Extreme foehn in Switzerland: a climatology and the relation to large scale flow

Abstract

Foehn is a well-known phenomenon in the Alpine region: a warm, dry, gusty and often severe lee-wind, descending from the crest into the valleys. South foehn is one possible type of foehn, forced by a large-scale flow with a southerly component. The process itself acts on a meso-bèta scale, while the local character can vary over short distances. Foehn can have a big impact on society. High wind velocities and strong wind gusts can cause damage to buildings and nature. A good understanding of foehn is necessary for reliable predictions to give out warnings in time, especially for extreme foehn. Since the 19th century, many theories have been published about the dynamics of foehn. Although the recent insights of a masked Bora provide improved dynamical understanding, the detailed forecasting remains problematic. A better understanding of, in particular, extreme foehn is necessary. To achieve this, a climatology of extreme foehn has been made, using 28 years of surface data from nine stations in Switzerland. The relation to the large scale forcing is studied by analyzing sounding profiles and ERA-Interim data. Foehn characteristics do not correlate and can all be used to identify extreme foehn. Based on extreme values, foehn stations can roughly be divided in three categories. A daily cycle is only present for stations close to the Alpine crest. Seasonal cycles are observed for all foehn characteristics and all station. An interannual periodicity is not found. Extreme foehn events are associated with so called ‘jet streaks’, regions of intense winds near the tropopause. Their location can vary. As long as the exit area is located nearby the Alps, upper air wind can come from the sector southwest to west, marking a difference between long and short extreme foehn events, respectively. The Po valley inversion layer is not found to be related to foehn.