Effects of Doppler shifting on the frequency spectra of atmospheric gravity waves

Abstract

Spectra of atmospheric motions as a function of observed frequency may depart significantly from the spectra as a function of intrinsic frequency due to a nonzero mean wind. In this paper we examine the effects of Doppler shifting on a model spectrum of atmospheric gravity waves. In order to gain insight into the effects of Doppler shifting, we have derived analytic solutions by approximation of the intrinsic frequency spectra and the gravity wave dispersion relation. Our results reveal that Doppler shifting can have major effects on the observed frequency spectrum of both horizontal and vertical gravity wave energy. For levels of Doppler shifting representative of the lower and middle atmosphere, possible effects include a substantial enhancement of horizontal energy density at higher observed frequencies, a corresponding reduction of the vertical energy density at higher frequencies, and a significant transfer of vertical energy to observed frequencies above the buoyancy frequency. The predicted effects are found to be consistent with some of the features of the observed frequency spectra.