What Controlled the Low-Level Moisture Transport during the Extreme Precipitation in Henan Province of China in July 2021?

Abstract A record-breaking precipitation event occurred in the Henan province of China in July 2021 (217HP). To identify the moisture source of the event, ensemble experiments with 120 members were conducted in this study. Results show that the precipitable water during this extreme event was primarily contributed by the low-level southeasterly (LLSE) water vapor transport. The LLSE was largely enhanced by the pressure gradient force maintained by the western Pacific subtropical high and further amplified by the latent heat release in the rainfall system over Henan. The positive moisture advection by the LLSE and evaporative water occurred below 950 hPa and was redistributed into higher levels by the LLSE jet-enhanced subgrid vertical turbulent transport. As a result, the combination of the enhanced LLSE centered around 950 hPa and the increase of moisture below 850 hPa were the main drivers for the continuous strengthening of LLSE moisture transport, with the former playing the dominant role. It is also found that not only the presence but also the intensity of the LLSE jet were important for reproducing the extreme rainfall. The impact of binary tropical cyclones In-Fa and Cempaka on the low-level moisture transport was also examined. We found that In-Fa (2021) presented an uncertain impact on 217HP, while Cempaka (2021) was found to be unfavorable for 217HP. Different from the LLSE water vapor transport, Cempaka mainly acted to weaken the southwesterly wind to the southwest of Henan by reducing the pressure gradient and impeding the water vapor transport toward Henan.