Abstract
Strong nocturnal downvalley winds can often be observed in the Isar Valley near Mittenwald (Bavarian Alps) under the influence of high pressure systems over Central Europe or in the presence of ambient southerly winds. The properties of this downvalley wind are similar to the Alpine foehn, bringing about a pronounced increase in temperature and a concomitant decrease in relative humidity. We will therefore refer to this wind as mini-foehn. Measurements with automatic surface stations as well as numerical simulations with the MM5 model show that the mini-foehn is a part of the strong nocturnal drainage flow in the adjacent Leutasch Valley in Tyrol. This flow is driven by the temperature difference between the nearby plateau of Seefeld and the free atmosphere above Mittenwald, lying some 250 m below Seefeld. The foehn effect arises from the fact that the cold low-level air leaves the Leutasch Valley through a narrow gorge while potentially warmer air from higher levels reaches Mittenwald via a small mountain ridge. A second jet-like downvalley wind is found in the Isar Valley between Scharnitz and Mittenwald. The temperature of this air mass is usually lower than that of the Leutasch Valley jet, leading to pronounced temperature contrasts around Mittenwald.
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