A concentric gravity wave event was captured by a photographer in Nagarzê County (90.28°N, 28.33°E) between 02:00 and 04:00 (local time) on May 11, 2019. This concentric gravity wave event was also observed by the Suomi National Polar-orbiting Partnership satellite and the all-sky airglow imager at Yangbajing station (90.5°E, 30.1°N). The temporal and spatial information on gravity waves from the photographs provided a rare opportunity to study the propagation of gravity waves over the Tibetan Plateau. According to wind and temperature data from the MERRA-2 reanalysis (Modern-Era Retrospective analysis for Research and Applications, Version 2) and empirical models (NRLMSISE-00 [Naval Research Laboratory Mass Spectrometer and Incoherent Scatter Radar Exosphere] and HWM [horizontal wind model]), we inversely derived the propagation trajectory from the observed wave pattern to the source region by using the ray-tracing method. The source of the concentric gravity wave was identified as deep convection in Bangladesh (90.6°E, 25.0°N). The maximum background wind speed in the propagation direction (31.05 m/s) was less than the phase speed of 53 m/s, which is consistent with the wind-filtering theory.
The global atmospheric static stability (N2) in the middle atmosphere and its relation to gravity waves (GWs) were investigated by using the temperature profiles measured by the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument from 2002 to 2018. At low latitudes, a layer with enhanced N2 occurs at an altitude of ~20 km and exhibits annual oscillations caused by tropopause inversion layers. Above an altitude of ~70 km, enhanced N2 exhibits semiannual oscillations at low latitudes caused by the mesosphere inversion layers and annual oscillations at high latitudes resulting from the downward shift of the summer mesopause. The correlation coefficients between N2 and GW amplitudes can be larger than 0.8 at latitudes poleward of ~40°N/S. This observation provides factual evidence that a large N2 supports large-amplitude GWs and indicates that N2 plays a dominant role in maintaining GWs at least at high latitudes of the middle atmosphere. This evidence also partially explains the previous results regarding the phase changes of annual oscillations of GWs at high latitudes.