Comparisons of Horizontal Winds Measured by Opposing Beams with the Flatland ST Radar and between Flatland Measurements and NMC Analyses

Abstract

This study examines the consistency between VHF horizontal wind measurements and those interpolated from routine objective analyses. First, the agreement between the two U components and between the two V components measured on opposing beams (here referred to as the beam-to-beam intercomparison) by the Flatland 49.8-MHz wind profiler is examined to determine the beam-to-beam consistency and relative precision of this radar. This part of the study demonstrates the ability of this technique to detect system problems affecting only one radar beam and provides a benchmark for comparison with radar systems operating near the Front Range of the Rockies and for the comparison in the second part of this study. This second comparison is between the Flatland observations and the spatially smooth winds from the National Meteorological Center's (NMC) regional objective analysis for July through November 1990. The location of the Flatland profiler near Champaign-Urbana, Illinois, is free of significant orographic features, in contrast to the proximity to the Colorado Rockies of many of the radars employed in earlier studies. The beam-to-beam intercomparison is presented in terms of the mean and standard deviation of the differences between the measurements made on opposing beams. The Flatland difference standard deviations of about 0.8 m s?1 are roughly one-third of those for radars in the lee of the Rocky Mountains, reflecting reduced vertical velocities. However, the mean differences are approximately ?0.25 m s?1 for both the U and V components, consistent with the tropospheric monthly mean downward motion of 2?6 cm s?1 indicated in the Flatland vertical beam measurements since its construction, including the period of this study. In fact, when the data were stratified into periods with and without precipitation based on estimates of latent heating from the NMC data, the precipitation periods showed standard deviations of about 1.3 m s?1, with mean differences two to three times that for nonprecipitation cases. This behavior is consistent with larger downward velocities during precipitation, whether from clear-air or hydrometeor scatterers. Thus, these vertical-motion biases, which the authors believe are of atmospheric origin (whether bulk motion or reflectivity effects), must be accounted for in long-term climatological studies. Finally, for the Flatland?NMC comparison, 4-h averages of Flatland winds were chosen to better correspond to the spatially smooth NMC winds. The correlation coefficients, larger than 0.95, indicate very good agreement, but not as good as the 0.99 found in the beam-to-beam intercomparison. The larger 2.3 m s?1 difference standard deviations are similar to those found in studies comparing profiler and rawinsonde winds near the Front Range of the Rockies, indicating the applicability of the Taylor hypothesis implicit in this comparison of the 4-h temporally averaged Flatland winds and the spatially avenged NMC-analyzed winds. The consistency between these two datasets implies that increases in accuracy of objective analyses may result more from the increased time resolution of the profiler data rather than from an inherent increase in accuracy of the observations.