FORM: Foehn in the Rhine Valley during MAP

The Rhine valley between Chur and the Lake of Constance was chosen by a joint CIG-SSC committee as the target area for the second scientific objective of MAP (Binder and Schär, 1997):

• To study Foehn and Foehn-related phenomena, including their three-dimensional structure and associated boundary layer processes.
• To improve the understanding of three-dimensional gravity wave breaking and associated wave drag in order to improve the parameterization of gravity wave drag effects in NWP and climate models.

The Rhine Valley was mainly chosen for the following reasons:

• The distinct three dimensional characteristics.
• It is well equipped with existing routine observation networks.
• It is immediately north of the Lago Maggiore target area. Therefore the observational sites near the main Alpine crest can be used for both target areas.
• Excellent opportunity for international cooperation because the Rhine valley is shared by three countries.

A small group of individuals who are particularly interested in this field and motivated to participate with research projects during the SOP met for a first meeting in Bregenz at the end of November 1997. This meeting was primarily devoted to the planning of observational instruments. Nevertheless, the participants took some time to discuss objectives of such a project:

• When and where does the Foehn hit the surface?
• Improve the forecasting of Foehn and Foehn-related phenomena e.g. turbulence and stratus in the Rhine valley.
• Interaction of Foehn and the boundary layer.
• Where is the source of the increased ozone concentration in the Rhine Valley during Foehn events?

Since it is obvious that Foehn does not stop on the shore of Lake Constance, the participants of the Bregenz meeting propose to bound the target area at 8.7 E in the West (Oberalppass), 10.3 E in the East (Arlbergpass), at 48 N and at 46.3 N (see map p. 11).

Quite a number of institutions already have agreed to participate in FORM and to provide observational equipment. Figure 1 gives an overview of the tentative location of the various devices. This arrangement has to be discussed and refined.

• All locations of the devices in the target area Rhine Valley have to be coordinated with the target area Lago Maggiore. This is particularly the case for the upper air stations (radiosounding and/or windprofiler) near the main Alpine crest.
• The location of the radiosounding stations will be verified with backwards trajectories for several Foehn cases.

The discussion of the observational strategy can be summarized as follows:

• Stations producing data of poor quality have to be known before the GOP starts. DAQUAMAP will help to achieve this goal.
• It would be favorable if all routine stations could be checked (calibration, parallel measurements) before and after the GOP. It should be assured that at least the yearly maintenance is properly scheduled.
• Some of the classic stations should be equipped with additional automatic instruments using a higher sampling rate.
• Seasonal stations should be operated during the whole GOP or at least during the SOP.

map: courtesy of Robert Benoit, Envirement Canada

Binder, Peter and Christoph Schär, 1997: MAP Design Proposal.