VHF Radar Measurements of In-Beam Incidence Angles and Associated Vertical-Beam Radial Velocity Corrections

Abstract

Atmospheric backscatter or partial reflections observed with VHF radars are strongly enhanced for angles within a few degrees of the vertical, although the strongest echoes are not necessarily exactly from the vertical direction. Consequently, a nominally vertical beam can actually receive the strongest echoes from a direction slightly off-vertical. The radial velocities measured in the vertical beam, then, are not true vertical velocities but have contributions from both the projection of the vertical and the horizontal velocity along the effective beam direction. We present measurements of the tropospheric and lower stratospheric incidence angles obtained over a period of four days with a spaced antenna radar system and calculate the true vertical velocities by means of a correction derived by combining the incidence-angle measurements, the radial velocities in the vertical beam, and the horizontal winds obtained by the standard spaced antenna method. Typical values for the incidence angles are less than 2° when averaged over 8 min or more. The vertical velocity corrections are typically 5%?200% of the magnitude of the vertical-beam radial velocity. The corrected vertical velocities are found to be in better qualitative agreement with expectations based on the meteorological conditions for the period.