The origin and dynamics of banner clouds - an analysis based on large eddy simulations

Banner clouds belong to the class of orographic clouds. They are known to occur when sufficiently moist air flows across steep mountain peaks or quasi 2D ridges. The cloud is confined to the immediate lee of the mountain, whereas the windward side remains cloud-free. Preferred places of occurrence are e.g. the Matterhorn in the Swiss Alps or Mount Zugspitze in the Bavarian Alps. The cloud has a banner-like appearance with a characteristic (horizontal) scale of 1 km. It is attached to the mountain, while its downwind end flickers in the wind like a flag. The cloud is quasi-stationary and can often be observed for several hours. The underlying mechanism of formation, as well as the relative importance of dynamics versus thermodynamics for its formation and maintenance, are not yet fully understood.

As a tool to investigate this phenomenon we developed a new Large Eddy Simulation (LES) model which can handle moist atmospheric flow above highly complex terrain. The model is able to treat orography with slopes up to 90° by applying the method of viscous topography. Furthermore, turbulent inflow conditions can be generated by using a modified perturbation recycling method. The model has been tested in detail and was validated against wind tunnel data.

Here we focus on the origin and the dynamics of banner clouds for 3D versus 2D geometries. Both dry and moist flow across an idealized pyramidal shaped obstacle and a quasi 2D ridge is investigated. These obstacle shapes may be considered as idealized approximations to the Matterhorn- and Zugspitz-geometry.

For the 3D (pyramidal shaped) case the dry simulations reveal that the flow field is highly asymmetric regarding the Lagrangian vertical displacement: air parcels which are lifted in the upward branch of the induced lee vortex originate at lower levels than air parcels which are lifted on the windward side. As we will show, the upward displacement in the lee is sufficient to explain the occurrence of banner clouds. Due to the strong asymmetry of the flow field this is even true for horizontally homogenous initial conditions regarding both moisture and temperature. Simulations with moist physics switched on show that our model is able to simulate realistically shaped banner clouds downwind of a pyramidal shaped mountain peak. A comparison of model runs with the cloud model switched on and off indicates that thermodynamics (i.e. latent heat release) do not have a significant impact on the mean flow, even though they moderately increase the intensity of leeward turbulence. For pyramidal shaped obstacles we conclude that banner clouds are primarily a dynamical phenomenon, with their dynamics only slightly modulated by thermodynamical processes.

Differences for 3D versus 2D geometries, regarding e.g. the relative importance of thermodynamics and the origin of the condensing leeward moisture will be highlighted, too.