Session 6.6 Gravity wave propagation through time-dependent shear

Tuesday, 18 August 2009: 9:15 AM
The Canyons (Sheraton Salt Lake City Hotel)
Julie C. Vanderhoff, Brigham Young University, Provo, UT

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Small-scale internal gravity waves can affect large scale atmospheric motions. A parameterization of their influence is necessary to understand the mesosphere and lower thermosphere thermal structure and constituent transport. Mixing induced by dissipating gravity waves in the atmosphere is important to the vertical transport of chemicals, energy, and momentum, which when transferred in large amounts to the local mean flow plays a central role in driving the mean meridional circulation. In addition, global circulation patterns in the middle atmosphere including the quasi-biennial oscillation of the equatorial lower stratosphere and the semiannual oscillations of the equatorial upper stratosphere and mesosphere are driven by the drag and diffusion caused by internal wave breaking. These internal waves can encounter many other phenomena, the most widely studied being a steady shear background wind. Although this is realistic, it is not the whole picture. Large-scale inertial frequency waves over a short time period act much like a steady shear, but are time-dependent and influence small-scale gravity wave propagation in a dynamically different way than a steady shear. The waves which may generally reach a critical level (where their relative frequency approaches zero) and distribute their momentum to the mean flow, do not because the critical level generated by the inertial wave is not steady. The effects of time-dependent shear are quantified through linear, Wentzel-Kramers-Brillouin (WKB) ray theory, with which thousands of waves are tested, and complemented by fully nonlinear numerical simulations of a few representative waves. It is found that internal gravity waves interacting with inertial waves are continually being shifted, altered, and break in different regions. These waves have the propensity to distribute their energy hundreds of kilometers from their expected location if that position is calculated only taking into account constant winds. The result is a significant shift in probable breaking locations of waves and changes in overall momentum flux of the waves throughout their propagation. Some waves gain energy, some lose energy, and some are unaffected, but shifted spatially. Others may even break during the interaction with the inertial wave, dissipating their energy prematurely. The investigation of momentum fluxes of smaller-scale internal waves within larger mesoscale inertial gravity waves is accomplished. Probable interactions are identified and results are quantified.
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