4.5 FOEHN FLOW IN THE AUSTRIAN ALPS INTERRUPTED BY A

4.5
FOEHN FLOW IN THE AUSTRIAN ALPS INTERRUPTED BY A COLD FRONT PASSAGE: PART II
Alexander Gohm ∗, Georg J. Mayr , Stephen Mobbs , Samantha Arnold ,
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Johannes Vergeiner , Lisa S. Darby , Robert M. Banta , and Scott Sandberg
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University of Innsbruck, Austria
University of Leeds, United Kingdom
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National Oceanic and Atmospheric Administration/Environmental Technology Laboratory, Boulder, CO
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1.
INTRODUCTION
During the field phase of the Mesoscale Alpine
Programme (MAP) in fall 1999, an intense network of
automatic weather stations and atmospheric sounding
systems was operated across the Brenner pass in the
Wipptal and Inn Valley (cf. Fig. 2) to investigate
southerly foehn winds on the lee side of the European
Alps which are observed as pronounced gap flows over
the Brenner Pass. Many international institutions
contributed to this unique dense instrumentation,
including the Universities of Innsbruck, Leeds, Munich,
Washington, and Agricultural Sciences Vienna (BOKU),
the Pacific Northwest National Laboratory (PNNL), the
Environmental Technology Laboratory (ETL) and the
National Severe Storm Laboratory (NSSL) – both from
the National Oceanic and Atmospheric Administration
NOAA –, the Austrian Weather service (ZAMG), and the
Aeronautical Meteorological Service (Austro-Control).
The 4 through 6 November 1999 event is chosen for a
case study of the break down of this southerly flow
caused by the passage of a cold front.
2.
SYNOPTIC OVERVIEW
With the approach of a mid-tropospheric pressure
trough towards the Alps from the 4 to 5 November 1999,
winds over the Austrian Alps turned to SW. The airflow
was forced to rise over the Alpine barrier and
descended as foehn wind into some valleys. At the
southeastern edge of the trough cyclogenesis took
place in the Gulf of Genova on 6 November at about 6
UTC. The cold front associated with the trough passed
western Austria about six hours later (Fig. 1) and
terminated the foehn event. The pressure gradient
across the Alps reversed its sign with the passage of the
cold front (see Fig 1); the upper-level winds turned to
NW.
3. MESOSCALE ANALYSIS
3.1 South foehn on 4 - 6 November
With the buildup of a northward directed pressure
gradient across the main Alpine ridge during 4
November, the southerly flow in the Wipptal started to
∗
Corresponding author address: Alexander Gohm,
Department of Meteorology and Geophysics, University
of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria;
e-mail: [email protected]
Figure 1: Hand-analysis of surface pressure with cold
front on 6 November 1999 12 UTC. Contour interval is 1
hPa. Alpine topography exceeding 1000 m MSL is
shaded. Indicated are Innsbruck (IBK) and Milan (MIL).
increase. The onset of south foehn in the Wipptal was
recorded at the weather station Ellboegen (EL, 1120 m
MSL), located 22 km north of the Brenner Pass, at
about 1000 UTC. The highest gusts observed there on
-1
this day were 16 m s . Winds at higher levels had
turned from N to S earlier: at Sattelberg (SA, 2150 m
MSL) located near the Brenner Pass at 0200 UTC and
Patscherkofel (PK, 2250m MSL) at the exit of the
Wipptal as early as 3 November 2100 UTC. However,
winds at these stations did not exceed the strength of
the flow measured at the valley bottom near EL.
After a weakening of the southerly flow during the
following night, foehn strengthened again on 5
-1
November with a maximal gust of 20 m s at EL. PK
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recorded gusts up to 26 m s at that day.
South foehn continued in the Wipptal through about
noon of the next day (Nov 6) with about the same
intensity.
Horizontal analysis
Figure 2 shows a contour plot of potential
temperature (θ) at the surface in the Inn Valley and
Wipptal on 6 November 1130 UTC derived from data of
the MAP Mesonet as well as operational weather
stations of the Austrian weather service. The potentially
warmest air was found in the middle and lower
(northern) part of the Wipptal due to the descent of the
foehn flow from higher elevations into the valley.
A comparison with potential temperatures from an
upstream rawinsounding at Sterzing (ST) on 0900 UTC
indicates that this air had its origin from between 2.4
and 2.7 km MSL. The air at the upper Wipptal was
cooler since it flew over the Brenner Pass (BR), which is
1.4 km high. This increase of θ from BR northward
towards EL is a typical feature of foehn in the Wipptal
(e.g. Seibert, 1985). The distribution of θ in the Inn
Valley, as well as the wind field, shows that cooler air
was advected from both (!) sides, W and E, towards
Innsbruck (IBK). The front advanced towards Innsbruck
through a gap in the mountain range north of Innsbruck
(cf. NOAA P-3 aircraft measurements in Darby et. al,
2000, this volume) and up the lower Inn valley as a
backdoor front. The strong gradient of θ and a wind shift
of 180° near the exit of the Wipptal in Fig. 2 shows the
sharp boundary where the southerly foehn flow collided
with the northward progressing cold air. The movement
of this frontal zone towards BR was impressively
reported by the NOAA/ETL Doppler lidar (Darby et al.,
2000).
3.2 Cold front passage on 6 November
Figure 2: Hand-analysis of potential temperature on 6
November 1999 1130 UTC. Contour interval is 1 K.
Topography is shaded with contour lines at 800, 1100,
and 1500 m. Wind barbs are drawn in knots. Indicated
are weather stations Seefeld (SE), Innsbruck (IBK),
Schwaz (SW), Patscherkofel (PK), Ellboegen (EL),
Sattelberg (SA), Brenner (BR), Sterzing (ST), as well as
potentially warm (W) and cold (C) regions.
Horizontal analysis
The frontal zone at the exit of the Wipptal in Fig. 2
at 1130 UTC was only the leading edge of a relatively
shallow cold density current approximately 1 km deep
(see Darby et al., 2000) moving northward. Figure 3
shows that the main cold front was located behind,
approaching IBK from the upper (western) Inn Valley
and passing it around 12 UTC. IBK reported 8.4 mm of
precipitation within the following 24 hours. While the
leading edge of the shallow cold air was progressing
upward the Wipptal and was located between Gedeir
(GE) and Tienzens (TI) at 12 UTC, the upper-level flow
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near the Wipptal exit was still southerly with 5 m s at
PK. In the Inn Valley, a potentially warm region
persisted on the northern slopes of PK.
Figure 4 shows the northward progress of the cold
front from IBK to ST over the Brenner Pass. The
passage of the cold front was indicated by a flow
reversal and a sudden drop of potential temperature
behind the front of approximately 7 K per hour. The
average speed of the front in the Wipptal derived from
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Fig. 4 was 6.7 m s , which agrees with the propagation
speed calculated from Doppler lidar data (cf. Darby et
al., 2000).
Vertical analysis
Figure 3: As in figure 2 but half an hour later, at 1200
UTC. Indicated are weather stations Gedeir (GE) and
Tienzens (TI).
The thermodynamic structure of the air mass in the
valley was well documented by measurements from 12
temperature loggers installed at the Wipptal exit along
the slope up to PK (Fig. 5). At 1100 UTC the mean
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vertical lapse rate of temperature was –8.8⋅10 K m ,
i.e. the valley atmosphere was nearly adiabatically
mixed caused by the descent of the foehn flow. After the
front passage the valley atmosphere was stabilized, with
Figure 4: Temporal evolution of potential temperature
and wind field along the Wipptal from IBK to ST during
the passage of the cold front on 6 November 1999
between 1000 UTC and 1600 UTC. Contour interval is 1
K. Data are taken from MAP Mesonet.
Figure 5: Temporal evolution of the vertical temperature
profile near the exit of the Wipptal on 6 November 1999
between 1100 UTC and 1300 UTC. Data are taken from
12 temperature loggers (deployed by PNNL) installed
along the western slope of PK between 708 m MSL and
2105 m MSL. Contour interval is 1 K.
Figure 6: Sounding at Gedeir on 6 November 1999 0900, 1200, and 1300 UTC. Left: Vertical profile of temperature
(solid) and dew point (dotted). Middle and right: wind barbs, as well as vertical profile of wind direction and speed (kn).
The sounding system belonging to NOAA/NSSL was operated by the Universities of Innsbruck and Washington.
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-1
a mean laps rate of –6.6⋅10 K m at 1200 UTC. At the
highest station (Pat12, 2105 m MSL) the temperature
dropped as the front arrived shortly after 12 UTC,
however, at the lowest station (Pat1, 708 m MSL)
almost half an hour earlier. This gives a mean slope of
the surface of the cold front of 12:100.
Three rawinsoundings from the ETL Doppler lidar
site at GE, located in the middle of the Wipptal, show in
detail the atmospheric structure before and after the
cold front passage (Fig. 6). At 0900 UTC the valley
atmosphere was well mixed within several layers below
4.5 km MSL. Two stable layers were found at 1.8 to 2.3
km MSL and at 2.8 to 3.2 km MSL, respectively. The air
in the lower troposphere was dry with relative humidity
between 60 % and 70 %. The southerly foehn flow was
strongest at about 2 km MSL with maximal wind speeds
-1
of 21 m s . Winds were turning to SW above the main
Alpine crest at about 3 km MSL, indicating that this
event was a shallow foehn. Three hours later, the
advection of cold air decreased the temperature
between 800 hPa and 700 hPa by almost 4 K. At the
surface, the cold front had already passed, indicated by
a flow reversal up to about 1.7 km MSL. However, on
the top of this northerly flow, foehn was still present,
with upper-level winds even more southerly due to the
approach of the synoptic pressure trough. One hour
later, at 1300 UTC, the cold air near the surface gained
vertical depth – up to 2.5 km MSL –, which was
impressively marked by a bow-shaped form of the
temperature curve in the thermodynamic diagram.
4. Summary
The break down of south foehn in the Wipptal due
to the passage of a cold front on 6 November 1999 has
been investigated with special emphasis on the analysis
of data from a dense network of automatic weather
stations, which were operated during the Special
Observing Period of the Mesoscale Alpine Programme.
The fine scale structure of the approaching cold front,
which lead to break down of foehn, could be
successfully investigated with the ETL Dopplar Lidar
(see Part I, Darby et al., 2000). This work is a
preliminary case study, which will be further expanded,
including additional surface and upper air data from
other sources.
Acknowledgements: The authors would like to thank
Dave Whiteman from PNNL, the Austrian weather
service (ZAMG), and the Aeronautical Meteorological
Service (Austro-Control) for providing their data. This
work was supported by the Austrian Science Fund
(FWF), grant 13489-TEC.
References
Darby, L. S., A. Gohm, L. B. Nance, S. Gabersek, R. M.
Banta, S. Sandberg, 2000: Foehn flow in the
Austrian Alps interrupted by a cold front passage:
Part I. This volume.
Seibert, P., 1985: Fallstudien und statistische
Untersuchungen zum Südföhn im Raum Tirol.
Dissertation, Univ. Innsbruck, Austria, 369 pp.