A Quasi-Linear Study of Gravity-Wave Saturation and Self-Acceleration

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  • 1 Geophysical Institute and Department of Space Physics and Atmospheric Sciences, University of Alaska, Fairbanks, AK 99701
  • | 2 Physical Dynamics, Inc., P.O. Box 3027, Bellevue, WA 98009
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Abstract

We present the results of quasi-linear simulations performed to illuminate the effects of saturation and self-acceleration on gravity waves prompting into the middle atmosphere. It is shown for transient, horizontally monochromatic wave packets that self-accelerations due to transient mean wind accelerations can be a significant factor in the evolution. Self-accelerations represent a possibly major change in the phase speed of the wave motion and permit larger vertical wavelengths and vertical group velocities than would otherwise occur. In some instances, permit gravity wave motions to propagate well beyond an initial critical level, a phenomenon we label “critical-level dislocation.” This phenomenon does not occur under the slowly-varying (WKB) and single phase speed assumptions. As such, it may be an intrinsically non-WKB effect.

Saturation was modeled using a relaxational convective adjustment scheme. This was found to limit wave amplitudes without radically affecting the structure of the primary wave, as anticipated in the linear saturation theory. Due to gradual adjustment, however, wave amplitudes and momentum fluxes were larger than predicted by linear theory. Local saturation was also found to reduce but not eliminate the effects of self-acceleration and to permit the excitation of harmonics of the primary wave motion in a coherent manner.

Abstract

We present the results of quasi-linear simulations performed to illuminate the effects of saturation and self-acceleration on gravity waves prompting into the middle atmosphere. It is shown for transient, horizontally monochromatic wave packets that self-accelerations due to transient mean wind accelerations can be a significant factor in the evolution. Self-accelerations represent a possibly major change in the phase speed of the wave motion and permit larger vertical wavelengths and vertical group velocities than would otherwise occur. In some instances, permit gravity wave motions to propagate well beyond an initial critical level, a phenomenon we label “critical-level dislocation.” This phenomenon does not occur under the slowly-varying (WKB) and single phase speed assumptions. As such, it may be an intrinsically non-WKB effect.

Saturation was modeled using a relaxational convective adjustment scheme. This was found to limit wave amplitudes without radically affecting the structure of the primary wave, as anticipated in the linear saturation theory. Due to gradual adjustment, however, wave amplitudes and momentum fluxes were larger than predicted by linear theory. Local saturation was also found to reduce but not eliminate the effects of self-acceleration and to permit the excitation of harmonics of the primary wave motion in a coherent manner.

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