Seasonal variations of gravity wave structures in OH airglow with a CCD imager at Shigaraki

A wideview CCD imager for OH airglow observations was operated at the MU radar site in Shigaraki, Japan (35°N, 136°E). From the 18 months’ observation, dominant gravity wave components in the OH images are extracted, and seasonal variation of the characteristics of the waves is investigated. These waves typically have short horizontal wavelengths (5 km–60 km) and short periods (5 min–30 min), with horizontal phase speeds of 0–100 m/s. All the wave events are separated into two groups by a boundary of a horizontal wavelength of 17.5 km, which is close to the boundary between ripples and bands. For the waves with larger horizontal wavelengths, the horizontal propagation direction showed clear seasonal variation with summer eastward and winter westward preferences, with a change of direction in mid-March and at the end of September. This suggests that these waves are propagated from the lower atmosphere and filtered in the middle atmosphere by the mean winds. However, the small scale waves propagate in almost all azimuths with a slight seasonal variation. Therefore, in-situ generation would be the major source of such waves although the wavelength as a physical boundary between the two groups could be smaller than 17.5 km. The seasonal variation of the wave parameters especially between summer/winter and equinoctial months is also discussed. The waves with small horizontal wavelengths (<15 km), longer periods (>10 min), and slow horizontal phase speeds (<30 m/s) are mainly seen in summer/winter.