Characteristics of gravity waves in the mesosphere observed with the middle and upper atmosphere radar: 1. Momentum flux

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Abstract
We observed wind motions from 60‐ to 90‐km altitudes with the MU radar (35°N, 136°E) in the four observation periods: October 1986, June 1987, July 1990, and January/February 1991. Mean wind profiles were fairly consistent with results of Kyoto meteor radar observations (35°N, 136°E) collected in 1983–1985 at 80‐ to 110‐km altitudes, and zonal mean winds generally agreed well with CIRA 1986 except for the profiles below 70 km in January/February 1991, although discrepancy of amplitudes sometimes ranged up to 10 to 20 m/s. Characteristics of frequency spectra of radial wind velocities were basically similar among results determined in four observation periods. That is, oblique spectra had a logarithmic slope of about −5/3 in the frequency range lower than 4–5 × 10−4 s−1. Gravity waves with periods longer than 20–50 min to 5 hours (the lowest limit of the spectral analysis) were found to carry a large part of the zonal momentum flux, while a dominant frequency component was not detected for the meridional flux. Zonal and meridional drag of mean winds induced by the gravity wave with periods from 5 min to 8–10 hours were +51 m/s/d and −4 m/s/d in June at 75 km and −4.0 m/s/d and +7.4 m/s/d in October at 70 km, respectively. The major part of the drag force was also found in the wave component with periods larger than 30 min. The day‐to‐day variation in the zonal momentum flux showed a good correlation with the vertical shear of the zonal mean wind, which suggests effects of the gravity wave activities on mean wind fields, or the effect of shear variation on the gravity wave activity.