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| Titel |
What flow conditions are conducive to banner cloud formation? |
| VerfasserIn |
Volkmar Wirth  , Isabelle Prestel |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250136682
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| Publikation (Nr.) |
 EGU/EGU2016-17778.pdf |
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| Zusammenfassung |
| Banner clouds are clouds that are attached to the leeward slope of a steep mountain. Their
formation is essentially due to strong Lagrangian uplift of air in the lee of the mountain.
However, little is known about the flow regime in which banner clouds can be expected to
occur. The present study addresses this question through numerical simulations of flow past
an idealized mountain. Systematic sets of simulations are carried out exploring the parameter
space spanned by two dimensionless numbers, which represent the aspect ratio of the
mountain and the stratification of the flow. The simulations include both two-dimensional
flow past a two-dimensional mountain and three-dimensional flow past a three-dimensional
mountain.
Regarding boundary layer separation, both the two- and the three-dimensional
simulations show the characteristic regime behavior which has previously been found in
laboratory experiments for two-dimensional flow. Boundary layer separation is observed in
two of the three regimes, namely in the “leeside separation regime”, which occurs preferably
for steep mountains in weakly stratified flow, and in the “post-wave separation regime”,
which requires increased stratification. The physical mechanism for the former is boundary
layer friction, while the latter may also occur for inviscid flow. However, boundary layer
separation is only a necessary, not sufficient condition for banner cloud formation.
Diagnosing the vertical uplift and its leeward-windward asymmetry it turns out that
banner clouds cannot form in the two-dimensional simulations. In addition, even
in the three-dimensional simulations they can only be expected in a small part of
the parameter space corresponding to steep mountains in weakly stratified flow. |
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