The dynamics of orographic banner clouds

Abstract

While being observer on Mount Zugspitze during the years 1945-1948, Joachim Kuettner observed and described (amongst many other things) the phenomenon of banner clouds. He also took measurements, which unfortunately got lost afterwards. Meeting with Joach many decades later motivated the author to start a project called "The Dynamics of Orographic Banner Clouds". This project involved continuous observations with a web camera, insitu measurements on both sides of Mount Zugspitze, as well as a measurement campaign with observations from a variety of instruments. In addition, a new large eddy simulation (LES) model was designed, which is particularly well suited to deal with moist air flow over and around an isolated steep mountain. Key scientific questions are the origin of the windward-leeward asymmetry, the underlying basic mechanism, the role of diabatic processes, and the observed unstable stratification. The results from our project indicate that there is a strong windward-leeward asymmetry in the Lagrangian vertical displacement when air flows over and around a tall and steep mountain. Three-dimensional geometry seems to be a prerequisite for banner cloud formation. Latent heat release leads to the observed unstable stratification at the top of the banner cloud, but it is of secondary importance for the banner cloud as a whole.