Gravity waves in the mesosphere observed with the middle and upper atmosphere radar

We have observed wind motions from 60 to 90 km altitudes with the middle and upper atmosphere (MU) radar during daylight hours (0800–1600 LT) from October 13 to 31, 1986. Gravity waves with fairly sinusoidal vertical structure were evident on 16 days of the 19 days of observations. They were characterized by a typical vertical wavelength of 5–15 km and intrinsic periods centered at about 8.6 hours. The propagation velocity of these waves was determined using the linear gravity wave theory. All of the waves propagated downward and had an equatorward component of the meridional propagation. The median direction of horizontal propagation was slightly east of south, and the mean horizontal phase speed was 35.3 m/s. The vertical wave number spectra of horizontal components of mesoscale wind fluctuations agree well with the theoretical spectrum of saturated gravity waves. At frequencies from 1 × 10⁻⁵ to about 4 × 10⁻⁵ c/s frequency spectra of vertical and radial wind components had logarithmic slopes of 1/3 and -5/3, respectively, which agree fairly well with a model gravity wave spectrum. The effects of Doppler shifting on frequency spectra are most obviously recognized at large frequencies near the Brunt Väisälä frequency. We have also determined the upward flux of horizontal momentum induced by waves with periods from 10 min to 8 hours and further estimated the westward and northward body force of 5.1 and 4.0 m/s/d, respectively.