Fohn in the Rhine Valley during MAP: A review of its ...FOHN IN THE RHINE VALLEY DURING MAP¨ 899 is...

QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETYQ. 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 itsmultiscale 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 Zanglna Institut Pierre Simon Laplace, Service d’Aeronomie, Paris, France

b Department of Meteorology and Geophysics, University of Vienna, Austriac Institute for Atmospheric and Climate Science, ETH Zurich, Switzerland

d Central Institute for Meteorology and Geodynamics, Vienna, Austriae Centre National de Recherches Meteorologiques, Meteo-France, Toulouse, France

f Laboratoire d’Aerologie, Toulouse, Franceg MeteoSwiss, Payerne, Switzerlandh MeteoSwiss, Zurich, Switzerland

i Observatoire de Neuchatel, Switzerlandj Paul Scherrer Institute, Villigen, Switzerland

k Institute of Geodesy and Photogrammetry, ETH Zurich, Switzerlandl Institut fur Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany

m Umweltinstitut des Landes Vorarlberg, Austrian Meteorologisches Institut der Universitat Munchen, Germany

ABSTRACT: This paper summarizes the findings of seven years of research on fohn conducted within the project ‘Fohn inthe Rhine Valley during MAP’ (FORM) of the Mesoscale Alpine Programme (MAP). It starts with a brief historical reviewof fohn research in the Alps, reaching back to the middle of the 19th century. Afterwards, it provides an overview of theexperimental and numerical challenges identified before the MAP field experiment and summarizes the key findings madeduring MAP in observation, simulation and theory. We specifically address the role of the upstream and cross-Alpine flowstructure on fohn at a local scale and the processes driving fohn propagation in the Rhine Valley. The crucial importanceof interactions between the fohn and cold-air pools frequently filling the lower Rhine Valley is highlighted. In addition, thedynamics of a low-level flow splitting occurring at a valley bifurcation between the Rhine Valley and the Seez Valley areexamined. 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 Valleyregion and point out some open research questions needing further investigation. Copyright 2007 Royal MeteorologicalSociety

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 Alpinefohn studies since 1850

Fohn is a generic term for strong downslope winds expe-riencing warming at the lee of a mountain ridge. Fohnis associated with a decrease in cloudiness in the lee,is strong and gusty, and is often channelled along gapsand valleys cut into the main ridge (Brinkmann, 1971;Seibert, 1990). Fohn may cause damage due to severestorms (Brinkmann, 1974), snow melting (Hoinka, 1985),

* Correspondence to: Philippe Drobinski, Service d’Aeronomie, Uni-versite Pierre et Marie Curie, Tour 15, Couloir 15-14, 4 Place Jussieu,75252 Paris Cedex 05, France.E-mail: [email protected]

or high pollution levels (Nkemdirim and Leggat, 1978;Hoinka and Rosler, 1987), so that it is of great practicalimportance to better understand and predict the structureof the fohn flow. In the Alps, fohn occurs most frequentlyin the presence of strong synoptic-scale flow perpendicu-lar to the Alpine crest, i.e. either northerly (north fohn) orsoutherly (south fohn) flow (Hoinka, 1980). While bothflow directions are similarly frequent, the vast majorityof the scientific literature on the Alpine fohn deals withthe south fohn (e.g. Frey, 1953; Seibert, 1985). This isprobably because south fohn usually has a much strongerimpact on the local temperature than north fohn (or evenwest and east fohns which are more infrequent, how-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 southfohn, polar air mass for north fohn). Usually, fohn flowis restricted to the respective lee side of the Alpine crest,but exceptions can be found in a few regions. For exam-ple, the Inn Valley (located north of the Alpine crest, seeFigure 1) can also experience north fohn in situationsof strong northerly or northwesterly flow (Hann, 1891;Zangl, 2006).

In the scientific literature on Alpine fohn, an impor-tant distinction is made between deep fohn and shallowfohn. A fohn is termed deep when the cross-Alpine syn-optic flow extends significantly above the height of theAlpine crest (e.g. Seibert, 1985, 1990). The dynamicsof deep fohn is strongly influenced by vertically prop-agating gravity waves. Due to their three-dimensional(3D) dispersion characteristics, gravity waves excitedover mountain ranges encompassing a valley can alsoaffect the flow dynamics in the valley proper, leadingto pronounced wind maxima under suitable topographicconditions (Zangl, 2003). On the other hand, shallowfohn flow is restricted to a relatively small number ofdeep valley transects in the Alpine crest. So far, shal-low fohn has been reported from southerly directionsonly, occurring under approximately westerly synoptic-scale flow conditions. Shallow fohn frequently precedesdeep fohn when the synoptic-scale flow direction grad-ually backs from west to south-west or south (Seibert,1990). However, there are also shallow fohn cases thatdo not develop into a deep fohn. The mesoscale dynam-ics of shallow fohn have been investigated by Sprengerand Schar (2001) and Zangl (2002a). They found thatthe synoptic-scale pressure gradient related to geostroph-ically balanced westerly flow, frictional flow deflectiontowards the lower pressure, and cross-Alpine tempera-ture contrasts with cold air lying in the south play animportant role in generating shallow fohn. On the localscale, shallow fohn flow is mainly governed by hydraulicdynamics (Flamant et al., 2002; Gohm and Mayr, 2004).

Fohn research has a long history in the Alps, reachingback to the middle of the 19th century. Already in 1866,Hann recognized that adiabatic warming in the lee of theAlpine crest is the main reason for the fohn being warmand dry, rejecting earlier hypotheses that assumed the

fohn to originate from the Sahara desert (Hann, 1866,reprinted in Kuhn, 1989). Hann also found that latentheat release related to orographic precipitation is oneimportant factor contributing to the temperature differ-ence between the windward side and the lee side of theAlps. However, in contrast to subsequent textbook ver-sions of the so-called thermodynamic fohn theory, he didnot claim that latent heat release is the only relevant fac-tor (Seibert, 1985). In fact, cold-air blocking over thePo basin may give rise to much larger cross-Alpine tem-perature differences than could be explained by latentheat release (Seibert, 1985, 1990). While the thermody-namic explanation for the warmth of the fohn was readilyaccepted in the scientific community, there was a lot ofdebate on dynamical aspects of the fohn, particularly thequestion how the fohn is able to penetrate into Alpinevalleys and to remove the denser cold air lying there.One of the earliest theories was proposed by Wild (1868),who hypothesized that the fohn ‘sucks’ the cold air outof the valleys in a way similar to a vacuum cleaner.This was questioned by Billwiller (1878) who ascribeda more passive role to the fohn flow, merely replacingthe cold air that had been driven away by some synoptic-scale pressure gradient. These theories survived severaldecades and gave rise to a remarkably heavy dispute (e.g.von Ficker, 1913; Streiff-Becker, 1931). Entirely differenthypotheses were brought up in the mid-twentieth century,for example by Roßmann (1950) who ascribed the downs-lope acceleration of the fohn to evaporation of cloudwater and spilled-over precipitation. (Seibert, 1985, gavea critical review of fohn theories; article reprints appearedin Kuhn, 1989.)

With the advance of the theories of orographic gravitywaves (Lyra, 1943; Queney, 1948) and shooting hydraulicflows (Schweitzer, 1953; Long, 1953), a deeper under-standing and more complete picture than provided by theearly fohn theories became available. Nevertheless, thevariety of different hypotheses reflects the important factthat local flow patterns related to fohn differ stronglybetween various Alpine valleys. Wild (1868) and Streiff-Becker (1931) observed that the southerly fohn startsclose to the Alpine crest and then gradually penetratestoward the north, whereas light opposing (northerly) flow

Hei

ght a

bove

sea

leve

l (km

)

0

0.5

1

1.5

2

2.5

3

3.5

8.5 9 9.5 10 10.5 11 11.5 12 12.546.5

47

47.5

Longitude (°E)

Lat

itude

(°N

)

Rhine valleytarget area

Wipp valleytarget area

Figure 1. Topography of the Alps. The two boxes indicate the Rhine Valley target area and the Wipp/Inn Valley target area for fohn studiesduring 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-willer (1878) and von Ficker (1905, 1913) observed out-flow of cold air into the Alpine foreland before the onsetof the fohn. Recently, Zangl (1999, 2003) demonstratedthat these different flow patterns are related to differ-ent orientations of the valleys. Valleys aligned with theimpinging flow tend to exhibit the former flow patternwith converging flow at the leading edge of the fohn zonebecause the gravity-wave pattern favours a local pressureminimum there. Cold-air outflow at low levels appears tobe more typical for valleys perpendicular to the imping-ing flow that are less favourable for a direct penetrationof the fohn.

Although fohn research in the Rhine Valley has afairly long tradition, research activities prior to MAP(Mesoscale Alpine Programme) largely concentrated onthe vicinity of Innsbruck, presumably because of the uni-versity institute located there (a comprehensive overviewis provided by Seibert, 1985; also Hoinka, 1990; Zangl,2003). Probably the first climatological evaluation offohn in the Rhine Valley was done by Hann (1882).Based on a 17-year record from Bludenz, he found abroad frequency maximum from autumn through spring(about 10 days per season) and a pronounced minimumin summer. Peppler (1930) investigated kite soundingsand pilot balloon ascents that were conducted opera-tionally at Friedrichshafen (located at the northern shoreof Lake Constance) for more than 20 years. Peppler

(1935) chose all cases with southerly wind directionsbetween 1 and 3 km above ground to detect typicalprofile structures during fohn conditions. Particularlyin spring, positive temperature deviations of 3 °C withrespect to the climatological average were found at 1to 2 km altitude. Near the ground, inversions were fre-quently observed in the kite soundings. Studying thespatial flow variability along the Rhine Valley, Guter-mann (1970) found that fohn air near Chur is gener-ally cooler than near Vaduz, which he explained withthe influence of katabatic outflow from tributaries. Fur-ther statistical investigations, including a comparison offohn frequencies in the Rhine Valley and the Reuss Val-ley (located in central Switzerland, Figure 2), were con-ducted by Waibel (1984). A more complete review offohn research in the Rhine Valley is provided by Richneret al. (2006).

Section 2 lists the reasons for selecting the Rhine val-ley as a target area for fohn studies during MAP andthe main scientific objectives. It also gives a summary ofthe fohn events in the Rhine Valley during MAP. Sec-tion 3 presents the main challenges identified before thefield experiment to be addressed in terms of observationand modelling. Section 4 synthesizes the main findingsin terms of observation, numerical simulation and the-ory of fohn. Finally Section 5 concludes this overviewof results and point out some open research questionsneeding further investigation.

AltenrheinDiepoldsau

Feldkirch

MaseinGütsch

Chur

Sargans

Kunkelspass

Zurich

Rhine valley

Rhi

ne v

alle

y

Walgau

Reuss valley

Seez valley

Praettigau

Val

Lum

nezi

a

Dom

leschg

Tam

ina

Weis

stannen

Lake of Zurich

Weis

stann

en

Tam

ina

SV

LRV

URV

Heiligkreuz

Vaduz

Buchs-Grabs

Malans

Weite

Sargans

Bad Ragaz

(a)

(b)

Rankweil

Julier

Bregenz

Rätikon range

Lake Constance

Hoher Kasten

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 themost instrumented area (see Section 3.1). The contour interval is 500 m from 500 m to 3000 m altitude. Italics indicate names of valleys, andcaptions 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 theeastern 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



2. Selection of the Rhine Valley as an avenue forfohn flows in a valley with complex geometry

2.1. Scientific motivation

Since the aforementioned findings strongly suggest thatfohn research should consider various regions of the Alps,it was decided to select the Alpine Rhine Valley as a tar-get area for fohn research during MAP (see also Volkertand Gutermann, 2007). The research area extends fromthe Alpine crest to the northern Alpine foreland and fea-tures several low passes in the main Alpine crest (seeFigure 2), which provides an excellent opportunity tostudy the development of shallow fohn. Moreover, themain valley and its tributaries cover a wide range of val-ley orientations, allowing investigation of the effects ofvalley orientation on the dynamics of fohn. The selectionof the Rhine Valley was also motivated by the facts that itis well equipped with operational meteorological measur-ing networks and that there is a high practical significancefor any improvement in forecasting the onset or cessationof fohn for storm warnings on Lake Constance. Com-pared to the other target area (the Wipp Valley/Innsbruckregion; project P4; Mayr et al., 2007), the Rhine Valleyhas a significantly more complex topographical structure.In particular, the valley axis of the Wipp Valley is almoststraight while the Rhine Valley has several marked kinks.Moreover, the Rhine Valley exits into the Alpine forelandwhereas the Wipp Valley ends in another inner-Alpinevalley (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-tal set-up did not allow for a coherent 3D documenta-tion of the fohn from the synoptic scale to the valleyscale, providing sufficient resolution in space and timeto analyze the processes governing the spatio-temporalevolution of fohn. This gap was attempted to be closedduring MAP with instrumentation of unprecedented den-sity, including surface and radiosonde stations, a varietyof remote-sensing instruments and several research air-craft. An overview of MAP, its strategy, the projects, andpreliminary results are given by Bougeault et al. (2001).Among the MAP objectives, the program FORM (Fohnin the Rhine Valley during MAP), was designed to study(Richner et al., 2006):

(1) the dynamics of that part of the blocked, potentiallycooler air mass that typically reaches up to the meancrest height on the windward side of the main ridgeand which flows through deep Alpine passes into thelee-side valleys (shallow fohn);

(2) the interaction between low-level and mid-tro-pospheric fohn flows on the scale of large Alpinevalleys including the improvement of understandingand forecasting of fohn-related phenomena like tur-bulence;

(3) the mechanism of temporal and spatial evolution andcessation of fohn flows in complex valley systems ona local scale;

Table I. Fohn episodes during the Special Observing Period ofthe MAP field experiment (7 Sept 1999 to 15 Nov 1999).

IOP Beginning End

1 15 Sep 1999 0750 15 Sep 1999 17002 19 Sep 1999 0050 20 Sep 1999 0700– 22 Sep 1999 0750 23 Sep 1999 1130– 30 Sep 1999 0420 30 Sep 1999 08405 2 Oct 1999 0810 3 Oct 1999 03207 18 Oct 1999 1340 18 Oct 1999 16508 19 Oct 1999 2320 21 Oct 1999 14109 22 Oct 1999 0640 23 Oct 1999 0330

10 24 Oct 1999 0210 24 Oct 1999 192012 30 Oct 1999 0900 31 Oct 1999 031013 1 Nov 1999 1200 2 Nov 1999 161015 5 Nov 1999 0610 6 Nov 1999 0940

The dates and times (UTC) indicate the onset and end of fohn eventsin the Rhine Valley detected by the multi-parameter algorithm ofGutermann (1970). Twelve fohn episodes were observed during 10IOPs; two fohn episodes occurred outside IOPs.

(4) the interaction of the fohn flow with the boundarylayer and the removal process of the cold-air pool;

(5) the interaction between the fohn and air blowing inthe side valleys;

(6) the dynamics of flow splitting at the bifurcationbetween the Rhine and Seez valleys; and

(7) the requirements to improve the forecast skills offohn in the Alpine region.

2.2. Summary of the fohn events in the Rhine Valleyduring MAP

The special observing period (SOP) of MAP took placein autumn from 7 September to 15 November 1999,corresponding to the maximum of the climatological fohnfrequency in the northern Alps. During the MAP SOP,twelve fohn episodes were observed. Ten fell into anintensive observing period (IOP) and two were outside.The start and end of each episode are given in Table I.The detection of fohn and the determination of the startand end time of the episodes were conducted accordingto the multi-parameter algorithm of Gutermann (1970),which was applied to numerous surface measurementsacquired in the Rhine Valley and its main tributaries.The algorithm is based on a thresholding of mean andgust valley winds, temperature increase >3 K and lowhumidity during the whole period. The fohn episodeswere determined for each station individually. A fohnevent is defined by the earliest onset and by the latestend at any of the considered stations on the valley floor.Altogether, fohn covered 244 hours 20 min, i.e. about15% of the whole SOP (1464 hours) or 29% of the IOPs(836 hours). In most events, the fohn flow first reachesthe valley floor in the area of the Flascherberg (halfwaybetween Sargans and Malans; Figure 2). The fohn alsoends later in this area. Frequently, the fohn flow aloft doesnot extend downwards to these well-exposed points in thevalley because the valley is filled by a stagnant cold-air



pool. Thus, fohn is observed more frequently on passes ormountaintops (‘pass fohn’) than in valleys (‘valley fohn’).

These MAP fohn events have been studied by Bau-mann et al. (2001, all IOPs, with IOPs 8, 9 and 10 indetail), Beffrey et al. (2004a, IOP 8), Drobinski et al.(2001, IOP 5; 2003a, IOP 12; 2003b, all IOPs; 2006,IOPs 2, 5, 8, 10, 12 and 15), Flamant et al. (2006,IOP 15), Frioud et al. (2004, IOPs 4 and 5), Gubserand Richner (2001, IOP 9), Jaubert and Stein (2003,IOP 2), Jaubert et al. (2005, IOP 15), Lothon et al.(2003, IOPs 2, 5, 8, 13, 15), Vogt and Jaubert (2004,IOP 15) and Zangl et al. (2004a, IOP 10). In spite oftheir specific features linked to fohn intensity, these stud-ies allowed the building of a comprehensive scheme ofthe temporal evolution of the fohn in a large valley, whichis useful to validate numerical simulations.

3. Challenges for new measurements and modellingtools at the beginning of MAP

3.1. Observation network

As already mentioned, the instrumental set-up usedin earlier experiments did not allow for detailed 3Ddocumentation of the fohn flow over the wide range ofrelevant spatial and temporal scales. During MAP, themain principle of the design of the observing systemwas to optimally combine the more or less continuouslyobserving remote-sensing systems with only sporadicallyactive, but usually more accurate, in situ observations.An extensive description of the instrument set-up can befound in Richner et al. (2006).

To address the scientific issues summarized above, thevery-fine-scale 3D structure of the fohn and its timeevolution have been documented by means of remotesensors operated continuously or during IOPs only. Atransportable wind lidar (TWL) located in Vilters nearBad Ragaz (Figure 2) provided radial wind velocity mea-surements along the line-of-sight. It proved to be akey instrument for investigating flow-splitting dynamicsat the bifurcation between the Rhine and Seez Valleys(Drobinski et al., 2001, 2003a, 2006) and validating high-resolution simulations (Beffrey et al., 2004a; Drobinskiet al., 2006). Five Doppler sodars and two wind profilingradars (one equipped with a radio acoustic sounding sys-tem, RASS) contributed to validate high-resolution simu-lations of the life cycle of fohn (Vogt and Jaubert, 2004).Despite its location outside the Rhine Valley target area,the UHF profiler set up at Julier Pass was a key instru-ment to study the dynamics at one of the main passes thatfeed the Rhine Valley network (Ruffieux et al., 2000).

The cross-Alpine flow structure of gravity wavesrelated to fohn has been investigated using constant-volume balloons launched from Ispra (Italy) locatednear Lago Maggiore. The balloons served to documentthe dynamics of the isopycnic airflow (pressure, windspeed and direction, temperature and humidity content;Benech et al., 2002) and particularly the characteristics

of vertically propagating and trapped gravity waves (loca-tion, amplitude, period, intensity) (e.g. Drobinski et al.,2003a). Two scintillometers located in the lower RhineValley (the transmitters at Triesenberg, the receivers atFlusa and Ergellen; Figure 2(b)) allowed for measuringthe vertical and horizontal wind components and for doc-umenting the gravity-wave penetration into the lowerRhine Valley during strong fohn windstorms (Furgeret al., 2001; Drobinski et al., 2003a). Closely connectedto the wave monitoring by the sodars was the operation ofmicrobarographs, which made possible the determinationof the relevant characteristics (direction of propagation,phase speed, and wavelength) of gravity waves on topof the cold pool by detecting the related pressure sig-nal on the ground (Flamant et al., 2006). Several aircraftflying over the Rhine Valley target area (French Mer-lin IV and ARAT Fokker-27, UK C-130, Swiss MetairDimona, USA NCAR Electra and NOAA P-3) measuredaveraged mean and turbulent variables (wind speed anddirection, vertical wind, temperature, pressure, humid-ity, turbulent kinetic energy, momentum and heat fluxes),providing information on gravity-wave activity in relationto cold-pool erosion (Gubser and Richner, 2001; Jaubertet al., 2005; Flamant et al., 2006). Finally, the cold-poolremoval was also monitored by three continuously run-ning cameras mounted at ∼1800 m amsl (above meansea level) on Hoher Kasten. Indeed, the turbidity in thecold pool is always significantly greater than in the fohnair, making the boundary between the two air masses andits variation visible over time.

To investigate the interaction of the fohn flow betweena main valley and a tributary, six surface stations wereset up along the Brandner Valley (in the Austrianprovince of Vorarlberg) and traverses were made withan instrumented car measuring pressure, temperature,humidity, and wind.

Finally, for MAP the already dense network of conven-tional meteorological surface stations in the Rhine Valleytarget area was completed with 14 additional surface sta-tions and 9 radiosonde stations. This extremely densestation network was necessary to resolve small-scale fea-tures and to achieve the same resolution for observationaldata as for numerical weather prediction (NWP) models.This was vital for the validation of the models. This net-work also contributed to the derivation of better initialconditions for research and NWP models (Zangl et al.,2004a; Jaubert et al., 2005) and the mesoscale analyzesextensively used for fohn investigations in the RhineValley (Drobinski et al., 2003a; Chimani et al., 2006; Fla-mant et al., 2006).

As fohn-related aspects, air quality issues were inves-tigated by the means of a tethered balloon operatedat Fussach and a cable car located at Bregenz, nearLake Constance, to obtain high-resolution vertical pro-files of meteorological variables and ozone (Baumannet al., 2001). In addition, a vertically pointing backscat-tering lidar located near Sargans monitored aerosol layerscontinuously with almost no interruption during the entirefield phase (Frioud et al., 2003, 2004).



3.2. Numerical modelling

The main objectives of the numerical modelling effortsmade in the context of MAP were

(1) to document the capability of state-of-the-art meso-γ -scale numerical models to simulate air flow andprecipitation fields in complex Alpine orography, and

(2) to improve the process understanding of the underly-ing interactions between atmosphere and orography.

For the FORM project, the specific research challengeswere to simulate the detailed characteristics and evolutionof fohn flows in complex valley systems and to betterunderstand the underlying flow dynamics. The practicalimportance of predicting meso-γ -scale aspects of fohninside Alpine valleys is very high since fohn representsa weather risk to all outdoor activities including airoperations, and also influences air quality. Moreover,the uniquely dense dataset collected during MAP wasexpected to offer one of the first opportunities to clearlydemonstrate the possible benefits of future real-time,meso-γ -scale NWP. Necessary ingredients for a highforecasting skill of high-resolution numerical models areexpected to be:

(1) an accurate simulation of larger-scale aspects, suchas the upstream and downstream wind and stability,

(2) a good initial analysis of the low-level atmosphericstate inside the valleys under consideration, and

(3) a high-resolution model with a proper representationof orography and sophisticated parametrizations forphysical processes like radiation and turbulence.

4. Key findings of the post-SOP period inobservation, simulation and theory

4.1. Upstream and cross-Alpine flow structure duringfohn events

The upstream conditions largely determine the temporal,horizontal and vertical extension of the fohn withinand over the Rhine Valley. The key parameters are:(1) �θ and �p, the potential temperature and mean-sea level pressure (mslp) differences across the Alps,(2) H = NH/U , the non-dimensional height, whereN is the Brunt–Vaisala frequency, H the dimensionalmountain height and U a characteristic upstream windspeed, and (3) the upstream flow direction. Generally,H ∼ O(1) implies that air parcels mainly flow over themountain and substantial nonlinear effects occur (suchas hydraulic jumps or large-amplitude gravity waves),whereas for H � 1 much of the airstream is divertedaround the flanks of the mountains and the perturbationenergy mainly appears in the horizontal with generationof vortices rather than vertical motions (Smolarkiewiczand Rotunno, 1989). The wind direction determinesif the fohn flow across the Alps is deep or shallow.Finally, experience shows that the occurrence of fohnis strongly correlated with the cross-Alpine pressure

difference (Seibert, 1985, 1990). For the Rhine Valley,it has been found that the fohn tends to reach thevalley bottom when the pressure difference betweenLugano and Vaduz exceeds about 5 hPa (Richner et al.,2006), corresponding to a cross-Alpine pressure gradientof about 4 hPa (100 km)−1. Table II summarizes theupstream key parameters for all the fohn IOPs whichcan be compared to their climatology detailed in Richneret al. (2006).

During typical fohn events, H decreases with time andthe fohn flow deepens with time as the ambient flowintensifies and gets more southerly. In the early stage,the flow regime is often a ‘flow around’ the Alps duringfohn with H > 3, switching to a ‘flow over’ regime asH ⇒ 1 and keeps its minimum value for several hoursdepending on the events. This fraction of time can be halfof the duration of the event. The jet associated with flowsplitting at the scale of the Alps and flowing along thewestern flank of the range may affect the altitude of thefohn jet in the Rhine Valley. It is capable of triggeringcold-air intrusions from the north into the Rhine Valley(Jaubert et al., 2005), lifting the fohn flow passing overthe Alpine range off the ground in the lower Rhine Valley.These intrusions are favoured when the strong flow-around jet is positioned close to the Rhine Valley outletand is oriented orthogonally to it. Local topography, i.e.the small mountain range east of the Lake Constancebasin, makes it easier for the air to enter the Rhine Valley,due to channelling (Figure 2).

The meridional extension of the fohn within the valleystrongly depends on the upstream conditions (Lothon,2002):

(1) in the southern part of the Rhine Valley (e.g. aroundChur), light downslope wind appears as soon asthe upstream conditions are favourable for fohn,and its intensity does not depend strongly on theseconditions;

(2) in the central region of the Rhine Valley (e.g. aroundVaduz), the downslope wind requires smaller H toreach the valley bottom, and its intensity is stronglycorrelated with the intensity of the upstream flow;

(3) finally, in the northern part of the valley (e.g. LakeConstance), the downslope wind occurrence dependson the upstream conditions but also on the local anddownstream conditions. The cold pool which is oftenpresent (see next subsections) in this region interactswith the fohn and only strong upstream flow and lowH can trigger a downslope wind that reaches theground in this area.

Downstream turbulence was measured using the Mer-lin IV aircraft during MAP (Lothon et al., 2003). Themeasurements showed that in addition to organized prop-agating or trapped gravity waves, at a 10-km scale tur-bulent plume exists in the wake of the mountain withlarge turbulent kinetic energy (TKE) and dissipation rate(Figure 3). This plume extends from 3000 m altitudeabove the southern part (maximum measured TKE) down



Tabl

eII

.Te

mpo

ral

mea

nva

lues

ofth

ere

leva

ntup

stre

am,

cros

s-A

lpin

ean

ddo

wns

trea

mflo

wpa

ram

eter

sfo

rth

efo

hnIn

tens

ive

Obs

erva

tion

Peri

ods.

IOP

1IO

P2

IOP

3IO

P4

IOP

5IO

P6

IOP

8IO

P9

IOP

10IO

P12

IOP

13IO

P15

Sept

embe

r→O

ctob

er→

Nov

embe

r→�

p(h

Pa)

4.8

8.2

5.7

6.5

7.1

4.0

10.5

10.7

9.7

8.0

4.7

5.8

�θ

(K)

4.7

6.8

4.3

−0.2

4.6

0.5

8.6

8.3

7.3

9.1

0.7

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late

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ndin

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yer)

.



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 IVaircraft 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 RhineValley (minimum measured intensity). The scale of theturbulence can be characterized by the dissipative length-scale Lε = e3/2/ε, where e is the TKE, and ε the TKEdissipation rate. This length characterizes the size of thelargest eddies that lie in the inertial subrange, and is ofhigh importance in TKE equation closure of mesoscalemodels. We found Lε = 1500 ± 500 m, with no signif-icant variation with altitude and height. This is typicalfor Lε measured in homogeneous convective boundarylayers, although TKE dissipation rate can vary by sev-eral orders of magnitude depending on the complexity ofthe terrain. This lends further support to this length-scaleas a robust key parameter to be used in the mesoscalemodels.

4.2. Dynamics of fohn in the Rhine Valley

4.2.1. Fohn propagation in the complex valley network

As mentioned previously, the life-cycle of a fohn usuallybegins with a shallow-fohn phase followed by a deep-fohn phase. One objective of FORM was to analyze thefohn propagation in the complex valley network in thearea of the Rhine Valley. In fact, previous studies onthe channelling effect of the fohn flow by major Alpinevalleys considered only north–south oriented portions ofAlpine valleys. In the FORM target area, the Rhine Valley

has a very complex shape: south of 46.8 °N, the RhineValley is oriented west–east, while north of 46.8 °N, ithas a more south–north orientation up to Lake Constance(Figure 2). Several major valleys merge with the RhineValley, namely the Val Lumnezia, the Domleschg, thePrattigau, the Seez Valley and the Walgau. At thebifurcation between the Rhine and Seez Valleys, theTamina gorge and the Weisstannen Valley also play animportant role. In other words, the Rhine Valley can notbe considered as a single transect in the Alpine ridgesince all these tributaries play a potential role in thecirculation of the fohn flow in the FORM target area.As a consequence, a second objective in this section is toexamine the role of the Alpine valley network in directingthe fohn flow towards the FORM target area from its earlystage of development (shallow fohn) until its breakdown.

During shallow fohn, the air which reaches the Rhinevalley area is potentially cooler and comes from thesouthern side of the Alps through Alpine gaps (passes).Due to the stably stratified air mass and/or the height ofthe cold pool lying south of the Alps, the height of thepasses is also crucial. As an example for the shallow-fohnphase, the wind field on 29 October 1999 at 1800 UTC(Figure 4) simulated with the mesoscale model Meso-NH provides evidence for weak southerly wind throughthe Reuss Valley and Domleschg which remains confinedbelow the crest line (Drobinski et al., 2003a).



(a)

Lat

itude

(N

)

47.43

46.428.34 10.05

3000.

2500.

2000.

1500.

1000.

500.0

(c)

Lat

itude

(N

)

47.43

46.428.34 10.05

3000.

2500.

2000.

1500.

1000.

500.0

Longitude (E)

(e)

Lat

itude

(N

)

47.43

46.428.34 10.05

3000.

2500.

2000.

1500.

1000.

500.0

(b)

47.43

46.428.34 10.05

3000.

2500.

2000.

1500.

1000.

500.0

(d)

47.43

46.428.34 10.05

3000.

2500.

2000.

1500.

1000.

500.0

(f)

Longitude (E)

47.43

46.428.34 10.05

3000.

2500.

2000.

1500.

1000.

500.0

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 29October 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 atChur is fed by fohn (combined with katabatic flow;Drobinski et al., 2003a,b) blowing in the upper RhineValley. At higher levels (Figure 4(c), (e)), the air fromthe Domleschg partly goes straight to the north overthe Kunkelspass (which is about 1300 m amsl), partlytowards Chur in the upper Rhine Valley. The reasonwhy Val Lumnezia does not channel the shallow fohnis that probably the air does not reach high enough tocross all passes feeding into the upper Rhine Valley.Indeed, Val Lumnezia seems to be closed off to the southby a pass much higher than the Domleschg valley. At1500 m amsl, katabatic drainage flow blows from thezonally oriented Prattigau. At the bifurcation betweenthe Rhine and Seez Valleys, the flow comes from theTamina gorge and the upper Rhine Valley (the flow first

splits between these two valleys near Kunkelspass), andsplits between the Seez and Rhine Valleys. One can notea strong intensification of the flow in the lower RhineValley near Feldkirch where the flow originating fromthe flow-splitting at the bifurcation of the Rhine and SeezValleys merges with flow blowing from the east–westoriented Walgau.

During the deep-fohn phase, when fohn reaches theRhine Valley, the upstream upper-level flow veers to thesouth/south–west and the stability decreases. The useof Lagrangian tracers in numerical models (Gheusi andStein, 2002) shows that the air reaching the ground atthe northern edge of the Alps originates from a level of2000 to 3500 m in the south (e.g. Lothon, 2002; Jaubertand Stein, 2003). The air mass accelerates as it flowsover the ridge and the Rhine Valley, and experiences



mountain waves and associated strong downward motionpenetrating down to the Rhine Valley (Furger et al.,2001 for IOP 8; Drobinski et al., 2003a for IOP 12). Thedownslope wind on the northern side generates turbulencein the Rhine Valley, as indicated by various numericalsimulations (Jaubert and Stein, 2003 for IOP 2; Drobinskiet al., 2003a for IOP 12; Zangl et al., 2004a for IOP 10;Beffrey et al., 2004a; Jaubert et al., 2005 for IOP 15)as well as observations (Lothon et al., 2003 for IOPs 2,5, 8, 13, and 15). As an example for the deep-fohnphase, the wind field on 30 October 1999 at 1200 UTC,simulated with Meso-NH, is shown in Figure 4. Asthe upstream upper-level flow is quasi-aligned with thetransverse valleys, the channelling efficiency of thesevalleys in directing the fohn flow towards the FORMtarget area increases. Val Lumnezia plays a significantlymore important role than during the shallow fohn whereasthe Prattigau does not channel the fohn flow.

To further illustrate the importance of orographicgravity waves for the low-level wind field in the RhineValley, Figure 5 displays the near-surface wind field anda vertical cross-section of wind and potential temperaturealong the lower Rhine Valley on 24 October 1999 at1100 UTC (IOP 10; Zangl et al., 2004a). As evidentfrom Figure 5(a), a pronounced surface wind maximum isfound in the valley segment between Sargans and Buchs-Grabs (Figure 2). The presence of this wind maximumis supported by observational data, and the surfaceobservations gathered during the full MAP SOP revealthat a wind maximum is frequently encountered in thisregion. Figure 5(b) indicates that vertically propagatingorographic gravity waves are responsible for this windmaximum. Due to their 3D dispersion characteristics,gravity waves excited over the adjacent mountain ridgesradiate toward the valley axis while propagating upwards,thus influencing the wind field over the valley axis in asimilar way as in the lee of the ridges.

4.2.2. Flow splitting at the bifurcation between theRhine and Seez Valleys

One of the objectives of FORM was to identify

(1) the factors determining whether flow splitting occursor not between the Rhine and Seez valleys, and

(2) the valley into which the fohn is directed.

The flow structure at the junction between the Rhineand Seez Valleys has been characterized statistically at thesynoptic scale and at the scale of the valley by Drobinskiet al. (2003b). The results reveal that the flow regimes inthe Rhine and Seez Valleys are, as expected, dominatedby the orography. In the Seez Valley, the wind directionis parallel to the valley axis in the vast majority of cases.In the lower Rhine Valley, the wind direction also fre-quently follows the main valley axis but is also influencedby the small pass to the west as well as by the orograph-ically undisturbed synoptic-flow direction. This impliesthat the lower Rhine Valley from Bad Ragaz to LakeConstance is less efficient in channelling the flow thanthe Seez Valley. Drobinski et al. (2003b) found five mainflow patterns: south-east/south, north-west/west, north-west/north, north-west/south, south-east/north, where thefirst (second) wind direction refers to the Seez Val-ley (lower Rhine Valley) (Figure 6). The flow splittingbetween the Rhine and Seez Valleys (south-east/southflow regime) prevails and occurs either during fohnevents or is due to katabatic flow. In fact, 75% of thesouth-east/south cases outside fohn events were observedbetween 1800 and 0600 UTC. The very high probabilityof flow splitting occurrence indicates that flow separationfrom the western wall of the lower Rhine Valley nearMalans (where the Rhine Valley makes a sharp turn fromsouth/north orientation to south–east/north–west orienta-tion when looking along the river flow) does not preventthe flow from being directed towards the Seez Valley,

7.0

6.0

5.0

4.0

3.0

2.0

1.0

0.0

Hei

ght (

km)

0 10 20 30 40 50S NDistance (km)

(a)

0 10 20 30 40 50 60 70

Distance (km)

70

60

50

40

30

20

10

0

316

(b)

314

312

310

296

298

300

302

304306

308

318

316 316

314314

312

310 310

308306

304

302

300

298 296

Figure 5. MM5 model results for 1100 UTC on 24 October 1999 (Zangl et al., 2004a): (a) Surface wind field (full barb = 5 m s−1; topographyis shaded at intervals of 600 m), and (b) vertical cross-section of potential temperature (contour interval 1 K) with wind component parallelto 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.



47.4

47.3

47.2

47.1

47

46.9

46.8

SE/S 44%

NW/S 8%

Lat

itude

(°N

)

47.4

47.3

47.2

47.1

47

46.9

46.8

Lat

itude

(°N

)

NW/N 13% NW/W 12%

9.3 9.4 9.5 9.6Longitude (°E)

9.3 9.4 9.5 9.6

Longitude (°E)

9.3 9.4 9.5 9.6

Longitude (°E)

SE/N 6%

10 m/s10 m/s 5 m/s

5 m/s 5 m/s

(a)

(d) (e)

(b) (c)

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 directionrefers 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 startingfrom the rawinsonde location, following the direction of the wind and having a length proportional to the wind speed. The scale is indicated bya 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 theSeez Valley on some occasions.

To investigate the small-scale dynamics of flow split-ting between the Rhine and Seez Valleys, Drobinski et al.(2001) simplified the problem by using the theory of2D, incompressible and irrotational potential flows flow-ing along sidewalls. They demonstrated the key roleplayed by the valley geometry (angles between val-leys, valley widths) and, in particular, the complemen-tary contribution of the deflection and blocking/splittingmechanisms when flow splitting occurs. However, theseresults did not account for surface friction, turbulent mix-ing, and for the channelling effect because the solutionsfor each sidewall were obtained independently of eachother. In a second step, Drobinski et al. (2006) conductednumerical simulations including the effects of channellingand turbulent mixing. However, surface friction wasstill neglected, and a highly idealized valley geome-try was used together with a single-layer approximationof the equations of motion, similar to the well-knownshallow-water model. Comparison with observations andfully 3D numerical simulations with MM5 indicated that

the non-dimensional valley depth �H = N�H/U (with�H being the dimensional valley depth) is the keyparameter for the validity of the idealized model andfor the occurrence of flow splitting in reality. For fohncases with �H � 1, the flow splitting at the bifurcationbetween the Rhine and Seez Valleys was found to be verysimilar to the predictions of the idealized model.

An example of the detailed flow structure is shownin Figure 7(a), which displays the Doppler lidar radialvelocity field at 1000 m amsl on 29 October 1999at about 2000 UTC and the corresponding wind fieldsimulated with the MM5 model. There is evidence of flowsplitting in both the simulated wind field and the Dopplerlidar measurements, which show air blowing away fromthe Doppler lidar in the Seez and lower Rhine Valleys(positive radial velocity) and blowing towards the lidarfrom the upper Rhine Valley (negative radial velocity). Across-valley velocity shear is visible in the Seez Valleywith a near-wall jet (about 8 m s−1) at the northern wallof the Seez Valley, while the wind speed is weaker at thesouthern wall of Seez Valley. Figure 7(a) also shows that,near the split point, there is a very sharp radial velocity



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 29October 1999 at about 2000 UTC. For the Doppler lidar data, positive (negative) radial velocities (indicated by the labels) denote air blowingaway from (towards) the Doppler lidar (absence of shading means no reliable data). The altitude contour interval is 500 m from 500 m to3000 m, and the marker × indicates the location of the Doppler lidar. The bold solid line indicates the section along which the along-valleywind from the Doppler lidar, the MM5 model and the idealized model are plotted in (b) and (c). (b) shows wind speed v�

r normalized by theinflow wind speed U , simulated and measured along the section shown in (a). The solid line depicts the idealized simulations and the dottedline the MM5 simulations on 24 October 1999 at 1200 UTC (IOP 8). The open and filled circles are the Doppler lidar measurements of theradial 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 29and 30 October 1999 (IOP 12). The solid line depicts the idealized simulations, and the dashed and dash-dotted lines the MM5 simulations at2000 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 downto zero within less than two kilometres.

Figure 7(b,c) displays the radial wind speed v�r along

the thick line indicated in Figure 7(a), which is a goodapproximation to the wind speed along the streamlineintersecting the split point. Wind speeds are normalizedby the inflow wind speed, U , in the upper Rhine Valleyin order to simplify the comparison of the different datasources and cases. The solid lines depict the idealizedmodel result. The horizontal wind gradient decreasesfrom about 1 at the entrance of the two tributaries(47.02 °N) down to 0 at the bifurcation point (47.06 °N)with two regimes:

(1) between 47.02 °N and 47.045 °N, v�r decreases

smoothly from 1 to 0.8;(2) between 47.045 °N and 47.06 °N (bifurcation point),

v�r drops sharply from 0.8 down to zero.

Drobinski et al. (2006) showed that the main mecha-nism occurring during regime (1) is flow deflection by theexternal valley sidewalls whereas the main mechanism asthe flow approaches the bifurcation point is blocking andsplitting (regime (2)).

On 2 October 1999 (IOP 5), 29 and 30 October1999 (IOP 12), the agreement between the Doppler lidarmeasurements, the MM5 simulations and the idealizedsimulations is very good (Figure 7(b,c). These cases werecharacterized by �H � 1 (Drobinski et al., 2006 and

Table III). In accordance with the idealized model, theDoppler lidar and the MM5 model (horizontal resolution200 m and 330 m, respectively) show a clear distinctionbetween the smooth and sharp gradient regions. However,the Doppler lidar measurements and MM5 simulationsdo not show evidence of a sharp gradient near thebifurcation point on 24 October 1999 at 1200 UTC(IOP 8) (Figure 7(b)). This can be traced back to the factthat �H was only about 0.8 on that day, correspondingto a ‘flow over’ regime without a stagnation point atthe valley bifurcation. Evidently, the idealized 2D modelceases to be valid in such a situation.

Another fundamental difference between the chan-nelled and unchannelled flow regimes becomes evidentfrom the along-valley mass fluxes. Table III shows themeasured mass flux in the Seez, SRsv, and lower Rhine,SRlrv, Valleys, normalized with the mass flux measuredin the upper Rhine Valley (called split ratio in thetable) for several IOPs (Drobinski et al., 2006). It canbe seen that the flow budget is approximately balanced(SRlrv + SRsv ≈ 1) for �H � 1 (IOPs 5, 8 and 12), con-firming that the flow is channelled and quasi-2D underthese circumstances. One must note that simulations ofthese IOPs indicate that the fohn extends only a fewkilometres north of the valley bifurcation and afterwardsgets lifted over the low-level cold pool, so the verticalcontraction of the fohn layer eliminates the need of adeceleration in response to the widening of the RhineValley bottom. Otherwise, for the other IOPs, there is



Table III. Split ratios in the lower Rhine (SRlrv) and Seez(SRsv) valleys..

IOP Split ratio Brunt– Normalized

SRlrv SRsv Vaisalafrequency

N (10−2 s−1)

valleydepth �H

2 (19 Sep 1999) 1.26 0.13 0.9 0.75 (2 Oct 1999) 0.70 0.35 2.0 2.18 (20 Oct 1999) 0.83 0.24 1.2 1.810 (24 Oct 1999) 1.08 0.84 1.2 0.812 (29 Oct 1999) 0.67 0.39 1.7 4.212 (30 Oct 1999) 0.52 0.37 2.3 5.715 (5 Nov 1999) 1.20 0.11 0.7 1.0

The split ratio is computed from the radiosounding (launched fromHeiligkreuz in the Seez Valley, Buchs-Grabs in the lower Rhine Valleyand Malans in the upper Rhine Valley) velocity measurements forIOP 2 and 15 (when the Doppler lidar was not in operation), or fromthe Doppler lidar velocity measurements for IOP 5, 8, 10 and 12. Thein-valley velocities are integrated vertically over the fohn jet depth.When the Doppler lidar data are used, the in-valley velocity is alsointegrated horizontally over the valley width. The values of N and �H

are computed from the radiosounding launched from Malans (upstreamsounding) or from Heiligkreuz when one was not launched from Malans(IOP 2 and 5).

a substantial excess flow rate in the lower Rhine Val-ley, which can be traced back to descending air masses.In an independent calculation, Jaubert and Stein (2003)and Beffrey et al. (2004b) found that the downward massflux can contribute more than 30% to the total flow bud-get during deep fohn events. The subsidence potentiallyaffects both the Seez Valley and the lower Rhine Valleybut usually tends to be more pronounced in the lowerRhine Valley.

As pointed out by Zangl et al. (2004a), the subsidencecan also have a profound impact on the temperature fieldin the adjacent valley segments. The simulated low-levelflow field for IOP 10 (24 October 1999 at 1200 UTC)is displayed in Figure 8 together with the corresponding

surface potential temperature field. As already mentioned,the flow splitting at the ridge separating the Seez Valleyfrom the lower Rhine Valley did not involve a stagnationpoint on that day (Figure 8(a)). Thus, the low-levelairflow originating from the upper Rhine Valley entersnot only the two valleys, but part of it even ascendsthe dividing mountain ridge. The strongest subsidence ofwarm air occurs in the lower Rhine Valley around Vaduz,where south-south-easterly flow reaches the valley fromthe adjacent Ratikon massif. The surface observationscollected at Vaduz indicate that this flow feature is quitetypical for deep fohn, showing a typical surface winddirection between 150° and 170°. In the case consideredhere, the model also predicts warm-air subsidence in theSeez Valley, particularly in its western part where the airenters the valley directly from the mountain range to thesouth. The latter feature cannot be verified due to thelack of a surface station. An important consequence ofthe warm-air subsidence downstream of the flow-splittingpoint is that the surface potential temperature in the lowerRhine Valley (and presumably also in the Seez Valley)can be substantially higher than in the upper Rhine Valley(Figure 8(b)). In fact, Vaduz was as much as 8 K warmerthan Chur in the morning of 24 October 1999 (Zanglet al., 2004a), and θ differences in excess of 4 K areobserved quite frequently. It remains to be pointed outthat turbulent vertical mixing of stably stratified air is alsocapable of inducing an along-valley increase of potentialtemperature. However, separating this effect from theimpact of direct warm-air advection is difficult becausenumerical models appear to have substantial deficienciesin representing turbulent mixing in narrow Alpine valleys(e.g. Zangl, 2003).

4.3. Fohn/cold-pool interaction

A cold surface layer or cold pool often fills the floor ofAlpine valleys and prevents the upper-level fohn flowfrom reaching the ground during most of the duration offohn episodes. The cold pool is either present from thepreceding (colder) weather situation or rebuilds at night

(a)

40

35

30

25

20

15

10

5

00 5 10 15 20 25 30 35 40

Distance (km)

40

35

30

25

20

15

10

5

00 5 10 15 20 25 30 35 40

Distance (km)

300 300

300

298300

298

296

298

298

300

298

294

(b)

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.



when the absence of clouds permits the ground to radiatefreely to space. Only when the fohn is sufficiently intensedoes the fohn flow touch the floor of Alpine valleys.Three mechanisms are likely to govern the penetration ofthe fohn flow to the valley floor:

(1) the diurnal heating of the cold pool by solar radiationmay diminish the stability and allow vertical mixing,

(2) turbulent entrainment induced by Kelvin–Helmholtzinstability at the top of the cold pool may erode andeventually destroy the pool (Nater et al., 1979), and

(3) the occasional intensification of a mountain wave athigher levels may force the fohn flow down to theground level and flush the cold pool downstream, forinstance in the case of a breaking wave aloft.

Indeed, gravity-wave breaking accelerates the flowbeneath the wave-breaking zone and thus can increase thecold-pool erosion due to turbulent mixing or due to anenhanced pressure drag force if the cold pool has a steeplateral edge. Downstream advection of the cold pool isalso possible in the absence of pronounced gravity-waveactivity when the mesoscale pressure field at the top ofthe cold pool imposes a favourable forcing (e.g. Zangl,2005). Of course, these cold-pool removal processesmay also occur simultaneously. The interaction betweenthe cold pool and the fohn in the Rhine Valley wasbest documented during IOPs 8 and 9 (21–22 October1999; Gubser and Richner, 2001) and IOP 15 (5 and 6November 1999; Vogt and Jaubert, 2004; Jaubert et al.,2005; Flamant et al., 2006).

The warming rate due to heat flux in the cold poolunder fohn conditions could be estimated only duringIOP 9, as there were no dedicated aircraft flights on otherfohn days. Gubser and Richner (2001) found that the heatfluxes at the surface and at the top of the cold pool werecomparable in magnitude (about 15 W m−2). Based onthese heat fluxes but without accounting for long-waveradiative cooling (which causes the air column to lose atleast part of the energy gained by the heat flux), theyestimated warming rate of about 25 K (day)−1, whichappears to be considerably too high. Gubser and Richner(2001) concluded that, with such a warming rate, anycold pool would disappear within less than one day, andfohn periods with more or less stationary and persistentcold pools could not occur. An important contribution tothis discrepancy might arise from cold-air advection fromthe Alpine foreland. Surface observations and numericalsimulations (Zangl et al., 2004a; Beffrey et al., 2004a;Jaubert et al., 2005) frequently indicate a light northerlyflow within the cold pool, particularly when the precedingweather evolution formed a significant cold-air pool inthe Alpine foreland. The pressure gradient driving thisinflow is most likely due to the fact that gravity-wavedynamics tends to form a local pressure minimum at theboundary between the fohn and the cold pool (Zangl et al.2004a). Moreover, a gradually decreasing cold-pool depthbetween the Lake Constance region and the southern edgeof the cold pool might play a role.

Numerical simulations of IOP 15 conducted by Jaubertet al. (2005) and an observational study by Flamant et al.(2006) indicate that the presence or absence of a cold poolin the lower Rhine Valley is of crucial importance for theflow evolution. Jaubert et al. (2005) simulated this eventusing the mesoscale model Meso-NH with great realism.They used a mesoscale analysis over the whole simu-lation domain, which turned out to improve the resultssignificantly. Specifically, the mesoscale analysis allowedintroduction of a realistic initial cold pool that was miss-ing in the large-scale analysis at noon. As an illustration,Figure 9 shows the vertical structure of the cold pool doc-umented through reflectivity measurements made alongthe Rhine Valley between 1445 and 1505 UTC with thenadir-pointing differential absorption lidar LEANDRE-2,and the potential temperature fields for two simulationsusing the Meso-NH model – with (A12) and without(REF12) mesoscale analysis as initial state – along theARAT flight track. In the reflectivity measurements, thecold pool corresponds to the region of high reflectivity(350 arbitrary units or higher). Above the cold pool andbelow 2 km amsl, the reflectivity is generally very low,indicating the presence of the dry fohn layer. The south-ern tip of the cold pool is located just north of Weite,in agreement with the surface measurements. The depthof the cold pool increases slowly towards the north, andreaches 250 m north of Rankweil, where the reflectivityalso increases, indicating moister conditions. This depthwas also observed over Lake Constance (not shown, seeFlamant et al., 2006). The structure of the cold pool, asdefined by the low potential temperature regions, bearsresemblance to that observed by lidar in simulation A12,but the cold pool is missing in simulation REF12 (Jaubertet al., 2005).

The effect of the mesoscale analysis as initial state ofthe A12 simulation was not limited to the first hours ofthe simulation, but was still effective 12 hours after thebeginning of the run; a realistic cold-pool height preventsthe fohn from touching the ground too early, and allowsfor simulating an accurate timing of the fohn onset. Aheat budget analysis of the interactions between the coldpool and the fohn jet above during the late afternoonand evening of 5 November 1999 indicated that theleading terms are the advection by the mean flow and theturbulent tendency, whereas radiation tendency is weak(also Flamant et al., 2006). The turbulent mixing occursmainly close to the terrain, in the regions where the fohnair descends in the lee of the mountain range and theninteracts with the cold pool. Jaubert et al.’s (2005) resultsare consistent with the analysis of IOP 4–5 by Frioudet al. (2004) and of IOP 10 by Zangl et al. (2004a), whoalso found that turbulent vertical mixing is important forthe erosion of the cold-air pool that initially fills the lowerRhine Valley.

4.3. Advances in numerical modelling of fohn in theRhine Valley during MAP

One major objective of MAP was to improve the perfor-mance of high-resolution NWP, hydrological and coupled



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Figure 9. (a) Atmospheric reflectivity at 732 nm obtained from theairborne lidar LEANDRE-2 between 1445 and 1505 UTC on 5November 1999 (IOP 15) along the lower Rhine Valley. Reflectivityunits are arbitrary. The continuous white line represents the orography.(b) Vertical cross-section along the lower Rhine Valley at 1500 UTC on5 November 1999 of potential temperature (with contour interval 1 K,light shading over 300 K, medium shading below 296 K, and darkershading below 292 K) and vertical velocity (dashed contours withinterval 0.25 m s−1) from the reference Meso-NH simulation (REF12)without mesoscale analysis. (c) As (b) for a simulation with mesoscale

analysis as initial state (A12).

models in mountainous terrain. The numerical modellingwork on fohn began in 1999 with the state-of-the-artmesoscale models. The models are non-hydrostatic andnested model domains are used, with different horizontal

meshes at the same time in a one-way or two-way inter-active mode. The highest model resolution was about akilometre. These real-case simulations were forced andinitialized with NWP analyzes and realistically repro-duced several MAP fohn events over their whole dura-tion (IOP 12, Drobinski et al., 2003a; IOP 2, Jaubert andStein, 2003; IOP 8, Lothon, 2002).

However, some limits appeared with respect to modelinitialization (since large-scale NWP analyzes do notallow for reproducing the low-level thermodynamic struc-ture in the valleys), grid resolution (since a grid size of afew kilometres is not fine enough to reproduce the com-plex orography of the Rhine Valley and its tributaries),and numerical diffusion over steep orography. All theseproblems could be solved using the FORM dataset, allow-ing more accurate and reliable numerical modelling andforecasting of the 3D structure of fohn and its time evo-lution in the Rhine Valley.

4.3.1. Initial state

The numerical studies by Jaubert and Stein (2003) andBeffrey et al. (2004a), using the mesoscale model Meso-NH, provides evidence that a satisfactory simulation oflarger-scale aspects as well as meso-γ aspects of fohncases (IOPs 2 and 8, respectively) can be achieved withoperational meteorological large-scale analyzes and amesoscale model with nested domains. However, in thesetwo studies, the depth of the cold pool was unrealistic,probably due to a deficiency of the initial analyzes. Usingthe MM5 model, Zangl et al. (2004a) used sounding mea-surements to modify the lower levels of the large-scaleanalysis in the Alpine region in order to get a morerealistic surface temperature, particularly in the RhineValley. Jaubert et al. (2005) went one step further andexplored the benefits of an operational mesoscale anal-ysis scheme, capable of introducing mesoscale featuressuch as cold pools at the scale of Alpine valleys. Theirsimulation of IOP 15 (5–6 November 1999) initializedwith the mesoscale analysis scheme improved signifi-cantly the cold-pool dynamics and the fohn life-cycle(especially onset) as shown by comparing the model out-puts with lidar (Figure 9) and wind profiler (Vogt andJaubert, 2004) measurements.

4.3.2. Numerical diffusion

Numerical simulations performed with the mesoscalemodel MM5 for 24 October 1999 (IOP 10, Zangl et al.,2004a) demonstrate that a proper treatment of numericaldiffusion is of crucial importance. As in several othermesoscale models, the numerical diffusion was origi-nally implemented as a fourth-order horizontal smooth-ing operator evaluated along the terrain-following sigmacoordinate surfaces without accounting for any metricterms. Over steep topography, this tends to induce largesystematic errors for variables having a strong verticalstratification (temperature and the water vapour mix-ing ratio). The original implementation was changed byZangl (2002b) into a truly horizontal computation of



the numerical diffusion of temperature and the mixingratios of water vapour and cloud water. As illustratedin Figure 10 for the station at Fussach, located at thesouthern shore of Lake Constance, the modified diffu-sion scheme (denoted as z-diffusion) enables the modelto reproduce the observed flow evolution essentially cor-rectly. Discrepancies between model results and observa-tions are largely restricted to small errors in the time offohn breakthrough or in the time of the cold front pas-sage (1–2 hours each). However, using the original MM5diffusion scheme (denoted as sigma-diffusion) greatlydegrades the results. The fohn breakthrough at Lake Con-stance is then simulated at least 5 hours too early, andthe agreement between simulated and observed surfacetemperatures worsens even in those parts of the valleywhere the occurrence of fohn is predicted correctly (notshown). This can be explained by the fact that computingthe numerical diffusion of temperature along the modelsurfaces effectively destroys local cold-air pools withinvalleys because the cold air is mixed with the warmerair over the adjacent side slopes and ridges. The sever-ity of the related numerical errors is further emphasizedby another sensitivity experiment in which the horizontalresolution was degraded from 1 to 3 km while retain-ing the improved diffusion scheme. As evident fromFigure 10, the reduced model resolution has much lessdetrimental effects on the model results than using theoriginal diffusion scheme at 1 km resolution.

4.3.3. Model validation

The deployment of innovative remote sensors to docu-ment at high temporal and spatial resolutions the 3D flowstructure was the instrumental core for new methods ofvalidation of high-resolution modelling. Fohn flow wasobserved with Doppler lidars, sodars and wind profil-ers, and the structure of the planetary boundary layer incomplex terrain with a backscatter lidar. During MAP,mesoscale simulations were performed with horizontalmesh sizes down to 200–600 m, implying that spatiallycontinuous measurements are crucial for a proper val-idation. The Rankweil RASS wind profiler operated at

30 min time resolution and 60 m vertical resolution (Vogtand Jaubert, 2004) and the TWL operated at about 1 mintime resolution and 250 m radial resolution (Beffreyet al., 2004a; Drobinski et al., 2006; Figure 7) allowedfor an unprecedented validation exercise. As an illustra-tion, the radial velocity field obtained from the TWLmeasurements on 20 October 1999 between 0837 and0943 UTC (IOP 8) is compared to the radial velocityfield simulated with Meso-NH (Figure 11) (Beffrey et al.,2004a). The simulated field is consistent with the obser-vations. A strong jet of incoming air (radial velocitiesup to 15 m s−1) can be found in the upper Rhine Val-ley. At the junction of the Seez and Rhine Valleys itsplits into two branches, one along the Seez Valley andthe other towards Lake Constance. In the Seez Valley, astrong, transverse gradient can be observed in the radial-velocity field with strong winds along the northern walland almost no wind in the south. At higher levels, radialvelocities are somewhat underestimated by the model (byabout 5 m s−1) due to an insufficient channelling by thesmoothed topography of the model (the resolution for theprescribed topography is 1 km). Another explanation isthat the direction of the simulated wind is slightly dif-ferent from the actual wind direction. Although there aresmall but significant discrepancies, it can be noted thatthe salient features of the dynamic field revealed by theTWL are well reproduced by the model.

A novel approach for validating high-resolution modelsagainst conventional station data was introduced by Zanglet al. (2004a). A model-independent 2D analysis (VERA,Vienna Enhanced Resolution Analysis; Chimani et al.,2006) was used as an alternative to interpolating modelfields to the locations of the stations. When the analysisapproximately equals the model resolution, this methodhelps to gain a better overview of the spatial distributionof the differences than just comparing point data.

4.4. FORM-related side project: Evaluation of fohnimpact on regional air quality during MAP

Fohn conditions impair human comfort and healthin Alpine regions, causing headache and circulatory

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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 kmand 3 km resolution in the finest domain (Zangl et al., 2004a). z-diffusion denotes the truly horizontal diffusion scheme developed by Zangl

(2002b).



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 byMeso-NH at 0900 UTC on 20 October 1999. Arrows represent the simulated wind field; the arrow at the bottom left-hand corner represents20 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). Besidesthese effects, air pollution can be strongly enhanced undersouth fohn conditions in several regions on the northernside of the Alps (Nkemdirim and Leggat, 1978; Hoinkaand Rosler, 1987). Indeed, in the cold-air pool, the ozoneconcentration is reduced below the free-troposphericbackground level due to chemical reactions with otherpollutants, mainly nitrogen oxides. Thus, fohn break-through goes along with an increase in the ozone con-centration whereas other pollutants are reduced. So ozoneconcentrations in the valley tend to increase at the onsetof fohn. High wind speeds and turbulence reduce theeffects of titration by nitric oxide and dry deposition onthe concentrations of ozone. This results in higher ozoneconcentrations in the valley at night and in the morninghours. These ozone concentrations are usually not as highas the highest ozone concentrations reached during photo-chemical smog episodes. This means that fohn events donot necessarily cause ozone peaks, but prolong the dura-tion of ozone stress in fohn areas. On the other hand, apenetration of the fohn to the ground may bring a suddenrelief to some valley segments when polluted air in thevalley is replaced by usually less polluted air from above,whilst other segments of the valley remain within a shal-low inversion without significant air-mass exchange. Airquality is thus of high interest in the densely populatedAlpine Rhine Valley. The set-up of ozone, nitrogen and

aerosol concentration measurements by in situ and remotesensors allowed investigation of the air-mass compositionwith high temporal and vertical resolution.

In the Rhine Valley, air quality strongly depends on theinteraction between fohn and the cold pool close to LakeConstance. Indeed, the cold-air pool often persists (alsoduring most south fohn conditions) leading to enhancedair pollution within the stagnant boundary layer whichproved to be an aerosol-rich layer from the backscatterlidar measurements (Frioud et al., 2004), whereas fohnbrings relief as clean air from above is mixed intothe boundary layer. The question arises whether thehigh ozone levels found north of the Alps, e.g. in theRhine Valley, during south fohn are originally producedin polluted areas south of the Alps or are transporteddownwards from the stratosphere. Baumann et al. (2001)showed that, during the MAP SOP, fohn-induced ozonepeaks in October and November were found to bemuch lower than in September. They found remarkablespatial differences in the ozone records over a relativelysmall area of the Rhine Valley, confirming the usually‘patchy’ distribution of ozone concentrations during fohnevents which reflect the separation of fohn-shielded fromfohn-exposed areas. The stratification within the lowestfew hundred metres, especially the presence of a cold-air pool, determines whether the air mass with higherozone concentrations advected by the fohn flow reaches



the ground or remains a few hundred metres aboveground without or before removing the near-groundcold-air pool. The trajectory calculations for the fohnperiod confirm the general experience from previousinvestigations (Seibert, 1990) that the air advected bysouth fohn often originates from around 2000 m abovethe Po basin. Nevertheless, the results of 22 October1999 (IOP 9) demonstrate that air masses from thelower free troposphere can be imported into the fohnflow due to changing meteorological conditions in thecourse of a longer fohn period. In this case, the fohnair originating from 4000 m agl (above ground level)caused the most distinct increase of ozone of this fohnphase at the monitoring stations in the Rhine Valley.No vertical transport of ozone from the stratosphere andupper troposphere was involved in the increase of theozone concentrations in the fohn valleys during the fohnphase.

5. Concluding remarks

Looking back from some distance, the design of thecomposite observing network may be assessed as quitepositive, even though lessons can always be learned.The composite observing system and the combination ofremote-sensing and in situ systems produced a wealthof data which allows unprecedented insight into thestructure of the fohn flow and a valley network withcomplex geometry. The combination of established andnovel remote-sensing instruments (e.g. Doppler and watervapour lidars) with conventional in situ measurements(dense surface network and radiosondes) allowed captureof previously unseen details of the fine structure offohn (Richner et al., 2006). This allows the validationof ultra-high-resolution numerical research and weatherprediction models.

The work conducted in the framework of FORM nowalso allows the comparison of the flow characteristicsof the fohn in the Rhine Valley with those observedin the Wipp Valley region, the second fohn target areaduring MAP (project P4; Mayr et al., 2007) (Figure 1).In accordance with our findings for the Rhine Valley,the importance of 3D gravity wave effects for the low-level wind field in the valley has also been pointedout for the Wipp Valley (Flamant et al., 2002; Zangl,2003; Zangl et al., 2004b). However, the Wipp Valleyappears to encounter shallow fohn flows more frequently,in which the essential flow dynamics can be alternativelyexplained with the conceptually simpler shallow-watermodel (Gohm and Mayr, 2004). Marked differencesalso occur for the low-level flow behaviour beforefohn breakthrough. The lower Rhine Valley frequentlyexperiences light upvalley flow within the cold pool,indicating a cold-air advection from the Alpine forelandopposing the fohn breakthrough. In the Wipp Valley,katabatic downvalley flow usually prevails in the pre-fohnphase, except perhaps for a very short period immediatelybefore fohn breakthrough (Zangl, 2003). Moreover, the

adjacent Inn Valley usually encounters downvalley flow,which has frequently been interpreted as cold-air outflowinto the Alpine foreland. Zangl (2003) showed that thewesterly downvalley flow is locally enhanced aroundInnsbruck due to an east–west asymmetry in the gravitywave activity. It is also important to note that the lowerRhine Valley tends to be more strongly affected bycold-air pools lying north of the Alps than the WippValley because the former valley exits directly into theAlpine foreland. Finally, it has been found that thedownvalley increase of the surface potential temperaturecan be significantly stronger for the Rhine Valley thanfor the Wipp Valley. In the Wipp Valley, the low-levelairflow essentially follows the valley axis, so that anincrease in surface potential temperature can occur onlydue to turbulent vertical mixing of stably stratified air(Seibert, 1985; Zangl, 2003). Since the Wipp Valleywidens considerably in the downvalley direction, thestable stratification might be reinforced by subsidence ofpotentially even warmer air from aloft into the valleyregion, so that the mixing-induced potential temperatureincrease can be quite appreciable (∼5 K). In the RhineValley, values up to 8 K have been observed because,in addition to mixing-induced warming, direct warm-airadvection from the adjacent mountain ranges into thevalley is possible, particularly in the region of Vaduz.

Finally, despite the significant progress made in fohnunderstanding, modelling and forecasting thanks to theMAP programme, several key issues are still at best partlyunderstood. Among those are:

(1) The fohn/cold-pool interaction: the interaction ofambient air flow with cold pools still poses amajor challenge to understanding and predictinglocal weather in mountainous regions. It is of cru-cial importance not only for fohn flows but alsofor wintertime warm-front passages, leading to pro-nounced horizontal temperature differences and pos-sibly large fluctuations in the height of the snow line.As already discussed above, the most important pro-cesses involved in the interaction between ambientflows and cold pools are turbulent erosion, radiativeheating/cooling, interaction with orographic gravitywaves and cold-air drainage related to an exter-nally imposed pressure gradient. Some of these pro-cesses are only partly understood so far and are dif-ficult to represent in a numerical model. Apart fromhigh resolution and accurate model numerics, repre-senting these processes requires highly sophisticatedparametrizations for turbulence, radiation and clouds.Regarding the model numerics, the implementationof numerical diffusion is particularly crucial becausesimple methods tend to destroy cold pools in nar-row valleys. A major weakness of present turbulencemodels is that the effects of the valley topography(increased turbulence due to sidewall friction) arenot properly accounted for, even when the turbulencemodel is 3D. Finally, interactions between fog and



radiation are potentially important, and most micro-physical parametrizations used in mesoscale modelstend to remove fog by converting the cloud water toorapidly into drizzle (Crewell et al., 2003).

(2) The role of scale interactions in the local responseof fohn to a large-scale forcing: the sensitivity ofthe local response of the fohn to any uncertainty inthe large-scale analysis still needs to be investigatedin more detail. For example, the simulation of someMAP fohn events (particularly IOP 8, Beffrey et al.,2004a) presumably failed due to an erroneous tim-ing in the large-scale forcing (inflow strength anddirection, cold front location). Improving the under-standing of scale interactions includes analyzing theinteraction of the synoptic-scale flow with the Alpinemassif as a whole, and its possible side effects on thelocal characteristics of fohn flow. At a smaller scale,the sensitivity of the amplitude, phase and temporalevolution of orographic gravity waves to the ambi-ent flow are still poorly understood, particularly inthe presence of wave breaking. (Lothon (2002) andLothon et al. (2003) showed evidence of enhancedturbulence around and above crest level; this couldbe taken as evidence for wave breaking, since it waspresumably not boundary-layer turbulence, but thereis no formal proof.) However, the local evolution offohn flow might depend very sensitively on the wavestructure. In addition, the interaction between fohnand cold pools again comes into play, as the coldpools might either be localized in some valley seg-ments or be fed from a larger-scale cold-air reservoirin the northern Alpine foreland.

(3) The model initialization issue: future data assimila-tion will need to be conducted on a high-resolutiongrid to allow for a proper use of the available data(particularly surface data). The range of validityof point measurements (or line measurements i.e.radiosondes) can be highly anisotropic in mountain-ous terrain, which will need to be accounted for infuture mesoscale data assimilation systems.

Acknowledgements

We are deeply indebted to many colleagues who con-tributed to this overview paper by providing text, figuresand other input. We also thank the many agencies of theparticipating countries which, by their financial support,contributed to the success of the field experiment and theprogress of fohn understanding in the complex Alpinevalleys.

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