Impact of Orography on Banner Cloud Occurrence at Mount Zugspitze

Banner clouds are clouds which appear to be attached to the leeward slope of a steep mountain. They are frequently observed e.g. at the Matterhorn in the Swiss Alps or at Mount Zugspitze in the Bavarian Alps.

Previously it has been shown that the occurrence of banner clouds at Mount Zugspitze depends on the wind direction, with a distinct preference for south-easterly flow. It is hypothesized that this dependence is due to the asymmetry of the underlying orography, which at Mount Zugspitze has a steep slope facing north, while the slope is significantly more gentle towars the south-east.

The hypothesis is tested with the help of large eddy simulations past both idealized and realistic orography. The potential for banner cloud formation is diagnosed in terms of Lagrangian vertical uplift induced by the mountain, which is obtained either from a passive tracer or through trajectories. The simulations with an idealized asymmetric mountain indicate a higher potential for banner cloud formation on the steeper slope. The simulations with realistic orography are more complex and somewhat harder to interpret, but they are consistent with the preference for the steep slope, i.e. the northward facing slope in the case of Mount Zugspitze. Taken together, the results are taken as evidence that the observed depencence of banner cloud occurrence on wind direction at Mount Zugspitze is at least partly due to the asymmetry of the orography.