Occurrence characteristics of medium-scale gravity waves observed in OH and OI nightglow over Adelaide (34.5°S, 138.5°E)

This paper presents a 7 year climatology describing medium‐scale gravity waves observed in the menopause region covering the years from 1995 to 2001. The data comes from the OI and OH airglow observations of the three‐field photometer employed at the University of Adelaide's Buckland Park, Australia (34.5°S, 138.5°E). About 1300 gravity wave events (AGW) are identified during the years 1995–2001. These AGW events usually persist for between 40 min and 4 hours. The magnitudes range from 1% to 14% of the background intensities and peak at 2% for OI observations and at 3% for OH observations. The observed periods range from 10 to 30 min, and the horizontal phase speeds range from 20 to 250 m s−1, with dominant wave scales of 17 min, 70 m s−1 for OI observations and 20 min, 40 m s−1 for OH observations. The intrinsic parameters are obtained by using medium‐frequency (MF) wind data observed at the same place. The occurrence frequency of AGW events peaks at 13 min, 40 m s−1 for both OI and OH observations. The occurrence rate of gravity waves has a major peak in summer and a minor peak in winter. There is an obvious dominating southeastward direction for gravity waves, with azimuths of 160° in summer and 130° in winter. Studies for gravity waves observed in various locations show a similar tendency of propagating meridionally toward the summer pole. This implies that the tendency of propagating toward the summer pole may be a global trend for medium‐scale gravity waves observed in the mesopause region. During summer, gravity waves propagate against winds measured by MF radar in their dominating direction. Using the ray tracing method, we found that the seasonal variation of winds limits the access of gravity waves to the observation height through reflection and critical coupling, which is one of the causes leading to the seasonal behavior of gravity waves observed over Adelaide.