Gravity Wave Breaking and Turbulence in the MLT: Implications for Transport, Diffusion, and the Turbulent Prandtl Number

Details Gravity Wave Breaking and Turbulence in the MLT: Implications for Transport, Diffusion, and the Turbulent Prandtl Number Gravity Wave Breaking and Turbulence in the MLT: Implications for Transport, Diffusion, and the Turbulent Prandtl Number

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsa54a..01f&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #SA54A-01

Physics

0341 Middle Atmosphere: Constituent Transport And Chemistry (3334), 0342 Middle Atmosphere: Energy Deposition (3334), 3334 Middle Atmosphere Dynamics (0341, 0342), 3379 Turbulence (4490), 3384 Acoustic-Gravity Waves

Scientific paper

Gravity waves (GWs) account for the majority of turbulence and transport in the MLT due to their amplitude growth with altitude and instabilities that occur for all wave amplitudes and intrinsic frequencies. Linear theory provides a useful guide to initial instability structures and growth rates, and also to likely consequences of turbulent mixing. Neither linear theory nor observations can readily address all of the contributions to mixing and transport in an evolving 3D motion field, however, and we must seek guidance from numerical studies that define these flows with high resolution and precision. Given evidence for large-amplitude GWs in the MLT, we have performed direct numerical simulations (DNS) that resolve the full GW and turbulence spectrum for GW parameters and Reynolds numbers relevant to the MLT. These simulations exhibit rapid instability development, a competition between 2D and 3D instability modes, large GW amplitude reductions, and spatially-localized turbulence. Turbulence is strongly correlated with the GW phase and highly variable in intensity. Both 2D GWs and 3D turbulence contribute to fluxes of heat and momentum, but the contributions are often of opposite sign. Turbulent mixing is strong, but impacts the velocity and thermal fields to different degrees. The turbulent Prandtl number, a key quantity in GCM descriptions of unresolved small-scale dynamics, is assessed for breaking GWs having subcritical and supercritical amplitudes and is found to be Pr ~ 3 - 8 during active breaking, confirming expectations of earlier linear theory and consistent with values required by large-scale models.

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