Banner Cloud Formation at the Matterhorn: Measurements versus Large-Eddy Simulations

Banner clouds are clouds in the lee of steep mountains or sharp ridges on otherwise cloud-free days. Previous studies identified key mechanisms and preferred flow conditions on the basis of idealized large-eddy simulations, but it remained unclear to what extent these findings apply to real-world conditions. This state of affairs motivated a campaign at the Matterhorn in the Swiss Alps in fall 2023, during which observations were made using a scanning Doppler lidar and radiosondes. This paper analyzes the measurements from two episodes of banner cloud occurrence during the campaign and puts them into context using large-eddy simulations. The observations confirm the previously hypothesized key mechanism of banner cloud formation, namely, recirculating flow and strong upwelling on the leeward side of the mountain. The model simulations are configured such as to replicate the ambient flow conditions as observed by the radiosondes, allowing a comparison between the observed lidar scans and synthetic lidar scans produced from the model data. The comparison reveals a considerable level of agreement. Subsequently, the model simulations are used to contextualize the lidar measurements within the complex ambient flow conditions. The analysis confirms that the Lagrangian vertical displacement is a suitable dry diagnostic for banner cloud occurrence and that distinct leeside vortex structures are not a necessary prerequisite for banner cloud formation. It is concluded that banner clouds at the Matterhorn are a robust phenomenon occurring under a wide range of ambient flow conditions.