The measurement of gravity wave momentum flux in the lower atmosphere using VHF radar

Radar measurements have often been used to estimate gravity wave momentum flux profiles in the troposphere, lower stratosphere, and mesosphere. The reliability of the measurements in the troposphere and lower stratosphere is investigated, with a comparison of three techniques employing various radar beam directions and zenith angles, including the standard ''symmetric-beam'' method. The study uses the 46.5 MHz MST radar system at Aberystwyth, Wales (52.4 degrees N, 4.1 degrees W). For short-period waves, errors in momentum flux profiles arising, for example, from differing data quality in the two radial beams sometimes prove difficult to eliminate in the symmetric-beam method. For long-period waves, including inertia-gravity waves, problems arise from the vertical wind measurement which is implicit in any estimate of momentum flux. Furthermore, a difference is sometimes found between the echo powers of the two symmetric radial beams, and this fluctuates with the horizontal wind perturbations from long-period waves. The possible in influence in this power difference of tilted anisotropic scattering layers, which might also bias radial velocity measurements, is discussed. A momentum flux method which measures the vertical wind with a vertical beam, and the horizontal wind with a pair of radial beams, avoids some of the problems at both short and long periods. Agreement of this method with the symmetric-beam results would allow improved confidence in momentum flux profiles.