, Volume 93, Issue 1-2, pp 79-95
Date: 19 Apr 2006

Small-scale dynamics of the south foehn in the lower Wipp Valley

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Summary

In this paper, very-high-resolution numerical simulations are presented to analyze the small-scale dynamics of the foehn in the lower Wipp Valley and the adjacent parts of the Inn Valley. This region was one of the target areas for foehn observations during the Mesoscale Alpine Programme (MAP). Our simulations consider two MAP cases that markedly differed in the depth of the foehn flow. To isolate the dynamical effect of the key orographic features in the Wipp Valley region, we performed sensitivity experiments with different topography modifications. These involve lowering or even removing the Nordkette range, which constitutes the northern side wall of the east–west-oriented Inn Valley, and closing the Stubai Valley, which is the northernmost and largest tributary of the Wipp Valley. A comparison with surface and lidar observations indicates that our present model resolution of 467 m significantly improves the realism of the simulations compared to a resolution of 800 m, as used in a previous study.

The Nordkette is found to have a twofold impact on the dynamics of foehn breakthrough into the Inn Valley. In reality, this mountain chain deflects part of the southerly foehn current coming from the Wipp Valley into the perpendicularly oriented Inn Valley. Our sensitivity tests indicate that this flow deflection tends to accelerate the foehn breakthrough into the Inn Valley, while upstream blocking effects induced by the Nordkette act to slow down the process of foehn breakthrough. The flow pattern in the Wipp Valley reveals that the upstream effects of the Nordkette are not quite far-reaching. The amplitude of the gravity waves over the lower Wipp Valley gets somewhat reduced by these upstream effects, but the overall flow pattern remains largely unaffected. Closing the Stubai Valley also has a minor effect of the wave structure and tends to reduce the cross-valley variability of the foehn flow in the lower Wipp Valley.