Measuring gravity wave momentum fluxes with airglow imagers

Abstract

Abstract. 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 twodimensional 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

Regional variations of mesospheric gravity-wave momentum flux over Antarctica

“…In addition, successfully validated the momentum fluxes derived using this imager-spectra technique against those measured directly by a dual-beam Na wind lidar. Due to the extended gravity-wave spectrum observed, Swenson et al (1999) found that the magnitudes of the imager-spectra and Na-lidar momentum fluxes reported by Gardner et al (1999) were larger than those derived from imaging specific monochromatic waves or the climatological values derived from radars (Vincent, 1984). Thus, only momentum-flux magnitudes derived using this same imagerspectra technique described by will be compared here.…”

“…The technique for estimating momentum fluxes using airglow imager data in this way has been described fully by Gardner et al (1999), and can be used to estimate the momentum fluxes associated with the full spectrum of both monochromatic as well as quasi-random wave perturbations. The technique has been validated against Na wind lidar measurements of momentum flux when applied to both monochromatic waves and the full wave spectrum , and has been used to infer the seasonal variation of the momentum flux in the Northern and Southern Hemispheres Espy et al, 2004) Briefly, the 3-D Fourier spectrum in frequency, ω, and the zonal, k, and meridional, l, horizontal wave numbers, was computed for the nightly sequence of all-sky Na images using the processing techniques detailed in Gardner et al (1996) and Coble et al (1998).…”

“…As a model frequency spectrum (spectral slope =2) is integrated from the inertial to the Brunt frequency, it is not necessary to compensate for the Doppler effects associated with the mean background wind. For the exact form of G and uncertainties of the zonal and meridional cross-correlation coefficients, the reader is referred to Gardner et al (1999). As the method assumes that all waves propagate upwards and are not ducted, it provides an upper limit to the momentum flux (Fritts, 2000).…”

“…Nightly sequences of these Na images were used to calculate the normalized momentum flux using the technique of Gardner et al (1999), which is described in the next section. To estimate the daily averaged upper-limit of the vertical flux of horizontal momentum due to gravity waves, these normalized values were multiplied by the RMS wind perturbation due to the gravity waves.…”

Gravity wave amplitudes and momentum fluxes inferred from OH airglow intensities and meteor radar winds during SpreadFEx

“…In this sense, there is considerable interest in measuring momentum fluxes carried by those waves (Holton, 1982;Gardner et al, 1999). Co-located optical imagers and radar wind instruments (mean horizontal wind information) at the same observation site are combined to provide intrinsic wave parameters (frequency, horizontal wavelength, direction of propagation), determined directly from the airglow Correspondence to: F. Vargas (fabio.vargas.br@gmail.com) images.…”