[1] Gravity waves in the mesopause region (80-105 km) may induce perturbations in OH Meinal Band emissions at $87 km. These perturbations can be observed by ground-based OH airglow imagers. In this paper, we present observations of concentric gravity waves (CGW) by the all-sky OH imager at Yucca Ridge Field Station (40.7°N, 104.9°W) near Fort Collins, Colorado. We find that expanding rings of concentric gravity waves were observed on 9 out of 723 clear nights from 2003 to 2008. In particular, on 11 May 2004, concentric rings were observed for $1.5 h, with nearly perfect circular rings entirely in the field of view during the first 30 min. The centers of the concentric rings occurred at the geographic locations of two strong convective plumes which were active in the troposphere $1 h earlier. We measured the horizontal wavelengths and periods of these gravity waves as functions of both radius and time. These results agreed reasonably well with the internal Boussinesq gravity wave dispersion relation with an assumed zero background wind. Similarly, for the other 8 cases, strong convective plumes occurred prior to the CGW observations near the apparent center of each of the arcs or rings. For the 7 out of the 9 cases, radiosonde data were available up to z = 30-35 km. These data showed that the wind speeds from the tropopause to $30-35 km were smaller than $20-30 m/s. Because 8 of the 9 cases occurred when the total horizontal mean winds were weak and because the horizontal winds below $87 km were less than $20 m/s on 11 May 2004 (according to radiosonde and TIME-GCM model data), we postulate that weak background horizontal winds are likely a necessary condition for gravity waves excited from convective overshooting to be observed as concentric arcs or rings in the OH layer.

The January 2022 Hunga Tonga–Hunga Ha’apai eruption was one of the most explosive volcanic events of the modern era1,2, producing a vertical plume that peaked more than 50 km above the Earth3. The initial explosion and subsequent plume triggered atmospheric waves that propagated around the world multiple times4. A global-scale wave response of this magnitude from a single source has not previously been observed. Here we show the details of this response, using a comprehensive set of satellite and ground-based observations to quantify it from surface to ionosphere. A broad spectrum of waves was triggered by the initial explosion, including Lamb waves5,6 propagating at phase speeds of 318.2 ± 6 m s−1 at surface level and between 308 ± 5 to 319 ± 4 m s−1 in the stratosphere, and gravity waves7 propagating at 238 ± 3 to 269 ± 3 m s−1 in the stratosphere. Gravity waves at sub-ionospheric heights have not previously been observed propagating at this speed or over the whole Earth from a single source8,9. Latent heat release from the plume remained the most significant individual gravity wave source worldwide for more than 12 h, producing circular wavefronts visible across the Pacific basin in satellite observations. A single source dominating such a large region is also unique in the observational record. The Hunga Tonga eruption represents a key natural experiment in how the atmosphere responds to a sudden point-source-driven state change, which will be of use for improving weather and climate models.

[1] Using a convective plume model and a ray trace model, we investigate the effects of winds on concentric rings of gravity waves (GWs) excited from a convective plume on 11 May 2004, near Fort Collins, Colorado. We find that winds can shift the apparent center of the concentric rings at z = 87 km from the plume location. We also find that critical level filtering (for GWs with small phase speeds propagating in the same direction as the wind) and wave reflection (for high-frequency GWs with small horizontal wavelengths propagating in the opposite direction to the wind) prevent many GWs from reaching the OH airglow layer. Additionally, we find that strong winds disrupt the concentric ring patterns, causing distorted ''squashed'' ring and arc-like patterns instead. Using a zero wind profile and a representative April mean zonal wind profile, we compare our model results with observations of concentric rings at the Yucca Ridge Field Station (40.7°N, 104.9°W). We find that the model horizontal wavelengths and periods agree reasonably well with the observed data. We also compare the model temperature perturbations with the temperature perturbations calculated from the intensity perturbations. Because the observations show less critical level filtering than from the April wind profile and more critical level filtering than from the zero wind profile, we conclude that the winds on 11 May were likely somewhat smaller than the April zonal wind profile assumed here.