Multisource Remote Sensing Assessment of Ocean Responses and Rainfall-Related Flood Hazards During Severe Tropical Storm Maysak

This study examines ocean conditions and rainfall-related flood candidates during Severe Tropical Storm Maysak using multisource observations in the northwestern South China Sea and Guangxi, China. Satellite products, atmospheric data, operational ocean products, published gauge summaries and official impact reports were analyzed with explicit observation windows and sensor provenance. Regional daily mean wind speed, significant wave height and mixed-layer depth peaked around 4 July 2026. Daily mean sea surface temperature (SST) on 5 July was 1.12 °C below the maximum during the study period. A daily first frontal analysis south of Hainan showed reductions of 15.3% in mean SST gradient and 25.6% in the mean daily strong front area between paired 15-day windows. Gradient decreases persisted across the tested windows and smoothing settings. Photosynthetically available radiation decreased by 24.1%, and diffuse attenuation at 490 nm increased by 7.9% across their respective windows. The merged chlorophyll-a retrievals increased descriptively by approximately 5.0%; sensor differences and limited common observations prevent interpreting this estimate as a bio-optical response independently corroborated across sensors. Integrated Multisatellite Retrievals for the Global Precipitation Measurement mission (IMERG) indicated a maximum accumulation of approximately 702 mm over the prolonged 2–16 July rainfall episode, including subsequent weather systems. Water-change masks identified 126.5 km2 of potential new inundation candidates, including 1.5 km2 of same-pixel multisensor overlap. The analysis documents marine changes, prolonged rainfall and regional water-change candidates within their stated observation windows.