A maximum diameter (Dmax) of raindrop should be assumed when rainfall rate (R) is estimates from the differential reflectivity (ZDR) and the horizontal reflectivity (ZH) measured with dual-polarization radar. If the assumed Dmax is different from actual Dmax, the estimated R contains errors. Using distrometer data, it was found that ZDR correlates with Dmax, and it was verified that when Dmax is adaptively determined by an empirical relationship between ZDR and Dmax, errors in estimated R can be reduced.

Radar signals fluctuate because of the incoherent scattering of raindrops. Dual-polarization radar estimates rainfall rates from differential reflectivity (ZDR) and horizontal reflectivity (ZH). Here, ZDR and ZH are extracted from fluctuating radar signals by averaging. Therefore, instrumentally measured ZDR and ZH always have errors, so that estimated rainfall rates also have errors. This paper evaluates rainfall rate errors caused by signal fluctuation. Computer simulation based on a physical raindrop model is used to investigate the standard deviation of rainfall rate. The simulation considers acquisition time, and uses both simultaneous and alternate sampling of horizontal and vertical polarizations for square law and logarithmic estimators at various rainfall rates and elevation angles. When measuring rainfall rates that range from 1.0 to 10.0mm/h with the alternate sampling method, using a logarithmic estimator at a relatively large elevation angle, the estimated rainfall rates have significant errors. The simultaneous sampling method is effective in reducing these errors.