Banner cloud dynamics at the Matterhorn: from idealized simulations to real-world observations
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Abstract
Banner clouds are a phenomenon that occur on the leeward side of steep mountains or sharp ridges on otherwise cloud free days. Investigating their formation mechanisms helps to improve our understanding of flow response in complex orography. Previous studies have identified key mechanisms and conducive atmospheric conditions primarily based on theoretical considerations and idealized simulations. However, it remains unclear to what extent these findings are valid in complex terrain under realistic atmospheric conditions. This thesis aims to bridge the gap between idealized model experiments and the real-world conditions by investigating the key mechanisms of banner cloud formation at the Matterhorn in the Swiss Alps. Therefore, Large-Eddy Simulations with a progressive transition from idealized to fully realistic Matterhorn orography are combined with observations from the dedicated MatterHEX field campaign. MatterHEX provides the first systematic observations of banner cloud conditions, including radiosonde upwind profiles, Doppler lidar scans of the lee-side flow, and webcam footage capturing cloud evolution. To ensure consistency in the direct comparison, the model inflow profiles were tuned to match the ambient flow to the radiosonde observations. The simulations show that increased terrain complexity alters flow symmetry, yet strong lee-side upwelling is still a predominant flow feature. The upwelling is no longer confined to the leeward side, but extends to the spanwise slopes, where most parcels ascend before entering the banner cloud. Weak stratification in the lee emerges as an important prerequisite for banner cloud formation at the Matterhorn, either reflecting ambient conditions or turbulence induced by terrain-flow interaction at high wind speeds or wind shear. The direct comparison of observed and simulated lidar scans during two contrasting banner cloud episodes confirms lee-side separation associated with recirculating flow and strong upwelling as the key mechanisms. Furthermore, a classification of all observed ambient flow conditions using a theoretical flow regime framework reveals that the necessary lee-side flow separation occurs more frequently than the actual banner cloud observation would suggest. These findings confirm that previous results from model-based investigations are applicable under realistic conditions, at least at a steep mountain like the Matterhorn. Moreover, lee-side separation seems to be a robust flow response that may occur under a wide range of ambient flow conditions, promoting banner cloud occurrence given suitable moisture conditions. This thesis highlights the value of combining high-resolution simulations with targeted observations to investigate turbulent flows in complex terrain.