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 (52.4øN, 4.1øW). 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 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. 1. Introduction Early estimates of the energy and momentum fluxes associated with gravity waves suggested that their effects at upper heights could be important [e.g., Gossard, 1962; Hines, 1972]. However, the role of such waves in balancing the momentum and energy budgets of the mesosphere awaited studies such as those of Houghton [1978] and Lindzen [1981]. Interest in the momentum budget at lower heights was inspired by the observation of substantial fluxes associated with mountain waves [e.g., Lilly and Kennedy, 1973] and the realization that the flux divergence could affect the large-scale circulation at the lower heights [Lilly, 1972]. Observational techniques to measure the vertical transport of horizontal momentum have included the use of aircraft-mounted systems [e.g., Lilly and Kennedy, 1973; Bougeault et al., 1993; Alexander and Pfister, 1995], radiosonde flights [e.g., Mobbs and Rees, 1989; Shutts et al., 1994], and mesospherestratosphere-troposphere (MST) radar [e.g., Fukao et al., 1988a; Fritts et al., 1990]. The symmetric-beam radar method of Vincent and Reid [1983] has been applied to measure momentum flux in both the lower atmosphere [e.g., Fukao et al., 1988a; McAfee et al., 1989; $ato, 1990, 1993; Fritts et al., 1990; Thomas et al., 1992; Prichard and Thomas, 1993] and the mesosphere [e.g., Tsuda et al., 1990; Reid and Vincent, 1987]. In this method, the momentum flux in a vertical plane is obtained using two radial radar beams in that plane, pointing at symmetric zenith angles +0 and -0: uw --r}ø -r2-ø 2 sin 20 (1) where u, w represent the horizont...

Abstract. Radar measurements at Aberystwyth (52.4°N, 4.1°W) of winds at tropospheric and lower stratospheric heights are shown for 12±13 March 1994 in a region of highly curved¯ow, downstream of the jet maximum. The perturbations of horizontal velocity have comparable amplitudes in the troposphere and lower stratosphere with downward and upward phase propagation, respectively, in these two height regions. The sense of rotation with increasing height in hodographs of horizontal perturbation velocity derived for hourly intervals show downwards propagation of energy in the troposphere and upward propagation in the lower stratosphere with vertical wavelengths of 1.7 to 2.3 km. The results indicate inertia-gravity waves propagating in a direction similar to that of the jet stream but at smaller velocities. Some of the features observed contrast with those of previous observations of inertia-gravity waves propagating transverse to the jet stream. The interpretation of the hodographs to derive wave parameters has taken account of the vertical shear of the background wind transverse to the direction of wave propagation.

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