Dynamics of orographic banner clouds

Banner clouds occur downwind of steep mountains or sharp ridges, even on otherwise cloudfree days. Systematic observations at Mount Zugspitze have shown that they can be a rather frequent phenomenon and occasionally persist for several hours.

This work investigates the dynamical conditions which are prerequisite to banner cloud formation. Large Eddy Simulations (LES) are carried through using the well-established EULAG-model. The model set-up consists of idealized mountains with the atmospheric flow characterized by different wind speed and stratification. Key diagnostic is the Lagrangian vertical displacement both on the windward and the leeward side of the mountain. Large upward displacement increases the likelihood for cloud formation.

For a steep isolated (pyramid-shaped) mountain one obtains a pronounced windward-leeward asymmetry with larger upward displacement on the leeward side. The large leeward upward displacement is associated with boundary layer separation and a rather complex lee-vortex geometry. It follows that banner clouds can be entirely due to orografic dynamics and that moisture asymmetries are not essential. The asymmetry in vertical displacement is lost and even reversed when the mountain becomes more ridge-shaped with the flow becoming increasingly two-dimensional. In that case ``flow around the mountain'' is replaced by ``flow across the mountain'' and gravity waves start to play a more dominant role. The latter may give rise to so-called cap clouds, which are different from banner clouds.