Moisture sources and isotopic composition of a record-breaking heavy Meiyu-Baiu rainfall in southwestern Japan in early July 2020

Torrential Meiyu-Baiu rainfall events have become more frequent in East Asia this decade. The moisture source and isotopic composition of precipitation are crucial for understanding the underlying dynamical and thermodynamic processes of extreme heavy Meiyu-Baiu rainfall. To clarify the moisture source and isotopic composition, we simulated a record-breaking heavy Meiyu-Baiu rainfall over Kyushu in southwestern Japan in early July 2020 using an isotopic regional spectral model. In addition, we observed the isotopic composition of the rainfall in Kyushu to validate the simulations. Moisture sources were classified into three groups: Asian monsoon (AM) moisture from the Indian Ocean, the South China Sea, and the East Asian Continent; North Pacific subtropical high (NPSH) moisture from the Pacific Ocean and the Philippine Sea; and local moisture from the East China Sea, the Kuroshio region, and the Sea of Japan. The remote AM and NPSH moisture was dominant. The NPSH moisture, transported by the NPSH at low levels, gradually increased in the early period. While AM moisture, transported by the monsoonal southwesterly flow at the mid-level, rapidly increased in association with a Baiu frontal depression and merged with the NPSH moisture in the middle period. Meanwhile, a quasi-stationary convective band (QSCB) formed and then triggered heavy rainfall. As for the isotopic composition, condensation exhibited lower 62H and higher d-excess with more AM moisture in the beginning and on the north side of the QSCB. Regarding the effects of hydrometeorological processes on the isotopic composition, the lower 62H and higher d-excess of AM moisture were principally attributable to more rainout and below-cloud evaporation during the transport process, rather than surface evaporation in moisture source regions. This study systematically clarified the major moisture sources and their corresponding isotopic dynamics with respect to the hydrometeorological processes of surface evaporation, moisture transport, rainout, and below-cloud evaporation for torrential Meiyu-Baiu rainfall. The findings may be useful in weather forecasting and disaster prevention for heavy Meiyu-Baiu rainfall in East Asia.