Beam broadening effect on oblique MST radar Doppler spectrum

Beam broadening effect on Doppler spectrum for an obliquely pointed meso-. strato-. troposphere (MST) radar beam is investigated theoretically in this article. The general behavior of radial velocity on the cross section of an oblique radar volume is analyzed first. It is found that the contour of radial velocity can be described perfectly by a set of concentric circles. The coordinate of the center of the circle is governed not only by the ratio of vertical w to horizontal U wind velocities, but also by the tilt angle of the radar beam. The contour of radial velocity is approximate to a straight line if the ratio of w to U is low (<0.35) and the tilt angle of the antenna beam is small (<30°). Under these conditions, the analytic expression of the corresponding beam broadening spectrum for an oblique radar beam with beamwidth of a few degrees is derived. The results show that the width of the beam broadening spectrum is not only determined by antenna beamwidth and horizontal wind velocity, but also governed by the tilt angle of antenna beam and the ratio of w to U. Quantitative calculation indicates that the spectral width at zenith direction may be greater that at a zenith angle of 30° by about 32%, provided the ratio of vertical to horizontal wind velocities is 0.4 and the horizontal wind is parallel to the antenna beam. Moreover, it is found that horizontal wind direction plays a crucial role in determining the beam broadening width. Theoretical calculation shows that a change in horizontal wind direction from 0° to 90° with respect to direction of the oblique radar beam can lead to a 15% change in beam broadening spectral width for a 20% zenith angle of antenna beam and 0.2 of w/U. These results suggest that the effects of tilt angle of antenna beam, horizontal wind direction, and the ratio of vertical to horizontal wind velocities on Doppler spectral width of an obliquely directed MST radar beam should be taken into account in analyzing the radar returns to estimate atmospheric parameters of interest from the observed Doppler spectral width.