13.6
Physically based foehn wind detection
Johannes M. Vergeiner, Univ. of Innsbruck, Innsbruck, Tyrol, Austria; and S. D. Mobbs and G. J. Mayr
An important issue in the analysis of the GAP- field data in the Brenner target area during the Mesoscale Alpine Programme (MAP) was the identification of foehn periods during the Special Observing Period (SOP). The classical foehn detection at a station is based on the assumption of a warm and dry wind. So the foehn detection would try to see a sharp increase in temperature and decrease in humidity at the beginning of a foehn period and vice versa and the end together the wind being aligned along the valley axis. However, a drawback of this method is, that a warming (and drying) due to foehn is not essential, but dependent on the air mass properties of the oncoming flow. In the Alps the restrictions of the above mentioned scheme are most obvious in summer (where the solar heating is strong enough to potentially make the air near the ground warmer without than with foehn) and for north foehn (where cold air masses approach the Alps from the north).
In this talk, a new scheme will be presented. It is based on the comparison of two stations and can be applied to every foehn wind around the world. Take a reference station at the top of the mountain/saddle, which is representative for the flow at crest level. Since the air descends dry adiabatically after passing the crest, potential temperature is a good tracer for the origin of the air mass. Foehn at the downstream station is observed, whenever its potential temperature is equal or higher than the one at the reference station. The wind criterion is kept unchanged, so additionally the wind has to be aligned along the valley axis.
Results of the two methods are compared and discussed for the main advantages and disadvantages. Detailed examples for different stations during the MAP-SOP are given and the dependance on distance to the crest and height of the station is discussed.
Additionally, the height origin of the foehn air can be calculated, if a temperature profile at the crest is available. Examples for selected stations are given and interpreted in combination with the pressure and wind term of the Bernoulli equation.
Session 13, Gap Winds and Foehn I
Thursday, 20 June 2002, 8:00 AM-10:30 AM
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