Extended Abstract (68K)P3.1
What can we learn of surface mesonets in foehn valleys?
Reinhold Steinacker, Univ. of Vienna, Vienna, Austria; and M. Spatzierer, B. Chimani, C. Haeberli, M. Dorninger, and S. Tschannett
During MAP-SOP the Rhine Valley and some of its tributaries have been heavily instrumented by in situ as well as mobile and remote sensing platforms to study unstationary aspects of foehn in a large Alpine valley complex. Several research groups from different countries worked together to investigate some specific phenomena like flow splitting, the role of cold air pools and the spatio-temporal evolution of foehn periods. In the present paper two questions are addressed namely if high resolution surface networks are able to create realistic fields of the mass and wind field in a complex valley system and if the physical process of foehn penetration to the valley floor is different in the main valley and a small side valley.
To answer the first question a variational analysis technique has been applied to create low level fields of pressure, temperature and the wind field. Using the raw data as available from the data base leads to a very noisy field. Sophisticated quality control and bias correction procedures have to be applied to end up with fields which can be interpreted dynamically. It can be shown that pressure gradients are not uniformly distributed along valley segements and even cross valley gradients are encountered, which are needed to change the flow direction in curved valley segments. The cold air pool is not behaving like a shallow front line but may form some isolated pools which remain stagnant in some portions of the valley.
The second question addressed is also investigated with the aid of very high resolution surface fields. It becomes evident that in the main Rhine Valley, like in other prominent Foehn valleys (Wipptal, Reusstal) the flow is strongly influenced by cool air coming from the South (windward) side of the Alps via passes in the main crest which produces a bora-like katabatic flow component. Hence, the penetration towards lower valley sections can be explained by gravity flow phenomena together with the forcing of the synoptic scale pressure gradient. In small side valleys like the Brandner Valley, however, in the lee of high secondary mountain ridges, without the possibility of cold air penetration from the South, the foehn flow seems to be rather determined by mountain waves which prohibit the penetration much beyond the leeside trough.
Poster Session 3, Topographic Flows (with Coffee Break)
Thursday, 20 June 2002, 2:45 PM-4:30 PM
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