Understanding and forecasting Alpine Foehn - what do we know about it today? (Invited)

For many decades, foehn was the outstanding example to explain thermodynamic processes and the role of latent heat in the atmosphere. As beautiful as this example is, in reality foehn winds often do not follow this classical textbook theory attributed to Hann (19th century). On the other hand, it would also be wrong to say that nature never obeys the classical theory, as examples show.

The talk centers on recent results of the Mesoscale Alpine Program (MAP) and on ongoing research activities mainly in Switzerland, but touches also on activities in Austria and France. The overview encompasses statistical analyses of foehn occurrence in different Alpine regions, on the interaction of foehn flow and the cold pool, and on current techniques for forecasting foehn-related windstorms.

Statistical analyses show that foehn occurrence is highly variable, and that no trend in the last 140 years is discernible. In addition, the regional and seasonal variability is considerable.

The dynamics of foehn flow and its interaction with the leeside cold pool are still not completely understood. In the past, an impressive variety of explanations for the descending of the foehn in the lee of the mountain was offered, however, no satisfactory mechanism that could be applied in all cases has been brought forward. For the first time, MAP produced data that indicates the major processes in the cold pool removal. The leeside cold pool in some areas remains for up to days while foehn moves over its surface. The impeded disappearance of cold pools is either caused by the topography that blocks the stagnant cold air or - particularly in wintertime - is due to lack of radiation which maintains high stability in the boundary layer.

For forecasting, both statistically based "foehn-tests" as well as models are used operationally. "Foehn tests" rely basically on pressure differences between the two sides of the mountain massive; some include also wind and/or temperature data. Naturally, there are hopes that the new generation of mesoscale models will improve the predictability of foehn. To what extent these expectations were met, e.g., by increasing the resolution from 7 to 2 km in the COSMO model will be shown.

There are various societal impacts of foehn winds. Still much debated are biometeorological effects supposedly caused by quasi-periodic pressure fluctuations initiated by gravity waves on the cold pool. Foehn winds were responsible for many devastating fires: In 1861, 600 houses of the city of Glarus were completely destroyed during a foehn storm, and recently, in 2001, a fire maintained by foehn winds in excess of 15 m/s destroyed 15 houses in Balzers (Principality of Liechtenstein). Foehn storms recurrently cause severe damage to boats and piers particularly on the Lake of Constance and on the Lake of Lucerne. Finally, severe accidents with aerial cable cars and, in rare cases, also railway cars occurred during foehnstorms.