Drag produced by waves trapped at a density interface in non-hydrostatic flow over an axisymmetric hill

Drag produced by waves trapped at a density interface in non-hydrostatic flow over an... AbstractLinear non-hydrostatic theory is used to evaluate the drag produced by 3D trapped lee waves forced by an axisymmetric hill at a density interface. These waves occur at atmospheric temperature inversions, for example at the top of the boundary layer, and contribute to low-level drag possibly misrepresented as turbulent form drag in large-scale numerical models. Unlike in 2D waves, the drag has contributions from a continuous range of wavenumbers forced by the topography, because the waves can vary their angle of incidence to match the resonance condition. This leads to non-zero drag for Froude numbers (Fr) both < 1 and > 1, and a drag maximum typically for Fr slightly below 1, with lower magnitude than in hydrostatic conditions due to wave dispersion. These features are in good agreement with laboratory experiments using two axisymmetric obstacles, particularly for the lower obstacle, if the effects of a rigid lid above the upper layer and friction are taken into account. Quantitative agreement is less satisfactory for the higher obstacle, as flow nonlinearity increases. However, even in that case the model still largely outperforms both 3D hydrostatic and 2D non-hydrostatic theories, emphasizing the importance of both 3D and non-hydrostatic effects. The associated wave signatures are dominated by transverse waves for Fr lower than at the drag maximum, a dispersive ‘Kelvin ship wave’ pattern near the maximum, and divergent waves for Fr beyond the maximum. The minimum elevation at the density interface depression existing immediately downstream of the obstacle is significantly correlated with the drag magnitude. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of the Atmospheric Sciences American Meteorological Society

Drag produced by waves trapped at a density interface in non-hydrostatic flow over an axisymmetric hill

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Publisher
American Meteorological Society
Copyright
Copyright © American Meteorological Society
ISSN
1520-0469
eISSN
1520-0469
D.O.I.
10.1175/JAS-D-16-0199.1
Publisher site
See Article on Publisher Site

Abstract

AbstractLinear non-hydrostatic theory is used to evaluate the drag produced by 3D trapped lee waves forced by an axisymmetric hill at a density interface. These waves occur at atmospheric temperature inversions, for example at the top of the boundary layer, and contribute to low-level drag possibly misrepresented as turbulent form drag in large-scale numerical models. Unlike in 2D waves, the drag has contributions from a continuous range of wavenumbers forced by the topography, because the waves can vary their angle of incidence to match the resonance condition. This leads to non-zero drag for Froude numbers (Fr) both < 1 and > 1, and a drag maximum typically for Fr slightly below 1, with lower magnitude than in hydrostatic conditions due to wave dispersion. These features are in good agreement with laboratory experiments using two axisymmetric obstacles, particularly for the lower obstacle, if the effects of a rigid lid above the upper layer and friction are taken into account. Quantitative agreement is less satisfactory for the higher obstacle, as flow nonlinearity increases. However, even in that case the model still largely outperforms both 3D hydrostatic and 2D non-hydrostatic theories, emphasizing the importance of both 3D and non-hydrostatic effects. The associated wave signatures are dominated by transverse waves for Fr lower than at the drag maximum, a dispersive ‘Kelvin ship wave’ pattern near the maximum, and divergent waves for Fr beyond the maximum. The minimum elevation at the density interface depression existing immediately downstream of the obstacle is significantly correlated with the drag magnitude.

Journal

Journal of the Atmospheric SciencesAmerican Meteorological Society

Published: Mar 17, 2017

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