Investigations on Concentric Gravity Wave Sources Over the Brazilian Equatorial Region

Observations of concentric gravity waves (CGWs) in the OH airglow emission over the Brazilian equatorial region are presented. An all-sky imager installed at Sao Joao do Cariri [7.4 degrees S, 36.5 degrees W] was used to acquire near-infrared OH images. Using over 19 years of observational data from 2000 to 2019, more than 1,052 clear sky nights of airglow data were acquired with a total of 25 observed CGW cases. Three CGW events were selected for this study. These waves had small-scale characteristics and well-defined concentric structures. The selected CGW events showed horizontal wavelengths lambda(H) similar to 31.9, 42.5, and 30.9 km, horizontal phase speeds c(H) similar to 49.2, 57.6, and 74.1 m/s, and periods tau similar to 10.8, 12.3, and 7.0 min, respectively. The CGW structures were well defined, with coherent wave patterns expanding concentrically from the source point, with the observed events having semi-circle or arc-like and semi-elliptical shapes. We found the occurrence of the CGWs to coincide with the seasons of intense tropospheric convective activity and low background winds. This suggests little or no wave breaking, critical level absorption, and filtering or reflection, thereby allowing the CGWs to propagate up to the mesosphere and lower thermosphere region. Using backward ray tracing, we also found the positions and times where and when the ray paths reached the tropopause. The tropopause positions and times of two of the CGW events coincide with active convective systems with cold cloud top temperatures. However, the tropopause position of one of the 3 CGW events did not coincide with any active convective system, suggesting that this CGW was most likely generated in-situ.