On the difference of raindrop spectra between coastal plain and mountain areas in Fujian province, Southeast China

To investigate the microphysical characteristics and differences of precipitation in the Fujian coastal plain and mountainous areas, raindrop size distributions at two different sites (plain site: Jinjiang, mountain site: Jiuxianshan) are analyzed and compared from the perspectives of the entire statistics, various rain intensities, and different rain types using laser disdrometer measurements from 2018 to 2022. Additionally, applicative empirical relations between radar quantities (including simple-polarization and dual-polarization variables) and rain rate are presented. Results indicate that the number concentrations of small raindrops (diameter (D)＜1 mm) and large raindrops (D≥3 mm) at the mountain site are significantly higher than that at the plain site, while the number concentration of medium raindrops (1≤D＜3 mm) at the plain site is slightly higher than that at the mountain site. As the rain rate increases, the number concentration of small raindrops exhibits a variation trend of “increase first, then decrease, and increase again”, while the number concentration of medium-to-large raindrops continuously increases. Overall, the mountain site has a smaller mass-weighted mean diameter (D_m) and a larger normalized intercept parameter (lgN_w) than the plain site. For the spectral shape ($\mu$) and slope ($\varLambda$) parameters, the plain site has a larger $\varLambda$ than the mountain site when under the same $\mu$, implying a rapid decrease of raindrop number concentration as the diameter increases. Actual radar data are used to verify the effect of the improved empirical relations of reflectivity factor (Z)-rain rate (R) and differential phase shift rate (${K}_{\mathrm{D}\mathrm{P}}$)-rain rate (R). Validation shows that the performances of the proposed empirical relations are better than that of the traditional relations.