A look at the ensemble-mean structure of a breaking mountain wave
Tingting Qian, Texas A&M Univ., College Station, TX; and C. C. Epifanio
The mean and turbulent structures in a breaking mountain wave are considered from an ensemble-averaged large-eddy simulation (LES) perspective.
Mean fields and covariances are computed from a set of 40 high-resolution wave-breaking simulations for the problem of 3D flow past a 2D ridge. The resulting TKE distribution is found to be localized primarily along the phase line for which the isentropes in the wave are most strongly displaced downward. The TKE is largest immediately after breaking and then decays (ultimately by a factor of 4) out to time Ut/L = 20, at which point the turbulent energy again begins to increase. The TKE generation is dominated by mechanical production that occurs along the shear line bounding the accelerated lee-slope flow. A TKE budget is computed and found to be closely approximated by a balance between mechanical production and viscous dissipation.
The dependence of the turbulent fluxes on grid resolution is considered by computing the area-averaged cross-stream PV flux at both the resolved and subgrid scales. As expected, the PV flux shifts from predominately resolved-scale to predominately parameterized as the grid spacing is increased from L/55 to L/10. However, the net PV flux remains surprisingly uniform over this range of grid spacings. At larger spacings the PV flux is almost entirely parameterized and begins to show significant error.
Finally, the role of turbulence in the wave-mean interaction process is considered by computing the momentum flux convergence at both the turbulent and wave (or ensemble-mean) scales. The stress convergence is found to be overwhelmingly dominated by the wave scale---that is, the role of the turbulent fluxes is apparently indirect.
Poster Session 2, Mountain Waves, Rotors, Foehn, Wakes and Blocking
Tuesday, 29 August 2006, 2:30 PM-4:00 PM, Ballroom North
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