Dynamik von Bannerwolken : Theoriebildung und Simulationen mit realistischer Orografie
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Abstract
Banner Clouds are a rather unknown but still interesting meteorological phenomena. This cloud type occurs exclusively on the leeward side of steep mountains or ridges on otherwise cloud free days. They can be frequently observed at Mt. Matterhorn in the Swiss Alps or at
Mt. Zugspitze in the Bavarian Alps.
The present work investigates under which flow conditions as well as mountain shapes Banner Clouds are most likely to occur. Therefore, we carried out Large-Eddy simulations of dry air past idealized orography in the first part of this work. In the second part, we focussed on a more realistic case by the use of the orography of Mt. Zugspitze. To gain more informations about the formation of Banner Clouds at this specific mountain, we carried out numerical Large-Eddy simulations with the orography of Zugspitzplatt. Additionally, we compared the
simulation results with the observations at Mt. Zugspitze.
In the first part of the work, we showed that Banner Clouds favour steep mountains in combination with a weakly stratified flow. Banner Clouds can be observed in the lee of the mountain, only if the flow is able to go rather around than over it. We also found that a strong surface friction is essential for Banner Cloud formation. The reason for this is, only the frictional generated lee vortex can produce an uplift of air on the leeward side of the
mountain that is characteristic for Banner Clouds. Concerning asymmetric mountains, we showed that Banner Clouds occur on obstacles with a steep slope on the leeward side while the windward side is relatively flat.
In the second part of the work we found, that the distribution of the humidity as well as the origin of the air parcels have a large impact on Banner Cloud formation at Mt. Zugspitze. Furthermore, we showed that there is a preference for Banner Clouds to occur for south easterly flows as long as the lifting of air on the leeward side is quite short. Additionally, we found that the height of the neutral boundary layer has to be around the height of the ridge
top, to produce a large vertical uplift on the leeward side.