Previous linear stability analyses of inertia-gravity waves approximated the velocity field as steady plane-parallel horizontal flow with different approximations of the spatial structure. We employ a Floquet method that accounts for the wave spatial periodicity, tilt and time dependence, and use physically relevant ratio of Coriolis to buoyancy frequency of the middle atmosphere. Our results for a constant large amplitude wave for varying wave frequency shows the general trend of how the fastest growing instability changes with frequency.
Such a scan of stability properties passes through (and below) the vertical overturning threshold as frequency approaches f. We find a dynamic oblique mode that dominates in sub-overturning wave amplitude at frequencies around f/0.7 to f/0.8. This result may help to interpret an instability found in non-linear simulations seen by Lelong and Dunkerton (1997), which does not correspond to any modes found in earlier linear stability studies. We also find a set of wave-scale dynamic modes, which becomes nearly isotropic in wavevector orientation as the frequency approaches f. These modes dominate for both sub-overturning and overturning amplitudes at very low frequencies, e.g. when frequency below f/0.95.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics