Measuring gravity wave momentum fluxes with airglow imagers

We describe a new technique for estimating the vertical fluxes of horizontal momentum in the mesopause region using airglow imager data. The azimuthal distribution of the relative airglow intensity or rotational temperature variance associated with both monochromatic and quasi-random waves is computed from the unambiguous two-dimensional horizontal wave number spectrum. The variance distribution is used in a gravity wave model to predict the cross-correlation coefficient between the vertical and horizontal wind perturbations (and hence the momentum flux) as a function of azimuth angle. The technique is applied to OH imager data collected during five nights in February and April 1995 at the Starfire Optical Range, New Mexico. To validate the technique, the cross-correlation coefficients and momentum fluxes derived from the imager data are compared with the zonal and meridional fluxes measured by a co-located Na wind/temperature lidar employing the standard dual-beam Doppler technique. The mean zonal and meridional cross-correlation coefficients and momentum fluxes inferred from the OH imager data are -0.27 ± 0.011 (-11.9 ± 0.49 m²/s²) and 0.009 ± 0.011 (0.44 ± 0.53 m²/s²), respectively. The corresponding zonal and meridional coefficients and fluxes measured by the lidar are -0.26 ± 0.09 (-11.9 ± 3.9 m²/s²) and -0.081 ± 0.10 (-3.76 ± 4.7 m²/s²), respectively. These results demonstrate that creditable estimates of momentum fluxes at mesopause heights can be derived from OH image data.