Isotopic characteristics of extreme “dragon-boat water” rainfall between mid-May and mid-June in 2022 in Fuzhou, southeastern China

With global warming, the surge in torrential “dragon-boat water” (DBW) events during mid-May and mid-June, following the onset of the South China Sea summer monsoon (SCSSM), poses escalating threats to southern China through severe flooding and impacts on human livelihoods and socio-economic development. To better understand their fundamental mechanisms, we investigated extreme DBW rain events that occurred in southeastern China in 2022. Analysis of stable isotopes in precipitation samples collected in Fuzhou before and after SCSSM onset uncovered a significant decrease in δ¹⁸O values following monsoon onset, indicating intensified regional convective activity. This decrease is attributed to the seasonal variation in moisture sources, with water vapor originating from distant oceans after SCSSM onset contributing to increased Rayleigh distillation and subsequent δ¹⁸O reduction. A negative correlation between upstream rainfall and δ¹⁸O values, particularly at 96 h along the backward trajectory, indicates the influences of accumulative convective activities on precipitation isotopes along water vapor transport paths. Individual rain events typically exhibit characteristic “V” or “W” shapes in δ¹⁸O, with the lowest values mostly observed in stratiform zone, governed by microphysical processes, such as mesoscale subsidence, vapor deposition of ice particles, and rain re-evaporation below cloud-base. Notably, events with low δ¹⁸O values often coincide with low initial values, suggesting decreased vapor δ¹⁸O before precipitation due to accumulative convection or upstream rainout. Our findings elucidate the mechanisms driving the long-term changes in precipitation isotopes and their evolution during DBW events, enriching our understanding of the interplay between precipitation isotopes, large-scale convection, and microphysical processes.