6.1 Weakly nonlinear internal waves in the middle atmosphere

Wednesday, 10 June 2009: 10:20 AM
Pinnacle A (Stoweflake Resort and Confernce Center)
Hayley V. Dosser, Univ. of Alberta, Edmonton, AB, Canada; and B. R. Sutherland

The diagnoses of internal wave propagation, anelastic growth and breaking in the middle atmosphere are assessed in general circulation models through heuristics based upon observations and the predictions of linear theory. Before wave breaking occurs, however, internal waves grow to moderately large amplitude and so the predictions of linear theory are drawn into question. In this talk we derive a nonlinear Schrodinger equation that assumes the dominant weakly nonlinear dynamics are determined by interactions between internal waves and the mean flow (the `Stokes drift' of internal waves) that they induce. The results are shown to agree with the results of fully nonlinear simulations of moderately large amplitude internal waves. The equations predict that high-frequency waves are modulationally unstable so that their amplitude grows faster than the linear theory anelastic growth rate. The amplitude of hydrostatic waves grows more slowly than linear theory predictions. Fully nonlinear simulations show, as a consequence, that high-frequency waves break at lower levels in the atmosphere than predicted by linear theory whereas hydrostatic waves deposit their momentum well above predicted breaking levels. The promising adaption of these results for efficient use in general circulation models is discussed.
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