Improved Understanding of the East Asian Monsoon Rainbands and Dynamics Via Lagrangian Moisture Tracking and Atmospheric Rivers Analysis

In East Asia, summer monsoon rainbands usually stretch for thousands of kilometers in the east-west direction and are capable of producing heavy rainfall intensity of 20-40 mm day-1 on average, rendering them the culprit of many devastating historical floods. Despite the previous endeavor to understand their formation dynamics and hazards, the atmospheric water cycle of these rainband systems remains surprisingly poorly understood. In this study, we leverage backward moisture tracking to demystify the dominant moisture pathways and sources that feed the East Asian rain belt events during the warm season (April to September) from 1981 to 2018. The simulations were conducted using a semi-Lagrangian dynamical recycling model (DRM) forced by hourly-0.25˚ ERA5 reanalysis. In virtue of an Expectation-Maximization (EM)-based curve clustering, we classify up to 15 moisture pathways along four main corridors reaching the Somali Jet, South Asia, the Bay of Bengal and the Pacific basin. Long-range moisture pathways turn out to dominate high-impact monsoon rainbands, coinciding well with the role of planetary-scale atmospheric rivers in triggering extreme rainfall over East Asia. The result also highlights the importance of terrestrial moisture pathways and sources in supplying rain belts. Back-tracing the moisture pathways and atmospheric rivers unravels interesting couplings of pre-existing weather systems. The terrestrial moisture pathways over South Asia turn out to link to circumglobal wave trains at the upper levels up to a two-week lead time. The findings here bridge the knowledge gap in the regional hydrological cycle of the disastrous East Asian rain belts. The pre-existing weather systems uncovered by tracking the moisture and atmospheric rivers also provide potential predictability of heavy precipitation in East Asia.