From compact to diffuse optical-depth distributions: life-cycle evolution of cloud properties in tropical cyclones over the South China Sea

Tropical cyclones (TCs) over the South China Sea (SCS) frequently produce heavy rainfall, yet their cloud physical structures remain poorly understood. Using daytime cloud property retrievals from the Himawari satellites, this study statistically examines the cloud optical and microphysical characteristics of TCs that affected the SCS during 2016-2023, classified according to the intensity and the life-cycle stage. We also investigate factors associated with their spatial distributions. Unlike weaker systems, strong TCs exhibit a "compact-diffuse" pattern in cloud optical depth (COD): during the mature stage, a pronounced COD maximum is tightly confined to the inner core, while COD weakens and diffuses outward as storms decay. The cloud-top particle effective radius (CER) in the inner core is comparatively small during the mature stage, consistent with rapid lofting and limited time for particle growth. Further analysis indicates that the spatial evolution of COD in strong TCs is closely associated with vertical motion. Strong updrafts during the mature stage can transport abundant low-level moisture upward, facilitating condensation, vapor deposition, and the formation and maintenance of liquid droplets and ice particles, thereby increasing cloud water and ice contents. By contrast, CER exhibits only a weak correlation with vertical velocity across all intensity categories and life-cycle stages. These findings elucidate the cloud physical structure of SCS TCs and provide new insights into the optimization of cloud microphysical parameterizations in numerical models.