Comparison of Dual-Polarization Radar Measurements of Rain with Ground-Based Disdrometer Measurements

Abstract

Dual-polarization radar measurements of ZH and ZDR, where ZDR = 10 logZH/ZV and ZH, ZV are the radar reflectivity factors for horizontal and vertical polarizations, respectively, are compared with values derived from raindrop-size distributions measured by a Joss-Waldvogel disdrometer. The measurements were made simultaneously, with the radar pulse volume situated 120 m above the ground-based disdrometer. Despite the difference in size of the respective sampling volumes, agreement between the radar and disdrometer measurements was generally good. The radar estimates of ZH exceeded the disdrometer estimates by 1.6 dB on average, well within the expected experimental error. The disdrometer estimates of ZDR exceeded the radar estimates by 0.3 dB on average. This was somewhat larger than could be accounted for by experimental error, and further analysis suggested that some modification to the theoretical ratios of horizontal and vertical backscattering cross sections was required, for drops of diameter <2.5 mm. An empirical modification to these ratios enabled the difference to be reduced to 0.15 dB on average.