pdf-Dokument

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
Föhn 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 Dürr,h Cyrille Flamant,a Max Frioud,i Markus Furger,j
Inga Gröhn,b Stefan Gubser,c Thomas Gutermann,h Christian Häberli,b,h
Esther Häller-Scharnhost,c,h Geneviève 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 Günther Zängln
a
Institut Pierre Simon Laplace, Service d’Aéronomie, Paris, France
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 Météorologiques, Météo-France, Toulouse, France
f Laboratoire d’Aérologie, Toulouse, France
g MeteoSwiss, Payerne, Switzerland
h MeteoSwiss, Zurich, Switzerland
i Observatoire de Neuchâtel, Switzerland
j Paul Scherrer Institute, Villigen, Switzerland
k Institute of Geodesy and Photogrammetry, ETH Zurich, Switzerland
Institut für Meteorologie und Klimaforschung, Forschungszentrum Karlsruhe, Germany
m Umweltinstitut des Landes Vorarlberg, Austria
n Meteorologisches Institut der Universität München, Germany
b
l
ABSTRACT: This paper summarizes the findings of seven years of research on föhn conducted within the project ‘Föhn in
the Rhine Valley during MAP’ (FORM) of the Mesoscale Alpine Programme (MAP). It starts with a brief historical review
of föhn 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 föhn at a local scale and the processes driving föhn propagation in the Rhine Valley. The crucial importance
of interactions between the föhn 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 föhn events in the Rhine Valley are also underlined.
Finally, we discuss the main differences between föhn 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
föhn studies since 1850
Föhn is a generic term for strong downslope winds experiencing warming at the lee of a mountain ridge. Föhn
is associated with a decrease in cloudiness in the lee,
is strong and gusty, and is often channelled along gaps
and valleys cut into the main ridge (Brinkmann, 1971;
Seibert, 1990). Föhn may cause damage due to severe
storms (Brinkmann, 1974), snow melting (Hoinka, 1985),
* Correspondence to: Philippe Drobinski, Service d’Aéronomie, Université Pierre et Marie Curie, Tour 15, Couloir 15-14, 4 Place Jussieu,
75252 Paris Cédex 05, France.
E-mail: [email protected]
Copyright  2007 Royal Meteorological Society
or high pollution levels (Nkemdirim and Leggat, 1978;
Hoinka and Rösler, 1987), so that it is of great practical
importance to better understand and predict the structure
of the föhn flow. In the Alps, föhn occurs most frequently
in the presence of strong synoptic-scale flow perpendicular to the Alpine crest, i.e. either northerly (north föhn) or
southerly (south föhn) flow (Hoinka, 1980). While both
flow directions are similarly frequent, the vast majority
of the scientific literature on the Alpine föhn deals with
the south föhn (e.g. Frey, 1953; Seibert, 1985). This is
probably because south föhn usually has a much stronger
impact on the local temperature than north föhn (or even
west and east föhns which are more infrequent, however), which is in turn related to the different origin
898
P. DROBINSKI ET AL.
of the air masses (moist subtropical air mass for south
föhn, polar air mass for north föhn). Usually, föhn flow
is restricted to the respective lee side of the Alpine crest,
but exceptions can be found in a few regions. For example, the Inn Valley (located north of the Alpine crest, see
Figure 1) can also experience north föhn in situations
of strong northerly or northwesterly flow (Hann, 1891;
Zängl, 2006).
In the scientific literature on Alpine föhn, an important distinction is made between deep föhn and shallow
föhn. A föhn is termed deep when the cross-Alpine synoptic flow extends significantly above the height of the
Alpine crest (e.g. Seibert, 1985, 1990). The dynamics
of deep föhn is strongly influenced by vertically propagating gravity waves. Due to their three-dimensional
(3D) dispersion characteristics, gravity waves excited
over mountain ranges encompassing a valley can also
affect the flow dynamics in the valley proper, leading
to pronounced wind maxima under suitable topographic
conditions (Zängl, 2003). On the other hand, shallow
föhn flow is restricted to a relatively small number of
deep valley transects in the Alpine crest. So far, shallow föhn has been reported from southerly directions
only, occurring under approximately westerly synopticscale flow conditions. Shallow föhn frequently precedes
deep föhn when the synoptic-scale flow direction gradually backs from west to south-west or south (Seibert,
1990). However, there are also shallow föhn cases that
do not develop into a deep föhn. The mesoscale dynamics of shallow föhn have been investigated by Sprenger
and Schär (2001) and Zängl (2002a). They found that
the synoptic-scale pressure gradient related to geostrophically balanced westerly flow, frictional flow deflection
towards the lower pressure, and cross-Alpine temperature contrasts with cold air lying in the south play an
important role in generating shallow föhn. On the local
scale, shallow föhn flow is mainly governed by hydraulic
dynamics (Flamant et al., 2002; Gohm and Mayr, 2004).
Föhn research has a long history in the Alps, reaching
back to the middle of the 19th century. Already in 1866,
Hann recognized that adiabatic warming in the lee of the
Alpine crest is the main reason for the föhn being warm
and dry, rejecting earlier hypotheses that assumed the
föhn to originate from the Sahara desert (Hann, 1866,
reprinted in Kuhn, 1989). Hann also found that latent
heat release related to orographic precipitation is one
important factor contributing to the temperature difference between the windward side and the lee side of the
Alps. However, in contrast to subsequent textbook versions of the so-called thermodynamic föhn theory, he did
not claim that latent heat release is the only relevant factor (Seibert, 1985). In fact, cold-air blocking over the
Po basin may give rise to much larger cross-Alpine temperature differences than could be explained by latent
heat release (Seibert, 1985, 1990). While the thermodynamic explanation for the warmth of the föhn was readily
accepted in the scientific community, there was a lot of
debate on dynamical aspects of the föhn, particularly the
question how the föhn is able to penetrate into Alpine
valleys and to remove the denser cold air lying there.
One of the earliest theories was proposed by Wild (1868),
who hypothesized that the föhn ‘sucks’ the cold air out
of the valleys in a way similar to a vacuum cleaner.
This was questioned by Billwiller (1878) who ascribed
a more passive role to the föhn flow, merely replacing
the cold air that had been driven away by some synopticscale pressure gradient. These theories survived several
decades and gave rise to a remarkably heavy dispute (e.g.
von Ficker, 1913; Streiff-Becker, 1931). Entirely different
hypotheses were brought up in the mid-twentieth century,
for example by Roßmann (1950) who ascribed the downslope acceleration of the föhn to evaporation of cloud
water and spilled-over precipitation. (Seibert, 1985, gave
a critical review of föhn theories; article reprints appeared
in Kuhn, 1989.)
With the advance of the theories of orographic gravity
waves (Lyra, 1943; Queney, 1948) and shooting hydraulic
flows (Schweitzer, 1953; Long, 1953), a deeper understanding and more complete picture than provided by the
early föhn theories became available. Nevertheless, the
variety of different hypotheses reflects the important fact
that local flow patterns related to föhn differ strongly
between various Alpine valleys. Wild (1868) and StreiffBecker (1931) observed that the southerly föhn starts
close to the Alpine crest and then gradually penetrates
toward the north, whereas light opposing (northerly) flow
3
Latitude (°N)
47.5
Rhine valley
target area
2.5
2
Wipp valley
target area
47
1.5
1
46.5
8.5
9
9.5
10
10.5
11
Longitude (°E)
11.5
12
12.5
0.5
Height above sea level (km)
3.5
0
Figure 1. Topography of the Alps. The two boxes indicate the Rhine Valley target area and the Wipp/Inn Valley target area for föhn studies
during MAP.
Copyright  2007 Royal Meteorological Society
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
899
FÖHN IN THE RHINE VALLEY DURING MAP
(1935) chose all cases with southerly wind directions
between 1 and 3 km above ground to detect typical
profile structures during föhn conditions. Particularly
in spring, positive temperature deviations of 3 ° C with
respect to the climatological average were found at 1
to 2 km altitude. Near the ground, inversions were frequently observed in the kite soundings. Studying the
spatial flow variability along the Rhine Valley, Gutermann (1970) found that föhn air near Chur is generally cooler than near Vaduz, which he explained with
the influence of katabatic outflow from tributaries. Further statistical investigations, including a comparison of
föhn frequencies in the Rhine Valley and the Reuss Valley (located in central Switzerland, Figure 2), were conducted by Waibel (1984). A more complete review of
föhn research in the Rhine Valley is provided by Richner
et al. (2006).
Section 2 lists the reasons for selecting the Rhine valley as a target area for föhn studies during MAP and
the main scientific objectives. It also gives a summary of
the föhn events in the Rhine Valley during MAP. Section 3 presents the main challenges identified before the
field experiment to be addressed in terms of observation
and modelling. Section 4 synthesizes the main findings
in terms of observation, numerical simulation and theory of föhn. Finally Section 5 concludes this overview
of results and point out some open research questions
needing further investigation.
is present in the cold-air pool. On the other hand, Billwiller (1878) and von Ficker (1905, 1913) observed outflow of cold air into the Alpine foreland before the onset
of the föhn. Recently, Zängl (1999, 2003) demonstrated
that these different flow patterns are related to different orientations of the valleys. Valleys aligned with the
impinging flow tend to exhibit the former flow pattern
with converging flow at the leading edge of the föhn zone
because the gravity-wave pattern favours a local pressure
minimum there. Cold-air outflow at low levels appears to
be more typical for valleys perpendicular to the impinging flow that are less favourable for a direct penetration
of the föhn.
Although föhn research in the Rhine Valley has a
fairly long tradition, research activities prior to MAP
(Mesoscale Alpine Programme) largely concentrated on
the vicinity of Innsbruck, presumably because of the university institute located there (a comprehensive overview
is provided by Seibert, 1985; also Hoinka, 1990; Zängl,
2003). Probably the first climatological evaluation of
föhn in the Rhine Valley was done by Hann (1882).
Based on a 17-year record from Bludenz, he found a
broad frequency maximum from autumn through spring
(about 10 days per season) and a pronounced minimum
in summer. Peppler (1930) investigated kite soundings
and pilot balloon ascents that were conducted operationally at Friedrichshafen (located at the northern shore
of Lake Constance) for more than 20 years. Peppler
(a)
Lake Constance
Zurich
Bregenz
Altenrhein
Diepoldsau
Hoher Kasten
Sargans
Seez valley
n
ne
Tam
i
Reu
W
Kunkelspass
ey
esc
zia
BuchsGrabs
LRV
Heiligkreuz
ne
n
hg
um
ne
Vaduz
Sargans
SV
ml
lL
Weite
Chur
Do
Masein
Tam
Julier
ina
Va
Bad Ragaz
Rä
tik
on
ran
ge
Pra
etti
gau (b)
all
ine v
Rh
y
alle
ss v
Gütsch
na
n
sta
eis
Feldkirch
lga
u
Wa
tan
rich
eis
s
of Zu
W
Lake
Rhine valley
Rankweil
Malans
URV
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
föhn flows in a valley with complex geometry
Table I. Föhn episodes during the Special Observing Period of
the MAP field experiment (7 Sept 1999 to 15 Nov 1999).
2.1.
IOP
Beginning
1
2
–
–
5
7
8
9
10
12
13
15
15 Sep 1999
19 Sep 1999
22 Sep 1999
30 Sep 1999
2 Oct 1999
18 Oct 1999
19 Oct 1999
22 Oct 1999
24 Oct 1999
30 Oct 1999
1 Nov 1999
5 Nov 1999
Scientific motivation
Since the aforementioned findings strongly suggest that
föhn research should consider various regions of the Alps,
it was decided to select the Alpine Rhine Valley as a target area for föhn research during MAP (see also Volkert
and Gutermann, 2007). The research area extends from
the Alpine crest to the northern Alpine foreland and features several low passes in the main Alpine crest (see
Figure 2), which provides an excellent opportunity to
study the development of shallow föhn. Moreover, the
main valley and its tributaries cover a wide range of valley orientations, allowing investigation of the effects of
valley orientation on the dynamics of föhn. The selection
of the Rhine Valley was also motivated by the facts that it
is well equipped with operational meteorological measuring networks and that there is a high practical significance
for any improvement in forecasting the onset or cessation
of föhn for storm warnings on Lake Constance. Compared to the other target area (the Wipp Valley/Innsbruck
region; project P4; Mayr et al., 2007), the Rhine Valley
has a significantly more complex topographical structure.
In particular, the valley axis of the Wipp Valley is almost
straight while the Rhine Valley has several marked kinks.
Moreover, the Rhine Valley exits into the Alpine foreland
whereas the Wipp Valley ends in another inner-Alpine
valley (the Inn). Thus, the two föhn research areas complement each other in an ideal way.
In the studies conducted prior to MAP, the instrumental set-up did not allow for a coherent 3D documentation of the föhn from the synoptic scale to the valley
scale, providing sufficient resolution in space and time
to analyze the processes governing the spatio-temporal
evolution of föhn. This gap was attempted to be closed
during MAP with instrumentation of unprecedented density, including surface and radiosonde stations, a variety
of remote-sensing instruments and several research aircraft. An overview of MAP, its strategy, the projects, and
preliminary results are given by Bougeault et al. (2001).
Among the MAP objectives, the program FORM (Föhn
in the Rhine Valley during MAP), was designed to study
(Richner et al., 2006):
(1) the dynamics of that part of the blocked, potentially
cooler air mass that typically reaches up to the mean
crest height on the windward side of the main ridge
and which flows through deep Alpine passes into the
lee-side valleys (shallow föhn);
(2) the interaction between low-level and mid-tropospheric föhn flows on the scale of large Alpine
valleys including the improvement of understanding
and forecasting of föhn-related phenomena like turbulence;
(3) the mechanism of temporal and spatial evolution and
cessation of föhn flows in complex valley systems on
a local scale;
Copyright  2007 Royal Meteorological Society
End
0750
0050
0750
0420
0810
1340
2320
0640
0210
0900
1200
0610
15 Sep 1999
20 Sep 1999
23 Sep 1999
30 Sep 1999
3 Oct 1999
18 Oct 1999
21 Oct 1999
23 Oct 1999
24 Oct 1999
31 Oct 1999
2 Nov 1999
6 Nov 1999
1700
0700
1130
0840
0320
1650
1410
0330
1920
0310
1610
0940
The dates and times (UTC) indicate the onset and end of föhn events
in the Rhine Valley detected by the multi-parameter algorithm of
Gutermann (1970). Twelve föhn episodes were observed during 10
IOPs; two föhn episodes occurred outside IOPs.
(4) the interaction of the föhn flow with the boundary
layer and the removal process of the cold-air pool;
(5) the interaction between the föhn and air blowing in
the side valleys;
(6) the dynamics of flow splitting at the bifurcation
between the Rhine and Seez valleys; and
(7) the requirements to improve the forecast skills of
föhn in the Alpine region.
2.2. Summary of the föhn events in the Rhine Valley
during MAP
The special observing period (SOP) of MAP took place
in autumn from 7 September to 15 November 1999,
corresponding to the maximum of the climatological föhn
frequency in the northern Alps. During the MAP SOP,
twelve föhn episodes were observed. Ten fell into an
intensive observing period (IOP) and two were outside.
The start and end of each episode are given in Table I.
The detection of föhn and the determination of the start
and end time of the episodes were conducted according
to the multi-parameter algorithm of Gutermann (1970),
which was applied to numerous surface measurements
acquired in the Rhine Valley and its main tributaries.
The algorithm is based on a thresholding of mean and
gust valley winds, temperature increase >3 K and low
humidity during the whole period. The föhn episodes
were determined for each station individually. A föhn
event is defined by the earliest onset and by the latest
end at any of the considered stations on the valley floor.
Altogether, föhn covered 244 hours 20 min, i.e. about
15% of the whole SOP (1464 hours) or 29% of the IOPs
(836 hours). In most events, the föhn flow first reaches
the valley floor in the area of the Fläscherberg (halfway
between Sargans and Malans; Figure 2). The föhn also
ends later in this area. Frequently, the föhn flow aloft does
not extend downwards to these well-exposed points in the
valley because the valley is filled by a stagnant cold-air
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
FÖHN IN THE RHINE VALLEY DURING MAP
pool. Thus, föhn is observed more frequently on passes or
mountaintops (‘pass föhn’) than in valleys (‘valley föhn’).
These MAP föhn events have been studied by Baumann et al. (2001, all IOPs, with IOPs 8, 9 and 10 in
detail), 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), Gubser
and 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 Zängl et al. (2004a, IOP 10). In spite of
their specific features linked to föhn intensity, these studies allowed the building of a comprehensive scheme of
the temporal evolution of the föhn in a large valley, which
is useful to validate numerical simulations.
3. Challenges for new measurements and modelling
tools at the beginning of MAP
3.1. Observation network
As already mentioned, the instrumental set-up used
in earlier experiments did not allow for detailed 3D
documentation of the föhn flow over the wide range of
relevant spatial and temporal scales. During MAP, the
main principle of the design of the observing system
was to optimally combine the more or less continuously
observing remote-sensing systems with only sporadically
active, but usually more accurate, in situ observations.
An extensive description of the instrument set-up can be
found in Richner et al. (2006).
To address the scientific issues summarized above, the
very-fine-scale 3D structure of the föhn and its time
evolution have been documented by means of remote
sensors operated continuously or during IOPs only. A
transportable wind lidar (TWL) located in Vilters near
Bad Ragaz (Figure 2) provided radial wind velocity measurements along the line-of-sight. It proved to be a
key instrument for investigating flow-splitting dynamics
at the bifurcation between the Rhine and Seez Valleys
(Drobinski et al., 2001, 2003a, 2006) and validating highresolution simulations (Beffrey et al., 2004a; Drobinski
et al., 2006). Five Doppler sodars and two wind profiling
radars (one equipped with a radio acoustic sounding system, RASS) contributed to validate high-resolution simulations of the life cycle of föhn (Vogt and Jaubert, 2004).
Despite its location outside the Rhine Valley target area,
the UHF profiler set up at Julier Pass was a key instrument to study the dynamics at one of the main passes that
feed the Rhine Valley network (Ruffieux et al., 2000).
The cross-Alpine flow structure of gravity waves
related to föhn has been investigated using constantvolume balloons launched from Ispra (Italy) located
near Lago Maggiore. The balloons served to document
the dynamics of the isopycnic airflow (pressure, wind
speed and direction, temperature and humidity content;
Bénech et al., 2002) and particularly the characteristics
Copyright  2007 Royal Meteorological Society
901
of vertically propagating and trapped gravity waves (location, amplitude, period, intensity) (e.g. Drobinski et al.,
2003a). Two scintillometers located in the lower Rhine
Valley (the transmitters at Triesenberg, the receivers at
Flusa and Ergellen; Figure 2(b)) allowed for measuring
the vertical and horizontal wind components and for documenting the gravity-wave penetration into the lower
Rhine Valley during strong föhn windstorms (Furger
et al., 2001; Drobinski et al., 2003a). Closely connected
to the wave monitoring by the sodars was the operation of
microbarographs, which made possible the determination
of the relevant characteristics (direction of propagation,
phase speed, and wavelength) of gravity waves on top
of the cold pool by detecting the related pressure signal on the ground (Flamant et al., 2006). Several aircraft
flying over the Rhine Valley target area (French Merlin IV and ARAT Fokker-27, UK C-130, Swiss Metair
Dimona, USA NCAR Electra and NOAA P-3) measured
averaged mean and turbulent variables (wind speed and
direction, vertical wind, temperature, pressure, humidity, turbulent kinetic energy, momentum and heat fluxes),
providing information on gravity-wave activity in relation
to cold-pool erosion (Gubser and Richner, 2001; Jaubert
et al., 2005; Flamant et al., 2006). Finally, the cold-pool
removal was also monitored by three continuously running cameras mounted at ∼1800 m amsl (above mean
sea level) on Hoher Kasten. Indeed, the turbidity in the
cold pool is always significantly greater than in the föhn
air, making the boundary between the two air masses and
its variation visible over time.
To investigate the interaction of the föhn flow between
a main valley and a tributary, six surface stations were
set up along the Brandner Valley (in the Austrian
province of Vorarlberg) and traverses were made with
an instrumented car measuring pressure, temperature,
humidity, and wind.
Finally, for MAP the already dense network of conventional meteorological surface stations in the Rhine Valley
target area was completed with 14 additional surface stations and 9 radiosonde stations. This extremely dense
station network was necessary to resolve small-scale features and to achieve the same resolution for observational
data as for numerical weather prediction (NWP) models.
This was vital for the validation of the models. This network also contributed to the derivation of better initial
conditions for research and NWP models (Zängl et al.,
2004a; Jaubert et al., 2005) and the mesoscale analyzes
extensively used for föhn investigations in the Rhine
Valley (Drobinski et al., 2003a; Chimani et al., 2006; Flamant et al., 2006).
As föhn-related aspects, air quality issues were investigated by the means of a tethered balloon operated
at Fussach and a cable car located at Bregenz, near
Lake Constance, to obtain high-resolution vertical profiles of meteorological variables and ozone (Baumann
et al., 2001). In addition, a vertically pointing backscattering lidar located near Sargans monitored aerosol layers
continuously with almost no interruption during the entire
field phase (Frioud et al., 2003, 2004).
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
902
3.2.
P. DROBINSKI ET AL.
Numerical modelling
The main objectives of the numerical modelling efforts
made in the context of MAP were
(1) to document the capability of state-of-the-art mesoγ -scale numerical models to simulate air flow and
precipitation fields in complex Alpine orography, and
(2) to improve the process understanding of the underlying interactions between atmosphere and orography.
For the FORM project, the specific research challenges
were to simulate the detailed characteristics and evolution
of föhn flows in complex valley systems and to better
understand the underlying flow dynamics. The practical
importance of predicting meso-γ -scale aspects of föhn
inside Alpine valleys is very high since föhn represents
a weather risk to all outdoor activities including air
operations, and also influences air quality. Moreover,
the uniquely dense dataset collected during MAP was
expected to offer one of the first opportunities to clearly
demonstrate the possible benefits of future real-time,
meso-γ -scale NWP. Necessary ingredients for a high
forecasting skill of high-resolution numerical models are
expected to be:
(1) an accurate simulation of larger-scale aspects, such
as the upstream and downstream wind and stability,
(2) a good initial analysis of the low-level atmospheric
state inside the valleys under consideration, and
(3) a high-resolution model with a proper representation
of orography and sophisticated parametrizations for
physical processes like radiation and turbulence.
4. Key findings of the post-SOP period in
observation, simulation and theory
4.1. Upstream and cross-Alpine flow structure during
föhn events
The upstream conditions largely determine the temporal,
horizontal and vertical extension of the föhn within
and over the Rhine Valley. The key parameters are:
(1) θ and p, the potential temperature and meansea level pressure (mslp) differences across the Alps,
= N H /U , the non-dimensional height, where
(2) H
N is the Brunt–Väisälä frequency, H the dimensional
mountain height and U a characteristic upstream wind
speed, and (3) the upstream flow direction. Generally,
∼ O(1) implies that air parcels mainly flow over the
H
mountain and substantial nonlinear effects occur (such
as hydraulic jumps or large-amplitude gravity waves),
1 much of the airstream is diverted
whereas for H
around the flanks of the mountains and the perturbation
energy mainly appears in the horizontal with generation
of vortices rather than vertical motions (Smolarkiewicz
and Rotunno, 1989). The wind direction determines
if the föhn flow across the Alps is deep or shallow.
Finally, experience shows that the occurrence of föhn
is strongly correlated with the cross-Alpine pressure
Copyright  2007 Royal Meteorological Society
difference (Seibert, 1985, 1990). For the Rhine Valley,
it has been found that the föhn tends to reach the
valley bottom when the pressure difference between
Lugano and Vaduz exceeds about 5 hPa (Richner et al.,
2006), corresponding to a cross-Alpine pressure gradient
of about 4 hPa (100 km)−1 . Table II summarizes the
upstream key parameters for all the föhn IOPs which
can be compared to their climatology detailed in Richner
et al. (2006).
decreases with time and
During typical föhn events, H
the föhn flow deepens with time as the ambient flow
intensifies and gets more southerly. In the early stage,
the flow regime is often a ‘flow around’ the Alps during
> 3, switching to a ‘flow over’ regime as
föhn with H
⇒ 1 and keeps its minimum value for several hours
H
depending on the events. This fraction of time can be half
of the duration of the event. The jet associated with flow
splitting at the scale of the Alps and flowing along the
western flank of the range may affect the altitude of the
föhn jet in the Rhine Valley. It is capable of triggering
cold-air intrusions from the north into the Rhine Valley
(Jaubert et al., 2005), lifting the föhn flow passing over
the Alpine range off the ground in the lower Rhine Valley.
These intrusions are favoured when the strong flowaround jet is positioned close to the Rhine Valley outlet
and is oriented orthogonally to it. Local topography, i.e.
the small mountain range east of the Lake Constance
basin, makes it easier for the air to enter the Rhine Valley,
due to channelling (Figure 2).
The meridional extension of the föhn within the valley
strongly depends on the upstream conditions (Lothon,
2002):
(1) in the southern part of the Rhine Valley (e.g. around
Chur), light downslope wind appears as soon as
the upstream conditions are favourable for föhn,
and its intensity does not depend strongly on these
conditions;
(2) in the central region of the Rhine Valley (e.g. around
to
Vaduz), the downslope wind requires smaller H
reach the valley bottom, and its intensity is strongly
correlated with the intensity of the upstream flow;
(3) finally, in the northern part of the valley (e.g. Lake
Constance), the downslope wind occurrence depends
on the upstream conditions but also on the local and
downstream conditions. The cold pool which is often
present (see next subsections) in this region interacts
with the föhn and only strong upstream flow and low
can trigger a downslope wind that reaches the
H
ground in this area.
Downstream turbulence was measured using the Merlin IV aircraft during MAP (Lothon et al., 2003). The
measurements showed that in addition to organized propagating or trapped gravity waves, at a 10-km scale turbulent plume exists in the wake of the mountain with
large turbulent kinetic energy (TKE) and dissipation rate
(Figure 3). This plume extends from 3000 m altitude
above the southern part (maximum measured TKE) down
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
Copyright  2007 Royal Meteorological Society
5.7
4.3
6.3
4.0/7.8
0.4/1.6
Lugano – rain rate (mm hr−1 )/accumulated rain (mm)
0/0
1.2/36.3
0/0
0.07/0.3
0.5/10.1
229/1.4
292/1.7
084/1.4
176/6.3
201/4.8
190/5.7
172/14.7
219/14.7
237/17.6
233/21.2
10.7
8.3
6.8
2.2/11.0
IOP 9
1.5/25.3
165/4.8
237/5.1
213/2.6
161/7.9
199/5.3
197/8.1
175/17.1
204/20.6
233/21.1
237/23.7
9.7
7.3
3.0
2.0/5.5
IOP 10
0.05/0.8
196/2.0
274/2.6
218/1.7
173/3.1
206/3.9
235/4.5
173/9.8
219/12.1
236/12.8
248/16.4
8.0
9.1
3.5
November→
IOP 12
0/0
199/1.6
208/1.1
133/1.1
105/1.8
218/3.6
244/4.8
168/11.1
198/9.8
239/9.9
241/9.4
4.7
0.7
8.8
4.5/12.5
IOP 13
0.1/1.9
213/1.9
245/2.3
047/1.4
249/3.3
199/4.1
221/4.4
173/12.1
209/12.2
232/10.3
242/11.4
5.8
2.1
5.0
2.9/8.0
IOP 15
= non-dimensional height (calculated from Milano
p = pressure difference between Lugano and Vaduz reduced to mean sea level. θ = potential temperature difference between Vaduz and Lugano. H
radiosoundings within a 2000–4500 m layer).
0.5/10.3
1.3/50.9
065/1.3
250/1.8
047/0.9
175/3.1
190/5.7
193/7.6
181/13.3
035/2.5
041/3.6
341/1.9
299/1.8
033/4.0
141/4.0
171/11.2
119/0.7
280/0.9
142/0.6
344/1.1
195/3.3
215/4.3
166/14.9
Ground stations – wind direction (° )/wind speed (m s−1 )
St. Gallen
024/1.1
148/4.3
165/1.3
Altenrhein
269/1.3
175/4.6
262/1.3
Lustenau
043/1.3
204/1.9
165/1.1
Vaduz
159/3.3
164/7.3
117/2.7
Chur
212/4.2
209/5.1
200/3.9
Säntis
184/3.4
185/7.2
213/2.4
Gütsch
169/10.7
169/22.0
177/9.5
220/1.9
112/1.5
033/1.1
185/4.0
210/4.4
213/4.5
173/14.8
195/16.0
10.5
8.6
3.9
2.0/10.4
IOP 8
Julier Pass wind profiler – wind direction (° )/wind speed (m s−1 ) at about 3600 m amsl
152/6.5
180/17.0
237/9.7
211/16.3
211/14.1
275/3.5
4.0
0.5
IOP 6
207/16.3
217/18.5
249/17.0
252/21.0
7.1
4.6
5.7
4.1/6.3
IOP 5
258/2.1
223/4.9
Milano/Linate wind profiler – wind direction (° )/wind speed (m s−1 ).
4000 m amsl
161/7.3
205/16.6
242/15.1
240/19.3
6000 m amsl
185/6.8
215/20.9
241/20.1
240/24.0
8.2
6.8
3.5
1.2/5.6
6.5
−0.2
4.8
4.7
IOP 4
p (hPa)
θ (K)
(mean)
H
(min/max)
H
IOP 3
October→
IOP 2
September→
IOP 1
Table II. Temporal mean values of the relevant upstream, cross-Alpine and downstream flow parameters for the föhn Intensive Observation Periods.
FÖHN IN THE RHINE VALLEY DURING MAP
903
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 föhn 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
Valley (minimum measured intensity). The scale of the
turbulence can be characterized by the dissipative lengthscale Lε = e3/2 /ε, where e is the TKE, and ε the TKE
dissipation rate. This length characterizes the size of the
largest eddies that lie in the inertial subrange, and is of
high importance in TKE equation closure of mesoscale
models. We found Lε = 1500 ± 500 m, with no significant variation with altitude and height. This is typical
for Lε measured in homogeneous convective boundary
layers, although TKE dissipation rate can vary by several orders of magnitude depending on the complexity of
the terrain. This lends further support to this length-scale
as a robust key parameter to be used in the mesoscale
models.
4.2.
Dynamics of föhn in the Rhine Valley
4.2.1. Föhn propagation in the complex valley network
As mentioned previously, the life-cycle of a föhn usually
begins with a shallow-föhn phase followed by a deepföhn phase. One objective of FORM was to analyze the
föhn propagation in the complex valley network in the
area of the Rhine Valley. In fact, previous studies on
the channelling effect of the föhn flow by major Alpine
valleys considered only north–south oriented portions of
Alpine valleys. In the FORM target area, the Rhine Valley
Copyright  2007 Royal Meteorological Society
has a very complex shape: south of 46.8 ° N, the Rhine
Valley is oriented west–east, while north of 46.8 ° N, it
has a more south–north orientation up to Lake Constance
(Figure 2). Several major valleys merge with the Rhine
Valley, namely the Val Lumnezia, the Domleschg, the
Prättigau, the Seez Valley and the Walgau. At the
bifurcation between the Rhine and Seez Valleys, the
Tamina gorge and the Weisstannen Valley also play an
important role. In other words, the Rhine Valley can not
be considered as a single transect in the Alpine ridge
since all these tributaries play a potential role in the
circulation of the föhn flow in the FORM target area.
As a consequence, a second objective in this section is to
examine the role of the Alpine valley network in directing
the föhn flow towards the FORM target area from its early
stage of development (shallow föhn) until its breakdown.
During shallow föhn, the air which reaches the Rhine
valley area is potentially cooler and comes from the
southern side of the Alps through Alpine gaps (passes).
Due to the stably stratified air mass and/or the height of
the cold pool lying south of the Alps, the height of the
passes is also crucial. As an example for the shallow-föhn
phase, the wind field on 29 October 1999 at 1800 UTC
(Figure 4) simulated with the mesoscale model MésoNH provides evidence for weak southerly wind through
the Reuss Valley and Domleschg which remains confined
below the crest line (Drobinski et al., 2003a).
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
905
FÖHN IN THE RHINE VALLEY DURING MAP
47.43
Latitude (N)
47.43
3000.
3000.
2500.
2500.
2000.
2000.
1500.
1500.
1000.
1000.
500.0
500.0
(a)
46.42
8.34
(b)
10.05
47.43
46.42
8.34
10.05
Latitude (N)
47.43
(c)
46.42
8.34
3000.
3000.
2500.
2500.
2000.
2000.
1500.
1500.
1000.
1000.
500.0
500.0
10.05
47.43
(d)
46.42
8.34
10.05
Latitude (N)
47.43
3000.
3000.
2500.
2500.
2000.
2000.
1500.
1500.
1000.
1000.
500.0
500.0
(f)
(e)
46.42
8.34
10.05
Longitude (E)
46.42
8.34
10.05
Longitude (E)
Figure 4. Horizontal wind field at 2 km resolution at 1000 m amsl as obtained from Méso-NH numerical simulations (a) at 1800 UTC on 29
October 1999 (shallow föhn phase of IOP 12), and (b) at 1200 UTC on 30 October 1999 (deep föhn 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
Chur is fed by föhn (combined with katabatic flow;
Drobinski et al., 2003a,b) blowing in the upper Rhine
Valley. At higher levels (Figure 4(c), (e)), the air from
the Domleschg partly goes straight to the north over
the Kunkelspass (which is about 1300 m amsl), partly
towards Chur in the upper Rhine Valley. The reason
why Val Lumnezia does not channel the shallow föhn
is that probably the air does not reach high enough to
cross all passes feeding into the upper Rhine Valley.
Indeed, Val Lumnezia seems to be closed off to the south
by a pass much higher than the Domleschg valley. At
1500 m amsl, katabatic drainage flow blows from the
zonally oriented Prättigau. At the bifurcation between
the Rhine and Seez Valleys, the flow comes from the
Tamina gorge and the upper Rhine Valley (the flow first
Copyright  2007 Royal Meteorological Society
splits between these two valleys near Kunkelspass), and
splits between the Seez and Rhine Valleys. One can note
a strong intensification of the flow in the lower Rhine
Valley near Feldkirch where the flow originating from
the flow-splitting at the bifurcation of the Rhine and Seez
Valleys merges with flow blowing from the east–west
oriented Walgau.
During the deep-föhn phase, when föhn reaches the
Rhine Valley, the upstream upper-level flow veers to the
south/south–west and the stability decreases. The use
of Lagrangian tracers in numerical models (Gheusi and
Stein, 2002) shows that the air reaching the ground at
the northern edge of the Alps originates from a level of
2000 to 3500 m in the south (e.g. Lothon, 2002; Jaubert
and Stein, 2003). The air mass accelerates as it flows
over the ridge and the Rhine Valley, and experiences
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
penetrating down to the Rhine Valley (Furger et al.,
2001 for IOP 8; Drobinski et al., 2003a for IOP 12). The
downslope wind on the northern side generates turbulence
in the Rhine Valley, as indicated by various numerical
simulations (Jaubert and Stein, 2003 for IOP 2; Drobinski
et al., 2003a for IOP 12; Zängl 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-föhn
phase, the wind field on 30 October 1999 at 1200 UTC,
simulated with Méso-NH, is shown in Figure 4. As
the upstream upper-level flow is quasi-aligned with the
transverse valleys, the channelling efficiency of these
valleys in directing the föhn flow towards the FORM
target area increases. Val Lumnezia plays a significantly
more important role than during the shallow föhn whereas
the Prättigau does not channel the föhn flow.
To further illustrate the importance of orographic
gravity waves for the low-level wind field in the Rhine
Valley, Figure 5 displays the near-surface wind field and
a vertical cross-section of wind and potential temperature
along the lower Rhine Valley on 24 October 1999 at
1100 UTC (IOP 10; Zängl et al., 2004a). As evident
from Figure 5(a), a pronounced surface wind maximum is
found in the valley segment between Sargans and BuchsGrabs (Figure 2). The presence of this wind maximum
is supported by observational data, and the surface
observations gathered during the full MAP SOP reveal
that a wind maximum is frequently encountered in this
region. Figure 5(b) indicates that vertically propagating
orographic gravity waves are responsible for this wind
maximum. Due to their 3D dispersion characteristics,
gravity waves excited over the adjacent mountain ridges
radiate toward the valley axis while propagating upwards,
thus influencing the wind field over the valley axis in a
similar way as in the lee of the ridges.
(a)
4.2.2. Flow splitting at the bifurcation between the
Rhine and Seez Valleys
One of the objectives of FORM was to identify
(1) the factors determining whether flow splitting occurs
or not between the Rhine and Seez valleys, and
(2) the valley into which the föhn is directed.
The flow structure at the junction between the Rhine
and Seez Valleys has been characterized statistically at the
synoptic scale and at the scale of the valley by Drobinski
et al. (2003b). The results reveal that the flow regimes in
the Rhine and Seez Valleys are, as expected, dominated
by the orography. In the Seez Valley, the wind direction
is parallel to the valley axis in the vast majority of cases.
In the lower Rhine Valley, the wind direction also frequently follows the main valley axis but is also influenced
by the small pass to the west as well as by the orographically undisturbed synoptic-flow direction. This implies
that the lower Rhine Valley from Bad Ragaz to Lake
Constance is less efficient in channelling the flow than
the Seez Valley. Drobinski et al. (2003b) found five main
flow patterns: south-east/south, north-west/west, northwest/north, north-west/south, south-east/north, where the
first (second) wind direction refers to the Seez Valley (lower Rhine Valley) (Figure 6). The flow splitting
between the Rhine and Seez Valleys (south-east/south
flow regime) prevails and occurs either during föhn
events or is due to katabatic flow. In fact, 75% of the
south-east/south cases outside föhn events were observed
between 1800 and 0600 UTC. The very high probability
of flow splitting occurrence indicates that flow separation
from the western wall of the lower Rhine Valley near
Malans (where the Rhine Valley makes a sharp turn from
south/north orientation to south–east/north–west orientation when looking along the river flow) does not prevent
the flow from being directed towards the Seez Valley,
(b) 7.0
318
316 316
316
70
6.0
314
314
60
312
5.0
40
30
312
0 3
10
31
310
308
4.0
306
8
30
304
3.0
302
302
2.0
20
3
10
1.0
0
0.0
296
0
10
20
30 40 50
Distance (km)
60
70
0
S
10
30
6
304
Height (km)
50
314
00
298
300
296
298
20
30
40
Distance (km)
50
N
Figure 5. MM5 model results for 1100 UTC on 24 October 1999 (Zängl et al., 2004a): (a) Surface wind field (full barb = 5 m s−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
907
FÖHN IN THE RHINE VALLEY DURING MAP
(a)
SE/S 44%
47.4
(b)
NW/N 13%
(c)
NW/W 12%
Latitude (°N)
47.3
47.2
47.1
47
46.9
10 m/s
46.8
9.3
NW/S 8%
(d)
10 m/s
5 m/s
9.4 9.5 9.6
Longitude (°E)
(e)
SE/N 6%
47.4
Latitude (°N)
47.3
47.2
47.1
47
46.9
5 m/s
46.8
9.3
9.4
9.5
9.6
Longitude (°E)
5 m/s
9.3
9.4
9.5
9.6
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
Seez Valley on some occasions.
To investigate the small-scale dynamics of flow splitting between the Rhine and Seez Valleys, Drobinski et al.
(2001) simplified the problem by using the theory of
2D, incompressible and irrotational potential flows flowing along sidewalls. They demonstrated the key role
played by the valley geometry (angles between valleys, valley widths) and, in particular, the complementary contribution of the deflection and blocking/splitting
mechanisms when flow splitting occurs. However, these
results did not account for surface friction, turbulent mixing, and for the channelling effect because the solutions
for each sidewall were obtained independently of each
other. In a second step, Drobinski et al. (2006) conducted
numerical simulations including the effects of channelling
and turbulent mixing. However, surface friction was
still neglected, and a highly idealized valley geometry was used together with a single-layer approximation
of the equations of motion, similar to the well-known
shallow-water model. Comparison with observations and
fully 3D numerical simulations with MM5 indicated that
Copyright  2007 Royal Meteorological Society
= N H /U (with
the non-dimensional valley depth H
H being the dimensional valley depth) is the key
parameter for the validity of the idealized model and
for the occurrence of flow splitting in reality. For föhn
1, the flow splitting at the bifurcation
cases with H
between the Rhine and Seez Valleys was found to be very
similar to the predictions of the idealized model.
An example of the detailed flow structure is shown
in Figure 7(a), which displays the Doppler lidar radial
velocity field at 1000 m amsl on 29 October 1999
at about 2000 UTC and the corresponding wind field
simulated with the MM5 model. There is evidence of flow
splitting in both the simulated wind field and the Doppler
lidar measurements, which show air blowing away from
the Doppler lidar in the Seez and lower Rhine Valleys
(positive radial velocity) and blowing towards the lidar
from the upper Rhine Valley (negative radial velocity). A
cross-valley velocity shear is visible in the Seez Valley
with a near-wall jet (about 8 m s−1 ) at the northern wall
of the Seez Valley, while the wind speed is weaker at the
southern wall of Seez Valley. Figure 7(a) also shows that,
near the split point, there is a very sharp radial velocity
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
to zero within less than two kilometres.
Figure 7(b,c) displays the radial wind speed vr along
the thick line indicated in Figure 7(a), which is a good
approximation to the wind speed along the streamline
intersecting the split point. Wind speeds are normalized
by the inflow wind speed, U , in the upper Rhine Valley
in order to simplify the comparison of the different data
sources and cases. The solid lines depict the idealized
model result. The horizontal wind gradient decreases
from 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, vr decreases
smoothly from 1 to 0.8;
(2) between 47.045 ° N and 47.06 ° N (bifurcation point),
vr drops sharply from 0.8 down to zero.
Drobinski et al. (2006) showed that the main mechanism occurring during regime (1) is flow deflection by the
external valley sidewalls whereas the main mechanism as
the flow approaches the bifurcation point is blocking and
splitting (regime (2)).
On 2 October 1999 (IOP 5), 29 and 30 October
1999 (IOP 12), the agreement between the Doppler lidar
measurements, the MM5 simulations and the idealized
simulations is very good (Figure 7(b,c). These cases were
1 (Drobinski et al., 2006 and
characterized by H
Copyright  2007 Royal Meteorological Society
Table III). In accordance with the idealized model, the
Doppler lidar and the MM5 model (horizontal resolution
200 m and 330 m, respectively) show a clear distinction
between the smooth and sharp gradient regions. However,
the Doppler lidar measurements and MM5 simulations
do not show evidence of a sharp gradient near the
bifurcation point on 24 October 1999 at 1200 UTC
(IOP 8) (Figure 7(b)). This can be traced back to the fact
was only about 0.8 on that day, corresponding
that H
to a ‘flow over’ regime without a stagnation point at
the valley bifurcation. Evidently, the idealized 2D model
ceases to be valid in such a situation.
Another fundamental difference between the channelled and unchannelled flow regimes becomes evident
from the along-valley mass fluxes. Table III shows the
measured mass flux in the Seez, SRsv , and lower Rhine,
SRlrv , Valleys, normalized with the mass flux measured
in the upper Rhine Valley (called split ratio in the
table) for several IOPs (Drobinski et al., 2006). It can
be seen that the flow budget is approximately balanced
1 (IOPs 5, 8 and 12), con(SRlrv + SRsv ≈ 1) for H
firming that the flow is channelled and quasi-2D under
these circumstances. One must note that simulations of
these IOPs indicate that the föhn extends only a few
kilometres north of the valley bifurcation and afterwards
gets lifted over the low-level cold pool, so the vertical
contraction of the föhn layer eliminates the need of a
deceleration in response to the widening of the Rhine
Valley bottom. Otherwise, for the other IOPs, there is
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
909
FÖHN IN THE RHINE VALLEY DURING MAP
2 (19 Sep 1999)
5 (2 Oct 1999)
8 (20 Oct 1999)
10 (24 Oct 1999)
12 (29 Oct 1999)
12 (30 Oct 1999)
15 (5 Nov 1999)
Split ratio
Brunt–
Normalized
SRlrv
SRsv
Väisälä
frequency
N (10−2 s−1 )
valley
depth H
1.26
0.70
0.83
1.08
0.67
0.52
1.20
0.13
0.35
0.24
0.84
0.39
0.37
0.11
0.9
2.0
1.2
1.2
1.7
2.3
0.7
0.7
2.1
1.8
0.8
4.2
5.7
1.0
The split ratio is computed from the radiosounding (launched from
Heiligkreuz in the Seez Valley, Buchs-Grabs in the lower Rhine Valley
and Malans in the upper Rhine Valley) velocity measurements for
IOP 2 and 15 (when the Doppler lidar was not in operation), or from
the Doppler lidar velocity measurements for IOP 5, 8, 10 and 12. The
in-valley velocities are integrated vertically over the föhn jet depth.
When the Doppler lidar data are used, the in-valley velocity is also
integrated horizontally over the valley width. The values of N and H
are computed from the radiosounding launched from Malans (upstream
sounding) or from Heiligkreuz when one was not launched from Malans
(IOP 2 and 5).
a substantial excess flow rate in the lower Rhine Valley, 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 mass
flux can contribute more than 30% to the total flow budget during deep föhn events. The subsidence potentially
affects both the Seez Valley and the lower Rhine Valley
but usually tends to be more pronounced in the lower
Rhine Valley.
As pointed out by Zängl et al. (2004a), the subsidence
can also have a profound impact on the temperature field
in the adjacent valley segments. The simulated low-level
flow field for IOP 10 (24 October 1999 at 1200 UTC)
is displayed in Figure 8 together with the corresponding
4.3. Föhn/cold-pool interaction
A cold surface layer or cold pool often fills the floor of
Alpine valleys and prevents the upper-level föhn flow
from reaching the ground during most of the duration of
föhn episodes. The cold pool is either present from the
preceding (colder) weather situation or rebuilds at night
35
35
30
30
25
25
20
20
15
15
10
10
5
5
300
0
5
10
15 20 25 30
Distance (km)
35
40
0
298
0
29
8
298
40
296
40
300
(b)
300
(a)
298
IOP
surface potential temperature field. As already mentioned,
the flow splitting at the ridge separating the Seez Valley
from the lower Rhine Valley did not involve a stagnation
point on that day (Figure 8(a)). Thus, the low-level
airflow originating from the upper Rhine Valley enters
not only the two valleys, but part of it even ascends
the dividing mountain ridge. The strongest subsidence of
warm air occurs in the lower Rhine Valley around Vaduz,
where south-south-easterly flow reaches the valley from
the adjacent Rätikon massif. The surface observations
collected at Vaduz indicate that this flow feature is quite
typical for deep föhn, showing a typical surface wind
direction between 150° and 170° . In the case considered
here, the model also predicts warm-air subsidence in the
Seez Valley, particularly in its western part where the air
enters the valley directly from the mountain range to the
south. The latter feature cannot be verified due to the
lack of a surface station. An important consequence of
the warm-air subsidence downstream of the flow-splitting
point is that the surface potential temperature in the lower
Rhine 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 warmer
than Chur in the morning of 24 October 1999 (Zängl
et al., 2004a), and θ differences in excess of 4 K are
observed quite frequently. It remains to be pointed out
that turbulent vertical mixing of stably stratified air is also
capable of inducing an along-valley increase of potential
temperature. However, separating this effect from the
impact of direct warm-air advection is difficult because
numerical models appear to have substantial deficiencies
in representing turbulent mixing in narrow Alpine valleys
(e.g. Zängl, 2003).
300
Table III. Split ratios in the lower Rhine (SRlrv ) and Seez
(SRsv ) valleys..
30
0
298
0
5
10
294
15 20 25 30
Distance (km)
35
40
Figure 8. MM5 model results for 1200 UTC on 24 October 1999 (Zängl 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
freely to space. Only when the föhn is sufficiently intense
does the föhn flow touch the floor of Alpine valleys.
Three mechanisms are likely to govern the penetration of
the föhn flow to the valley floor:
(1) the diurnal heating of the cold pool by solar radiation
may diminish the stability and allow vertical mixing,
(2) turbulent entrainment induced by Kelvin–Helmholtz
instability at the top of the cold pool may erode and
eventually destroy the pool (Nater et al., 1979), and
(3) the occasional intensification of a mountain wave at
higher levels may force the föhn flow down to the
ground level and flush the cold pool downstream, for
instance in the case of a breaking wave aloft.
Indeed, gravity-wave breaking accelerates the flow
beneath the wave-breaking zone and thus can increase the
cold-pool erosion due to turbulent mixing or due to an
enhanced pressure drag force if the cold pool has a steep
lateral edge. Downstream advection of the cold pool is
also possible in the absence of pronounced gravity-wave
activity when the mesoscale pressure field at the top of
the cold pool imposes a favourable forcing (e.g. Zängl,
2005). Of course, these cold-pool removal processes
may also occur simultaneously. The interaction between
the cold pool and the föhn in the Rhine Valley was
best documented during IOPs 8 and 9 (21–22 October
1999; Gubser and Richner, 2001) and IOP 15 (5 and 6
November 1999; Vogt and Jaubert, 2004; Jaubert et al.,
2005; Flamant et al., 2006).
The warming rate due to heat flux in the cold pool
under föhn conditions could be estimated only during
IOP 9, as there were no dedicated aircraft flights on other
föhn days. Gubser and Richner (2001) found that the heat
fluxes at the surface and at the top of the cold pool were
comparable in magnitude (about 15 W m−2 ). Based on
these heat fluxes but without accounting for long-wave
radiative cooling (which causes the air column to lose at
least part of the energy gained by the heat flux), they
estimated warming rate of about 25 K (day)−1 , which
appears to be considerably too high. Gubser and Richner
(2001) concluded that, with such a warming rate, any
cold pool would disappear within less than one day, and
föhn periods with more or less stationary and persistent
cold pools could not occur. An important contribution to
this discrepancy might arise from cold-air advection from
the Alpine foreland. Surface observations and numerical
simulations (Zängl et al., 2004a; Beffrey et al., 2004a;
Jaubert et al., 2005) frequently indicate a light northerly
flow within the cold pool, particularly when the preceding
weather evolution formed a significant cold-air pool in
the Alpine foreland. The pressure gradient driving this
inflow is most likely due to the fact that gravity-wave
dynamics tends to form a local pressure minimum at the
boundary between the föhn and the cold pool (Zängl et al.
2004a). Moreover, a gradually decreasing cold-pool depth
between the Lake Constance region and the southern edge
of the cold pool might play a role.
Copyright  2007 Royal Meteorological Society
Numerical simulations of IOP 15 conducted by Jaubert
et al. (2005) and an observational study by Flamant et al.
(2006) indicate that the presence or absence of a cold pool
in the lower Rhine Valley is of crucial importance for the
flow evolution. Jaubert et al. (2005) simulated this event
using the mesoscale model Méso-NH with great realism.
They used a mesoscale analysis over the whole simulation domain, which turned out to improve the results
significantly. Specifically, the mesoscale analysis allowed
introduction of a realistic initial cold pool that was missing in the large-scale analysis at noon. As an illustration,
Figure 9 shows the vertical structure of the cold pool documented through reflectivity measurements made along
the Rhine Valley between 1445 and 1505 UTC with the
nadir-pointing differential absorption lidar LEANDRE-2,
and the potential temperature fields for two simulations
using the Méso-NH model – with (A12) and without
(REF12) mesoscale analysis as initial state – along the
ARAT flight track. In the reflectivity measurements, the
cold pool corresponds to the region of high reflectivity
(350 arbitrary units or higher). Above the cold pool and
below 2 km amsl, the reflectivity is generally very low,
indicating the presence of the dry föhn layer. The southern tip of the cold pool is located just north of Weite,
in agreement with the surface measurements. The depth
of the cold pool increases slowly towards the north, and
reaches 250 m north of Rankweil, where the reflectivity
also increases, indicating moister conditions. This depth
was also observed over Lake Constance (not shown, see
Flamant et al., 2006). The structure of the cold pool, as
defined by the low potential temperature regions, bears
resemblance to that observed by lidar in simulation A12,
but the cold pool is missing in simulation REF12 (Jaubert
et al., 2005).
The effect of the mesoscale analysis as initial state of
the A12 simulation was not limited to the first hours of
the simulation, but was still effective 12 hours after the
beginning of the run; a realistic cold-pool height prevents
the föhn from touching the ground too early, and allows
for simulating an accurate timing of the föhn onset. A
heat budget analysis of the interactions between the cold
pool and the föhn jet above during the late afternoon
and evening of 5 November 1999 indicated that the
leading terms are the advection by the mean flow and the
turbulent tendency, whereas radiation tendency is weak
(also Flamant et al., 2006). The turbulent mixing occurs
mainly close to the terrain, in the regions where the föhn
air descends in the lee of the mountain range and then
interacts with the cold pool. Jaubert et al.’s (2005) results
are consistent with the analysis of IOP 4–5 by Frioud
et al. (2004) and of IOP 10 by Zängl et al. (2004a), who
also found that turbulent vertical mixing is important for
the erosion of the cold-air pool that initially fills the lower
Rhine Valley.
4.3. Advances in numerical modelling of föhn in the
Rhine Valley during MAP
One major objective of MAP was to improve the performance of high-resolution NWP, hydrological and coupled
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
911
FÖHN IN THE RHINE VALLEY DURING MAP
(a)
3
Lidar reflectivity
430
Altitude (km)
410
2
390
370
350
1
330
0
Altitude (km)
(b)
0.4
3
Pot.Temp. (K) REF12
2
4.3.1. Initial state
1
Altitude (km)
(c)
meshes at the same time in a one-way or two-way interactive mode. The highest model resolution was about a
kilometre. These real-case simulations were forced and
initialized with NWP analyzes and realistically reproduced several MAP föhn events over their whole duration (IOP 12, Drobinski et al., 2003a; IOP 2, Jaubert and
Stein, 2003; IOP 8, Lothon, 2002).
However, some limits appeared with respect to model
initialization (since large-scale NWP analyzes do not
allow for reproducing the low-level thermodynamic structure in the valleys), grid resolution (since a grid size of a
few kilometres is not fine enough to reproduce the complex orography of the Rhine Valley and its tributaries),
and numerical diffusion over steep orography. All these
problems could be solved using the FORM dataset, allowing more accurate and reliable numerical modelling and
forecasting of the 3D structure of föhn and its time evolution in the Rhine Valley.
0.4
3
Pot.Temp. (K) A12
2
1
0.4
0
20
Weite
X (km)
40
60
Rankweil
Altenrhein
Figure 9. (a) Atmospheric reflectivity at 732 nm obtained from the
airborne lidar LEANDRE-2 between 1445 and 1505 UTC on 5
November 1999 (IOP 15) along the lower Rhine Valley. Reflectivity
units are arbitrary. The continuous white line represents the orography.
(b) Vertical cross-section along the lower Rhine Valley at 1500 UTC on
5 November 1999 of potential temperature (with contour interval 1 K,
light shading over 300 K, medium shading below 296 K, and darker
shading below 292 K) and vertical velocity (dashed contours with
interval 0.25 m s−1 ) from the reference Méso-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 modelling
work on föhn began in 1999 with the state-of-the-art
mesoscale models. The models are non-hydrostatic and
nested model domains are used, with different horizontal
Copyright  2007 Royal Meteorological Society
The numerical studies by Jaubert and Stein (2003) and
Beffrey et al. (2004a), using the mesoscale model MésoNH, provides evidence that a satisfactory simulation of
larger-scale aspects as well as meso-γ aspects of föhn
cases (IOPs 2 and 8, respectively) can be achieved with
operational meteorological large-scale analyzes and a
mesoscale model with nested domains. However, in these
two studies, the depth of the cold pool was unrealistic,
probably due to a deficiency of the initial analyzes. Using
the MM5 model, Zängl et al. (2004a) used sounding measurements to modify the lower levels of the large-scale
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
explored the benefits of an operational mesoscale analysis scheme, capable of introducing mesoscale features
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 significantly the cold-pool dynamics and the föhn life-cycle
(especially onset) as shown by comparing the model outputs with lidar (Figure 9) and wind profiler (Vogt and
Jaubert, 2004) measurements.
4.3.2. Numerical diffusion
Numerical simulations performed with the mesoscale
model MM5 for 24 October 1999 (IOP 10, Zängl et al.,
2004a) demonstrate that a proper treatment of numerical
diffusion is of crucial importance. As in several other
mesoscale models, the numerical diffusion was originally implemented as a fourth-order horizontal smoothing operator evaluated along the terrain-following sigma
coordinate surfaces without accounting for any metric
terms. Over steep topography, this tends to induce large
systematic errors for variables having a strong vertical
stratification (temperature and the water vapour mixing ratio). The original implementation was changed by
Zängl (2002b) into a truly horizontal computation of
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
ratios of water vapour and cloud water. As illustrated
in Figure 10 for the station at Fussach, located at the
southern shore of Lake Constance, the modified diffusion scheme (denoted as z-diffusion) enables the model
to reproduce the observed flow evolution essentially correctly. Discrepancies between model results and observations are largely restricted to small errors in the time of
föhn breakthrough or in the time of the cold front passage (1–2 hours each). However, using the original MM5
diffusion scheme (denoted as sigma-diffusion) greatly
degrades the results. The föhn breakthrough at Lake Constance is then simulated at least 5 hours too early, and
the agreement between simulated and observed surface
temperatures worsens even in those parts of the valley
where the occurrence of föhn is predicted correctly (not
shown). This can be explained by the fact that computing
the numerical diffusion of temperature along the model
surfaces effectively destroys local cold-air pools within
valleys because the cold air is mixed with the warmer
air over the adjacent side slopes and ridges. The severity of the related numerical errors is further emphasized
by another sensitivity experiment in which the horizontal
resolution was degraded from 1 to 3 km while retaining the improved diffusion scheme. As evident from
Figure 10, the reduced model resolution has much less
detrimental effects on the model results than using the
original diffusion scheme at 1 km resolution.
4.3.3. Model validation
The deployment of innovative remote sensors to document at high temporal and spatial resolutions the 3D flow
structure was the instrumental core for new methods of
validation of high-resolution modelling. Föhn flow was
observed with Doppler lidars, sodars and wind profilers, and the structure of the planetary boundary layer in
complex terrain with a backscatter lidar. During MAP,
mesoscale simulations were performed with horizontal
mesh sizes down to 200–600 m, implying that spatially
continuous measurements are crucial for a proper validation. The Rankweil RASS wind profiler operated at
(a) 300
Föhn conditions impair human comfort and health
in Alpine regions, causing headache and circulatory
(b) 16
Fussach, wind speed
14
296
Wind speed (m s-1)
Potential temperature (K)
4.4. FORM-related side project: Evaluation of föhn
impact on regional air quality during MAP
Fussach, temperature
298
294
292
290
288
Observation
1 km, z-diffusion
1 km, sigma-diffusion
3 km, z-diffusion
286
284
282
30 min time resolution and 60 m vertical resolution (Vogt
and Jaubert, 2004) and the TWL operated at about 1 min
time resolution and 250 m radial resolution (Beffrey
et al., 2004a; Drobinski et al., 2006; Figure 7) allowed
for an unprecedented validation exercise. As an illustration, the radial velocity field obtained from the TWL
measurements on 20 October 1999 between 0837 and
0943 UTC (IOP 8) is compared to the radial velocity
field simulated with Méso-NH (Figure 11) (Beffrey et al.,
2004a). The simulated field is consistent with the observations. A strong jet of incoming air (radial velocities
up to 15 m s−1 ) can be found in the upper Rhine Valley. At the junction of the Seez and Rhine Valleys it
splits into two branches, one along the Seez Valley and
the other towards Lake Constance. In the Seez Valley, a
strong, transverse gradient can be observed in the radialvelocity field with strong winds along the northern wall
and almost no wind in the south. At higher levels, radial
velocities are somewhat underestimated by the model (by
about 5 m s−1 ) due to an insufficient channelling by the
smoothed topography of the model (the resolution for the
prescribed topography is 1 km). Another explanation is
that the direction of the simulated wind is slightly different from the actual wind direction. Although there are
small but significant discrepancies, it can be noted that
the salient features of the dynamic field revealed by the
TWL are well reproduced by the model.
A novel approach for validating high-resolution models
against conventional station data was introduced by Zängl
et al. (2004a). A model-independent 2D analysis (VERA,
Vienna Enhanced Resolution Analysis; Chimani et al.,
2006) was used as an alternative to interpolating model
fields to the locations of the stations. When the analysis
approximately equals the model resolution, this method
helps to gain a better overview of the spatial distribution
of the differences than just comparing point data.
6
8
10
12 14 16
Time (h)
18
12
10
8
6
4
2
20
0
6
8
10
12 14 16
Time (h)
18
20
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 (Zängl et al., 2004a). z-diffusion denotes the truly horizontal diffusion scheme developed by Zängl
(2002b).
Copyright  2007 Royal Meteorological Society
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
FÖHN 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
Méso-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
these effects, air pollution can be strongly enhanced under
south föhn conditions in several regions on the northern
side of the Alps (Nkemdirim and Leggat, 1978; Hoinka
and Rösler, 1987). Indeed, in the cold-air pool, the ozone
concentration is reduced below the free-tropospheric
background level due to chemical reactions with other
pollutants, mainly nitrogen oxides. Thus, föhn breakthrough goes along with an increase in the ozone concentration whereas other pollutants are reduced. So ozone
concentrations in the valley tend to increase at the onset
of föhn. High wind speeds and turbulence reduce the
effects of titration by nitric oxide and dry deposition on
the concentrations of ozone. This results in higher ozone
concentrations in the valley at night and in the morning
hours. These ozone concentrations are usually not as high
as the highest ozone concentrations reached during photochemical smog episodes. This means that föhn events do
not necessarily cause ozone peaks, but prolong the duration of ozone stress in föhn areas. On the other hand, a
penetration of the föhn to the ground may bring a sudden
relief to some valley segments when polluted air in the
valley is replaced by usually less polluted air from above,
whilst other segments of the valley remain within a shallow inversion without significant air-mass exchange. Air
quality is thus of high interest in the densely populated
Alpine Rhine Valley. The set-up of ozone, nitrogen and
Copyright  2007 Royal Meteorological Society
aerosol concentration measurements by in situ and remote
sensors allowed investigation of the air-mass composition
with high temporal and vertical resolution.
In the Rhine Valley, air quality strongly depends on the
interaction between föhn and the cold pool close to Lake
Constance. Indeed, the cold-air pool often persists (also
during most south föhn conditions) leading to enhanced
air pollution within the stagnant boundary layer which
proved to be an aerosol-rich layer from the backscatter
lidar measurements (Frioud et al., 2004), whereas föhn
brings relief as clean air from above is mixed into
the boundary layer. The question arises whether the
high ozone levels found north of the Alps, e.g. in the
Rhine Valley, during south föhn are originally produced
in polluted areas south of the Alps or are transported
downwards from the stratosphere. Baumann et al. (2001)
showed that, during the MAP SOP, föhn-induced ozone
peaks in October and November were found to be
much lower than in September. They found remarkable
spatial differences in the ozone records over a relatively
small area of the Rhine Valley, confirming the usually
‘patchy’ distribution of ozone concentrations during föhn
events which reflect the separation of föhn-shielded from
föhn-exposed areas. The stratification within the lowest
few hundred metres, especially the presence of a coldair pool, determines whether the air mass with higher
ozone concentrations advected by the föhn flow reaches
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
ground without or before removing the near-ground
cold-air pool. The trajectory calculations for the föhn
period confirm the general experience from previous
investigations (Seibert, 1990) that the air advected by
south föhn often originates from around 2000 m above
the Po basin. Nevertheless, the results of 22 October
1999 (IOP 9) demonstrate that air masses from the
lower free troposphere can be imported into the föhn
flow due to changing meteorological conditions in the
course of a longer föhn period. In this case, the föhn
air originating from 4000 m agl (above ground level)
caused the most distinct increase of ozone of this föhn
phase at the monitoring stations in the Rhine Valley.
No vertical transport of ozone from the stratosphere and
upper troposphere was involved in the increase of the
ozone concentrations in the föhn valleys during the föhn
phase.
5.
Concluding remarks
Looking back from some distance, the design of the
composite observing network may be assessed as quite
positive, even though lessons can always be learned.
The composite observing system and the combination of
remote-sensing and in situ systems produced a wealth
of data which allows unprecedented insight into the
structure of the föhn flow and a valley network with
complex geometry. The combination of established and
novel remote-sensing instruments (e.g. Doppler and water
vapour lidars) with conventional in situ measurements
(dense surface network and radiosondes) allowed capture
of previously unseen details of the fine structure of
föhn (Richner et al., 2006). This allows the validation
of ultra-high-resolution numerical research and weather
prediction models.
The work conducted in the framework of FORM now
also allows the comparison of the flow characteristics
of the föhn in the Rhine Valley with those observed
in the Wipp Valley region, the second föhn target area
during 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 lowlevel wind field in the valley has also been pointed
out for the Wipp Valley (Flamant et al., 2002; Zängl,
2003; Zängl et al., 2004b). However, the Wipp Valley
appears to encounter shallow föhn flows more frequently,
in which the essential flow dynamics can be alternatively
explained with the conceptually simpler shallow-water
model (Gohm and Mayr, 2004). Marked differences
also occur for the low-level flow behaviour before
föhn breakthrough. The lower Rhine Valley frequently
experiences light upvalley flow within the cold pool,
indicating a cold-air advection from the Alpine foreland
opposing the föhn breakthrough. In the Wipp Valley,
katabatic downvalley flow usually prevails in the pre-föhn
phase, except perhaps for a very short period immediately
before föhn breakthrough (Zängl, 2003). Moreover, the
Copyright  2007 Royal Meteorological Society
adjacent Inn Valley usually encounters downvalley flow,
which has frequently been interpreted as cold-air outflow
into the Alpine foreland. Zängl (2003) showed that the
westerly downvalley flow is locally enhanced around
Innsbruck due to an east–west asymmetry in the gravity
wave activity. It is also important to note that the lower
Rhine Valley tends to be more strongly affected by
cold-air pools lying north of the Alps than the Wipp
Valley because the former valley exits directly into the
Alpine foreland. Finally, it has been found that the
downvalley increase of the surface potential temperature
can be significantly stronger for the Rhine Valley than
for the Wipp Valley. In the Wipp Valley, the low-level
airflow essentially follows the valley axis, so that an
increase in surface potential temperature can occur only
due to turbulent vertical mixing of stably stratified air
(Seibert, 1985; Zängl, 2003). Since the Wipp Valley
widens considerably in the downvalley direction, the
stable stratification might be reinforced by subsidence of
potentially even warmer air from aloft into the valley
region, so that the mixing-induced potential temperature
increase can be quite appreciable (∼5 K). In the Rhine
Valley, values up to 8 K have been observed because,
in addition to mixing-induced warming, direct warm-air
advection from the adjacent mountain ranges into the
valley is possible, particularly in the region of Vaduz.
Finally, despite the significant progress made in föhn
understanding, modelling and forecasting thanks to the
MAP programme, several key issues are still at best partly
understood. Among those are:
(1) The föhn/cold-pool interaction: the interaction of
ambient air flow with cold pools still poses a
major challenge to understanding and predicting
local weather in mountainous regions. It is of crucial importance not only for föhn flows but also
for wintertime warm-front passages, leading to pronounced horizontal temperature differences and possibly large fluctuations in the height of the snow line.
As already discussed above, the most important processes involved in the interaction between ambient
flows and cold pools are turbulent erosion, radiative
heating/cooling, interaction with orographic gravity
waves and cold-air drainage related to an externally imposed pressure gradient. Some of these processes are only partly understood so far and are difficult to represent in a numerical model. Apart from
high resolution and accurate model numerics, representing these processes requires highly sophisticated
parametrizations for turbulence, radiation and clouds.
Regarding the model numerics, the implementation
of numerical diffusion is particularly crucial because
simple methods tend to destroy cold pools in narrow valleys. A major weakness of present turbulence
models is that the effects of the valley topography
(increased turbulence due to sidewall friction) are
not properly accounted for, even when the turbulence
model is 3D. Finally, interactions between fog and
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
FÖHN IN THE RHINE VALLEY DURING MAP
radiation are potentially important, and most microphysical parametrizations used in mesoscale models
tend to remove fog by converting the cloud water too
rapidly into drizzle (Crewell et al., 2003).
(2) The role of scale interactions in the local response
of föhn to a large-scale forcing: the sensitivity of
the local response of the föhn to any uncertainty in
the large-scale analysis still needs to be investigated
in more detail. For example, the simulation of some
MAP föhn events (particularly IOP 8, Beffrey et al.,
2004a) presumably failed due to an erroneous timing in the large-scale forcing (inflow strength and
direction, cold front location). Improving the understanding of scale interactions includes analyzing the
interaction of the synoptic-scale flow with the Alpine
massif as a whole, and its possible side effects on the
local characteristics of föhn flow. At a smaller scale,
the sensitivity of the amplitude, phase and temporal
evolution of orographic gravity waves to the ambient flow are still poorly understood, particularly in
the presence of wave breaking. (Lothon (2002) and
Lothon et al. (2003) showed evidence of enhanced
turbulence around and above crest level; this could
be taken as evidence for wave breaking, since it was
presumably not boundary-layer turbulence, but there
is no formal proof.) However, the local evolution of
föhn flow might depend very sensitively on the wave
structure. In addition, the interaction between föhn
and cold pools again comes into play, as the cold
pools might either be localized in some valley segments or be fed from a larger-scale cold-air reservoir
in the northern Alpine foreland.
(3) The model initialization issue: future data assimilation will need to be conducted on a high-resolution
grid to allow for a proper use of the available data
(particularly surface data). The range of validity
of point measurements (or line measurements i.e.
radiosondes) can be highly anisotropic in mountainous terrain, which will need to be accounted for in
future mesoscale data assimilation systems.
Acknowledgements
We are deeply indebted to many colleagues who contributed to this overview paper by providing text, figures
and other input. We also thank the many agencies of the
participating countries which, by their financial support,
contributed to the success of the field experiment and the
progress of föhn understanding in the complex Alpine
valleys.
References
Baumann K, Maurer H, Rau G, Piringer M, Pechinger U, Prévôt A,
Furger M, Neininger B, Pellegrini U. 2001. The influence of south
föhn on the ozone distribution in the Alpine Rhine Valley – results
from the MAP field phase. Atmos. Env. 35: 6379–6390.
Beffrey G, Jaubert G, Dabas AM. 2004a. Föhn flow and stable air
mass in the Rhine Valley: the beginning of a MAP event. Q. J. R.
Meteorol. Soc. 130: 541–560.
Copyright  2007 Royal Meteorological Society
915
Beffrey G, Jaubert G, Dabas AM. 2004b. Spatial evolution of föhn
flows in the Rhine Valley area: Quantification using high-resolution
simulations. Meteorol. Z. 13: 77–82.
Bénech B, Lothon M, Berger H. 2002. ‘Analysis of the constant-volume
balloons flights above the Rhine Valley during föhn events (MAP
experiment)’. Proceedings of the 10th AMS Conference on Mountain
Meteorology, Utah.
Billwiller R. 1878. Report on ‘Étude sur les grands mouvements de
l’atmosphère et sur le Föhn et le Sirocco pendant l’hiver 1876-77’.
Z. Meteorol. 13: 317–320.
Bougeault P, Binder P, Buzzi A, Dirks R, Houze R, Kuettner J, Smith
RB, Steinacker R, Volkert H. 2001. The MAP special observing
period. Bull. Am. Meteorol. Soc. 82: 433–462.
Brinkmann WAR. 1971. What is a föhn? Weather 26: 230–239.
Brinkmann WAR. 1974. Strong downslope winds at Boulder, Colorado.
Mon. Weather Rev. 102: 592–602.
Chimani B, Steinacker R, Häberli C, Dorninger M, Tschannett S. 2006.
Objective mesoscale analyses in complex terrain: application to föhn
cases during MAP. Meteorol. Z. 15: 117–125.
Crewell S, Simmer C, Feijt A, van Meijgaard E. 2003. ‘CLIWANET Baltex Bridge cloud liquid water network’. Final Report,
International Baltex Secretariat, Publ. No. 26.
Drobinski P, Dabas AM, Häberli C, Flamant PH. 2001. On the smallscale dynamics of flow splitting in the Rhine Valley during a shallow
föhn event. Boundary-Layer Meteorol. 99: 277–296.
Drobinski P, Häberli C, Richard E, Lothon M, Dabas AM, Flamant PH,
Furger M, Steinacker R. 2003a. Scale interaction processes during
the MAP IOP 12 south föhn event in the Rhine Valley. Q. J. R.
Meteorol. Soc. 129: 729–753.
Drobinski P, Dabas AM, Häberli C, Flamant PH. 2003b. Statistical
characterization of the flow structure in the Rhine Valley. BoundaryLayer Meteorol. 106: 483–505.
Drobinski P, Bastin S, Dusek J, Zängl G, Flamant PH. 2006.
Idealized simulations of flow splitting at the bifurcation between two
valleys: comparison with the Mesoscale Alpine Program experiment.
Meteorol. Atmos. Phys. 92: 285–306.
von Ficker H. 1905. Einige Ergebnisse von Föhnbeobachtungen im
Gebiet um Innsbruck im Jahr 1904. Meteorol. Z. 22: 324–327.
von Ficker H. 1913. Ballonaufstiege bei Föhn. Meteorol. Z. 30:
213–216.
Flamant C, Drobinski P, Nance L, Banta RM, Darby L, Dusek J,
Hardesty RM, Pelon J, Richard E. 2002. Gap flow in an Alpine
valley during a shallow south föhn event: Observations, numerical
simulations and hydraulic analogue. Q. J. R. Meteorol. Soc. 128:
1173–1210.
Flamant C, Drobinski P, Protat A, Chimani B, Furger M, Richner H,
Steinacker R, Tschannett S, Gubser S, Häberli C. 2006. Föhn/coldpool interactions in the Rhine Valley during MAP IOP 15. Q. J. R.
Meteorol. Soc. 132: 3035–3058.
Florida-James G, Donaldson K, Stone V, Budgett R. 2004. Athens
2004: the pollution climate and athletic performance. Commentary.
J. Sports Sci. 22: 967–980.
Frey K. 1953. Die Entwicklung des Süd- und des Nordföhns. Arch.
Meteorol. Geophys. Biokl. A5(:): 432–477.
Frioud M, Mitev V, Matthey R, Häberli C, Richner H, Werner R, Vogt
S. 2003. Elevated aerosol stratification above the Rhine Valley under
strong anticyclonic conditions. Atmos. Environ. 37: 1785–1797.
Frioud M, Mitev V, Matthey R, Richner H, Furger M, Gubser S.
2004. Variation of the aerosol stratification over the Rhine Valley
during Föhn development: a backscatter lidar study. Meteorol. Z.
13: 175–181.
Furger M, Drobinski P, Prévôt ASH, Weber RO, Graber WK, Neininger
B. 2001. Comparison of horizontal and vertical scintillometer
crosswinds during strong föhn with lidar and aircraft measurements.
J. Atmos. Oceanic Technol. 18: 1975–1988.
Gheusi F, Stein J. 2002. Lagrangian description of air flows using
Eulerian passive tracers. Q. J. R. Meteorol. Soc. 128: 337–360.
Gohm A, Mayr GJ. 2004. Hydraulic aspects of föhn winds in an Alpine
valley. Q. J. R. Meteorol. Soc. 131: 449–480.
Gubser S, Richner H. 2001. ‘Investigations into mechanisms leading
to the removal of the cold pool in föhn situations’. MAP
Newsletter 15: available on http://www.map.meteoswiss.ch/mapdoc/newsletter.htm.
Gutermann T. 1970. Vergleichende Untersuchungen zur Föhnhäufigkeit
im Rheintal zwischen Chur und Bodensee. Veröffentlichungen der
Schweiz. Meteorol. Zentralanstalt 18: 1–64.
Hann J. 1866. Zur Frage Über den Ursprung des Föhn. Z. Meteorol. 1:
257–263.
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj
916
P. DROBINSKI ET AL.
Hann J. 1891. Nordföhn in Innsbruck. Meteorol. Z. 26: 239.
Hann J. 1882. Über den Föhn in Bludenz. Sitzungsber. Math.Naturwiss. Klasse der kaiserlichen Akad. der Wiss.
Hoinka KP. 1980. Synoptic-scale atmospheric features and föhn. Beitr.
Phys. Atmos. 53: 486–507.
Hoinka KP. 1985. Observations of the airflow over the Alps during a
föhn event. Q. J. R. Meteorol. Soc. 111: 199–224.
Hoinka KP. 1990. ‘Untersuchung der alpinen Gebirgsüberströmung bei
Südföhn’. DLR-FB 90-30.
Hoinka KP, Rösler F. 1987. The surface layer on the leeside of the
Alps during föhn. Meteorol. Atmos. Phys. 111: 199–224.
Jaubert G, Stein J. 2003. Multiscale and unsteady aspects of a deep
föhn event during MAP. Q. J. R. Meteorol. Soc. 129: 755–776.
Jaubert G, Bougeault P, Berger H, Chimani B, Flamant C, Häberli C,
Lothon M, Nuret M, Vogt S. 2005. Numerical simulation of mesogamma scale features of föhn at ground level in the Rhine Valley.
Q. J. R. Meteorol. Soc. 131: 1339–1361.
Kuhn M. (ed) 1989. Föhnstudien. Wissenschaftl. Buchgesellschaft:
Darmstadt.
Long RR. 1953. Some aspects of the flow of stratified fluids. I. A
theoretical investigation. Tellus 5: 42–58.
Lothon M. 2002. ‘Etude phénoménologique du föhn dans la vallée
du Rhin dans le cadre de l’expérience MAP (Mesoscale Alpine
Programme)’. PhD thesis. Université Toulouse III – Paul Sabatier,
France.
Lothon M, Druilhet A, Bénech B, Campistron B, Bernard S, Saı̈d F.
2003. Experimental study of five föhn events during the Mesoscale
Alpine Programme: From synoptic scale to turbulence. Q. J. R.
Meteorol. Soc. 129: 2171–2194.
Lyra G. 1943. Theorie der stationären Leewellenströmung in freier
Atmosphäre. Z. angew. Math. Mech. 23: 1–28.
Nater W, Richner H, Phillips PD. 1979. Shear instabilities and their
characteristics during föhn. Geophys. Astrophys. Fluid Dynamics 13:
215–223.
Nkemdirim LC, Leggat K. 1978. The effect of chinook weather on
urban heat islands and air pollution. Water, Air and Soil Pollution 9:
53–67.
Peppler W. 1930. Schema eines Föhnausbruchs aus dem Rheintal. Das
Wetter 47: 78–84.
Peppler W. 1935. Über die südlichen Luftströmungen auf dem Säntis
und in der freien Atmosphäre über dem Bodensee. Beitr. Phys. freien
Atmos. 22: 1–11.
Queney P. 1948. The problem of airflow over mountains. A summary
of theoretical studies. Bull. Am. Meteorol. Soc. 29: 16–26.
Richner H, Baumann-Stanzer K, Bénech B, Berger H, Chimani B,
Dorninger M, Drobinski P, Furger M, Gubser S, Gutermann T,
Häberli C, Häller E, Lothon M, Mitev V, Ruffieux D, Seiz G,
Steinacker R, Tschannett S, Vogt S, Werner R. 2006. Unstationary
aspects of föhn in a large valley. Part I: operational setup, scientific
objectives and analysis of the cases during the special observing
period of the MAP subprogramme FORM. Meteorol. Atmos. Phys.
92: 255–284.
Roßmann F. 1950. ‘Über das Absteigen des Föhns in die Täler’.
Berichte des deutschen Wetterdienstes der US-Zone 12, S. 94–98.
Copyright  2007 Royal Meteorological Society
Ruffieux D, Berger H, Stübi R, Wolfe DE, Welsh DC, Webber BL.
2000. ‘Extreme weather events measured by ground-based remote
sensing systems at the crest of the Swiss Alps’. Pp 165–167 in
Procedings of the Fifth International Symposium on Tropospheric
Profiling: Needs and Technology. 4–8 December 2000, Adelaide,
Australia.
Schweitzer H. 1953. Versuch einer Erklärung des Föhns als
Luftströmung mit überkritischer Geschwindigkeit. Arch. Meteorol.
Geophys. Biokl. A5(:): 350–371.
Seibert P. 1985. ‘Fallstudien und statistische Untersuchungen zum
Südföhn im Raum Tirol’. PhD thesis, University of Innsbruck,
Austria.
Seibert P. 1990. South föhn studies since the ALPEX experiment.
Meteorol. Atmos. Phys. 43: 91–103.
Smolarkiewicz PK, Rotunno R. 1989. Low Froude number flow past
three-dimensional obstacles. Part I: Baroclinically generated lee
vortices. J. Atmos. Sci. 46: 1154–1164.
Sprenger M, Schär C. 2001. Rotational aspects of stratified gap flows
and shallow föhn. Q. J. R. Meteorol. Soc. 127: 161–188.
Streiff-Becker R. 1931. Zur Dynamik des Föhns. Meteorol. Z. 48:
149–152.
Vogt S, Jaubert G. 2004. Föhn in the Rhine Valley as seen by a WindProfiler-RASS System and comparison with the non-hydrostatic
model Meso-NH. Meteorol. Z. 13: 165–174.
Volkert H, Gutermann T. 2007. Inter-domain cooperation for mesoscale
atmospheric laboratories: The Mesoscale Alpine Programme as a rich
study case. Q.J.R. Meteorol. Soc. 133: 949–967.
Waibel K. 1984. ‘Statistik des Föhns im Bodenseeraum im Vergleich
mit dem Reusstal (Altdorf)’. Arbeitsbericht der Schweizerischen
Meteorologischen Zentralanstalt, No. 118.
Wild H. 1868. Über Föhn und Eiszeit: Der Schweizerföhn. Zeitschr.
Schweiz. Statistik .
Zängl G. 1999. Three-dimensional idealized simulations of the föhn in
the region of Innsbruck. Contrib. Atmos. Phys. 72: 243–266.
Zängl G. 2002a. Idealized numerical simulations of shallow föhn. Q. J.
Meteorol. Soc. 128: 431–450.
Zängl G. 2002b. An improved method for computing horizontal
diffusion in a sigma-coordinate model and its application to
simulations over mountainous topography. Mon. Weather Rev. 130:
1423–1432.
Zängl G. 2003. Deep and shallow föhn in the region of Innsbruck:
typical features and semi-idealized numerical simulations. Meteorol.
Atmos. Phys. 83: 237–261.
Zängl G. 2005. Dynamical aspects of wintertime cold-air pools in an
Alpine valley system. Mon. Weather Rev. 133: 2721–2740.
Zängl G. 2006. North föhn in the Austrian Inn Valley: A case study and
idealized numerical simulations. Meteorol. Atmos. Phys. 91: 85–105.
Zängl G, Chimani B, Häberli C. 2004a. Numerical simulations of the
föhn in the Rhine Valley on 24 October 1999 (MAP IOP 10). Mon.
Weather Rev. 132: 368–389.
Zängl G, Gohm A, Geier G. 2004b. South föhn in the Wipp
Valley – Innsbruck region: Numerical simulations of the 24 October
1999 case (MAP IOP 10). Meteorol. Atmos. Phys. 86: 213–243.
Q. J. R. Meteorol. Soc. 133: 897–916 (2007)
DOI: 10.1002/qj

Download Report

pdf-Dokument

Paperzz.com

Your Paperzz