Seasonal variation of gravity waves observed with an OH CCD imager at Shigaraki (35°N,136°E), Japan

Abstract

A wide field CCD imager (FOV: diagonal 128°, N-S 89°, E-W 88°) for OH airglow observation was installed at the MU radar site in Shigaraki (34.9°N, 136.1°E) in November 1996, and has been operated continuously since January 29, 1997. From the observational data in 161 nights from November 1996 to May 1998, gravity wave patterns shown in the OH images are analyzed. Typical horizontal wavelength, horizontal phase speed and observed period are about 5–50 km, 15–70m/s and 5–20 minutes, respectively. Seasonal variations of the parameters of these short period or small scale gravity waves are investigated. For the waves with horizontal wavelength larger than 17.5km, horizontal propagation direction shows clear seasonal change of eastward propagation in summer and westward in winter with a transition in the middle of March and the end of September. These results are consistent with that the horizontal propagation directions of these gravity waves are strongly affected by filtering effect due to the mean wind in the middle atmosphere. On the other hand, for the waves with horizontal wavelength smaller than 17.5km, seasonal variations of these parameters are not clear. Therefore, these waves are probably generated in situ in the mesosphere.