Shear instabiiities and their characteristics during Foehn

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Abstract
During southern Foehn, there occurs within the planetary boundary layer a well-defined surface with a wind shear of typically 15 m/s and a temperature jump of about 15°C within a vertical distance of 100 m. These conditions are nearly ideal for studying shear waves and stability in the atmospheric boundary layer. The stratification and the dynamics of the boundary layer were investigated using radiosonde, pilot and tethered balloon ascents, automatic hill-top weather stations, and an array of microbarovariographs. The experimentally determined gravity wave characteristics are discussed and it is shown that they fit well into the theoretical k-ω diagnostic diagram. It is shown that under these extreme shear conditions the simple three-layer model (Gossard, 1975) gives results which compare quite well with the measurements. Shear instabilities (braids) are identified and it is demonstrated that descending parcels of air are present within these instabilities. Detailed measurements of temperature and wind in the boundary layer allow the Richardson numbers to be derived; the criterion Ri<0.25 for the occurrence of shear waves is fulfilled.

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