Dual-beam measurements of gravity waves over Arecibo: Reevaluation of wave structure, dynamics, and momentum fluxes

A previous study by Zhou and Morton (2006) employed dual-beam incoherent scatter radar measurements of radial velocities at the Arecibo Observatory to study the structure, dynamics, and momentum fluxes of gravity waves in the mesosphere and lower thermosphere for similar to 8 h on 28 July 2001. Because of erroneous assumptions about wave character and inferences of the relationship between radial velocities, however, the advertised results of this previous study are largely in error. The purposes of the present study are both to point out these errors to help avoid such pitfalls in the future and to provide a new interpretation of these data, which represent a very interesting case study of gravity wave dynamics at these altitudes. Specific findings of the present study (largely in contradiction to the previous analysis) include (1) the similar to 15-min oscillation was apparently a large-amplitude Doppler-ducted gravity wave structure propagating at one or two maxima of the westward large-scale wind present during the event; (2) the gravity wave exhibited a deep and coherent vertical phase structure, except between the two westward wind maxima at later times, entirely inconsistent with proximity to a critical level; (3) the dominant motions within the gravity wavefield were vertical velocities up to similar to 10 m s(-1), except for inferred horizontal motions where the vertical motions changed phase and above and below the vertical velocity maxima, as dictated by the continuity equation; (4) there were likely no regions of dynamical instability accompanying these ducted wave motions; and (5) momentum fluxes due to this wave motion were small, despite its very large amplitude.