Deciphering Divergent Atmospheric River Environments in Extreme and Non-Extreme Precipitation over the Lower Reach of Yangtze River Basin

While atmospheric rivers (ARs) are major drivers of extreme precipitation (EP) in East Asia and hold promise for improving forecasts, this predictive potential is limited because not all ARs result in EP. This raises a key question: what distinguishes the environments of ARs that lead to EP? This study addresses this issue by comparing the environments of ARs associated with EP (AR&EP) with those without EP (AR&NONEP) during summer seasons spanning 32 years, focusing on the lower Yangtze River Basin (LYRB). Results show that while moisture sources for both AR types are spatially similar, AR&EP events exhibit enhanced moisture originating from regions west and north of the LYRB. This indicates that AR&EP events depend more heavily on excess moisture supplied from mid- to high-latitudes. Key large-scale circulation anomalies were also identified during AR&EP events, including a strengthened and westward-extended western North Pacific subtropical high, an enhanced westerly jet, an intensified and eastward-extended South Asian high, and a deepened upper-level trough. Among these, the deep EAT and strengthened westerly jets emerge as the primary factors differentiating AR&EP from AR&NONEP events. Locally, significant differences in convective activity were observed, underscoring the essential role of the Mei-yu front in triggering AR&EP. In this context, the synergy among the deepened upper-level trough, moisture availability, and localized uplift provided by the Mei-yu front is crucial for accurately predicting AR&EP in the LYRB region.