EFFECT OF BACKGROUND WINDS ON VERTICAL WAVE-NUMBER SPECTRA OF ATMOSPHERIC GRAVITY-WAVES

Observations of quasi-invariant m(-3) power spectra of various atmospheric perturbations at large vertical wavenumbers m have received considerable theoretical attention. Yet some other observations, most notably in the stratosphere, have revealed significant departures from this shape and much reduced spectral densities at large m. Here it is argued that these changes arise when a spectrum of gravity waves encounter mean wind changes which cause intrinsic horizontal phase speeds to increase. Concomitant changes in vertical wavenumbers, amplitudes, and shear variance can produce changes in spectral character qualitatively similar to observations. The two important dynamical processes governing the spectral response are refraction of vertical wavenumbers and conservation of Eliassen-Palm flux for nondissipating waves, processes already encapsulated in both single-wave and spectral parameterizations of gravity-wave processes. Retrospective application of these ideas to various observations in the literature leads to successful ''prediction'' (based on the background wind profile) of either quasi-invariant m(-3) spectra or attenuated spectra at large m. However, while explaining their occurrence, the precise changes in spectral shape predicted by simple models of wind-shifted spectra often differ from those observed.