Multi-timescale water vapor transport for an extraordinary rainstorm in Zhengzhou, China, impacted by remote tropical cyclones on July 20, 2021

An extraordinary tropical cyclone-remote rainstorm with a 24-hour precipitation amount of 624.1 mm occurred in Zhengzhou, China, on July 20, 2021, during which a severe hourly precipitation with the amount of 201.9 mm at 1700 LST caused significant economic losses and casualties. Observational analysis and the backward trajectory model showed that low-level water vapor for this extraordinary rainstorm was transported by the southeasterly jet below 900 hPa from the intensifying Typhoon In-Fa in the western North Pacific (low-level southeasterly channel). Although the southerly flow between 900 and 800 hPa brought the water vapor from the developing Typhoon Cempaka in the South China Sea (low-level southerly channel), it did not converge over Zhengzhou. Further multi-timescale analysis showed that these two low-level water vapor channels both exhibited a feature of an intraseasonal timescale. Typhoon In-Fa supplied abundant water vapor to Zhengzhou through a low-level southeasterly channel in the northern part of the monsoon trough on a timescale longer than 30 days. The low-level southerly channel was associated with an anticyclonic circulation in a wave train on a 10–20-day timescale to the southeast of Henan Province, transporting water vapor from Typhoons In-Fa and Cempaka. However, the water vapor in the low-level southerly channel did not converge over Zhengzhou; Thus, Typhoon In-Fa as the water vapor source and the low-level southeasterly flow on the timescale longer than 30 days below 900 hPa as the water vapor channel played an essential role in water vapor transport during the extraordinary rainstorm on July 20.