Remote Effects of double Typhoons on Record-breaking Rainfall: A case Study in North China

During 18-22 July 2021, an extreme precipitation event occurred in Henan, China, with maximum daily precipitation reached 657 mm, broking their historical records and resulting in severe disaster. This heavy rainstorm occurred with the background of two typhoon (In-fa and Cempaka) activities, but resent studies have only considered the effects of individual typhoons, ignoring their combined effects. The Dynamic Recycling Model and Weather Research and Forecasting Model were applied to investigate the contributions of two typhoons on water vapor transportation. Two transportation paths were determined: the eastern path for In-fa and the southwestern path for Cempaka. These paths contributed up to 78% of the total water vapor over Henan. Two typhoons jointly led to extreme precipitation, and Cempaka showed greater impact than In-fa on the vapor transportation. The precipitation was 39.0% of that in the control test when only Cempaka existed and 34.4% when only considering In-fa. The sensitivity results also indicate that Cempaka mainly induced the early stage of precipitation, while Infa controlled the later stage. In addition to the effect of water vapor transportation, In-fa also influenced the local atmospheric stability. The pseudo equivalent potential temperature near the surface was 6 K higher when only In-fa existed relative to the scenario in which no typhoon was considered, indicating that In-fa strengthened the convective instability. Therefore, both Cempaka and In-fa contributed to this extreme precipitation event. Cempaka transferred more water vapor than In-fa, and In-fa acted to increase the instability. Their joint contribution amplified the precipitation intensity beyond the cumulative impact of individual effects.