Evolution and Causes of the Convective Structure and Microphysical Characteristics of Super Typhoon Saola

Guangdong Province experienced heavy to extremely heavy rainstorms during Super Typhoon Saola (2023). Areas such as Yunfu, which are typically drier, experienced anomalously extreme rainfall during this event. We analyzed the convective structure and microphysical characteristics of the typhoon using dual-polarization radar, disdrometer, and ERA5 reanalysis data. The results show that following Saola's second landfall, convection redeveloped on the northern side, which was characterized by rich mid-level ice particles and low rain mixing ratios. This resulted in a phase difference in the precipitation particles along Saola's path. Stations that recorded heavy rain were categorized into four types based on raindrop spectral, spatial, and temporal variables by employing the K-means clustering algorithm. Specifically, the particles observed at Doumen (DM) were classified into a different category from those observed at Yangchun (YC) and Luoding (LD). The microphysical characteristics of raindrop particles evolve under the influence of vertical motion. At DM, the convective structure exhibited a single center of vertical motion with fewer high-altitude ice particles, resulting in a uniform oceanic raindrop type. By contrast, YC was sequentially influenced by two vertically separated centers of vertical motion in the upper and lower atmospheres. These centers alternately dominated the ice-phase and warm-rain processes, resulting in a complex raindrop evolution that transitioned from an intermediate state to a continental type before rapidly reverting to an oceanic type. At LD, the structure showed more pronounced baroclinicity with a higher concentration of water vapor in lower levels, with raindrops tending 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 Province compared with that in the Pearl River Estuary. The upper-level vertical-motion center, induced by the uplifting effect of the frontal zone associated with cold air, promoted the development of ice-phase processes and an increase in raindrop size. Subsequently, the lower-level upward-motion center became dominant, which enhanced precipitation concentration. A unique circulation pattern created by the interaction of twin typhoons with each other and with cold air intensified the 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.