Extreme south foehn in Switzerland: its relation to forced large-scale vertical motion.

Abstract

South foehn in the Alps is a warm, dry and gusty wind from the south that is potentially strong enough to cause serious damage in the northern valleys. The start of a foehn event is characterized by a sudden increase in temperature and velocity and a sudden decrease in relative humidity. The foehn can last a few hours, but also more than 24 hours. At the end of the foehn event the variables again make a jump to a more ”normal” value. For a long time it has been thought that latent heat release and subsequent precipitation to the south of the Alps is responsible for the warm and dry wind in the north, but nowadays scientists think that latent heat release is just one of the contributing factors to the formation of a foehn wind. Other factors include the origin of the air, a secondary circulation due to the presence of a jetstreak and the subsequent latent heat release in the region with updraught. In 2012, a long dataset of meteorological observations at Altdorf, Switzerland, was explored to identify more than 1000 foehn events in the period June 1981 - May 2011. The 10% of the foehn events with the highest mean wind speed were defined as extreme foehn events. This research focuses on the relation between forced large-scale vertical motion and the occurrence of extreme foehn in Altdorf. First a case study is done to get an idea of what happens during an extreme foehn case. After that, a few meteorological variables are studied during extreme foehn. Then the quasi-geostrophic omega equation is solved to see whether the secondary circulation of the jetstreak can be found. And finally, a 2-layer shallow water model with flow over a mountain range is developed and solved. In this model, an upward air flow to the north of the Alps is included to simulate the latent heat release at these places.