Foehn study in the Rhine valley during MAP in Austria

In the Mesoscale Alpine Programme (MAP) the study of Foehn winds is one of the primary objectives. The Rhine valley, from its origin at the Passes in the Alpine divide to the Lake of Constance, including its tributary valleys has been chosen by the international MAP community as a target area. This region will be heavily instrumented in a joint international effort and aircraft will sample the atmosphere during meteorologically interesting situations. The research project proposed here, will be one of the Austrian contributions to these international efforts. It is embedded in such a way as to support and complement other groups with additional data, on the other hand it has its own specific scientific objectives which will be studied based on data gathered within this project plus all the data available from other research groups. This project centres on two major objectives namely (1) the small scale variability and life cycle of the Foehn flow close to the ground in a cascade of valleys of different scale and (2) the interaction of Foehn flows in valleys of different scale in the lower and mid troposphere. A secondary objective is the determination of the data quality by means of sophisticated methods before its release to the international MAP data base. The "Alpine Rhine valley region" has been chosen as target area because it shows a favourable statistics for the occurrence of Foehn during the MAP Special Observing Period (MAP-SOP, mid August till mid November) and it is of interest for Switzerland and Germany since these countries are directly affected by Foehn in this area. Only little is known about the spatial and temporal structure of the Foehn flow in the area where observations are planned within the proposed research project, namely the Walgau from Bludenz to Feldkirch and the Brandner Tal, south of Bludenz. Any improvement in the prediction of Foehn related weather is of great practical importance, primarily because Foehn wind storms are a major threat to that area, they cause substantial damage in property every few years and present a severe hazard to aeronautical activities. During three months of the MAP SOP (15 th August to 15th November 1999) a range of stationary observing systems will be deployed and operated by several institutions from Switzerland, Germany, France and Austria. In situations likely to produce Foehn, the intensity of observations will be increased and mobile ground based and airborne observing platforms will come to operation such as to obtain a data set on the three dimensional fields of wind, temperature and humidity as complete as possible. The data will be analyzed to identify the mechanisms which are ultimately responsible for the evolution and the penetration of the Foehn flows to the surface and its retreat. The results should enable operators of high resolution prediction models to test and improve the capabilities of the models in order to increase the accuracy in time and space of Foehn wind storms in this region and to other Foehn areas of the world.

The aim of the research project "FORMAT" (FOehn study in the Rhine valley during MAP in AusTria), as a part of the so far largest international Alpine Weather research programme MAP (Mesoscale Alpine Programme), was the generation of a spatial and temporal highly resolved meteorological data set in selected foehn areas. Such an information, which did not exist in comparable detail so far is necessary to document the onset, the fluctuations and the cessation of a foehn flow and to validate high resolution numerical simulation models. A careful validation of such models is necessary to qualify their ability for an operational foehn forecast in the future. The measurements took place on the one hand in the Alpine Rhine valley between Bad Ragaz and Feldkirch and in the Vorarlberg Brandnertal on the other hand. Besides a series of automatic surface weather stations also a SODAR-Instrument (SOund Detection And Ranging) were continuously operating between September and November 1999, a period with several foehn episods. During special field campaigns with foehn additional measurements with a radiosonde, with pilot balloons and with mobile equipment (car traverses) were taken. During several hours measurements with a meteorologically instrumented motor glider werde conducted in addition. With the exception of the SODAR, which gave only useful data for part of the time due to an unfavorable location (close to a highway, trouble with neighbors), without a possibility of relocation due to the necessary power supply, all other instruments gave very satisfying results. It was an important prerequisite for the high quality to maintain and contol the instruments several times during the field campaign. After the storage of data at the own Institute and at the international MAP data center in Zurich and the removal of the instruments at the end of 1999 a unique raw data existed which had to be carefully quality controlled and evaluated. This very important task was a tedious one. The necessity of the data to be of high quality made it conditional to compare the numerous own data with the data collected by other research groups in the frame of the whole programme. It could be shown that several inconsistencies with regard to meta information, time information (universal time, Central European time, daylight saving time) or concerning biasses of several station data existed. The evaluation of the corrected data led to very interesting results: It could be e. g. shown that the temperature and pressure fields in the Rhine valley during foehn is very structured. In contrast to the suspicion the boundary of foehn is not front-like but some shallow cold air pools remain isolated for a considerable time. An other sensational finding was the explanation for the rather strange behavior of the foehn in the Brandenertal. Whereas foehn flow is commonly present in the Innerdorf, foehn is rarely observed in the Ausserdorf, just a few km off and at about the same elevation. A nearly stationary pronounced pressure minimum over the Innerdorf is present due to a leewave. The evaluation of the data, especially with regard to the model evaluation is still in progress in the course of students diploma theses. Hence, further interesting insights into the mesoscale structure of the foehn flow may be expected which will help to further increase the quality of local foehn forecasts.