Doppler-shifting Effects on Frequency Spectra of Gravity Waves Observed near the Summer Mesopause at High Latitude

Abstract

Frequency spectra of horizontal and vertical velocities were inferred from MST radar observations near the summer mesopause at Poker Flat, Alaska. Height variations of the frequency spectra revealed only slight growth of wave amplitudes with increasing height. However, the observed vertical velocity spectra were influenced strongly by Doppler-shifting effects. Whereas horizontal velocity spectra were found to be relatively insensitive to horizontal wind speed, vertical velocity spectra acquired more negative slopes and larger energy densities at lower frequencies as the horizontal wind speed increased. Observed spectra were compared with several Doppler-shifted model spectra, consisting of separable intrinsic frequency and wavenumber spectra. The observed spectrum of horizontal velocity provides a measure of p, the negative slope of the intrinsic frequency spectrum, while the observed vertical velocity spectrum is more sensitive to the form of the vertical wavenumber spectrum, wavefield anisotropy, and the degree of Doppler shifting. These observations suggest consistency with a gravity wave interpretation of atmospheric motions, an intrinsic frequency spectrum of horizontal velocity with p ? 4/3, a vertical wavenumber spectrum with variance concentrated near a characteristic vertical scale, and a high degree of anisotropy with gravity waves propagating predominantly against the mean flow.