-wave pattern favours a local pressure mann (1970) found that fohn¨ air near Chur is gener- minimum there. Cold-air outflow at low levels appears to ally cooler than near Vaduz, which he explained with be more typical for valleys perpendicular to the imping- the influence of katabatic outflow from tributaries. Fur- ing flow that are less favourable for a direct penetration ther statistical investigations, including a comparison of of the fohn.¨ fohn¨ frequencies in the Rhine Valley and the Reuss Val- Although fohn¨ research in the Rhine Valley has a ley (located in central Switzerland, Figure 2), were con- fairly long tradition, research activities prior to MAP ducted by Waibel (1984). A more complete review of (Mesoscale Alpine Programme) largely concentrated on fohn¨ research in the Rhine Valley is provided by Richner the vicinity of Innsbruck, presumably because of the uni- et al. (2006). versity institute located there (a comprehensive overview Section 2 lists the reasons for selecting the Rhine val- is provided by Seibert, 1985; also Hoinka, 1990; Zangl,¨ ley as a target area for fohn¨ studies during MAP and 2003). Probably the first climatological evaluation of the main scientific objectives. It also gives a summary of fohn¨ in the Rhine Valley was done by Hann (1882). the fohn¨ events in the Rhine Valley during MAP. Sec- Based on a 17-year record from Bludenz, he found a tion 3 presents the main challenges identified before the broad frequency maximum from autumn through spring field experiment to be addressed in terms of observation (about 10 days per season) and a pronounced minimum and modelling. Section 4 synthesizes the main findings in summer. Peppler (1930) investigated kite soundings in terms of observation, numerical simulation and the- and pilot balloon ascents that were conducted opera- ory of fohn.¨ Finally Section 5 concludes this overview tionally at Friedrichshafen (located at the northern shore of results and point out some open research questions of Lake Constance) for more than 20 years. Peppler needing further investigation.

(a) Lake Constance Zurich Bregenz Altenrhein Diepoldsau Hoher Kasten Rankweil

Lake of Zurich Feldkirch Bad Ragaz Sargans Walgau Weite Seez valley Rätikon range Rhine valley Vaduz

Praettigau Buchs- Reuss valley (b) Grabs Weisstannen

Tamina Kunkelspass Chur LRV RhineDomleschg valley Sargans SV

Heiligkreuz Gütsch Val LumneziaMasein Julier Weisstannen

Tamina URV Malans

Figure 2. (a) Topography of the FORM target area with (b) an expansion of the region of interest (9.3–9.6 °E; 46.9–47.2 °N) which is also the most instrumented area (see Section 3.1). The contour interval is 500 m from 500 m to 3000 m altitude. Italics indicate names of valleys, and captions in boxes are the names of towns. SV, LRV and URV denote the Seez Valley, the lower Rhine Valley and the upper Rhine Valley, respectively. The dashed lines in (b) indicate the scintillometer light beams. The scintillometer transmitters are located at Triesenberg (on the eastern side of the LRV), and the receivers at Flusa and Ergellen (on the western side of LRV). This figure is available in colour online at www.interscience.wiley.com/qj

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 900 P. DROBINSKI ET AL.

2. Selection of the Rhine Valley as an avenue for Table I. Fohn¨ episodes during the Special Observing Period of fohn¨ flows in a valley with complex geometry the MAP field experiment (7 Sept 1999 to 15 Nov 1999).

2.1. Scientific motivation IOP Beginning End

Since the aforementioned findings strongly suggest that 1 15 Sep 1999 0750 15 Sep 1999 1700 fohn¨ research should consider various regions of the Alps, 2 19 Sep 1999 0050 20 Sep 1999 0700 it was decided to select the Alpine Rhine Valley as a tar- – 22 Sep 1999 0750 23 Sep 1999 1130 get area for fohn¨ research during MAP (see also Volkert – 30 Sep 1999 0420 30 Sep 1999 0840 and Gutermann, 2007). The research area extends from 5 2 Oct 1999 0810 3 Oct 1999 0320 the Alpine crest to the northern Alpine foreland and fea- 7 18 Oct 1999 1340 18 Oct 1999 1650 8 19 Oct 1999 2320 21 Oct 1999 1410 tures several low passes in the main Alpine crest (see 9 22 Oct 1999 0640 23 Oct 1999 0330 Figure 2), which provides an excellent opportunity to 10 24 Oct 1999 0210 24 Oct 1999 1920 study the development of shallow fohn.¨ Moreover, the 12 30 Oct 1999 0900 31 Oct 1999 0310 main valley and its tributaries cover a wide range of val- 13 1 Nov 1999 1200 2 Nov 1999 1610 ley orientations, allowing investigation of the effects of 15 5 Nov 1999 0610 6 Nov 1999 0940 valley orientation on the dynamics of fohn.¨ The selection of the Rhine Valley was also motivated by the facts that it The dates and times (UTC) indicate the onset and end of fohn¨ events is well equipped with operational meteorological measur- in the Rhine Valley detected by the multi-parameter algorithm of ing networks and that there is a high practical significance Gutermann (1970). Twelve fohn¨ episodes were observed during 10 IOPs; two fohn¨ episodes occurred outside IOPs. for any improvement in forecasting the onset or cessation of fohn¨ for storm warnings on Lake Constance. Com- pared to the other target area (the Wipp Valley/Innsbruck (4) the interaction of the fohn¨ flow with the boundary region; project P4; Mayr et al., 2007), the Rhine Valley layer and the removal process of the cold-air pool; has a significantly more complex topographical structure. (5) the interaction between the fohn¨ and air blowing in In particular, the valley axis of the Wipp Valley is almost the side valleys; straight while the Rhine Valley has several marked kinks. (6) the dynamics of flow splitting at the bifurcation Moreover, the Rhine Valley exits into the Alpine foreland between the Rhine and Seez valleys; and whereas the Wipp Valley ends in another inner-Alpine (7) the requirements to improve the forecast skills of fohn¨ in the Alpine region. valley (the Inn). Thus, the two fohn¨ research areas com- plement each other in an ideal way. In the studies conducted prior to MAP, the instrumen- 2.2. Summary of the fohn¨ events in the Rhine Valley tal set-up did not allow for a coherent 3D documenta- during MAP tion of the fohn¨ from the synoptic scale to the valley The special observing period (SOP) of MAP took place scale, providing sufficient resolution in space and time in autumn from 7 September to 15 November 1999, to analyze the processes governing the spatio-temporal corresponding to the maximum of the climatological fohn¨ evolution of fohn.¨ This gap was attempted to be closed frequency in the northern Alps. During the MAP SOP, during MAP with instrumentation of unprecedented den- twelve fohn¨ episodes were observed. Ten fell into an sity, including surface and radiosonde stations, a variety intensive observing period (IOP) and two were outside. of remote-sensing instruments and several research air- The start and end of each episode are given in Table I. craft. An overview of MAP, its strategy, the projects, and The detection of fohn¨ and the determination of the start preliminary results are given by Bougeault et al. (2001). and end time of the episodes were conducted according Among the MAP objectives, the program FORM (Fohn¨ to the multi-parameter algorithm of Gutermann (1970), in the Rhine Valley during MAP), was designed to study which was applied to numerous surface measurements (Richner et al., 2006): acquired in the Rhine Valley and its main tributaries. The algorithm is based on a thresholding of mean and (1) the dynamics of that part of the blocked, potentially gust valley winds, temperature increase >3Kandlow cooler air mass that typically reaches up to the mean humidity during the whole period. The fohn¨ episodes crest height on the windward side of the main ridge were determined for each station individually. A fohn¨ and which flows through deep Alpine passes into the event is defined by the earliest onset and by the latest lee-side valleys (shallow fohn);¨ end at any of the considered stations on the valley floor. (2) the interaction between low-level and mid-tro- Altogether, fohn¨ covered 244 hours 20 min, i.e. about pospheric fohn¨ flows on the scale of large Alpine 15% of the whole SOP (1464 hours) or 29% of the IOPs valleys including the improvement of understanding (836 hours). In most events, the fohn¨ flow first reaches and forecasting of fohn-related¨ phenomena like tur- the valley floor in the area of the Flascherberg¨ (halfway bulence; between Sargans and Malans; Figure 2). The fohn¨ also (3) the mechanism of temporal and spatial evolution and ends later in this area. Frequently, the fohn¨ flow aloft does cessation of fohn¨ flows in complex valley systems on not extend downwards to these well-exposed points in the a local scale; valley because the valley is filled by a stagnant cold-air

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 901 pool. Thus, fohn¨ is observed more frequently on passes or of vertically propagating and trapped gravity waves (loca- mountaintops (‘pass fohn’)¨ than in valleys (‘valley fohn’).¨ tion, amplitude, period, intensity) (e.g. Drobinski et al., These MAP fohn¨ events have been studied by Bau- 2003a). Two scintillometers located in the lower Rhine mann et al. (2001, all IOPs, with IOPs 8, 9 and 10 in Valley (the transmitters at Triesenberg, the receivers at detail), Beffrey et al. (2004a, IOP 8), Drobinski et al. Flusa and Ergellen; Figure 2(b)) allowed for measuring (2001, IOP 5; 2003a, IOP 12; 2003b, all IOPs; 2006, the vertical and horizontal wind components and for doc- IOPs 2, 5, 8, 10, 12 and 15), Flamant et al. (2006, umenting the gravity-wave penetration into the lower IOP 15), Frioud et al. (2004, IOPs 4 and 5), Gubser Rhine Valley during strong fohn¨ windstorms (Furger and Richner (2001, IOP 9), Jaubert and Stein (2003, et al., 2001; Drobinski et al., 2003a). Closely connected IOP 2), Jaubert et al. (2005, IOP 15), Lothon et al. to the wave monitoring by the sodars was the operation of (2003, IOPs 2, 5, 8, 13, 15), Vogt and Jaubert (2004, microbarographs, which made possible the determination IOP 15) and Zangl¨ et al. (2004a, IOP 10). In spite of of the relevant characteristics (direction of propagation, their specific features linked to fohn¨ intensity, these stud- phase speed, and wavelength) of gravity waves on top ies allowed the building of a comprehensive scheme of of the cold pool by detecting the related pressure sig- the temporal evolution of the fohn¨ in a large valley, which nal on the ground (Flamant et al., 2006). Several aircraft is useful to validate numerical simulations. flying over the Rhine Valley target area (French Mer- lin IV and ARAT Fokker-27, UK C-130, Swiss Metair Dimona, USA NCAR Electra and NOAA P-3) measured 3. Challenges for new measurements and modelling averaged mean and turbulent variables (wind speed and tools at the beginning of MAP direction, vertical wind, temperature, pressure, humid- ity, turbulent kinetic energy, momentum and heat fluxes), 3.1. Observation network providing information on gravity-wave activity in relation to cold-pool erosion (Gubser and Richner, 2001; Jaubert As already mentioned, the instrumental set-up used et al., 2005; Flamant et al., 2006). Finally, the cold-pool in earlier experiments did not allow for detailed 3D removal was also monitored by three continuously run- documentation of the fohn¨ flow over the wide range of ning cameras mounted at ∼1800 m amsl (above mean relevant spatial and temporal scales. During MAP, the sea level) on Hoher Kasten. Indeed, the turbidity in the main principle of the design of the observing system cold pool is always significantly greater than in the fohn¨ was to optimally combine the more or less continuously air, making the boundary between the two air masses and observing remote-sensing systems with only sporadically its variation visible over time. active, but usually more accurate, in situ observations. To investigate the interaction of the fohn¨ flow between An extensive description of the instrument set-up can be a main valley and a tributary, six surface stations were found in Richner et al. (2006). set up along the Brandner Valley (in the Austrian To address the scientific issues summarized above, the province of Vorarlberg) and traverses were made with very-fine-scale 3D structure of the fohn¨ and its time an instrumented car measuring pressure, temperature, evolution have been documented by means of remote humidity, and wind. sensors operated continuously or during IOPs only. A Finally, for MAP the already dense network of conven- transportable wind lidar (TWL) located in Vilters near tional meteorological surface stations in the Rhine Valley Bad Ragaz (Figure 2) provided radial wind velocity mea- target area was completed with 14 additional surface sta- surements along the line-of-sight. It proved to be a tions and 9 radiosonde stations. This extremely dense key instrument for investigating flow-splitting dynamics station network was necessary to resolve small-scale fea- at the bifurcation between the Rhine and Seez Valleys tures and to achieve the same resolution for observational (Drobinski et al., 2001, 2003a, 2006) and validating high- data as for numerical weather prediction (NWP) models. resolution simulations (Beffrey et al., 2004a; Drobinski This was vital for the validation of the models. This net- et al., 2006). Five Doppler sodars and two wind profiling work also contributed to the derivation of better initial radars (one equipped with a radio acoustic sounding sys- conditions for research and NWP models (Zangl¨ et al., tem, RASS) contributed to validate high-resolution simu- 2004a; Jaubert et al., 2005) and the mesoscale analyzes lations of the life cycle of fohn¨ (Vogt and Jaubert, 2004). extensively used for fohn¨ investigations in the Rhine Despite its location outside the Rhine Valley target area, Valley (Drobinski et al., 2003a; Chimani et al., 2006; Fla- the UHF profiler set up at Julier Pass was a key instru- mant et al., 2006). ment to study the dynamics at one of the main passes that As fohn-related¨ aspects, air quality issues were inves- feed the Rhine Valley network (Ruffieux et al., 2000). tigated by the means of a tethered balloon operated The cross-Alpine flow structure of gravity waves at Fussach and a cable car located at Bregenz, near related to fohn¨ has been investigated using constant- Lake Constance, to obtain high-resolution vertical pro- volume balloons launched from Ispra (Italy) located files of meteorological variables and ozone (Baumann near Lago Maggiore. The balloons served to document et al., 2001). In addition, a vertically pointing backscat- the dynamics of the isopycnic airflow (pressure, wind tering lidar located near Sargans monitored aerosol layers speed and direction, temperature and humidity content; continuously with almost no interruption during the entire Benech´ et al., 2002) and particularly the characteristics field phase (Frioud et al., 2003, 2004).

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 902 P. DROBINSKI ET AL.

3.2. Numerical modelling difference (Seibert, 1985, 1990). For the Rhine Valley, it has been found that the fohn¨ tends to reach the The main objectives of the numerical modelling efforts valley bottom when the pressure difference between made in the context of MAP were Lugano and Vaduz exceeds about 5 hPa (Richner et al., 2006), corresponding to a cross-Alpine pressure gradient (1) to document the capability of state-of-the-art meso- − of about 4 hPa (100 km) 1. Table II summarizes the γ -scale numerical models to simulate air flow and upstream key parameters for all the fohn¨ IOPs which precipitation fields in complex Alpine orography, and can be compared to their climatology detailed in Richner (2) to improve the process understanding of the underly- et al. (2006). ing interactions between atmosphere and orography. During typical fohn¨ events, H
decreases with time and the fohn flow deepens with time as the ambient flow For the FORM project, the specific research challenges ¨ were to simulate the detailed characteristics and evolution intensifies and gets more southerly. In the early stage, the flow regime is often a ‘flow around’ the Alps during of fohn¨ flows in complex valley systems and to better H>
understand the underlying flow dynamics. The practical fohn¨ with 3, switching to a ‘flow over’ regime as H
⇒ 1 and keeps its minimum value for several hours importance of predicting meso-γ -scale aspects of fohn¨ depending on the events. This fraction of time can be half inside Alpine valleys is very high since fohn¨ represents a weather risk to all outdoor activities including air of the duration of the event. The jet associated with flow operations, and also influences air quality. Moreover, splitting at the scale of the Alps and flowing along the the uniquely dense dataset collected during MAP was western flank of the range may affect the altitude of the expected to offer one of the first opportunities to clearly fohn¨ jet in the Rhine Valley. It is capable of triggering demonstrate the possible benefits of future real-time, cold-air intrusions from the north into the Rhine Valley et al meso-γ -scale NWP. Necessary ingredients for a high (Jaubert ., 2005), lifting the fohn¨ flow passing over forecasting skill of high-resolution numerical models are the Alpine range off the ground in the lower Rhine Valley. expected to be: These intrusions are favoured when the strong flow- around jet is positioned close to the Rhine Valley outlet (1) an accurate simulation of larger-scale aspects, such and is oriented orthogonally to it. Local topography, i.e. as the upstream and downstream wind and stability, the small mountain range east of the Lake Constance (2) a good initial analysis of the low-level atmospheric basin, makes it easier for the air to enter the Rhine Valley, state inside the valleys under consideration, and due to channelling (Figure 2). (3) a high-resolution model with a proper representation The meridional extension of the fohn¨ within the valley of orography and sophisticated parametrizations for strongly depends on the upstream conditions (Lothon, physical processes like radiation and turbulence. 2002): (1) in the southern part of the Rhine Valley (e.g. around 4. Key findings of the post-SOP period in Chur), light downslope wind appears as soon as observation, simulation and theory the upstream conditions are favourable for fohn,¨ 4.1. Upstream and cross-Alpine flow structure during and its intensity does not depend strongly on these conditions; fohn¨ events (2) in the central region of the Rhine Valley (e.g. around
The upstream conditions largely determine the temporal, Vaduz), the downslope wind requires smaller H to horizontal and vertical extension of the fohn¨ within reach the valley bottom, and its intensity is strongly and over the Rhine Valley. The key parameters are: correlated with the intensity of the upstream flow; (1) θ and p, the potential temperature and mean- (3) finally, in the northern part of the valley (e.g. Lake sea level pressure (mslp) differences across the Alps, Constance), the downslope wind occurrence depends (2) H
= NH/U, the non-dimensional height, where on the upstream conditions but also on the local and N is the Brunt–Vais¨ al¨ a¨ frequency, H the dimensional downstream conditions. The cold pool which is often mountain height and U a characteristic upstream wind present (see next subsections) in this region interacts speed, and (3) the upstream flow direction. Generally, with the fohn¨ and only strong upstream flow and low
H
∼ O(1) implies that air parcels mainly flow over the H can trigger a downslope wind that reaches the mountain and substantial nonlinear effects occur (such ground in this area. as hydraulic jumps or large-amplitude gravity waves), whereas for H
 1 much of the airstream is diverted Downstream turbulence was measured using the Mer- around the flanks of the mountains and the perturbation lin IV aircraft during MAP (Lothon et al., 2003). The energy mainly appears in the horizontal with generation measurements showed that in addition to organized prop- of vortices rather than vertical motions (Smolarkiewicz agating or trapped gravity waves, at a 10-km scale tur- and Rotunno, 1989). The wind direction determines bulent plume exists in the wake of the mountain with if the fohn¨ flow across the Alps is deep or shallow. large turbulent kinetic energy (TKE) and dissipation rate Finally, experience shows that the occurrence of fohn¨ (Figure 3). This plume extends from 3000 m altitude is strongly correlated with the cross-Alpine pressure above the southern part (maximum measured TKE) down

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 903 → November non-dimensional height (calculated from Milano 7 248/16.4 241/9.4 242/11.4 1 236/12.8 239/9.9 232/10.3 3 0.05/0.8 0/0 0.1/1.9 6 218/1.7 133/1.1 047/1.4 =
H 199/5.3 206/3.9 218/3.6 199/4.1 ¨ ohn Intensive Observation Periods. 7 237/5.1 274/2.6 208/1.1 245/2.3 10.4 2.2/11.0 2.0/5.5 4.5/12.5 2.9/8.0 /50.9 0.5/10.1 1.5/25. 0 193/7.6 190/5.7 197/8.1 235/4.5 244/4.8 221/4.4 8 175/3.1 176/6.3 161/7.9 173/3.1 105/1.8 249/3.3 4.9 217/18.5 233/21.2 237/23. 2.1 207/16.3 237/17.6 233/21. 1.9 047/0.9 084/1.4 213/2. 14.1 275/3.5 195/16.0 219/14.7 204/20.6 219/12.1 198/9.8 209/12.2 potential temperature difference between Vaduz and Lugano. = , cross-Alpine and downstream flow parameters for the f θ → 9 173/14.8 171/11.2 181/13.3 172/14.7 175/17.1 173/9.8 168/11.1 173/12.1 3 210/4.4 033/4.0 190/5.7 201/4.8 ). 1 − 0.2 4.6 0.5 8.6 8.3 7.3 9.1 0.7 2.1 ) at about 3600 m amsl 1 − − 119/0.7280/0.9 220/1.9 112/1.5 035/2.5 041/3.6 065/1.3 250/1.8 229/1.4 292/1. 165/4.8 196/2.0 199/1.6 213/1.9 October ) 1 − )/wind speed (m s ° )/wind speed (m s ° and Vaduz reduced to mean sea level. )/wind speed (m s ° )/accumulated rain (mm) 1 − 0/0 1.2/36.3 0/0 0.4/1.6 0.5/10.3 0.07/0.3 1.3 IOP 1 IOP 2 IOP 3 IOP 4 IOP 5 IOP 6 IOP 8 IOP 9 IOP 10 IOP 12 IOP 13 IOP 15 152/6.5 180/17.0 237/9.7 211/16.3 211/ Table II. Temporal mean values of the relevant upstream → pressure difference between Lugano (hPa) 4.8 8.2 5.7 6.5 7.1 4.0 10.5 10.7 9.7 8.0 4.7 5.8 (K) 4.7 6.8 4.3 = (mean)(min/max) 1.2/5.6 3.5 4.0/7.8 6.3 4.1/6.3 5.7 2.0/ 3.9 6.8 3.0 3.5 8.8 5.0 ¨ utsch 169/10.7 169/22.0 177/9.5 166/14. ¨ antis 184/3.4 185/7.2 213/2.4 215/4.3 213/4.5 141/4.

September p θ H H Milano/Linate wind profiler – wind direction ( Julier Pass wind profiler – wind direction ( Ground stations – wind direction ( Lugano – rain rate (mm hr p LustenauS 043/1.3G 204/1.9 165/1.1 142/0.6 033/1.1 341/ St. GallenAltenrhein 024/1.1 269/1.3 148/4.3 175/4.6 165/1.3 262/1.3 VaduzChur 159/3.3 212/4.2 164/7.3 209/5.1 117/2.7 200/3.9 344/1.1 195/3. 185/4.0 299/1. 6000 m amsl 185/6.8 215/20.9 241/20.1 240/24.0 252/21.0 223/ 4000 m amsl 161/7.3 205/16.6 242/15.1 240/19.3 249/17.0 258/ radiosoundings within a 2000–4500 m layer).

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 904 P. DROBINSKI ET AL.

Figure 3. (a) Mean turbulent kinetic energy (m2 s−2) and (b) mean dissipation rate (10−3 m2 s−3) of the TKE, computed from the Merlin IV aircraft data averaged over five fohn¨ flights. The grey bars indicate the 1 km averaged topography along the flight tracks. to the lowest layers above the northern part of the Rhine has a very complex shape: south of 46.8 °N, the Rhine Valley (minimum measured intensity). The scale of the Valley is oriented west–east, while north of 46.8 °N, it turbulence can be characterized by the dissipative length- has a more south–north orientation up to Lake Constance 3/2 scale Lε = e /ε,wheree is the TKE, and ε the TKE (Figure 2). Several major valleys merge with the Rhine dissipation rate. This length characterizes the size of the Valley, namely the Val Lumnezia, the Domleschg, the largest eddies that lie in the inertial subrange, and is of Prattigau,¨ the Seez Valley and the Walgau. At the high importance in TKE equation closure of mesoscale bifurcation between the Rhine and Seez Valleys, the models. We found Lε = 1500 ± 500 m, with no signif- Tamina gorge and the Weisstannen Valley also play an icant variation with altitude and height. This is typical important role. In other words, the Rhine Valley can not for Lε measured in homogeneous convective boundary be considered as a single transect in the Alpine ridge layers, although TKE dissipation rate can vary by sev- since all these tributaries play a potential role in the eral orders of magnitude depending on the complexity of circulation of the fohn¨ flow in the FORM target area. the terrain. This lends further support to this length-scale As a consequence, a second objective in this section is to as a robust key parameter to be used in the mesoscale examine the role of the Alpine valley network in directing models. the fohn¨ flow towards the FORM target area from its early stage of development (shallow fohn)¨ until its breakdown. 4.2. Dynamics of fohn¨ in the Rhine Valley During shallow fohn,¨ the air which reaches the Rhine valley area is potentially cooler and comes from the 4.2.1. F¨ohn propagation in the complex valley network southern side of the Alps through Alpine gaps (passes). As mentioned previously, the life-cycle of a fohn¨ usually Due to the stably stratified air mass and/or the height of begins with a shallow-fohn¨ phase followed by a deep- the cold pool lying south of the Alps, the height of the fohn¨ phase. One objective of FORM was to analyze the passes is also crucial. As an example for the shallow-fohn¨ fohn¨ propagation in the complex valley network in the phase, the wind field on 29 October 1999 at 1800 UTC area of the Rhine Valley. In fact, previous studies on (Figure 4) simulated with the mesoscale model Meso-´ the channelling effect of the fohn¨ flow by major Alpine NH provides evidence for weak southerly wind through valleys considered only north–south oriented portions of the Reuss Valley and Domleschg which remains confined Alpine valleys. In the FORM target area, the Rhine Valley below the crest line (Drobinski et al., 2003a).

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 905

47.43 47.43

3000. 3000.

2500. 2500.

2000. 2000.

1500. 1500. Latitude (N) 1000. 1000.

500.0 500.0 (a) (b) 46.42 46.42 8.34 10.05 8.34 10.05

47.43 47.43

3000. 3000.

2500. 2500.

2000. 2000.

1500. 1500. Latitude (N) 1000. 1000.

500.0 500.0 (c) (d) 46.42 46.42 8.34 10.05 8.34 10.05

47.43 47.43

3000. 3000.

2500. 2500.

2000. 2000.

1500. 1500. Latitude (N) 1000. 1000.

500.0 500.0 (e) (f) 46.42 46.42 8.34 10.05 8.34 10.05 Longitude (E) Longitude (E)

Figure 4. Horizontal wind field at 2 km resolution at 1000 m amsl as obtained from Meso-NH´ numerical simulations (a) at 1800 UTC on 29 October 1999 (shallow fohn¨ phase of IOP 12), and (b) at 1200 UTC on 30 October 1999 (deep fohn¨ phase of IOP 12). (c, d) and (e, f) are as (a, b), but for 1500 m amsl and 2000 m amsl, respectively.

Figure 4 shows that, near the surface, the flow at splits between these two valleys near Kunkelspass), and Chur is fed by fohn¨ (combined with katabatic flow; splits between the Seez and Rhine Valleys. One can note Drobinski et al., 2003a,b) blowing in the upper Rhine a strong intensification of the flow in the lower Rhine Valley. At higher levels (Figure 4(c), (e)), the air from Valley near Feldkirch where the flow originating from the Domleschg partly goes straight to the north over the flow-splitting at the bifurcation of the Rhine and Seez the Kunkelspass (which is about 1300 m amsl), partly Valleys merges with flow blowing from the east–west towards Chur in the upper Rhine Valley. The reason oriented Walgau. why Val Lumnezia does not channel the shallow fohn¨ During the deep-fohn¨ phase, when fohn¨ reaches the is that probably the air does not reach high enough to Rhine Valley, the upstream upper-level flow veers to the cross all passes feeding into the upper Rhine Valley. south/south–west and the stability decreases. The use Indeed, Val Lumnezia seems to be closed off to the south of Lagrangian tracers in numerical models (Gheusi and by a pass much higher than the Domleschg valley. At Stein, 2002) shows that the air reaching the ground at 1500 m amsl, katabatic drainage flow blows from the the northern edge of the Alps originates from a level of zonally oriented Prattigau.¨ At the bifurcation between 2000 to 3500 m in the south (e.g. Lothon, 2002; Jaubert the Rhine and Seez Valleys, the flow comes from the and Stein, 2003). The air mass accelerates as it flows Tamina gorge and the upper Rhine Valley (the flow first over the ridge and the Rhine Valley, and experiences

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 906 P. DROBINSKI ET AL. mountain waves and associated strong downward motion 4.2.2. Flow splitting at the bifurcation between the penetrating down to the Rhine Valley (Furger et al., Rhine and Seez Valleys 2001 for IOP 8; Drobinski et al., 2003a for IOP 12). The downslope wind on the northern side generates turbulence One of the objectives of FORM was to identify in the Rhine Valley, as indicated by various numerical simulations (Jaubert and Stein, 2003 for IOP 2; Drobinski (1) the factors determining whether flow splitting occurs et al., 2003a for IOP 12; Zangl¨ et al., 2004a for IOP 10; or not between the Rhine and Seez valleys, and Beffrey et al., 2004a; Jaubert et al., 2005 for IOP 15) (2) the valley into which the fohn¨ is directed. as well as observations (Lothon et al., 2003 for IOPs 2, 5, 8, 13, and 15). As an example for the deep-fohn¨ The flow structure at the junction between the Rhine phase, the wind field on 30 October 1999 at 1200 UTC, and Seez Valleys has been characterized statistically at the simulated with Meso-NH,´ is shown in Figure 4. As synoptic scale and at the scale of the valley by Drobinski the upstream upper-level flow is quasi-aligned with the et al. (2003b). The results reveal that the flow regimes in transverse valleys, the channelling efficiency of these the Rhine and Seez Valleys are, as expected, dominated by the orography. In the Seez Valley, the wind direction valleys in directing the fohn¨ flow towards the FORM is parallel to the valley axis in the vast majority of cases. target area increases. Val Lumnezia plays a significantly In the lower Rhine Valley, the wind direction also fre- more important role than during the shallow fohn¨ whereas quently follows the main valley axis but is also influenced the Prattigau¨ does not channel the fohn¨ flow. by the small pass to the west as well as by the orograph- To further illustrate the importance of orographic ically undisturbed synoptic-flow direction. This implies gravity waves for the low-level wind field in the Rhine that the lower Rhine Valley from Bad Ragaz to Lake Valley, Figure 5 displays the near-surface wind field and Constance is less efficient in channelling the flow than a vertical cross-section of wind and potential temperature the Seez Valley. Drobinski et al. (2003b) found five main along the lower Rhine Valley on 24 October 1999 at flow patterns: south-east/south, north-west/west, north- 1100 UTC (IOP 10; Zangl¨ et al., 2004a). As evident west/north, north-west/south, south-east/north, where the from Figure 5(a), a pronounced surface wind maximum is first (second) wind direction refers to the Seez Val- found in the valley segment between Sargans and Buchs- ley (lower Rhine Valley) (Figure 6). The flow splitting Grabs (Figure 2). The presence of this wind maximum between the Rhine and Seez Valleys (south-east/south is supported by observational data, and the surface flow regime) prevails and occurs either during fohn¨ observations gathered during the full MAP SOP reveal events or is due to katabatic flow. In fact, 75% of the that a wind maximum is frequently encountered in this south-east/south cases outside fohn¨ events were observed region. Figure 5(b) indicates that vertically propagating between 1800 and 0600 UTC. The very high probability orographic gravity waves are responsible for this wind of flow splitting occurrence indicates that flow separation maximum. Due to their 3D dispersion characteristics, from the western wall of the lower Rhine Valley near gravity waves excited over the adjacent mountain ridges Malans (where the Rhine Valley makes a sharp turn from radiate toward the valley axis while propagating upwards, south/north orientation to south–east/north–west orienta- thus influencing the wind field over the valley axis in a tion when looking along the river flow) does not prevent similar way as in the lee of the ridges. the flow from being directed towards the Seez Valley,

(a) (b) 7.0 318 70 316 316 316 6.0 314 314 314 60 312 5.0 312 310 310 310 50 308 308 4.0 306 40 306 304 304 3.0

30 Height (km) 302 302 2.0 20 300 300

10 1.0 298 296 298 296 0 0.0 0 10203040506070 0 1020304050 Distance (km) SNDistance (km)

Figure 5. MM5 model results for 1100 UTC on 24 October 1999 (Zangl¨ et al., 2004a): (a) Surface wind field (full barb = 5ms−1; topography is shaded at intervals of 600 m), and (b) vertical cross-section of potential temperature (contour interval 1 K) with wind component parallel to the cross-section (arrows and shading, shading increment 5 m s−1, white below 10 m s−1) along the bold line indicated in (a). The vertical arrow indicates the location of the kink in the cross-section.

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 907

(a) SE/S 44% (b) NW/N 13% (c) NW/W 12% 47.4

47.3

47.2 N) ° 47.1

Latitude ( 47

46.9 10 m/s 5 m/s 10 m/s 46.8 9.3 9.4 9.5 9.6 Longitude (°E) (d) NW/S 8% (e) SE/N 6% 47.4

47.3

47.2 N) ° 47.1

Latitude ( 47

46.9 5 m/s 5 m/s 46.8 9.3 9.4 9.5 9.6 9.3 9.4 9.5 9.6 Longitude (°E) Longitude (°E)

Figure 6. Five main flow regimes at the bifurcation between the Rhine and Seez valleys: (a) south–east/south (SE/S), (b) north-west/north (NW/N), (c) north-west/west (NW/W), (d) north-west/south (NW/S) and (e) south–east/north (SE/N), where the first (second) wind direction refers to the flow direction in the Seez Valley (lower Rhine Valley). The winds at 500 m agl are measured simultaneously at Heiligkreuz, Buchs-Grabs and Malans, or at Heiligkreuz and Buchs-Grabs only, for each wind regime. Each measurement is represented by a line starting from the rawinsonde location, following the direction of the wind and having a length proportional to the wind speed. The scale is indicated by a line at the bottom left corner of each panel. Above the graph, a title indicates the wind regime under consideration by giving the directions of wind at Heiligkreuz and Buchs-Grabs. The following percentage is the probability of occurrence of the regime during the MAP SOP. even though only a very thin jet can penetrate into the the non-dimensional valley depth H = NH/U (with Seez Valley on some occasions. H being the dimensional valley depth) is the key To investigate the small-scale dynamics of flow split- parameter for the validity of the idealized model and ting between the Rhine and Seez Valleys, Drobinski et al. for the occurrence of flow splitting in reality. For fohn¨ (2001) simplified the problem by using the theory of cases with H  1, the flow splitting at the bifurcation 2D, incompressible and irrotational potential flows flow- between the Rhine and Seez Valleys was found to be very ing along sidewalls. They demonstrated the key role similar to the predictions of the idealized model. played by the valley geometry (angles between val- An example of the detailed flow structure is shown leys, valley widths) and, in particular, the complemen- in Figure 7(a), which displays the Doppler lidar radial tary contribution of the deflection and blocking/splitting velocity field at 1000 m amsl on 29 October 1999 mechanisms when flow splitting occurs. However, these at about 2000 UTC and the corresponding wind field results did not account for surface friction, turbulent mix- simulated with the MM5 model. There is evidence of flow ing, and for the channelling effect because the solutions splitting in both the simulated wind field and the Doppler for each sidewall were obtained independently of each lidar measurements, which show air blowing away from other. In a second step, Drobinski et al. (2006) conducted the Doppler lidar in the Seez and lower Rhine Valleys numerical simulations including the effects of channelling (positive radial velocity) and blowing towards the lidar and turbulent mixing. However, surface friction was from the upper Rhine Valley (negative radial velocity). A still neglected, and a highly idealized valley geome- cross-valley velocity shear is visible in the Seez Valley try was used together with a single-layer approximation with a near-wall jet (about 8 m s−1) at the northern wall of the equations of motion, similar to the well-known of the Seez Valley, while the wind speed is weaker at the shallow-water model. Comparison with observations and southern wall of Seez Valley. Figure 7(a) also shows that, fully 3D numerical simulations with MM5 indicated that near the split point, there is a very sharp radial velocity

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 908 P. DROBINSKI ET AL.

Figure 7. (a) Horizontal cross-section of Doppler lidar radial velocity (grey scale) and of MM5 wind field (arrows) at about 1000 m amsl on 29 October 1999 at about 2000 UTC. For the Doppler lidar data, positive (negative) radial velocities (indicated by the labels) denote air blowing away from (towards) the Doppler lidar (absence of shading means no reliable data). The altitude contour interval is 500 m from 500 m to 3000 m, and the marker × indicates the location of the Doppler lidar. The bold solid line indicates the section along which the along-valley  wind from the Doppler lidar, the MM5 model and the idealized model are plotted in (b) and (c). (b) shows wind speed vr normalized by the inflow wind speed U, simulated and measured along the section shown in (a). The solid line depicts the idealized simulations and the dotted line the MM5 simulations on 24 October 1999 at 1200 UTC (IOP 8). The open and filled circles are the Doppler lidar measurements of the radial velocity at 1000 UTC on 2 October 1999 (IOP 5) and at 1200 UTC on 24 October 1999 (IOP 8), respectively. (c) is as (b), but for 29 and 30 October 1999 (IOP 12). The solid line depicts the idealized simulations, and the dashed and dash-dotted lines the MM5 simulations at 2000 UTC on 29 October 1999 and at 1200 UTC 30 October 1999 (MAP IOP 12), respectively. The squares and filled squares are the Doppler lidar measurements of the along-valley wind at 2000 UTC on 29 and at 1200 UTC on 30 October 1999, respectively. gradient. The wind decelerates from about 5 m s−1 down Table III). In accordance with the idealized model, the to zero within less than two kilometres. Doppler lidar and the MM5 model (horizontal resolution  Figure 7(b,c) displays the radial wind speed vr along 200 m and 330 m, respectively) show a clear distinction the thick line indicated in Figure 7(a), which is a good between the smooth and sharp gradient regions. However, approximation to the wind speed along the streamline the Doppler lidar measurements and MM5 simulations intersecting the split point. Wind speeds are normalized do not show evidence of a sharp gradient near the by the inflow wind speed, U, in the upper Rhine Valley bifurcation point on 24 October 1999 at 1200 UTC in order to simplify the comparison of the different data (IOP 8) (Figure 7(b)). This can be traced back to the fact  sources and cases. The solid lines depict the idealized that H was only about 0.8 on that day, corresponding model result. The horizontal wind gradient decreases to a ‘flow over’ regime without a stagnation point at from about 1 at the entrance of the two tributaries the valley bifurcation. Evidently, the idealized 2D model (47.02 °N) down to 0 at the bifurcation point (47.06 °N) ceases to be valid in such a situation. with two regimes: Another fundamental difference between the chan- nelled and unchannelled flow regimes becomes evident ° °  (1) between 47.02 N and 47.045 N, vr decreases from the along-valley mass fluxes. Table III shows the smoothly from 1 to 0.8; measured mass flux in the Seez, SRsv, and lower Rhine, ° ° (2) between 47.045 N and 47.06 N (bifurcation point), SRlrv, Valleys, normalized with the mass flux measured  vr drops sharply from 0.8 down to zero. in the upper Rhine Valley (called split ratio in the table) for several IOPs (Drobinski et al., 2006). It can Drobinski et al. (2006) showed that the main mecha- be seen that the flow budget is approximately balanced  nism occurring during regime (1) is flow deflection by the (SRlrv + SRsv ≈ 1) for H  1 (IOPs 5, 8 and 12), con- external valley sidewalls whereas the main mechanism as firming that the flow is channelled and quasi-2D under the flow approaches the bifurcation point is blocking and these circumstances. One must note that simulations of splitting (regime (2)). these IOPs indicate that the fohn¨ extends only a few On 2 October 1999 (IOP 5), 29 and 30 October kilometres north of the valley bifurcation and afterwards 1999 (IOP 12), the agreement between the Doppler lidar gets lifted over the low-level cold pool, so the vertical measurements, the MM5 simulations and the idealized contraction of the fohn¨ layer eliminates the need of a simulations is very good (Figure 7(b,c). These cases were deceleration in response to the widening of the Rhine characterized by H  1 (Drobinski et al., 2006 and Valley bottom. Otherwise, for the other IOPs, there is

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 909

Table III. Split ratios in the lower Rhine (SRlrv) and Seez surface potential temperature field. As already mentioned, (SRsv) valleys.. the flow splitting at the ridge separating the Seez Valley from the lower Rhine Valley did not involve a stagnation IOP Split ratio Brunt– Normalized point on that day (Figure 8(a)). Thus, the low-level

SRlrv SRsv Vais¨ al¨ a¨ valley airflow originating from the upper Rhine Valley enters frequency depth H not only the two valleys, but part of it even ascends N (10−2 s−1) the dividing mountain ridge. The strongest subsidence of warm air occurs in the lower Rhine Valley around Vaduz, 2 (19 Sep 1999) 1.26 0.13 0.9 0.7 where south-south-easterly flow reaches the valley from 5 (2 Oct 1999) 0.70 0.35 2.0 2.1 the adjacent Ratikon¨ massif. The surface observations 8 (20 Oct 1999) 0.83 0.24 1.2 1.8 collected at Vaduz indicate that this flow feature is quite 10 (24 Oct 1999) 1.08 0.84 1.2 0.8 typical for deep fohn,¨ showing a typical surface wind 12 (29 Oct 1999) 0.67 0.39 1.7 4.2 ° ° 12 (30 Oct 1999) 0.52 0.37 2.3 5.7 direction between 150 and 170 . In the case considered 15 (5 Nov 1999) 1.20 0.11 0.7 1.0 here, the model also predicts warm-air subsidence in the Seez Valley, particularly in its western part where the air The split ratio is computed from the radiosounding (launched from enters the valley directly from the mountain range to the Heiligkreuz in the Seez Valley, Buchs-Grabs in the lower Rhine Valley south. The latter feature cannot be verified due to the and Malans in the upper Rhine Valley) velocity measurements for lack of a surface station. An important consequence of IOP 2 and 15 (when the Doppler lidar was not in operation), or from the warm-air subsidence downstream of the flow-splitting the Doppler lidar velocity measurements for IOP 5, 8, 10 and 12. The in-valley velocities are integrated vertically over the fohn¨ jet depth. point is that the surface potential temperature in the lower When the Doppler lidar data are used, the in-valley velocity is also RhineValley(andpresumablyalsointheSeezValley) integrated horizontally over the valley width. The values of N and H can be substantially higher than in the upper Rhine Valley are computed from the radiosounding launched from Malans (upstream (Figure 8(b)). In fact, Vaduz was as much as 8 K warmer sounding) or from Heiligkreuz when one was not launched from Malans than Chur in the morning of 24 October 1999 (Zangl¨ (IOP 2 and 5). et al., 2004a), and θ differences in excess of 4 K are observed quite frequently. It remains to be pointed out a substantial excess flow rate in the lower Rhine Val- that turbulent vertical mixing of stably stratified air is also ley, which can be traced back to descending air masses. capable of inducing an along-valley increase of potential In an independent calculation, Jaubert and Stein (2003) temperature. However, separating this effect from the and Beffrey et al. (2004b) found that the downward mass impact of direct warm-air advection is difficult because flux can contribute more than 30% to the total flow bud- numerical models appear to have substantial deficiencies get during deep fohn¨ events. The subsidence potentially in representing turbulent mixing in narrow Alpine valleys affects both the Seez Valley and the lower Rhine Valley (e.g. Zangl,¨ 2003). but usually tends to be more pronounced in the lower Rhine Valley. 4.3. F¨ohn/cold-pool interaction As pointed out by Zangl¨ et al. (2004a), the subsidence A cold surface layer or cold pool often fills the floor of can also have a profound impact on the temperature field Alpine valleys and prevents the upper-level fohn¨ flow in the adjacent valley segments. The simulated low-level from reaching the ground during most of the duration of flow field for IOP 10 (24 October 1999 at 1200 UTC) fohn¨ episodes. The cold pool is either present from the is displayed in Figure 8 together with the corresponding preceding (colder) weather situation or rebuilds at night

(a) (b)

300 40 40 300 298 35 35

30 30 300 298 25 25 300 298 296 20 20

298 15 15

10 10 300 5 5 298 294

0 0 0 5 10 15 20 25 30 35 40 0 5 10 15 20 25 30 35 40 Distance (km) Distance (km)

Figure 8. MM5 model results for 1200 UTC on 24 October 1999 (Zangl¨ et al., 2004a): (a) surface streamlines, (b) surface potential temperature (contour interval 1 K). In both panels, topography is shaded with an increment of 600 m.

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 910 P. DROBINSKI ET AL. when the absence of clouds permits the ground to radiate Numerical simulations of IOP 15 conducted by Jaubert freely to space. Only when the fohn¨ is sufficiently intense et al. (2005) and an observational study by Flamant et al. does the fohn¨ flow touch the floor of Alpine valleys. (2006) indicate that the presence or absence of a cold pool Three mechanisms are likely to govern the penetration of in the lower Rhine Valley is of crucial importance for the the fohn¨ flow to the valley floor: flow evolution. Jaubert et al. (2005) simulated this event using the mesoscale model Meso-NH´ with great realism. (1) the diurnal heating of the cold pool by solar radiation They used a mesoscale analysis over the whole simu- may diminish the stability and allow vertical mixing, lation domain, which turned out to improve the results (2) turbulent entrainment induced by Kelvin–Helmholtz significantly. Specifically, the mesoscale analysis allowed instability at the top of the cold pool may erode and introduction of a realistic initial cold pool that was miss- eventually destroy the pool (Nater et al., 1979), and ing in the large-scale analysis at noon. As an illustration, (3) the occasional intensification of a mountain wave at Figure 9 shows the vertical structure of the cold pool doc- higher levels may force the fohn¨ flow down to the umented through reflectivity measurements made along ground level and flush the cold pool downstream, for the Rhine Valley between 1445 and 1505 UTC with the instance in the case of a breaking wave aloft. nadir-pointing differential absorption lidar LEANDRE-2, and the potential temperature fields for two simulations Indeed, gravity-wave breaking accelerates the flow using the Meso-NH´ model – with (A12) and without beneath the wave-breaking zone and thus can increase the (REF12) mesoscale analysis as initial state – along the cold-pool erosion due to turbulent mixing or due to an ARAT flight track. In the reflectivity measurements, the enhanced pressure drag force if the cold pool has a steep cold pool corresponds to the region of high reflectivity lateral edge. Downstream advection of the cold pool is (350 arbitrary units or higher). Above the cold pool and also possible in the absence of pronounced gravity-wave below 2 km amsl, the reflectivity is generally very low, activity when the mesoscale pressure field at the top of indicating the presence of the dry fohn¨ layer. The south- the cold pool imposes a favourable forcing (e.g. Zangl,¨ ern tip of the cold pool is located just north of Weite, 2005). Of course, these cold-pool removal processes in agreement with the surface measurements. The depth may also occur simultaneously. The interaction between of the cold pool increases slowly towards the north, and the cold pool and the fohn¨ in the Rhine Valley was reaches 250 m north of Rankweil, where the reflectivity best documented during IOPs 8 and 9 (21–22 October also increases, indicating moister conditions. This depth 1999; Gubser and Richner, 2001) and IOP 15 (5 and 6 was also observed over Lake Constance (not shown, see November 1999; Vogt and Jaubert, 2004; Jaubert et al., Flamant et al., 2006). The structure of the cold pool, as 2005; Flamant et al., 2006). defined by the low potential temperature regions, bears The warming rate due to heat flux in the cold pool resemblance to that observed by lidar in simulation A12, under fohn¨ conditions could be estimated only during but the cold pool is missing in simulation REF12 (Jaubert IOP 9, as there were no dedicated aircraft flights on other et al., 2005). fohn¨ days. Gubser and Richner (2001) found that the heat The effect of the mesoscale analysis as initial state of fluxes at the surface and at the top of the cold pool were the A12 simulation was not limited to the first hours of comparable in magnitude (about 15 W m−2). Based on the simulation, but was still effective 12 hours after the these heat fluxes but without accounting for long-wave beginning of the run; a realistic cold-pool height prevents radiative cooling (which causes the air column to lose at the fohn¨ from touching the ground too early, and allows least part of the energy gained by the heat flux), they for simulating an accurate timing of the fohn¨ onset. A estimated warming rate of about 25 K (day)−1,which heat budget analysis of the interactions between the cold appears to be considerably too high. Gubser and Richner pool and the fohn¨ jet above during the late afternoon (2001) concluded that, with such a warming rate, any and evening of 5 November 1999 indicated that the cold pool would disappear within less than one day, and leading terms are the advection by the mean flow and the fohn¨ periods with more or less stationary and persistent turbulent tendency, whereas radiation tendency is weak cold pools could not occur. An important contribution to (also Flamant et al., 2006). The turbulent mixing occurs this discrepancy might arise from cold-air advection from mainly close to the terrain, in the regions where the fohn¨ the Alpine foreland. Surface observations and numerical air descends in the lee of the mountain range and then simulations (Zangl¨ et al., 2004a; Beffrey et al., 2004a; interacts with the cold pool. Jaubert et al.’s (2005) results Jaubert et al., 2005) frequently indicate a light northerly are consistent with the analysis of IOP 4–5 by Frioud flow within the cold pool, particularly when the preceding et al. (2004) and of IOP 10 by Zangl¨ et al. (2004a), who weather evolution formed a significant cold-air pool in also found that turbulent vertical mixing is important for the Alpine foreland. The pressure gradient driving this the erosion of the cold-air pool that initially fills the lower inflow is most likely due to the fact that gravity-wave Rhine Valley. dynamics tends to form a local pressure minimum at the 4.3. Advances in numerical modelling of fohn¨ in the boundary between the fohn¨ and the cold pool (Zangl¨ et al. Rhine Valley during MAP 2004a). Moreover, a gradually decreasing cold-pool depth between the Lake Constance region and the southern edge One major objective of MAP was to improve the perfor- of the cold pool might play a role. mance of high-resolution NWP, hydrological and coupled

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 911

meshes at the same time in a one-way or two-way inter- (a) 3 Lidar reflectivity active mode. The highest model resolution was about a 430 kilometre. These real-case simulations were forced and initialized with NWP analyzes and realistically repro- 410 duced several MAP fohn¨ events over their whole dura- 2 tion (IOP 12, Drobinski et al., 2003a; IOP 2, Jaubert and 390 Stein, 2003; IOP 8, Lothon, 2002). 370 However, some limits appeared with respect to model

Altitude (km) initialization (since large-scale NWP analyzes do not 350 allow for reproducing the low-level thermodynamic struc- 1 ture in the valleys), grid resolution (since a grid size of a 330 few kilometres is not fine enough to reproduce the com- 0 plex orography of the Rhine Valley and its tributaries), 0.4 and numerical diffusion over steep orography. All these (b) Pot.Temp. (K) REF12 3 problems could be solved using the FORM dataset, allow- ing more accurate and reliable numerical modelling and forecasting of the 3D structure of fohn¨ and its time evo- lution in the Rhine Valley. 2 4.3.1. Initial state The numerical studies by Jaubert and Stein (2003) and Altitude (km) Beffrey et al. (2004a), using the mesoscale model Meso-´ NH, provides evidence that a satisfactory simulation of 1 larger-scale aspects as well as meso-γ aspects of fohn¨ cases (IOPs 2 and 8, respectively) can be achieved with operational meteorological large-scale analyzes and a 0.4 (c) Pot.Temp. (K) A12 mesoscale model with nested domains. However, in these 3 two studies, the depth of the cold pool was unrealistic, probably due to a deficiency of the initial analyzes. Using the MM5 model, Zangl¨ et al. (2004a) used sounding mea- surements to modify the lower levels of the large-scale 2 analysis in the Alpine region in order to get a more realistic surface temperature, particularly in the Rhine Valley. Jaubert et al. (2005) went one step further and

Altitude (km) explored the benefits of an operational mesoscale anal- ysis scheme, capable of introducing mesoscale features 1 such as cold pools at the scale of Alpine valleys. Their simulation of IOP 15 (5–6 November 1999) initialized with the mesoscale analysis scheme improved signifi- 0.4 cantly the cold-pool dynamics and the fohn¨ life-cycle 0 20 40 60 (especially onset) as shown by comparing the model out- WeiteX (km) Rankweil Altenrhein puts with lidar (Figure 9) and wind profiler (Vogt and Jaubert, 2004) measurements. Figure 9. (a) Atmospheric reflectivity at 732 nm obtained from the airborne lidar LEANDRE-2 between 1445 and 1505 UTC on 5 4.3.2. Numerical diffusion November 1999 (IOP 15) along the lower Rhine Valley. Reflectivity units are arbitrary. The continuous white line represents the orography. Numerical simulations performed with the mesoscale (b) Vertical cross-section along the lower Rhine Valley at 1500 UTC on model MM5 for 24 October 1999 (IOP 10, Zangl¨ et al., 5 November 1999 of potential temperature (with contour interval 1 K, 2004a) demonstrate that a proper treatment of numerical light shading over 300 K, medium shading below 296 K, and darker shading below 292 K) and vertical velocity (dashed contours with diffusion is of crucial importance. As in several other interval 0.25 m s−1) from the reference Meso-NH´ simulation (REF12) mesoscale models, the numerical diffusion was origi- without mesoscale analysis. (c) As (b) for a simulation with mesoscale nally implemented as a fourth-order horizontal smooth- analysis as initial state (A12). ing operator evaluated along the terrain-following sigma coordinate surfaces without accounting for any metric terms. Over steep topography, this tends to induce large models in mountainous terrain. The numerical modelling systematic errors for variables having a strong vertical work on fohn¨ began in 1999 with the state-of-the-art stratification (temperature and the water vapour mix- mesoscale models. The models are non-hydrostatic and ing ratio). The original implementation was changed by nested model domains are used, with different horizontal Zangl¨ (2002b) into a truly horizontal computation of

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 912 P. DROBINSKI ET AL. the numerical diffusion of temperature and the mixing 30 min time resolution and 60 m vertical resolution (Vogt ratios of water vapour and cloud water. As illustrated and Jaubert, 2004) and the TWL operated at about 1 min in Figure 10 for the station at Fussach, located at the time resolution and 250 m radial resolution (Beffrey southern shore of Lake Constance, the modified diffu- et al., 2004a; Drobinski et al., 2006; Figure 7) allowed sion scheme (denoted as z-diffusion) enables the model for an unprecedented validation exercise. As an illustra- to reproduce the observed flow evolution essentially cor- tion, the radial velocity field obtained from the TWL rectly. Discrepancies between model results and observa- measurements on 20 October 1999 between 0837 and tions are largely restricted to small errors in the time of 0943 UTC (IOP 8) is compared to the radial velocity fohn¨ breakthrough or in the time of the cold front pas- field simulated with Meso-NH´ (Figure 11) (Beffrey et al., sage (1–2 hours each). However, using the original MM5 2004a). The simulated field is consistent with the obser- diffusion scheme (denoted as sigma-diffusion) greatly vations. A strong jet of incoming air (radial velocities degrades the results. The fohn¨ breakthrough at Lake Con- up to 15 m s−1) can be found in the upper Rhine Val- stance is then simulated at least 5 hours too early, and ley. At the junction of the Seez and Rhine Valleys it the agreement between simulated and observed surface splits into two branches, one along the Seez Valley and temperatures worsens even in those parts of the valley the other towards Lake Constance. In the Seez Valley, a where the occurrence of fohn¨ is predicted correctly (not strong, transverse gradient can be observed in the radial- shown). This can be explained by the fact that computing velocity field with strong winds along the northern wall the numerical diffusion of temperature along the model and almost no wind in the south. At higher levels, radial surfaces effectively destroys local cold-air pools within velocities are somewhat underestimated by the model (by valleys because the cold air is mixed with the warmer about 5 m s−1) due to an insufficient channelling by the air over the adjacent side slopes and ridges. The sever- smoothed topography of the model (the resolution for the ity of the related numerical errors is further emphasized prescribed topography is 1 km). Another explanation is by another sensitivity experiment in which the horizontal that the direction of the simulated wind is slightly dif- resolution was degraded from 1 to 3 km while retain- ferent from the actual wind direction. Although there are ing the improved diffusion scheme. As evident from small but significant discrepancies, it can be noted that Figure 10, the reduced model resolution has much less the salient features of the dynamic field revealed by the detrimental effects on the model results than using the TWL are well reproduced by the model. original diffusion scheme at 1 km resolution. A novel approach for validating high-resolution models against conventional station data was introduced by Zangl¨ 4.3.3. Model validation et al. (2004a). A model-independent 2D analysis (VERA, Vienna Enhanced Resolution Analysis; Chimani et al., The deployment of innovative remote sensors to docu- 2006) was used as an alternative to interpolating model ment at high temporal and spatial resolutions the 3D flow fields to the locations of the stations. When the analysis structure was the instrumental core for new methods of approximately equals the model resolution, this method validation of high-resolution modelling. Fohn¨ flow was helps to gain a better overview of the spatial distribution observed with Doppler lidars, sodars and wind profil- of the differences than just comparing point data. ers, and the structure of the planetary boundary layer in complex terrain with a backscatter lidar. During MAP, 4.4. FORM-related side project: Evaluation of fohn¨ mesoscale simulations were performed with horizontal impact on regional air quality during MAP mesh sizes down to 200–600 m, implying that spatially continuous measurements are crucial for a proper val- Fohn¨ conditions impair human comfort and health idation. The Rankweil RASS wind profiler operated at in Alpine regions, causing headache and circulatory

(a)300 (b) 16 Fussach, temperature Fussach, wind speed 298 14

296 ) 12 -1 294 10 292 8 290 6 288 Wind speed (m s 286 Observation 4 Potential temperature (K) 1 km, z-diffusion 284 1 km, sigma-diffusion 2 3 km, z-diffusion 282 0 6 8 10 12 14 16 18 20 6 8 10 12 14 16 18 20 Time (h) Time (h)

Figure 10. Temporal evolution of (a) surface temperature and (b) surface wind speed at Fußach (near Bregenz on Lake Constance, see Figure 2) on 24 October 1999. The simulations were conducted with MM5 with either four or three interactively nested domains, corresponding to 1 km and 3 km resolution in the finest domain (Zangl¨ et al., 2004a). z-diffusion denotes the truly horizontal diffusion scheme developed by Zangl¨ (2002b).

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 913

Figure 11. Horizontal cross-section at (a), (b) 1000 m amsl and (c), (d) 1600 m amsl of the radial velocities (colour shading, m s−1). (a) and (c) are from the transportable wind lidar (TWL) between 0837 and 0943 UTC on 20 October 1999 (IOP 8); (b) and (d) are simulated by Meso-NH´ at 0900 UTC on 20 October 1999. Arrows represent the simulated wind field; the arrow at the bottom left-hand corner represents 20 m s−1. The topography is shown by the contours (600 to 2200 m amsl) with intervals of 200 m. This figure is available in colour online at www.interscience.wiley.com/qj disturbances (e.g. Florida-James et al., 2004). Besides aerosol concentration measurements by in situ and remote these effects, air pollution can be strongly enhanced under sensors allowed investigation of the air-mass composition south fohn¨ conditions in several regions on the northern with high temporal and vertical resolution. side of the Alps (Nkemdirim and Leggat, 1978; Hoinka In the Rhine Valley, air quality strongly depends on the and Rosler,¨ 1987). Indeed, in the cold-air pool, the ozone interaction between fohn¨ and the cold pool close to Lake concentration is reduced below the free-tropospheric Constance. Indeed, the cold-air pool often persists (also background level due to chemical reactions with other during most south fohn¨ conditions) leading to enhanced pollutants, mainly nitrogen oxides. Thus, fohn¨ break- air pollution within the stagnant boundary layer which through goes along with an increase in the ozone con- proved to be an aerosol-rich layer from the backscatter centration whereas other pollutants are reduced. So ozone lidar measurements (Frioud et al., 2004), whereas fohn¨ concentrations in the valley tend to increase at the onset brings relief as clean air from above is mixed into of fohn.¨ High wind speeds and turbulence reduce the the boundary layer. The question arises whether the effects of titration by nitric oxide and dry deposition on high ozone levels found north of the Alps, e.g. in the the concentrations of ozone. This results in higher ozone Rhine Valley, during south fohn¨ are originally produced concentrations in the valley at night and in the morning in polluted areas south of the Alps or are transported hours. These ozone concentrations are usually not as high downwards from the stratosphere. Baumann et al. (2001) as the highest ozone concentrations reached during photo- showed that, during the MAP SOP, fohn-induced¨ ozone chemical smog episodes. This means that fohn¨ events do peaks in October and November were found to be not necessarily cause ozone peaks, but prolong the dura- much lower than in September. They found remarkable tion of ozone stress in fohn¨ areas. On the other hand, a spatial differences in the ozone records over a relatively penetration of the fohn¨ to the ground may bring a sudden small area of the Rhine Valley, confirming the usually relief to some valley segments when polluted air in the ‘patchy’ distribution of ozone concentrations during fohn¨ valley is replaced by usually less polluted air from above, events which reflect the separation of fohn-shielded¨ from whilst other segments of the valley remain within a shal- fohn-exposed¨ areas. The stratification within the lowest low inversion without significant air-mass exchange. Air few hundred metres, especially the presence of a cold- quality is thus of high interest in the densely populated air pool, determines whether the air mass with higher Alpine Rhine Valley. The set-up of ozone, nitrogen and ozone concentrations advected by the fohn¨ flow reaches

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 914 P. DROBINSKI ET AL. the ground or remains a few hundred metres above adjacent Inn Valley usually encounters downvalley flow, ground without or before removing the near-ground which has frequently been interpreted as cold-air outflow cold-air pool. The trajectory calculations for the fohn¨ into the Alpine foreland. Zangl¨ (2003) showed that the period confirm the general experience from previous westerly downvalley flow is locally enhanced around investigations (Seibert, 1990) that the air advected by Innsbruck due to an east–west asymmetry in the gravity south fohn¨ often originates from around 2000 m above wave activity. It is also important to note that the lower the Po basin. Nevertheless, the results of 22 October Rhine Valley tends to be more strongly affected by 1999 (IOP 9) demonstrate that air masses from the cold-air pools lying north of the Alps than the Wipp lower free troposphere can be imported into the fohn¨ Valley because the former valley exits directly into the flow due to changing meteorological conditions in the Alpine foreland. Finally, it has been found that the course of a longer fohn¨ period. In this case, the fohn¨ downvalley increase of the surface potential temperature air originating from 4000 m agl (above ground level) can be significantly stronger for the Rhine Valley than caused the most distinct increase of ozone of this fohn¨ for the Wipp Valley. In the Wipp Valley, the low-level phase at the monitoring stations in the Rhine Valley. airflow essentially follows the valley axis, so that an No vertical transport of ozone from the stratosphere and increase in surface potential temperature can occur only upper troposphere was involved in the increase of the due to turbulent vertical mixing of stably stratified air ozone concentrations in the fohn¨ valleys during the fohn¨ (Seibert, 1985; Zangl,¨ 2003). Since the Wipp Valley phase. widens considerably in the downvalley direction, the stable stratification might be reinforced by subsidence of potentially even warmer air from aloft into the valley 5. Concluding remarks region, so that the mixing-induced potential temperature increase can be quite appreciable (∼5 K). In the Rhine Looking back from some distance, the design of the Valley, values up to 8 K have been observed because, composite observing network may be assessed as quite in addition to mixing-induced warming, direct warm-air positive, even though lessons can always be learned. advection from the adjacent mountain ranges into the The composite observing system and the combination of valley is possible, particularly in the region of Vaduz. remote-sensing and in situ systems produced a wealth Finally, despite the significant progress made in fohn¨ of data which allows unprecedented insight into the understanding, modelling and forecasting thanks to the structure of the fohn¨ flow and a valley network with MAP programme, several key issues are still at best partly complex geometry. The combination of established and understood. Among those are: novel remote-sensing instruments (e.g. Doppler and water vapour lidars) with conventional in situ measurements (dense surface network and radiosondes) allowed capture (1) The fohn/cold-pool¨ interaction: the interaction of of previously unseen details of the fine structure of ambient air flow with cold pools still poses a fohn¨ (Richner et al., 2006). This allows the validation major challenge to understanding and predicting of ultra-high-resolution numerical research and weather local weather in mountainous regions. It is of cru- prediction models. cial importance not only for fohn¨ flows but also The work conducted in the framework of FORM now for wintertime warm-front passages, leading to pro- also allows the comparison of the flow characteristics nounced horizontal temperature differences and pos- of the fohn¨ in the Rhine Valley with those observed sibly large fluctuations in the height of the snow line. in the Wipp Valley region, the second fohn¨ target area As already discussed above, the most important pro- during MAP (project P4; Mayr et al., 2007) (Figure 1). cesses involved in the interaction between ambient In accordance with our findings for the Rhine Valley, flows and cold pools are turbulent erosion, radiative the importance of 3D gravity wave effects for the low- heating/cooling, interaction with orographic gravity level wind field in the valley has also been pointed waves and cold-air drainage related to an exter- out for the Wipp Valley (Flamant et al., 2002; Zangl,¨ nally imposed pressure gradient. Some of these pro- 2003; Zangl¨ et al., 2004b). However, the Wipp Valley cesses are only partly understood so far and are dif- appears to encounter shallow fohn¨ flows more frequently, ficult to represent in a numerical model. Apart from in which the essential flow dynamics can be alternatively high resolution and accurate model numerics, repre- explained with the conceptually simpler shallow-water senting these processes requires highly sophisticated model (Gohm and Mayr, 2004). Marked differences parametrizations for turbulence, radiation and clouds. also occur for the low-level flow behaviour before Regarding the model numerics, the implementation fohn¨ breakthrough. The lower Rhine Valley frequently of numerical diffusion is particularly crucial because experiences light upvalley flow within the cold pool, simple methods tend to destroy cold pools in nar- indicating a cold-air advection from the Alpine foreland row valleys. A major weakness of present turbulence opposing the fohn¨ breakthrough. In the Wipp Valley, models is that the effects of the valley topography katabatic downvalley flow usually prevails in the pre-fohn¨ (increased turbulence due to sidewall friction) are phase, except perhaps for a very short period immediately not properly accounted for, even when the turbulence before fohn¨ breakthrough (Zangl,¨ 2003). Moreover, the model is 3D. Finally, interactions between fog and

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 915

radiation are potentially important, and most micro- Beffrey G, Jaubert G, Dabas AM. 2004b. Spatial evolution of fohn¨ physical parametrizations used in mesoscale models flows in the Rhine Valley area: Quantification using high-resolution simulations. Meteorol. Z. 13: 77–82. tend to remove fog by converting the cloud water too Benech´ B, Lothon M, Berger H. 2002. ‘Analysis of the constant-volume rapidly into drizzle (Crewell et al., 2003). balloons flights above the Rhine Valley during fohn¨ events (MAP (2) The role of scale interactions in the local response experiment)’. Proceedings of the 10th AMS Conference on Mountain Meteorology, Utah. of fohn¨ to a large-scale forcing: the sensitivity of Billwiller R. 1878. Report on ‘Etude´ sur les grands mouvements de the local response of the fohn¨ to any uncertainty in l’atmosphere` et sur le Fohn¨ et le Sirocco pendant l’hiver 1876-77’. the large-scale analysis still needs to be investigated Z. Meteorol. 13: 317–320. Bougeault P, Binder P, Buzzi A, Dirks R, Houze R, Kuettner J, Smith in more detail. For example, the simulation of some RB, Steinacker R, Volkert H. 2001. The MAP special observing MAP fohn¨ events (particularly IOP 8, Beffrey et al., period. Bull. Am. Meteorol. Soc. 82: 433–462. 2004a) presumably failed due to an erroneous tim- Brinkmann WAR. 1971. What is a fohn?¨ Weather 26: 230–239. Brinkmann WAR. 1974. Strong downslope winds at Boulder, Colorado. ing in the large-scale forcing (inflow strength and Mon. Weather Rev. 102: 592–602. direction, cold front location). Improving the under- Chimani B, Steinacker R, Haberli¨ C, Dorninger M, Tschannett S. 2006. standing of scale interactions includes analyzing the Objective mesoscale analyses in complex terrain: application to fohn¨ cases during MAP. Meteorol. Z. 15: 117–125. interaction of the synoptic-scale flow with the Alpine Crewell S, Simmer C, Feijt A, van Meijgaard E. 2003. ‘CLIWA- massif as a whole, and its possible side effects on the NET Baltex Bridge cloud liquid water network’. Final Report, local characteristics of fohn¨ flow. At a smaller scale, International Baltex Secretariat, Publ. No. 26. Drobinski P, Dabas AM, Haberli¨ C, Flamant PH. 2001. On the small- the sensitivity of the amplitude, phase and temporal scale dynamics of flow splitting in the Rhine Valley during a shallow evolution of orographic gravity waves to the ambi- fohn¨ event. Boundary-Layer Meteorol. 99: 277–296. ent flow are still poorly understood, particularly in Drobinski P, Haberli¨ C, Richard E, Lothon M, Dabas AM, Flamant PH, Furger M, Steinacker R. 2003a. Scale interaction processes during the presence of wave breaking. (Lothon (2002) and the MAP IOP 12 south fohn¨ event in the Rhine Valley. Q. J. R. Lothon et al. (2003) showed evidence of enhanced Meteorol. Soc. 129: 729–753. turbulence around and above crest level; this could Drobinski P, Dabas AM, Haberli¨ C, Flamant PH. 2003b. Statistical characterization of the flow structure in the Rhine Valley. Boundary- be taken as evidence for wave breaking, since it was Layer Meteorol. 106: 483–505. presumably not boundary-layer turbulence, but there Drobinski P, Bastin S, Dusek J, Zangl¨ G, Flamant PH. 2006. is no formal proof.) However, the local evolution of Idealized simulations of flow splitting at the bifurcation between two valleys: comparison with the Mesoscale Alpine Program experiment. fohn¨ flow might depend very sensitively on the wave Meteorol. Atmos. Phys. 92: 285–306. structure. In addition, the interaction between fohn¨ von Ficker H. 1905. Einige Ergebnisse von Fohnbeobachtungen¨ im and cold pools again comes into play, as the cold Gebiet um Innsbruck im Jahr 1904. Meteorol. Z. 22: 324–327. von Ficker H. 1913. Ballonaufstiege bei Fohn.¨ Meteorol. Z. 30: pools might either be localized in some valley seg- 213–216. ments or be fed from a larger-scale cold-air reservoir Flamant C, Drobinski P, Nance L, Banta RM, Darby L, Dusek J, in the northern Alpine foreland. Hardesty RM, Pelon J, Richard E. 2002. Gap flow in an Alpine valley during a shallow south fohn¨ event: Observations, numerical (3) The model initialization issue: future data assimila- simulations and hydraulic analogue. Q. J. R. Meteorol. Soc. 128: tion will need to be conducted on a high-resolution 1173–1210. grid to allow for a proper use of the available data Flamant C, Drobinski P, Protat A, Chimani B, Furger M, Richner H, Steinacker R, Tschannett S, Gubser S, Haberli¨ C. 2006. Fohn/cold-¨ (particularly surface data). The range of validity pool interactions in the Rhine Valley during MAP IOP 15. Q. J. R. of point measurements (or line measurements i.e. Meteorol. Soc. 132: 3035–3058. radiosondes) can be highly anisotropic in mountain- Florida-James G, Donaldson K, Stone V, Budgett R. 2004. Athens 2004: the pollution climate and athletic performance. Commentary. ous terrain, which will need to be accounted for in J. Sports Sci. 22: 967–980. future mesoscale data assimilation systems. Frey K. 1953. Die Entwicklung des Sud-¨ und des Nordfohns.¨ Arch. Meteorol. Geophys. Biokl. A5(:): 432–477. Frioud M, Mitev V, Matthey R, Haberli¨ C, Richner H, Werner R, Vogt S. 2003. Elevated aerosol stratification above the Rhine Valley under Acknowledgements strong anticyclonic conditions. Atmos. Environ. 37: 1785–1797. Frioud M, Mitev V, Matthey R, Richner H, Furger M, Gubser S. We are deeply indebted to many colleagues who con- 2004. Variation of the aerosol stratification over the Rhine Valley tributed to this overview paper by providing text, figures during Fohn¨ development: a backscatter lidar study. Meteorol. Z. 13: 175–181. and other input. We also thank the many agencies of the Furger M, Drobinski P, Prev´ otˆ ASH, Weber RO, Graber WK, Neininger participating countries which, by their financial support, B. 2001. Comparison of horizontal and vertical scintillometer contributed to the success of the field experiment and the crosswinds during strong fohn¨ with lidar and aircraft measurements. J. Atmos. Oceanic Technol. 18: 1975–1988. progress of fohn¨ understanding in the complex Alpine Gheusi F, Stein J. 2002. Lagrangian description of air flows using valleys. Eulerian passive tracers. Q. J. R. Meteorol. Soc. 128: 337–360. Gohm A, Mayr GJ. 2004. Hydraulic aspects of fohn¨ winds in an Alpine valley. Q. J. R. Meteorol. Soc. 131: 449–480. References Gubser S, Richner H. 2001. ‘Investigations into mechanisms leading to the removal of the cold pool in fohn¨ situations’. MAP Baumann K, Maurer H, Rau G, Piringer M, Pechinger U, Prev´ otˆ A, Newsletter 15: available on http://www.map.meteoswiss.ch/map- Furger M, Neininger B, Pellegrini U. 2001. The influence of south doc/newsletter.htm. fohn¨ on the ozone distribution in the Alpine Rhine Valley – results Gutermann T. 1970. Vergleichende Untersuchungen zur Fohnh¨ aufigkeit¨ from the MAP field phase. Atmos. Env. 35: 6379–6390. im Rheintal zwischen Chur und Bodensee. Veroffentlichungen ¨ der Beffrey G, Jaubert G, Dabas AM. 2004a. Fohn¨ flow and stable air Schweiz. Meteorol. Zentralanstalt 18: 1–64. mass in the Rhine Valley: the beginning of a MAP event. Q. J. R. Hann J. 1866. Zur Frage Uber¨ den Ursprung des Fohn.¨ Z. Meteorol. 1: Meteorol. Soc. 130: 541–560. 257–263.

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj 916 P. DROBINSKI ET AL.

Hann J. 1891. Nordfohn¨ in Innsbruck. Meteorol. Z. 26: 239. Ruffieux D, Berger H, Stubi¨ R, Wolfe DE, Welsh DC, Webber BL. Hann J. 1882. Uber¨ den Fohn¨ in Bludenz. Sitzungsber. Math.- 2000. ‘Extreme weather events measured by ground-based remote Naturwiss. Klasse der kaiserlichen Akad. der Wiss. sensing systems at the crest of the Swiss Alps’. Pp 165–167 in Hoinka KP. 1980. Synoptic-scale atmospheric features and fohn.¨ Beitr. Procedings of the Fifth International Symposium on Tropospheric Phys. Atmos. 53: 486–507. Profiling: Needs and Technology. 4–8 December 2000, Adelaide, Hoinka KP. 1985. Observations of the airflow over the Alps during a Australia. fohn¨ event. Q. J. R. Meteorol. Soc. 111: 199–224. Schweitzer H. 1953. Versuch einer Erklarung¨ des Fohns¨ als Hoinka KP. 1990. ‘Untersuchung der alpinen Gebirgsuberstr¨ omung¨ bei Luftstromung¨ mit uberkritischer¨ Geschwindigkeit. Arch. Meteorol. Sudf¨ ohn’.¨ DLR-FB 90-30. Geophys. Biokl. A5(:): 350–371. Hoinka KP, Rosler¨ F. 1987. The surface layer on the leeside of the Seibert P. 1985. ‘Fallstudien und statistische Untersuchungen zum Alps during fohn.¨ Meteorol. Atmos. Phys. 111: 199–224. Sudf¨ ohn¨ im Raum Tirol’. PhD thesis, University of Innsbruck, Jaubert G, Stein J. 2003. Multiscale and unsteady aspects of a deep Austria. fohn¨ event during MAP. Q. J. R. Meteorol. Soc. 129: 755–776. Seibert P. 1990. South fohn¨ studies since the ALPEX experiment. Jaubert G, Bougeault P, Berger H, Chimani B, Flamant C, Haberli¨ C, Meteorol. Atmos. Phys. 43: 91–103. Lothon M, Nuret M, Vogt S. 2005. Numerical simulation of meso- Smolarkiewicz PK, Rotunno R. 1989. Low Froude number flow past gamma scale features of fohn¨ at ground level in the Rhine Valley. three-dimensional obstacles. Part I: Baroclinically generated lee Q. J. R. Meteorol. Soc. 131: 1339–1361. vortices. J. Atmos. Sci. 46: 1154–1164. Kuhn M. (ed) 1989. F¨ohnstudien. Wissenschaftl. Buchgesellschaft: Sprenger M, Schar¨ C. 2001. Rotational aspects of stratified gap flows Darmstadt. and shallow fohn.¨ Q. J. R. Meteorol. Soc. 127: 161–188. Long RR. 1953. Some aspects of the flow of stratified fluids. I. A Streiff-Becker R. 1931. Zur Dynamik des Fohns.¨ Meteorol. Z. 48: theoretical investigation. Tellus 5: 42–58. 149–152. Lothon M. 2002. ‘Etude phenom´ enologique´ du fohn¨ dans la vallee´ Vogt S, Jaubert G. 2004. Fohn¨ in the Rhine Valley as seen by a Wind- du Rhin dans le cadre de l’experience´ MAP (Mesoscale Alpine Profiler-RASS System and comparison with the non-hydrostatic Programme)’. PhD thesis. Universite´ Toulouse III – Paul Sabatier, model Meso-NH. Meteorol. Z. 13: 165–174. France. Volkert H, Gutermann T. 2007. Inter-domain cooperation for mesoscale Lothon M, Druilhet A, Benech´ B, Campistron B, Bernard S, Sa¨ıd F. atmospheric laboratories: The Mesoscale Alpine Programme as a rich 2003. Experimental study of five fohn¨ events during the Mesoscale study case. Q.J.R. Meteorol. Soc. 133: 949–967. Alpine Programme: From synoptic scale to turbulence. Q. J. R. Waibel K. 1984. ‘Statistik des Fohns¨ im Bodenseeraum im Vergleich Meteorol. Soc. 129: 2171–2194. mit dem Reusstal (Altdorf)’. Arbeitsbericht der Schweizerischen Lyra G. 1943. Theorie der stationaren¨ Leewellenstromung¨ in freier Meteorologischen Zentralanstalt, No. 118. Atmosphare.¨ Z. angew. Math. Mech. 23: 1–28. Wild H. 1868. Uber¨ Fohn¨ und Eiszeit: Der Schweizerfohn.¨ Zeitschr. Nater W, Richner H, Phillips PD. 1979. Shear instabilities and their Schweiz. Statistik. characteristics during fohn.¨ Geophys. Astrophys. Fluid Dynamics 13: Zangl¨ G. 1999. Three-dimensional idealized simulations of the fohn¨ in 215–223. the region of Innsbruck. Contrib. Atmos. Phys. 72: 243–266. Nkemdirim LC, Leggat K. 1978. The effect of chinook weather on Zangl¨ G. 2002a. Idealized numerical simulations of shallow fohn.¨ Q. J. urban heat islands and air pollution. Water, Air and Soil Pollution 9: Meteorol. Soc. 128: 431–450. 53–67. Zangl¨ G. 2002b. An improved method for computing horizontal Peppler W. 1930. Schema eines Fohnausbruchs¨ aus dem Rheintal. Das diffusion in a sigma-coordinate model and its application to Wetter 47: 78–84. simulations over mountainous topography. Mon. Weather Rev. 130: Peppler W. 1935. Uber¨ die sudlichen¨ Luftstromungen¨ auf dem Santis¨ 1423–1432. und in der freien Atmosphare¨ uber¨ dem Bodensee. Beitr. Phys. freien Zangl¨ G. 2003. Deep and shallow fohn¨ in the region of Innsbruck: Atmos. 22: 1–11. typical features and semi-idealized numerical simulations. Meteorol. Queney P. 1948. The problem of airflow over mountains. A summary Atmos. Phys. 83: 237–261. of theoretical studies. Bull. Am. Meteorol. Soc. 29: 16–26. Zangl¨ G. 2005. Dynamical aspects of wintertime cold-air pools in an Richner H, Baumann-Stanzer K, Benech´ B, Berger H, Chimani B, Alpine valley system. Mon. Weather Rev. 133: 2721–2740. Dorninger M, Drobinski P, Furger M, Gubser S, Gutermann T, Zangl¨ G. 2006. North fohn¨ in the Austrian Inn Valley: A case study and Haberli¨ C, Haller¨ E, Lothon M, Mitev V, Ruffieux D, Seiz G, idealized numerical simulations. Meteorol. Atmos. Phys. 91: 85–105. Steinacker R, Tschannett S, Vogt S, Werner R. 2006. Unstationary Zangl¨ G, Chimani B, Haberli¨ C. 2004a. Numerical simulations of the aspects of fohn¨ in a large valley. Part I: operational setup, scientific fohn¨ in the Rhine Valley on 24 October 1999 (MAP IOP 10). Mon. objectives and analysis of the cases during the special observing Weather Rev. 132: 368–389. period of the MAP subprogramme FORM. Meteorol. Atmos. Phys. Zangl¨ G, Gohm A, Geier G. 2004b. South fohn¨ in the Wipp 92: 255–284. Valley – Innsbruck region: Numerical simulations of the 24 October Roßmann F. 1950. ‘Uber¨ das Absteigen des Fohns¨ in die Taler’.¨ 1999 case (MAP IOP 10). Meteorol. Atmos. Phys. 86: 213–243. Berichte des deutschen Wetterdienstes der US-Zone 12, S. 94–98.

Copyright  2007 Royal Meteorological Society Q. J. R. Meteorol. Soc. 133: 897–916 (2007) DOI: 10.1002/qj