Flow Past a Steep Mountain: Testing Hypotheses by Combining Numerical Simulations with Observations
Final Report Abstract
Flow past a steep mountain can lead to winds that deviate greatly from the ambient flow. This has far-reaching consequences, e.g., for local aviation (mountain rescue) or the transport of snow (avalanches). In this project, various hypotheses regarding the flow past a steep mountain were tested, combining high-resolution numerical simulations with the findings from a dedicated measurement campaign. The focus was on the Matterhorn in the Swiss Alps, which is not only characterized by its extreme steepness, but also has near-optimal conditions for a measurement campaign. The simulations initially showed that the flow structures in the vicinity of the Matterhorn are significantly more complex than suggested by previous work, which was based on idealized pyramid-shaped mountains. At the same time, however, certain phenomena are largely independent of the exact shape of the mountain. This includes the fact that under certain conditions the strongest updrafts do not occur upwind of the mountain, but rather downwind or on its lateral flanks. Accordingly, a so-called banner cloud can occasionally be seen, which appears to be attached to the leeward side of the mountain. An essential part of the project was the MatterHEX measurement campaign, carried through in autumn 2023. With the help of a modern lidar measuring device, the smallscale flow structures in the immediate vicinity of the Matterhorn could be measured for the first time; at the same time, the ambient flow was determined with the help of radiosondes launched from a neighboring mountain hut. For the purpose of campaign data analysis, synthetic lidar measurements were simulated using the numerical model and compared with the actual measured data. This comparison showed extensive agreement with regard to the main flow structures. In this way, previous model-based work on the subject was corroborated by measurement data for the first time. At the same time, the model simulations proved to be useful for placing the measurement data into a three-dimensional context.
Publications
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Sensitivity of Banner Cloud Formation to Orography and the Ambient Atmosphere: Transition from Idealized to More Realistic Scenarios. Journal of the Atmospheric Sciences, 80(11), 2653-2668.
Thomas, Marius Levin & Wirth, Volkmar
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Banner Cloud Formation at the Matterhorn: Measurements versus Large-Eddy Simulations. Journal of the Atmospheric Sciences, 82(8), 1661-1675.
Thomas, Marius Levin; Hoch, Sebastian W.; Huwald, Hendrik; Lehning, Michael; van Schaik, Brandon J. A.; Rentel, Dominique S.; Imbert, Paul & Wirth, Volkmar
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The MatterHEX Experiment—Investigating Atmospheric Flow Patterns in Highly Complex Terrain Related to Banner Cloud Formation. Bulletin of the American Meteorological Society, 106(8), E1687-E1702.
Hoch, Sebastian W.; Thomas, Marius L.; Huwald, Hendrik; Lehning, Michael; van Schaik, Brandon J. A.; Imbert, Paul; Rentel, Dominique S. & Wirth, Volkmar
