A study of OH imager observed concentric gravity waves near Fort Collins on 11 May 2004

Expanding concentric rings of gravity waves were observed on the night of 11 May 2004 by the all-sky OH imager at Yucca Ridge Field Station (40.7°N, 104.9°W) near Fort Collins, Colorado. The pattern was observed for about 1.5 hours, with the rings encompassing nearly 360° for the first 30 minutes. The centers of the rings were observed at the geographic locations of two convective plumes. We measure the horizontal wavelengths and periods of these gravity waves as functions of both radius and observation time. The observations compare favorably with predictions from a ray-tracing program using the internal gravity wave dispersion relation with assumed zero wind. Since all the 5 events of concentric patterns among 760 nights of image were observed in May or late August/early September, we hypothesize that the weak mean background zonal wind near equinoxes is a necessary condition for gravity waves excited from convective overshoots near the tropopause to be observed as concentric rings in the OH layer.