Titre/Title : On two classes of downslope flows : Bora and katabatic wind
Contact : Christophe Brun (équipe MEIGE)
Résumé/Abstract :
A type of severe downslope windstorm, called Bora, will be addressed in the first, larger part of the talk, and a thin katabatic flow shall be tackled next ; the former is driven dynamically, while the latter is thermally driven. Bora is a gusty wind that blows at the eastern Adriatic coast from the NE quadrant with sustained speeds between 5-20 ms-1, its gusts surpassing 50-70 ms-1 (corresponding to downslope windstorms with hurricane speeds) in the lee of the mountains. Similar Bora-like flows occur elsewhere in mountainous areas where flow transcriticality (regarding “vertical” Froude number, or its inverse, dimensionless mountain height) is the dominant flow property. There are a few types of Bora depending on the background airflow (synoptic setup), orographic details and occasionally the upstream vs. downstream surface temperature differences. Severe Bora is always related to the mountain wave breaking. On the finer mesoscale, the Bora jet/wake pattern relates to the mountain pass/top distribution. Current efforts to study Bora gradually moves toward progressively smaller spatio-temporal scales.
The second part of the talk addresses the ability of a mesoscale model, with a very fine vertical resolution and a decent turbulence parameterization scheme, to simulate a thin katabatic flow that is only 10 m deep. A comparison with an observed case, in addition to large-eddy simulations using three different subgrid-scale models performed by other authors, is provided. The short answer to the question whether a mesoscale model can simulate thin katabatic flows adequately is - yes, to a certain degree, but not in all details of the flow.