Wind Gradients and Variance of Doppler Spectra in Showers Viewed Horizontally

Louis J. Battan Institute of Atmospheric Physics, The University of Arizona Tucson 85721

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John B. Theiss Institute of Atmospheric Physics, The University of Arizona Tucson 85721

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Abstract

An X-band pulsed-Doppler radar having its beam fixed at an elevation angle of 3°, has supplied new data on radial velocity spectra in two showers. It was found that the mean Doppler velocity, variance of the Doppler spectrum, and radar reflectivity varied markedly over distances of less than a kilometer. The observed variances were less than about 1 m2 sec−2 in more than 80% of the observations and exceeded 3 m2 sec−2 in about 4% of the cases. An analysis of ΔV̄/Δr, the radial gradient of the mean Doppler velocity, yielded a nearly Gaussian curve having a mean of 0.2 × 10−2 sec−1 and a standard deviation of 5.8 × 10−3 sec−1. The effects of the radial gradient of the radial wind apparently can explain less than about 15% of the observed variance of the Doppler spectrum. It appears that a major part of the variance is attributable to scales of motion smaller than the dimensions of the sampling volume.

Abstract

An X-band pulsed-Doppler radar having its beam fixed at an elevation angle of 3°, has supplied new data on radial velocity spectra in two showers. It was found that the mean Doppler velocity, variance of the Doppler spectrum, and radar reflectivity varied markedly over distances of less than a kilometer. The observed variances were less than about 1 m2 sec−2 in more than 80% of the observations and exceeded 3 m2 sec−2 in about 4% of the cases. An analysis of ΔV̄/Δr, the radial gradient of the mean Doppler velocity, yielded a nearly Gaussian curve having a mean of 0.2 × 10−2 sec−1 and a standard deviation of 5.8 × 10−3 sec−1. The effects of the radial gradient of the radial wind apparently can explain less than about 15% of the observed variance of the Doppler spectrum. It appears that a major part of the variance is attributable to scales of motion smaller than the dimensions of the sampling volume.

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