Can trapped gravity waves be relevant for severe foehn windstorms? A case study

Abstract

This study presents 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−1 caused substantial damage to buildings and vegetation. For this event, the simulations indicate that the extreme low-level wind speeds were related to a large-amplitude trapped gravity wave, 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. 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. A sensitivity test in which the neutral layer was removed exhibits significantly weaker surface winds in the region of interest, suggesting that wave trapping made an important contribution to this windstorm event.