Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 2. Propagation direction

Abstract

We studied the characteristics of dominant component of gravity waves (λz ∼ 10 km) in the vertical profile of the wind velocities observed in the mesosphere (at around 70–75 km) with the middle and upper atmosphere (MU) radar in the four observation campaigns (June 1987, July 1990, October 1986, and January/February 1991) and the monthly mesosphere observations carried out 4–5 days a month in December 1985 to December 1988. By assuming linear dispersion relations of gravity waves, both vertical and horizontal propagation directions of gravity waves were determined. Most of the dominant waves propagated upward, except that they sometimes propagated downward mainly in summer. A typical vertical wavelength and intrinsic period of the dominant gravity waves were 4–15 km and 4–15 hours, respectively. The mean values of the vertical wavelength, horizontal wavelength, and intrinsic periods were 8.2 km, 1200 km, and 10.2 hours, respectively, which did not show significant seasonal variations. Most of the waves were found to be dissipated by the eddy diffusion of 15–100 m2/s, which does not agree with MF radar observations (10‐ to 120‐min observed periods) but is rather consistent with sodium lidar observations (36–300 m2/s). The mean horizontal phase velocity was 33 m/s. The horizontal propagation direction was generally eastward throughout a year except for the westward propagation in early winter (November to December). This suggests that the dominant gravity waves are less sensitive to a change in the mean zonal wind direction in the middle atmosphere than the short‐period gravity waves (2 hours to 5 min). Momentum flux carried by the dominant gravity waves was estimated to be 0.5–1 m2/s2, which was generally smaller than the momentum flux of gravity waves with periods ranging from 8 hours to 5 min. The amplitude of the momentum flux of the dominant gravity waves had an autumn maximum, while the momentum flux with periods of 8 hours to 5 min had equinoctial minima. Thus in September and October the momentum flux due to the dominant gravity waves became significant. We further pointed out that the possibility that / depends on the wave periods, and therefore the major frequency component of is not easily determined from simple dispersion relation only.