HF Doppler measurements of mesospheric gravity wave momentum fluxes

Abstract

Recent theoretical studies have emphasized the probable importance of internal gravity waves in balancing the momentum budget of the mesosphere. In this paper, we propose a method by which the vertical flux of horizontal momentum can be measured by ground based radars. The method uses two or more radar beams each offset from the vertical to measure the atmospheric motions by the Doppler technique. Provided there is horizontal homogeneity, the momentum flux is proportional to the difference of the variances of the Doppler velocities measured in each beam. The flux convergence and, hence, the associated body force acting on the atmosphere can be inferred by measuring the flux as a function of height. It is shown that mean wind components can also be measured by this method and, under certain circumstances, so can the horizontal wavelengths and phase velocities of the internal waves. Observations of the vertical flux of zonal momentum made with this technique using an HF radar located near Adelaide, Australia (35°S, 138°E) in May 1981 are discussed. At heights in the range of 80–90 km, a mean upward flux of westward momentum was measured, and from the flux convergence a body force equivalent to an acceleration of the order of −20 m s⁻¹ day⁻¹ is inferred. There wore, however, significant variations in the flux both in direction and magnitude on the scales of the order of hours. Measurements using cross-spectral techniques of the gravity wave horizontal wavelengths and phase velocities suggest that values of ~50 km and 50 m s⁻¹, respectively, are dominant at periods of less than one hour.