Abstract: A Lagrangian model—the Hybrid Single-Particle Lagrangian Integrated Trajectory model (HYSPLIT)—is used to quantify changes in moisture sources and paths for precipitation over North China’s Henan Province associated with tropical cyclone (TC) over the western North Pacific (WNP) during July–August of 1979–2021. During TC-active periods, an anomalous cyclone over the WNP enhances southeasterly and reduces southwesterly moisture transport to Henan. Accordingly, compared to TC-inactive periods, moisture contributions from the Pacific Ocean (PO), eastern China (EC), and the local area (Local) are significantly enhanced by 48.32% (16.73% versus 11.28%), 20.42% (9.44% versus 7.84%), and 2.89% (4.91% versus 4.77%), respectively, while moisture contributions from the Indian Ocean (IO), Southwestern China (SWC), Eurasia (EA), and the South China Sea (SCS) are significantly reduced by −31.90% (8.61% versus 12.64%), −16.27% (4.60% versus 5.50%), −8.81% (19.10% versus 20.95%), and −6.92% (12.18% versus 13.09%). Furthermore, the moisture transport for a catastrophic extreme rainfall event during 17–22 July (“21⋅7” event) influenced by Typhoon Infa is investigated. Compared to the mean state during TC-active periods, the moisture contribution from the PO was substantially increased by 126.32% (37.87% versus 16.73%), while that from IO significantly decreased by −98.26% (0.15% versus 8.61%) during the “21⋅7” event. Analyses with a bootstrap resampling method show that moisture contributions from the PO fall outside the +6σ range, for both the TC-active and TC-inactive probability distributions. Thus, the “21⋅7” event is rare and extreme in terms of the moisture contribution from the PO, with the occurrence probability being less than 1 in 1 million times.
Significance Statement
Henan, one of the most populated provinces in China, experienced a catastrophic extreme precipitation event in July 2021 (the “21⋅7” event), coinciding with the activity of a tropical cyclone (TC) over the western North Pacific, which helps establish the moisture channel. Using a Lagrangian model, we provide a better understanding of how moisture transport changes associated with TC for the mean state of 1979–2021, and reveal how extreme is the moisture transport for the “21⋅7” event with the bootstrap technique. It is found that during active TC periods, the moisture contribution from the Pacific Ocean (the Indian Ocean) is significantly enhanced (reduced). For every 1 000 000 six-day events, less than one instance like the “21⋅7” event should be expected.

Abstract: Two catastrophic extreme precipitation events in July 2021 and August 1975 caused tremendous damages and deaths in Henan, one of the most populated provinces in China. Revealing the relationship between large-scale circulation patterns and precipitation extremes is vital for understanding the physical mechanisms and providing potential value for improving prediction and hence reducing impacts. Here, nine large-scale circulation patterns are identified for July-August using the self-organizing map. We find daily precipitation extremes under the fifth pattern (P5), characterized with the strongest easterly wind anomalies in Henan, feature the highest frequency and the largest intensity. Seven out of total 11 days in the two catastrophic extreme precipitation events belong to P5, and the top two maximum hourly precipitation extremes over continental China occurred under P5. The larger intensity of precipitation extremes is attributed to the dynamical contribution, suggesting more-intense precipitation extremes under P5 are largely dominated by stronger ascending motions.