Abstract:
The foehn effect is well known as the warming, drying, and cloud clearance experienced on the lee side of mountain ranges during “flow over” conditions. Foehn flows were first described more than a century ago when two mechanisms for this warming effect were postulated. In this talk I will present the direct quantitative contribution of these and other foehn warming mechanisms. Our results suggest a new paradigm is required after it is demonstrated that a third mechanism, mechanical mixing of the foehn flow by turbulence, is significant. In fact, depending on the flow dynamics, any of the three warming mechanisms can dominate. A novel Lagrangian heat budget model, back trajectories, high-resolution numerical model output, and aircraft observations are all employed. The study focuses on a unique natural laboratory namely, the Antarctic Peninsula and Larsen C Ice Shelf. In this area foehn flows are prone to jets, which are relatively cool and moist, and wakes, which are relatively warm and dry, i.e. they have an enhanced foehn effect. These foehn jets are a type of gap wind, so funnelled through gaps (or inlets) in the mountain range. Foehn events can be divided into two types, depending on the non-dimensional mountain height. During a non-linear foehn flow, mountain wave breaking and strong down-slope acceleration leads to strong leeside warming, but a hydraulic jump prevents it being widespread. In contrast during a linear foehn event, there is no hydraulic jump and the foehn warming is widespread. Recent research by colleagues into wintertime foehn events and on the surface energy budget of foehn events will also be discussed.