The evolution and causes of the convective structure and microphysical characteristics of Super Typhoon Saola

Guangdong Province experienced heavy to extremely heavy rainstorms due to Super Typhoon Saola (2023). Areas like Yunfu, typically drier, witnessed anomalous extreme rainfall. We analyzed the convective structure and microphysical characteristics using dual-polarization radar, disdrometers, and ERA5 reanalysis data. The results show that following Saola's second landfall, convection redeveloped on the northern side, characterized by rich mid-level ice water and low-level rain mixing ratios. This results in a phased difference in precipitation particles along Saola's path. The stations that recorded heavy rain were categorized into four types based on raindrop spectral variables, spatial and temporal variables using K-means clustering algorithm. Specifically, the particles in Doumen (DM) were classified as Type 2, while those in Yangchun(YC) and Luoding(LD) belong to Type 3 and Type 4. The microphysics changes of raindrop particles evolve under the influence of vertical motion. DM exhibited a single center of vertical motion, with fewer high-altitude ice particles, resulting in a uniform oceanic raindrop type. In contrast, YC was sequentially influenced by two vertically separated centers of vertical velocity in the upper and lower atmospheres. These centers alternately dominated ice-phase and warm-rain processes, resulting in complex raindrop evolution that transitions from an intermediate state to a continental type before rapidly reverting to an oceanic type. LD showed more pronounced baroclinicity, with a higher concentration of low-level water vapor, leaning towards an intermediate continental type. The evolution of vertical motion was driven by changes in environmental conditions. The addition of cold air complicated raindrop evolution in western Guangdong compared to the Pearl River Estuary. The upper-level vertical motion center, caused by the uplifting effect of the frontal zone associated with cold air, promoted the development of ice-phase processes and increased raindrop size. Subsequently, the lower-level rising center becomes dominant, which enhanced precipitation concentration. Unique circulation pattern, created by the interaction of cold air and twin typhoons, intensified low-level easterly winds and moisture influx, contributing to extreme rainfall in a typically drier region. Orographic convergence and uplift also played a role in the microphysical evolution of raindrops.