Abstract : G.1
Indications that trapped gravity waves have a potential for generating extreme foehn windstorms

Günther Zängl, Matthias Hornsteiner
Meteorological Institute, University of Munich

We present high-resolution numerical simulations of an Alpine south foehn case (14-16 November 2002) during which extremely strong surface winds occurred in several regions of the Alps. The specific focus is on a storm event in the upper Isar Valley in the Bavarian Alps, where gusts estimated to about 45 m/s caused substantial damage to buildings and vegetation. For this event, the simulations indicate that a large-amplitude trapped gravity wave was responsible for the development of the extreme low-level wind speeds, contrasting previous findings that violent surface winds are usually related to vertically propagating gravity waves, especially in the presence of low-level wave breaking, or to shooting hydraulic flow. According to the model results, gravity waves were excited over the mountain ranges adjacent to the valley and propagated towards the valley axis due to their three-dimensional dispersion characteristics. Wave trapping was caused by a deep neutral layer in the upper troposphere. The low-level wind maximum formed beneath the wave trough, reflecting the dynamical structure of trapped waves for which wind and temperature perturbations are in phase. The temporal evolution suggests that the formation of the large-amplitude trapped wave started once the atmospheric conditions were suitable for wave resonance. Having reached its maximum amplitude, the trapped wave started to propagate downstream due to a gradual increase of the ambient southerly flow. The wave amplitude then decayed because the forcing from the adjacent surface topography got out of phase. A comparison of the model results with surface wind measurements shows that the simulated flow evolution is consistent with reality except for a minor timing error, indicating that trapped gravity waves are another possible mechanism for the generation of extreme surface winds.