Dual-polarization radar measurements of rain (using horizontal and vertical polarizations with copolar transmit and receive) are compared with simultaneous measurements using a Joss-Waldvogel distrometer and a rapid response rain gauge. The radar pulse volume was situated 200 m above the ground-based instruments, at a range of 8 km. The parameters compared are absolute and differential radar reflectivity factors and rainfall rate. A correlation of 0.95 was found between the radar and distrometer measurements of absolute and differential reflectivity factor, with the radar estimate of absolute reflectivity exceeding the distrometer estimate by 1.6 dB on average, and the distrometer estimate of differential reflectivity exceeding the radar estimate (assuming Pruppacher and Pitter drop shapes) by 0.1 dB on average. The latter difference is small but significant and is possibly attributable to drop canting or oscillation. A new relationship between drop shape and drop size is proposed, reducing the mean difference in differential reflectivity to ±0.02 dB. Radar estimates of rainfall rate, assuming an exponential drop-size distribution, tend to produce significant overestimates compared with the direct measurements of the two ground-based devices. This is shown to be largely attributable to departure from a strictly exponential distribution of drop sizes, and a correction factor is derived to compensate for this.
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