A Numerical Simulation on the Impacts of the Offshore Typhoons on Water Vapor Flux, Dynamic and Thermal Conditions of the Extreme Rainstorm Event in Henan Province in July 2021

This study uses the WRF model to numerically simulate the influence of offshore typhoon In-Fa and typhoon Cempaka on the extreme rainstorm process in Henan on 19–21 July 2021. The control (CTL) experiment reasonably captures the tracks and intensities of the two typhoons, the large-scale circulation pattern, the intensity, and spatial distribution pattern of the rainstorm event in Henan, basically reproducing the extreme rainstorm process in Henan. In addition, the sensitivity experiments indicate that after the removal of typhoon In-Fa, the subtropical high extends southward and forms the southeast wind jet in the south, causing the south (east) wind component to obtain weakened (strengthened) around Henan. The water vapor transport in the zonal direction becomes dominant, which is conducive to the transition of the rainfall distribution from the south–north orientation in CTL to the east–west orientation. On the other hand, given that the southeast wind at the low level is weaker than the easterly jet before the removal of typhoon In-Fa, the local convergence at the Henan rainfall area is weakened, and the net water vapor flux is reduced by 5.81% compared with that in CTL experiment. The slowdown in the westward movement of the mid-latitude cold air causes the reduction of the local equivalent potential temperature gradient. Therefore, the rainfall intensity in the removal of typhoon In-Fa experiment is relatively weaker than that in CTL. After the removal of typhoon Cempaka, the large-scale circulation characteristics are almost unaffected, and the water vapor transport on the south side of Henan is slightly weakened. Therefore, the distribution of heavy rainfall is similar to that in CTL, with a slight decrease in rainfall peak. Compared with typhoon In-Fa, typhoon Cempaka has less effect on the rainstorm event in Henan.