This raytracing study is part of an effort to understand the role of time dependence in nonlinear interactions within the atmospheric gravity wave spectrum. By including wave motions in the effective background seen by a particular member of the spectrum, the technique employed represents strongly nonlinear interactions in which the frequency and wavenumber of the test wave vary significantly through the advective nonlinearity. Our calculations go beyond the analysis at the heart of current Doppler-spread theory, by including the temporal as well as spatial variations that exist in the background fields. This addition removes the inverse relationship between wave action density and vertical group velocity, which has been used in previous studies to restrict wave dissipation to critical approaches. Here, Hayes' conservation equations for wave action density are used to directly calculate wave amplitude variation over the ray paths. Careful attention is paid to WKB considerations. We show that as the model is made more realistic in representing the complexity typically observed in the gravity wave spectrum, wave action density variations become increasingly independent of wavenumber variations; the time-varying background fluctuations focus wave energy both when the wavenumber is increasing and when it is decreasing. We examine ray solutions in a variety of background models to identify commonly observed features of the wind field that contribute to this focusing. These results suggest that wave dissipation may not be primarily restricted to critical approaches (even those of the spectrum-induced kind); and that temporal focusing may contribute significantly to wave dissipation in a broad spectrum such as that found in the middle atmosphere.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics