A Comparison of foehn flow in a major valley and its tributaries, a contribution to MAP

One objective of MAP was and still is the investigation of the temporal development and the three dimensional structure of foehn flow in a large valley. Besides the issue of predictability and model validation the observational data should answer the question how the mechanism of penetration of the foehn flow down the valley and the removal of cold air pools can be understood and explained. In complex large Alpine valleys, like the Alpine Rhine valley, with segments of different valley axis orientation, valley splittings, narrow and wide portions, the role of tributary valleys has not been investigated in detail so far. The Department of Meteorology and Geophysics of the University of Vienna as part of a consortium of international research groups concentrated some of their measurements to a comparatively short and narrow, south-north oriented side valley, the Brandner Valley in Vorarlberg, some 15 km long and well known for its severe foehn wind storms. The special situation of this valley is given by a mountain chain (Raetikon) at its origin, which is oriented approximately perpendicular to the the main direction of the Rhine valley and hence acts as a secondary barrier to the foehn flow.

Fortunately the weather during MAP-SOP (Early September till mid November 1999) brought us a considerable number of foehn events, more than could be expected from climatology. Different types of foehn, summertype foehn, involving unstable airmasses, shallow and „sandwich“ type, influencing only some layers of the valley atmosphere and deep, dry as well as rather moist foehn events with a variety of intensities occurred. An extremely dense surface automatic station network and upper air stations, conventional radiosoundings and remote sensing instruments together with aircraft measurements allow to analyse the temporal evolution and the spatial structure in a way, never achieved before. The paper will focus on the comparison of the evolution of the wind and mass field in the main Rhine valley and the Brandner valley.