Working Group Foehn-Research Valley/ (AGF) ICAM 2007 K. Burri, B. Dürr, Th. Gutermann, A. Neururer, R. Werner; E. Zala (MeteoSwiss)

Handout to the Poster Foehn Verification with the COSMO Model

The working group Foehn-Research Rhine Valley/Lake Constance (AGF) was formed by the meteorological services of Germany, and to study the Foehn in the Rhine Valley and to improve the Foehn forecast (also gale warning) for the Rhine Valley and Lake Constance. AGF is gathering meteorological data in the Rhine Valley since 1980s. During this time several case studies had been made and 1999 a paper was published (MeteoSwiss) about a special Foehn case in April 1993.

The Foehn Case of the 8th December 2006 The number of operational automatic surface stations in the upper Rhine Valley and Lake Constance region has considerably increased since MAP (1999), and 61 sites (46 of them are shown on the map below) are now available for validation purposes. In this case study forecasts and analyses of the spatial and temporal Foehn pattern from the COSMO-2 NWP model at MeteoSwiss with 2.2km spatial resolution are compared to different surface site based Foehn analyses on 8th of December 2006. Temperature and wind speed forecasts and analyses from COSMO were compared to real- time temperature analyses from INCA (Integrated Nowcasting through Comprehensive Analysis) and to surface based temperature and wind speed analyses compiled by AGF. As not all data sources were available for the same time, a 3h interval (11–13 UTC) was chosen, where the Foehn pattern and magnitude was quasi-stationary. Additionally time series of temperature and wind speed were compared at two dedicated sites to investigate the temporal Foehn evolution. Finally the origin of the Foehn air masses was examined by using backward trajectories.

The Gauging Stations Name Abbr. TT FF Longitude Latitude Height Date °C kn °East °North meter ASL yyyymmddhh Alberschwende ALS 17.4 3 9.85 47.46 721 2006120813 Altenrhein ARH 19.1 24 9.57 47.48 398 2006120813 Bludenz BLU 19.9 13 9.82 47.15 558 2006120813 BLZ 17.6 20 9.52 47.08 471 2006120813 Brand BRD 13.9 17 9.74 47.10 1046 2006120813 Bregenz BRE 17.9 11 9.75 47.50 424 2006120813 Berneck BRK 18.2 9.62 47.43 404 2006120813 Chur CHU 16.6 10 9.53 46.87 555 2006120813 Dornbirn DOB 18.7 9 9.72 47.43 410 2006120813 Ebenalp EAL 7.0 53 9.40 47.28 1640 2006120813 Feldkirch FEK 17.5 8 9.60 47.27 440 2006120813 Flaesch FLA 17.1 9.52 47.03 518 2006120813 Friedrichshafen FRD 16 9.44 47.68 400 2006120813

page 1 | 5 Working Group Foehn-Research Rhine Valley/Lake Constance (AGF) ICAM 2007 K. Burri, B. Dürr, Th. Gutermann, A. Neururer, R. Werner; E. Zala (MeteoSwiss)

Handout to the Poster Foehn Verification with the COSMO Model

Name Abbr. TT FF Longitude Latitude Height Date °C kn °East °North meter ASL yyyymmddhh Fruemsen FRU 16.9 9.45 47.23 456 2006120813 Fraxern FRX 16.1 12 9.68 47.32 809 2006120813 Gais GAI 11.7 18 9.48 47.38 1151 2006120813 Goetzis GOE 20.2 24 9.65 47.35 412 2006120813 Grabs GRB 17.1 9.45 47.19 469 2006120813 Guettingen GUT 11.0 7 9.28 47.60 440 2006120813 Haeggenschwil HAG 17.2 9.33 47.47 561 2006120813 Heerbrugg HBG 19.0 22 9.63 47.42 408 2006120813 Herisau HER 16.9 9.28 47.38 783 2006120813 Horn HRN 18.6 14 9.45 47.50 397 2006120813 Karren KAR 15.4 27 9.75 47.39 960 2006120813 Kressbronn KBR 18.0 21 9.62 47.60 460 2006120813 Lindau LID 27 9.68 47.55 400 2006120813 Lindau LIN 16.7 24 9.67 47.57 400 2006120813 Lustenau LUN 19.0 22 9.65 47.41 402 2006120813 Malbun MAL 8.1 16 9.62 47.10 1596 2006120813 Malans MAL 15.9 9.58 46.98 590 2006120813 PLK 15.4 12 9.55 47.18 795 2006120813 Bad Ragaz RAG 17.4 9.50 47.00 493 2006120813 Rohrspitz ROP 18.8 34 9.63 47.50 398 2006120813 Ruggell RUG 17.9 11 9.53 47.23 433 2006120813 Saentis SAE 0.2 20 9.34 47.25 2490 2006120813 Schwaegalp SAL 11.1 17 9.32 47.25 1350 2006120813 Sargans SAR 16.8 9.41 47.06 495 2006120813 Steg STE 10.8 22 9.57 47.12 1295 2006120813 St.Gallen STG 15.7 20 9.40 47.43 779 2006120813 Sulzberg SUG 15.2 18 9.90 47.53 1015 2006120813 Trogen TRO 14.6 7 9.47 47.42 890 2006120813 VAD 18.8 25 9.52 47.13 460 2006120813 Wald WAL 13.2 31 9.52 47.42 1061 2006120813 Wasserauen WAU 14.1 19 9.43 47.28 868 2006120813 Walenstadt WST 18.6 9.31 47.10 484 2006120813 Zizers ZIZ 16.2 3 9.58 46.94 564 2006120813

46 out of 61 gauging stations evaluated by AGF are shown in the map below. Temperatures are given in degrees Celsius, wind speed in knots, station height in meters and date in time UTC. Missing values are left blank. The sites are spread all over the investigation area and are, therefore, helpful for verification purposes. Although there are no upper air data available, the higher stations, e.g. Säntis (SAE) or Karren (KAR), give us an idea of this layer. Because there are sites from 400m above sea level up to 2500m, we can get a

page 2 | 5 Working Group Foehn-Research Rhine Valley/Lake Constance (AGF) ICAM 2007 K. Burri, B. Dürr, Th. Gutermann, A. Neururer, R. Werner; E. Zala (MeteoSwiss)

Handout to the Poster Foehn Verification with the COSMO Model

representative cross section through the Foehn air mass and use this information to verify the models (INCA, COSMO). The temperature is dependent upon the sea level and changes within the Foehn air mass almost with the 'dry-adiabatic lapse rate' (DALR). Therefore, the higher gauging station, e.g. Säntis (SAE, 2490m above sea level [ASL]: 0.2°C) must be around 11.4°C colder than Schwägalp (SAL, 1350m ASL: +11.1°C), which is nearly matched for this case. The exception is Güttingen (GUT, 440m ASL: 11°C) which was not touched by the Foehn, whereas Altenrhein (ALT, 398m ASL: 19°C), at the about the same sea level, is 8°C warmer. The wind force is dominated by the meso- and micro-scale orography. If the Foehn passes through a cut in a ridge (Sulzberg, SUG) and/or hits directly a summit (Säntis, SAE), then it is accelerated. On the other hand, the Foehn is deviated by a steep, southerly inclined slope (at least 40° inclination and higher than 500m) and, therefore, the wind at the ridge is slowed down. In an overview, the map shows clearly the spread of the Foehn air mass along the Rhine Valley and further on over the Lake Constance (mainly orange colour = 16–20°C) at the gauging stations. Güttingen (green = 8–12°C), which is situated further west, is obviously not covered by the Foehn air mass.

Results The Foehn flow within most of the typical Foehn valleys is now explicitly resolved by COSMO. The COSMO forecast and analysis fields correspond very well both for temperature and wind speed except for the north-west of the investigation area, where the forecast indicates the borderline of the Foehn. Both fields severely overestimate the spatial Foehn extension and widely underestimate temperature and wind speed compared to the AGF analysis. Comparisons for the Alpine crest site Gütsch (not shown here) revealed that firstly the modelled air temperature was 1.5°–2° too low, and secondly the modelled temperature gradient between the Rhine Valley and the Alpine crest never reached dry-adiabatic lapse rate conditions in contrast to measured gradients. Overall, modelled Foehn temperatures are underestimated by 4°–6°, which is also shown by the time series for Altenrhein (ARH) and Vaduz (VAD). The maximum wind speed modelled at ARH and VAD is approximately 20%– 30% lower than measured. The onset of the Foehn is 1h–2h too early at VAD, and even 3h– 4h at ARH. An important factor is the surface temperature of the Lake of Constance, which is probably too high, and eases the breakthrough of the Foehn over the Lake of Constance. The modelled Foehn end, however, is 1h–2h too late at ARH, and even 2h–3h at VAD. Very few surface sites in the upper Rhine Valley region are incorporated in the current INCA-CH version, which leads to an obvious underestimation and poor spatial representation of the 2m temperature field.

4th of June 2007, AGF

page 3 | 5 Working Group Foehn-Research Rhine Valley/Lake Constance (AGF) ICAM 2007 K. Burri, B. Dürr, Th. Gutermann, A. Neururer, R. Werner; E. Zala (MeteoSwiss)

Handout to the Poster Foehn Verification with the COSMO Model

page 4 | 5 Working Group Foehn-Research Rhine Valley/Lake Constance (AGF) ICAM 2007 K. Burri, B. Dürr, Th. Gutermann, A. Neururer, R. Werner; E. Zala (MeteoSwiss)

Handout to the Poster Foehn Verification with the COSMO Model

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