Variations of Raindrop Size Distribution and Radar Retrieval in Outer Rainbands of Typhoon Mangkhut (2018)

The evolution of the microphysical properties of raindrops from Typhoon Mangkhut’s outer rainbands as the storm made landfall in South China in September 2018 was investigated. The observations by three two-dimensional video disdrometers deployed in central Guangdong Province were analyzed concurrently. It was found that the radial distribution of the median volume diameter (D₀) and normalized intercept parameter (N_w) varied in different stages, and that raindrops smaller than 3.0 mm contributed more than 99% of the total precipitation. Considering the characteristics of precipitation in the typhoon outer rainband, a modified stratiform rain (SR)–convective rain (CR) separator line is proposed based on D₀ and N_w scatterplots. Meanwhile, an “S–C likelihood index” is introduced, which was used to classify three rain types (SR, CR, and mixed rain). The CR results were highly consistent with those of the improved typhoon precipitation classification method based on rain rate. By calculating effectively the radar reflectivity factor (Z_e) in the Ku and Ka bands, D₀–Z_e and N_w–D₀ empirical relations were thereby derived for improving the accuracy of rainfall retrieval. Among the four quantitative precipitation estimators using S-band dual-polarimetric radar parameters simulated by the T-matrix method, the estimator that adopted the specific differential phase and differential reflectivity was found to be the most effective for both SR and CR.