 |
| Titel |
Dynamics of orographic banner clouds |
| VerfasserIn |
M. Voigt, V. Wirth |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250061024
|
|
|
|
|
|
| Zusammenfassung |
| 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. Key scientific
questions are the origin of the windward-leeward asymmetry, the underlying basic
mechanism, the role of diabatic processes, the role of stratification, and the impact of the
mountain shape. Despite many decades of active research in mountain meteorology, the
processes and spatial scales relevant for banner cloud dynamics have not recieved particular
attention in the past.
This contribution adresses some of the fundamental issues based on numerical
simulations. These were carried through with a Large Eddy Simulation model using idealized
orography. 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 orographic
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 over the mountain" and gravity waves start to play a more
dominant role. The latter may give rise to so-called cap clouds, which are conceptually
different from banner clouds. Nevertheless, with the help of trajectories it can be
shown that there is a quasi-continuous transition between a banner cloud and a cap
cloud. |
|
|
|
|
|
|