The Role of Wind Speed and Wind Shear for Banner Cloud Formation

Author:

Wirth Volkmar1,Bubel Pascal1,Eichhorn Joachim1,Schömer Elmar1,Kremer Tobias1,Erbes Rainer1,Schappert Sebastian2,Prestel Isabelle3

Affiliation:

1. Johannes Gutenberg University, Mainz, Germany

2. Deutscher Wetterdienst, Offenbach, Germany

3. Carl von Ossietzky Universität, Oldenburg, Germany

Abstract

Abstract Banner clouds are clouds that appear to be attached to the leeward face of a steep mountain. This paper investigates the role of wind speed and wind shear for the formation of banner clouds. Large-eddy simulations are performed to simulate the flow of dry air past an idealized pyramid-shaped mountain. The potential for cloud formation is diagnosed through the Lagrangian vertical parcel displacement, which in the case of a banner cloud shows a plume of large values in the lee of the mountain. In addition, vortical structures are visualized through streamlines and their curvature. A series of sensitivity experiments indicates that both the flow and the banner cloud occurrence are largely independent of the ambient wind speed U. On the other hand, the shear of the ambient wind has a profound impact on the location of the stagnation point on the windward face as well as on the flow geometry in the lee of the mountain. The relevant measure for shear is H/Hs, where H denotes the height of the mountain and Hs = U/Uz is the scale height of the shear (with Uz denoting the scale of the shear). The simulations are also used to compute the line-of-sight velocity component seen by a hypothetical Doppler wind lidar positioned in the lee of the mountain; the analysis suggests that such sensitivities can potentially be detected using modern wind lidar technology.

Funder

German Research Foundation

Publisher

American Meteorological Society

Subject

Atmospheric Science

Reference17 articles.

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