Long-Term Average Vertical Motions Observed by VHF Wind Profilers: The Effect of Slight Antenna-Pointing Inaccuracies

Abstract

This paper shows that a very slight tilt of the vertically directed antenna beam of a VHF wind profiler can produce a measurable change in the observed long-term averaged ?vertical? velocity profiles. The results are based primarily on data obtained using the NOAA/CU profiler at Piurn, Peru, where phase measurements of individual antenna elements made in 1992 showed that the calculated angle of the 3°-wide vertical beam was skewed only by about 0.06° from true vertical. This small error was corrected in February 1993 by carefully rephasing some of the array feedpoints. Mean vertical velocity profiles obtained prior to the correction were adjusted to account for the slight contamination by the horizontal wind. These corrected vertical profiles compare favorably with vertical profiles obtained after rephasing the antenna, as well as with mean vertical wind profiles from other profiler sites in our tropical Pacific profiler network. The results show that in order to be confident in long-term averaged vertical wind profiles using VHF profilers in the Tropics, the vertically directed antenna needs to be very carefully phased. The results also suggest that long-term averaging tends to nullify any possible effects of apparent variations of the vertical beam that might arise from short-term echo specularity. In addition, asymmetric biases in the turbulent-scattering process thought to contaminate mean vertical velocity measurements at midlalitudes are not at all apparent in our tropical profiles. This final factor may be due to the much smaller average vertical velocity variances observed at low latitudes.