The South-Foehn of March 3 - Portraits by MC2 and
observations
Thomas Exner and Georg J. Mayr - Department of Meteorology and Geophysics - University of Innsbruck - 6020 Innsbruck Austria - email: [email protected]
Introduction:
The results of a comparison of a hindcast by the MC2 model in preparation for its operational use during MAP SOP and
observational data are presented. The nonhydrostatic MC2 model was run with a grid mesh of 3km one-way nested within the
Swiss model. The MC2-topography of the Brenner target area has all important features (Fig. 1) such as the Brenner Pass,
Wipp valley, Inn valley, and Karwendel mountains north of Innsbruck. Nevertheless the differences between model and real
altitude are significant. The model topography is approximately 400-500 m too high for Innsbruck, Brenner pass and Sterzing
(10 km south of the pass). This means though, that the elevation difference between Brenner and Innsbruck is the same in
MC2 and reality.
March 3 1999 was a deep foehn case. A pronounced trough, which extended far south to Spain, approached the Alps from the
west and caused a turning of the flow from westerly to southerly in all levels of the troposphere.
The observational data set consists of the sparse network of GTS stations augmented by the extensive network of regional
services on both sides of the Brenner Pass, and routine rawin soundings in Milano (six-hourly) and Munich (00 and 12 UTC
only). The paper focuses on vertical cross sections and surface values along the valley axis between Verona and Munich.
Results:
Potential temperature cross section
Lower-level isentropes in both MC2 simulations and observations slant upwards from Verona to Brenner by more than 500
m. There are not enough observations above 2 km ASL to see smaller-scale wave patterns like the ones in the model.
Suspicious is the forward-tilt with height of the wave crest upstream of the Sarnthein mountains in the model since it would
imply unphysical energy propagation towards the obstacle. It is there though, that the cross section changes from a SW-NE to
a N-S orientation. Observations show a drop of the isentropic surfaces before they reach the pass, which is an indication of a
hydraulic behavior of the flow. In contrast, the drop in the model before the pass is barely noticeable. But then the descent of
isentropes downstream of the pass in the model is weaker, too. The 290 K isentrope descends approximately 1500 m from
2500 m to the surface halfway between Brenner and Innsbruck in the observations, but only by half as much (from 2000 m to
1200 m) in the model. Similar discrepancies hold for the 292.5 K and 295 K isentropes. This means that the nonlinearity of
the flow is weaker in the model. Accordingly, the distribution of potential temperature along the valley axis of the Wipptal
Valley at ground level (Fig. 3) is nearly flat in the model. None of the typically observed increases in potential temperature
immediately downstream of the Brenner and near the region of the wind speed maximum halfway to Innsbruck are included.
Therefore the model-Innsbruck is potentially only 1 K warmer than Brenner compared to a 5 K difference in reality. Even a 3
km grid distance seems too coarse to elucidate the salient flow features in the Wipp valley. Simulations with 800 m distance
(Zängl, 1999, this issue) fare much better but are beyond the current computer capacity for real-time forecasts.
The standing wave over the Karwendel Mountains north of Innsbruck is similar in model and observations. The model shows
a hydraulic-jump like feature further to the north in Southern Baravia, which could not be verified in the observations due to
the lack of observational data.
Pressure
The model pressure difference between Verona and Munich is 10hPa (observed: 8hPa). The strongest pressure gradients
appear in both cases slightly south of the main alpine crest. The weaker descent of the flow into the Wipp valley in the model
is also reflected in the surface pressure (Fig. 4). The observed difference between Innsbruck and the Brenner Pass is 2.5 hPa
instead of 4hPa, and the observed smaller-scale features are absent.
Evolution of temperature and relative humidity in the vicinity of Innsbruck
Since the model forecast started at 06 UTC there had not been time to produce the fine-scale thermodynamic structure in the
Inn valley, which poses another modeling challenge. An only few hundred meter thick cold layer forms through nocturnal
cooling in the Inn valley. While there is still foehn in the at least 200 m higher (at its exit) Wipp valley, foehn in the Inn
valley is usually lifted off during the night. Foehn began in Ellbögen (15km south of Innsbruck) at 7:30 UTC, in Innsbruck
accordingly later at 10 UTC. Foehn in the model started at both locations nearly at the same time, 10 UTC (model output was
available hourly). Since air in the model does not descend from as high as in reality (Innsbruck is 500 m too high), the foehn
warming and temperatures during foehn are about 5 degrees lower. But the typical foehn signature of temperature increase
and relative humidity decrease is there. There is, however, no change of specific humidity in the model over the whole
period.
Unfortunately, no comparison of observed and simulated end of the foehn period could be made, since the simulations end at
18 UTC.
Wind speed
Magnitudes of observed and modeled wind speeds are the same, e.g. maximum of 15 m s-1 approximately 10 km south of
Innsbruck and up to 25 m s-1 at the surrounding mountain peaks.
Fig.1: MC2 model topography (isospacing:
300m); the thick black line shows the location of
the vertical Θ cross section;
Fig 2a: Vertical cross section of potential temperature
from observations within a 0.8° swath from Verona to
Munich. Maximum, average and minimum (valley
axis) elevation within the swath are also shown.
Fig. 2b: As a) but for MC2 data for the cross section along
the thick line in Fig.1
5.0
2 92
4.5
O b se rva tio n s
MC2
observation
4.0
pressure difference [hPa]
theta [K]
2 91
2 90
2 89
2 88
MC2
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
2 87
-0.5
5
10
15
Innsbruck
20
25
0
30
Fig 3: Potential temperature along the valley axis of the
Wipp valley at surface level
5
10
Innsbruck
B renner P ass
d is ta n c e [km ]
15
25
30
35
Brenner Pass
Fig 4: Pressure distribution along the Wipp valley
relative to the appropriate value for Innsbruck
O B S E R V A T IO N S
MC2
100
13
12
10 0
12
90
11
10
80
9
60
4
50
r e l. h u m id ity
tempertature [°C]
70
6
rel. H. [%]
8
2
90
10
te m p e r a tu r e
temperature [°C]
20
distance [km]
re l. h u m id ity
80
8
7
70
6
5
60
rel. H. [%]
0
4
3
50
2
40
0
1
te m p e ra tu re
40
0
800
1000
1200
1400
1600
1800
8
UTC
10
12
14
16
18
U TC
Fig. 5a
Fig. 5b
Evolution of temperature and relative humidity in Innsbruck: a) observations, b) MC2.
Acknowledgements: Thanks to R. Benoit and S. Chamberland (MC2), C. Zingerle (Hydrogr. Services Bozen), R. Mair
(LWD Tirol), , P.Parson (ZAMG), Inst. for Meteorology Vienna (Vera) for providing their data.