Seasonal variation of momentum flux in the mesosphere observed with the MU radar

We have regularly monitored mesospheric winds at 60–85km for three years with the MU radar (35°N, 136°E), and have found that the variance of wind velocity fluctuations for periods from 5 min to 2 hr shows a semi‐annual variation with a significant peak in summer centered in July and irregular enhancements in winter. The zonal momentum flux induced by gravity waves shows eastward and westward maxima in July–August and December–February, respectively. Typical eastward accelerations at the 70km altitude range from 7 to 13 m/s/day and from −8 to −11 m/s/day in summer and winter, respectively, which agree fairly well with theoretical predictions.