Concentric gravity waves in the mesosphere generated by deep convective plumes in the lower atmosphere near Fort Collins, Colorado

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.