Using Radar-Measured Radial Vertical Velocities to Distinguish Precipitation Scattering from Clear-Air Scattering

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  • 1 NOAA/ERL Environmental Technology Laboratory, Boulder, Colorado
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Abstract

Thresholds separating regimes for which Rayleigh scattering from precipitation is likely to dominate over Bragg scattering from clear air are 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 Cn2 in the troposphere. 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 rain help substantiate the applicability of the calculated thresholds, even 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 motions outside convective cells, profiler-observed radial vertical velocities are a robust indicator of the presence of rain in profiler data. Although snow can also often be identified in this manner, the V* thresholds (i.e., 0.5–1.2 m s−1) are small enough to increase the probability that mesoscale clear-air vertical motions can mask or resemble its signature. The technique developed here allows rain, and to a lesser extent snow, to be identified in radar wind profiler data under most conditions without having to examine the entire Doppler power spectrum, even when the profiler is not calibrated to measure reflectivity factor.

Abstract

Thresholds separating regimes for which Rayleigh scattering from precipitation is likely to dominate over Bragg scattering from clear air are 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 Cn2 in the troposphere. 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 rain help substantiate the applicability of the calculated thresholds, even 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 motions outside convective cells, profiler-observed radial vertical velocities are a robust indicator of the presence of rain in profiler data. Although snow can also often be identified in this manner, the V* thresholds (i.e., 0.5–1.2 m s−1) are small enough to increase the probability that mesoscale clear-air vertical motions can mask or resemble its signature. The technique developed here allows rain, and to a lesser extent snow, to be identified in radar wind profiler data under most conditions without having to examine the entire Doppler power spectrum, even when the profiler is not calibrated to measure reflectivity factor.

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