A spectral analysis was made of the radial (line of sight) wind velocity observed in the mesosphere by the middle and upper atmosphere radar at Shigaraki (34.9¿N, 136.1¿E), Japan, during the daytime of September 5 and October 6, 1988. The radial velocity was measured every 1 min at 300-m range resolution along the three beam directions of 0¿, 10¿, and 20¿ from the zenith. In this paper we first give an overview of the analytical results, in particular, on the height dependence of frequency spectra obtained in a wide range of altitudes in the mesosphere. The frequency spectrum of the vertical velocity shows an almost flat response at frequencies less than the Brunt--V¿is¿l¿ frequency, and the spectral shape hardly changes over the entire height range of the analysis. On the other hand, the frequency spectra of the oblique velocities considerably change their shape and magnitude with height. Although the oblique spectra show a rather flat response at low frequencies when they are taken at the lowest altitudes in this analysis, the overall shape asymptotically approaches a -2 power law with increasing height.

It is further shown that the height dependence of the oblique spectra is closely associated with the vertical propagation of an inertio-gravity wave with a period of ~12 hours. In addition, the overall f-2 dependence observed at around 76 km indicates a saturation of the oblique spectra as the upper limit. Finally, it is suggested that the frequency spectra for the vertical and oblique velocities can be well interpreted in terms of gravity wave theory. ¿ American Geophysical Union 1994