13.5
Gap Winds Forced by Flows with Simple Vertical Structure
Sasa Gabersek, University of Washington, Seattle, WA; and D. R. Durran
This paper investigates the dynamics of the flow through a gap in a ridge of finite length. Except in those simulations with a surface boundary layer, the flow upstream of the ridge has a uniform Brunt-Vaisala frequency and a uniform wind speed. The resulting gap winds are examined as a function of (1) the angle of incidence of the flow upon the ridge, (2) the nondimensional mountain height, (3) the Rossby number, (4) the surface roughness, and (5) various topographic shape parameters.
The mechanisms of gap flow acceleration are analyzed and compared with those processes leading to strong downslope winds using control-volume budgets and trajectory analyses. The Bernoulli function is evaluated along trajectories to relate the lee-side wind speed to the dissipation history of each air parcel. Surface friction is shown to have a first-order effect on the gap flow when the large scale pressure gradient has a significant component parallel to the gap.
Mass budgets for a control volume within the gap show considerable variation in the source of the air flowing through the gap as a function of nondimensional mountain height. Flow through a vertical face at the upstream end of the gap provides the dominant mass flux into this control volume for small values of the nondimensional mountain height (H). As H increases, the relative contributions from flow through the lateral faces and the top of the control volume increase dramatically.
Mass budgets for a low-level flow just upstream of the ridge show the expected increase in deflection around the ridge as H increases, but increasing H is not found to increase the proportion of the incoming low-level flow that passes through the gap.
Session 13, Gap Winds and Foehn I
Thursday, 20 June 2002, 8:00 AM-10:30 AM
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