Shallow foehn studies

The two arguably most famous locations for windstorms downstream of major mountain ranges are the Colorado Front Range (Boulder) and the Alps (e.g. Altdorf, Innsbruck). An extensive body of literature and also of understanding of its dynamics has accumulated for the Boulder-type windstorm. Parts of the dynamics of the Alpine foehn are less clear. During ALPEX it was (re)discovered that shallow foehn, a channeling of air through gaps in the Alps, was superimposed on deep foehn, and sometimes even existed in its pure form without any significant cross-mountain component above the crest. Little more than a phenomenological description from several case studies is known about shallow foehn. A comprehensive, three-pronged approach will thus be taken in order to better describe shallow foehn and to understand its dynamics. A 10-year climatology of shallow foehn in the Wipptal and Innsbruck will provide information about its frequency, surface and upper air characteristics and the difference to deep foehn. An understanding of the dynamics of shallow foehn will be gained through analytical studies and idealized numerical simulations. Since pure shallow foehn is decoupled from the flow aloft, application of hydraulic theory is possible to extract the salient features of shallow foehn dynamics. Idealized numerical simulations will then be used to investigate what the conditions leading to pure shallow foehn are, whether shallow foehn is a mixture between downslope windstorm and gap flow, and what role critical levels, wave-breaking, nonhydrostatic effects, friction and the Coriolis force play. Measurements from recently installed automatic weather stations and additional data sets from the international Mesoscale Alpine Programme (MAP) will be used to test the findings from hydraulic theory and numerical simulations. This proposal is an integral part of MAP, one of whose scientific objectives is the investigation of gap flow and shallow foehn along the Brenner cross section.

Shallow foehn is a strong, gusty wind that forms downwind of gaps and passes in mountain ridges like the Alps. It can affect air quality and aircraft safety and occurs as frequently as one third of the time in certain Alpine valleys. Foehn winds crossing mountain ridges have been studied for more than a century. Little attention has been paid to foehn where only air below a mountain crest crosses from one side to the other through gaps in the mountain. As with the "deep" foehn, where air crosses also above the ridge, the foehn air descends on the lee side and is warmed by compression. This study found that foehn (both the deep and shallow variety) occurs an astonishing one third of the time in the Wipp Valley north of the Brenner Pass in all seasons with the exception of summer. The occurrence and frequency of shallow foehn may vary drastically over a distance of several kilometers. For example, Innsbruck, 200 m below the northern end of the Wipp Valley gets foehn in less than 10% of the time and very rarely not only in summer but also in winter. One of the driving forces behind of the shallow foehn was found to be the temperature difference of the air below the Alpine crest on the northern and southern side. Shallow south foehn in the Wipp Valley occurs, when the air south of the Alps is colder than air to the north. This constellation baffles at first look but conforms to a frequent sequence of weather events where a cold front sweeps southwards across the Alps to leave cold air in the Po Valley while the next warm front brings relatively warmer air first to the north of the Alps. Computer simulations of the shallow foehn flow helped to identify the essential ingredients of shallow foehn. They confirmed that the exact shape, height and location of the topography crucially determine where the shallow foehn winds are strongest.