Reverse ray tracing of the mesospheric gravity waves observed at 23°S (Brazil) and 7°S (Indonesia) in airglow imagers

Gravity wave observations using all-sky CCD imager to measure the airglow OH, O2, and OI emissions were made at Cachoeira Paulista (22.7°S, 45°W) (CP), at the low-middle latitude in Brazil, from October 1998 to September 1999. Near the equator, at Tanjungsari observatory (6.9°S, 107.9°E) (TJS), Indonesia, another wide angle CCD imager measuring the OH airglow emission layer has been operated from September 2000 to September 2001. With these data sets, a reverse ray tracing method was used to study propagation of gravity waves through the middle atmosphere and to estimate the source region. The CIRA-86 reference zonal wind and temperature models and the GSWM-02 tidal wind model were used in the present analysis. Both observation sites showed apparently similar wave characteristics, except for the horizontal phase speed, which was much faster in the equatorial region. From the inverse ray tracing calculations, it was found that at CP, only about 15% of the wave events originated in the troposphere. Most of the events were located over the continent within an area of less than 400 km from the observation site. The identified source region in the troposphere showed a seasonal anisotropy, being mainly to the northwest of CP during summer and to the southeast and northwest in winter. This corresponds to the region of high tropospheric convection activity in the South American continent during summer and high meteorological front activity on the southwestern side during winter. At CP more than 70% of the waves showed that the ray tracing paths stopped in the mesosphere. At TJS the ray tracing indicates that 60% of the gravity wave sources might be located in the troposphere with an area of less than 200 km from the observation site, much closer than the case of CP. This corresponds to convective cloud activities on the north and northwest side of TJS. However, during southern hemisphere summer at both observation sites, most of the ray tracing paths stopped at the mesosphere owing to a condition of m2<0 or critical level interactions.