## 1. Introduction

[2] Since the first Doppler radar measurements of the density normalized upward flux of horizontal momentum in the mesosphere were obtained by [*Vincent and Reid*, 1983], mesospheric measurements have been obtained using the same technique at Adelaide at MF [*Reid and Vincent*, 1987a; *Fritts and Vincent*, 1987], and at other sites using powerful atmospheric radars operating at VHF [e.g., *Reid et al.*, 1989; *Fritts and Yuan*, 1989; *Rüster and Reid*, 1990; *Tsuda et al.*, 1990]. Indirect measurements of the same parameters have been obtained using other techniques by [e.g., *Thorsen et al.*, 1997; *Tang et al.*, 2002; *Fritts et al.*, 2002], but the dual beam technique still appears to be the most readily applied and least ambiguous. Furthermore, new MF/HF radars designed to utilize the Vincent and Reid technique and to investigate turbulence in the MLT region have recently been installed in northern Norway, and in Indonesia. The technique may also be applied to Doppler Lidars. It is therefore of some interest to consider another aspect of the measurement, the effect of finite pulse volumes.

[3] The density normalized upward flux of horizontal momentum is (, ) where (*u*′, *v*′, *w*′) are the zonal, meridional and vertical perturbation velocities, respectively. The technique described by [*Vincent and Reid*, 1983] provides a direct measure of or from the difference between the mean square radial velocities measured in Doppler radar beams directed at equal and opposite angles to the zenith in the east-west and north-south planes, respectively. Given horizontal homogeneity of the statistics of the motions, the technique will correctly measure the contributions to the and terms from both gravity wave and turbulent motions. Analyzing radial velocities, however, yields values of the flux terms that refer to scales of motion larger than the radar pulse volume. In the case of the Buckland Park MF Doppler radar [*Reid et al.*, 1995], the radar sample volume can be represented by a roughly disk shaped volume of radius ∼6 km and thickness ∼2 km at a range of 80 km. At VHF the radius would typically be ∼2 km and the thickness ∼300 m. These volumes could be expected to contain turbulence, and when compared with the scales of commonly observed high frequency mesospheric gravity waves, they are seen to be comparable in horizontal extent [e.g., *Reid*, 1986]. In fact, since it is the shortest horizontal scale, highest frequency gravity waves that should make the largest contribution to the flux terms [e.g., *Fritts*, 1984], existing measurements may underestimate the magnitude of the total flux.

[4] In this paper, we follow a suggestion by [*Reid*, 1987] and use the mean square spectral widths measured in beams directed at 11.6° off-zenith towards the east and west to calculate the term for scales smaller than the radar pulse volume. In the interests of ease of comparison, and to reassess the impact of this term on previously presented observations, we reanalyze data used to determine the term for scales larger than the pulse volume by [*Reid and Vincent*, 1987a].