A relationship between horizontal flow gradients, in-beam incidence angles, and vertical velocities

摘要

Interferometric radar measurements using vertically pointing beams generally show that the dominant scatter is coming from small off-vertical angles within the beam. At longer wavelengths, such as those in the VHF frequency range, the effects are often attributed to aspect-sensitive layers inclined at small angles with respect to horizontal. We show that off-vertical in-beam incidence angles can also be produced by gradients in the flow due to divergence and vorticity, for example, irrespective of whether aspect-sensitivity effects are present. Since the divergence is intrinsically linked to the vertical velocity through the mass continuity equation, our results imply that vertical velocity measurements will always be biased to some extent if the off-vertical incidence angles are not taken into account explicitly by making use of interferometric or extended variable azimuth display (EVAD) measurement techniques to correct for those biases.