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Gravity waves, scale asymptotics and the pseudo-incompressible equations

Published online by Cambridge University Press:  27 September 2010

ULRICH ACHATZ ,
R. KLEIN  and
F. SENF
ULRICH ACHATZ*
Affiliation:
Institut für Atmosphäre und Umwelt, Goethe-Universität Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany
R. KLEIN
Affiliation:
Institut für Mathematik, Freie Universität Berlin, Arnimallee 6, 14195 Berlin-Dahlem, Germany
F. SENF
Affiliation:
Leibniz-Institut für Atmosphärenphysik an der Universität Rostock, Schlossstraße 6, 18225 Kühlungsborn, Germany
*
†Email address for correspondence: achatz@iau.uni-frankfurt.de
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Abstract

Multiple-scale asymptotics is used to analyse the Euler equations for the dynamical situation of a gravity wave (GW) near breaking level. A simple saturation argument in combination with linear theory is used to obtain the relevant dynamical scales. As a small expansion parameter, the ratio of the inverse of the vertical wavenumber and potential temperature and pressure scale heights is used, which we allow to be of the same order of magnitude here. It is shown that the resulting equation hierarchy is consistent with that obtained from the pseudo-incompressible equations, both for non-hydrostatic and hydrostatic GWs, while this is not the case for the anelastic equations unless the additional assumption of sufficiently weak stratification is adopted. To describe vertical propagation of wavepackets over several atmospheric-scale heights, Wentzel–Kramers–Brillouin (WKB) theory is used to show that the pseudo-incompressible flow divergence generates the same amplitude equation that also obtains from the full Euler equations. This gives a mathematical justification for the use of the pseudo-incompressible equations in the study of GW breaking in the atmosphere for arbitrary background stratification. The WKB theory interestingly even holds at wave amplitudes close to static instability. In the mean-flow equations, we obtain in addition to the classic wave-induced momentum-flux divergences a wave-induced correction of hydrostatic balance in the vertical momentum equation, which cannot be obtained from Boussinesq or anelastic dynamics.

JFM classification

Geophysical and Geological Flows: Atmospheric flows Geophysical and Geological Flows: Internal waves Geophysical and Geological Flows: Stratified flows
Type
Papers
Information
Journal of Fluid Mechanics , Volume 663 , 25 November 2010 , pp. 120 - 147
Copyright
Copyright © Cambridge University Press 2010

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References

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