A-TREX

Strong and gusty windstorms downwind from mountain ranges occur all over the world. Some are so frequent that the local population has given them names like foehn (in the Alps), bura (Dinaric Alps), Chinook (Rocky Mountains). The Sierra Nevada in the western USA with its regular shape and a nearly 3 km drop of terrain from its peaks into the Owens Valley poses as ideal a research lab as nature provides it. Air flowing over the Sierra Nevada resembles water streaming across a weir: It accelerates on the way down, becomes thinner, and forms violent swirls at the bottom. These swirls, called "rotors" pose a severe hazard for aviation. Especially over dry regions there are no visible clues (clouds) for pilots to warn them of the danger. Especially at risk are take-off and landing procedures and smaller aircraft. A Boeing 747 lost an engine in Alaska in such a situation. And an aerobatic soaring plane spectacularly disintegrated over Owens Valley, CA, after encountering a rotor with estimated accelerations of 14 times earth`s gravity. An international measurement project will study the underlying atmospheric conditions and mechanisms for rotors with the hope of improving forecast capabilities of such situations. The Austrian contribution to the Terrain-induced Rotor EXperiment (A-TREX) supports both the measurements and the analyses of theses windstorms with their associated features like rotors. A-TREX will use an instrumented (rental) car, a weather station on wheels (WOW), to measure the footprints of the windstorms. It will also co- finance a Doppler laser instrument from the German research institution DLR, which can measure air speed along the laser beam similar to the police laser guns that measure vehicle speeds. For the first time in studies of mountain winds, several Doppler lidars will jointly measure the air flow thus allowing to compute the complete wind field (not just the speed in the direction where the laser points). Our project will help develop the algorithms for these computations. We will also use all available data provided by the many instruments from the international research community to study some of the windstorms in detail. We will use these data to fine-tune computer models that can simulate atmospheric flow. These simulations will provide then the flow details even in regions where no measurements could be made. Furthermore, the allow to isolate the important mechanisms for the windstorms and rotors by playing "what if ..." (e.g. there are no clouds; the oncoming wind speed upstream of the Sierra Nevada is only half as strong; ...).

Strong and gusty windstorms downwind from mountain ranges occur all over the world. Some are so frequent that the local population has given them names like foehn (in the Alps), bura (Dinaric Alps), Chinook (Rocky Mountains). The Sierra Nevada in the western USA with its regular shape and a nearly 3 km drop of terrain from its peaks into the Owens Valley poses as ideal a research lab as nature provides it. Air flowing over the Sierra Nevada resembles water streaming across a weir: It accelerates on the way down, becomes thinner, and forms violent swirls at the bottom. These swirls, called "rotors" pose a severe hazard for aviation. Especially over dry regions there are no visible clues (clouds) for pilots to warn them of the danger. Especially at risk are take-off and landing procedures and smaller aircraft. A Boeing 747 lost an engine in Alaska in such a situation. And an aerobatic soaring plane spectacularly disintegrated over Owens Valley, CA, after encountering a rotor with estimated accelerations of 14 times earth`s gravity. An international measurement project will study the underlying atmospheric conditions and mechanisms for rotors with the hope of improving forecast capabilities of such situations. The Austrian contribution to the Terrain- induced Rotor EXperiment (A-TREX) supports both the measurements and the analyses of theses windstorms with their associated features like rotors. A-TREX will use an instrumented (rental) car, a weather station on wheels (WOW), to measure the footprints of the windstorms. It will also co-finance a Doppler laser instrument from the German research institution DLR, which can measure air speed along the laser beam similar to the police laser guns that measure vehicle speeds. For the first time in studies of mountain winds, several Doppler lidars will jointly measure the air flow thus allowing to compute the complete wind field (not just the speed in the direction where the laser points). Our project will help develop the algorithms for these computations. We will also use all available data provided by the many instruments from the international research community to study some of the windstorms in detail. We will use these data to fine-tune computer models that can simulate atmospheric flow. These simulations will provide then the flow details even in regions where no measurements could be made. Furthermore, the allow to isolate the important mechanisms for the windstorms and rotors by playing "what if ..." (e.g. there are no clouds; the oncoming wind speed upstream of the Sierra Nevada is only half as strong; ...).