Indirect Effects of Binary Typhoons on an Extreme Rainfall Event in Henan Province, China From 19 to 21 July 2021: 2. Numerical Study

An extreme precipitation event over Henan province, China from 19 to 21 July 2021 led to flood disasters in this region and widespread concern about the subsequent loss of life and livelihoods. We conducted numerical simulations to examine the impacts of typhoons In-fa (2021) and Cempaka (2021) on this extreme rainfall event. The control simulation reasonably reproduced the motion of typhoons In-fa and Cempaka and the associated distribution and amount of extreme rainfall. Sensitivity experiments were conducted in which typhoon In-fa was artificially moved in both northerly and westerly directions in the initial conditions. The results indicated that the southerly flow between typhoon Cempaka and Henan, which determined the structure and distribution of the extreme rainfall event, was sensitive to the motion of typhoon In-fa. Numerical experiments that removed typhoon Cempaka confirmed that both the movement of In-fa and its interaction with Cempaka were closely associated with southerly flows and had a significant effect on the extreme precipitation event. In the absence of typhoon Cempaka, although typhoon In-fa still had a remote effect on precipitation, the effect was much smaller than in the presence of Cempaka. Our simulation of this extreme rainfall event in Henan and the associated sensitivity experiments are consistent with the results of previous studies of multiple tropical cyclones, which showed that interactions among multiple tropical cyclones can lead to changes in their outer circulation that affect extreme precipitation events.