Impact of Double Low-Level Jets on the Extreme Rainstormin Henan Province in July 2021

The impact of the double low-level jets on the extreme rainstorm in Henan Province during 19–22 July 2021 is analyzed using precipitation data from the national automatic weather station of China and the ERA-5 reanalysis data from the European Centre for Medium-Range Weather Forecasts (ECMWF). The analysis shows that the SLLJ (synoptic-system-related low-level jet) moved from the southwest to the northeast with the development of the vortex, with the wind direction changing from southeast to southerly. The direction of the BLJ (boundary layer jet) changed from easterly to southeast and had a diurnal variation characterized by strong night-time and weak daytime intensities. There were two types of coupling modes during the precipitation process. During the first stage (from 2000 BJT 19 July to 1800 BJT 20 July), the coupling mode was that of the E-BLJ (easterly BLJ) and the SE-SLLJ (southeast SLLJ). During the second stage (from 1800 BJT 20 July to 0800 BJT 22 July), the coupling mode was that of the SE-BLJ (southeast BLJ) and the S-SLLJ (southerly SLLJ). The precipitation occurred in the outlet of BLJ and the inlet of SLLJ. When E-BLJ and SE-SLLJ were coupled, the dynamic uplift of the strong convergence superimposed mountains at the outlet of E-BLJ converging with the lower troposphere in the precipitation zone, and the inlet of SLLJ and synoptic scale vortex system diverged the middle troposphere, providing a strong uplift trigger mechanism for the development of mesoscale convection. Owing to the small range and weak intensity of E-BLJ, the coupling between E-BLJ and SE-SLLJ was relatively weak, and the precipitation was significantly affected by SLLJ. When SE-BLJ and S-SLLJ were coupled, the convergence of the outlet of SE-BLJ and the divergence of the inlet of S-SLLJ enhanced the ascending motion of the lower troposphere, while the convergence of mesoscale in the middle troposphere and the divergence on the right side of the inlet of upper-level jet enhanced the ascending motion of the upper troposphere. Under the joint influence of the double low-level jets, the intensity and height of the mesoscale convective system developed strongly, promoting heavy precipitation. Under the two coupling modes, the water vapor transport of the double low-level jets played an important role in heavy rainfall. Although the water vapor transport of SLLJ in the first stage was obvious, the role of BLJ in the second stage was more important.