Radar observations of the mesosphere and lower thermosphere over Kauai, Hawaii, have provided measurements of the horizontal wind field since September 1990. Horizontal velocity variances in the gravity wave band of frequencies were computed for several averaging intervals over the entire dataset. The results, correlated with tidal and lower-frequency winds, show evidence of filtering of gravity waves by tidal and other lower-frequency motions. Spectra of gravity wave variances for two years of data display peaks at planetary wave as well as tidal periods, primarily near 12-, 24-, 48-h, and 16-day periods.
The modulation of gravity wave variance during four distinct background regimes is examined: one in which the diurnal tide predominates; another in which the 2-day wave is large, a third in which the zonal mean wind and tidal amplitudes are large; and a fourth in which the zonal mean, tidal, and 2-day amplitudes are all small. Intervals in which the diurnal tide or the 2-day wave predominates show evidence of gravity wave modulation at semidiurnal, diurnal, and 2-day periods. Intervals in which the mean wind and lower-frequency amplitudes are small have much reduced gravity wave activity relative to intervals when they are large.