Mountain Waves in Two-Layer Sheared Flows : Critical-Level Effects, Wave Reflection, and Drag Enhancement

Internal gravity waves generated in two-layer stratified shear flows over mountains are investigated here using linear theory and numerical simulations. The impact on the gravity wave drag of wind profiles with constant unidirectional or directional shear up to a certain height and zero shear above,...

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Bibliographic Details
Published in:Journal of the atmospheric sciences 2008-06, Vol.65 (6), p.1912-1926
Main Authors: TEIXEIRA, Miguel A. C, MIRANDA, Pedro M. A, ARGAIN, José L
Format: Article
Language:English
Subjects:
Atmosphere
Earth, ocean, space
Exact sciences and technology
External geophysics
Fourier transforms
Gravity
Gravity waves
Height
Internal waves
Meteorology
Mountains
Physics of the high neutral atmosphere
Theory
Velocity
Wind
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Summary:Internal gravity waves generated in two-layer stratified shear flows over mountains are investigated here using linear theory and numerical simulations. The impact on the gravity wave drag of wind profiles with constant unidirectional or directional shear up to a certain height and zero shear above, with and without critical levels, is evaluated. This kind of wind profile, which is more realistic than the constant shear extending indefinitely assumed in many analytical studies, leads to important modifications in the drag behavior due to wave reflection at the shear discontinuity and wave filtering by critical levels. In inviscid, nonrotating, and hydrostatic conditions, linear theory predicts that the drag behaves asymmetrically for backward and forward shear flows. These differences primarily depend on the fraction of wavenumbers that pass through their critical level before they are reflected by the shear discontinuity. If this fraction is large, the drag variation is not too different from that predicted for an unbounded shear layer, while if it is small the differences are marked, with the drag being enhanced by a considerable factor at low Richardson numbers (Ri). The drag may be further enhanced by nonlinear processes, but its qualitative variation for relatively low Ri is essentially unchanged. However, nonlinear processes seem to interact constructively with shear, so that the drag for a noninfinite but relatively high Ri is considerably larger than the drag without any shear at all.
ISSN:0022-4928
1520-0469
DOI:10.1175/2007JAS2577.1