Mountain Meteorology

Dynamics of orographic banner clouds

Banner clouds are clouds that appear to be attached to the leeward face of a steep mountain. They can be observed e.g. at Matterhorn in the Swiss Alps or at Mount Zugspitze in the Bavarian Alps. An introduction into this phenomenon can be found in Wirth (2016).
Banner cloud at Matterhorn in the Swiss Alps. The wind is blowing from the right, but the cloud is on the left side of the mountain. Photo courtesy (and copyright) Isabelle Prestel.
We mounted a webcam at the weather station on the summit of Mount Zugspitze and took time lapse movies during four years. These movies allowed us to provide a detailed description as well as a definition of banner clouds (Schween et al. 2007). In this online publication we also included several of our time lapse movies in the form of mpeg-movies.

In October 2005 we carried out a measurement campaign allowing us to collect a wealth of additional data. The key results from our observational studies are summarized in Wirth et al. (2012). Amongst others we obtained results about the diurnal and the seasonal cycle of banner cloud occurrence as well as their dependence on wind speed and wind direction.

Vertical wind (left panel, color, in m/s) and vertical displacement of air parcels (right panel, color, in m) in a vertical section through the center of an idealized mountain. The overall wind direction is from left to right. The blue lines on the left panel indicate streamlines. One can clearly see the lee vortex in the lee of the mountain (left panel) and the associated plume of large values of uplift (right panel). Given suitable moisture conditions, this implies that a cloud should occur preferentially on the leeward side of the mountain.
Trajectories of air parcels as they pass the mountain from left to right. The grey shaded encloses a volume where the curvature of the trajectories exceeds a certain threshold. It transpires that the flow in the lee is characterized by a bow-shaped vortex.
In parallel we configured a large eddy simulation model in order to perform numerical simulations of flow past steep orography. The earliest successful simulation of a banner cloud is reported in Reinert and Wirth (2009). Later we investigated the mechanisms of banner cloud formation (Voigt and Wirth 2013) and obtained information about parcel trajectories as they pass the mountain (Schappert and Wirth 2015). The study of Prestel and Wirth (2016) showed the banner cloud occur preferentially for weakly stratified flow past a steep mountain.
Regime diagram of the vertical displacement in 12 different simulations for 12 different combination of the stability parameter Nh/U and the mountain steepness h/L (N = Brunt Vaissala frequency, h = mountain height, U = wind speed, L = mountain width). Mountain steepness increases from bottom to top and stratification increases from left to right. Apparently, only the top left panel shows a plume of large positive values in the lee of the mountain. This suggests that banner cloud require weak stratification and a steep mountain for their existence.