LOW-FREQUENCY INERTIA-GRAVITY WAVES IN THE STRATOSPHERE REVEALED BY 3-WEEK CONTINUOUS OBSERVATION WITH THE MU RADAR

Continuous observations over three weeks were performed with the MU ra dar at Shigaraki, Japan (35N, 136E) in April of 1995. By integrating r adar echo spectra over 30 min after removing severe interferences, win ds were successfully estimated offline up to 24 km in the lower strato sphere. It was found that inertia-gravity waves with a period of about 20 h and a vertical wavelength of 3.5 km are dominant near 22 km in a region of weak background winds. Four wave packets were fitted to pla ne waves by a least-squares method and wave parameters were estimated theoretically. The inertia-gravity waves propagate energy upward, and their horizontal phase velocity is westward in the range -10 to -20 m s(-1). The waves have large momentum fluxes of about -0.002 Nm(-2). Th is result suggests that inertia-gravity wave drag may contribute to th e formation of easterlies in the summer stratosphere.