USING RADAR-MEASURED RADIAL VERTICAL VELOCITIES TO DISTINGUISH PRECIPITATION SCATTERING FROM CLEAR-AIR SCATTERING

Thresholds separating regimes for which Rayleigh scattering from preci pitation is likely to dominate over Bragg scattering from clear air ar e established for several common radar wind profiler frequencies. The thresholds are first determined for radar reflectivity factor Z based on observed values of the structure parameter C-n(2) in the tropospher e. These thresholds for Z are then transformed to thresholds for modal terminal velocities V of rain and snow for exponential particle size distributions. Measurements at 915, 404, and 50 MHz in stratiform rai n help substantiate the applicability of the calculated thresholds, ev en though fall velocities, rather than V, are measured. Because the V thresholds for rain at wind profiler frequencies (i.e., >2.5-5.6 m s (-1) at 404 MHz) are substantially greater than typical vertical air m otions outside convective cells, profiler-observed radial vertical vel ocities are a robust indicator of the presence of rain in profiler dat a. Although snow can also often be identified in this manner, the V t hresholds (i.e., 0.5-1.2 m s(-1)) are small enough to increase the pro bability that mesoscale clear-air vertical motions can mask or resembl e its signature. The technique developed here allows rain, and to a le sser extent snow, to be identified in radar wind profiler data under m ost conditions without having to examine the entire Doppler power spec trum, even when the profiler is not calibrated to measure reflectivity factor.