Evolution of the Upper-Level Jet and Sub-jet and Their Relationships to the "21·7" Henan Rainstorm in 2021

The hourly precipitation data provided by the China Meteorological Administration, and the hourly 0.25^o(latitude)×0.25^o(longitude) reanalysis data provided by the European Centre for Medium-Range Weather Forecasts (ECMWF) were used to investigate the key weather systems that generated the extreme hourly precipitation in Zhengzhou, Henan Province ("21 · 7" torrential rainfall event during 17−22 Jul 2021), and to diagnose the kinetic energy budget of the Upper-Level Jet (ULJ) and sub jet. It was found that, the "21·7" torrential rainfall event in Zhengzhou showed significant phased characteristics. The heavy rainfall mainly occurred in the mountainous areas in the north of Henan during the period of the extreme hourly precipitation in Zhengzhou and around the windward slope of the mountainous areas, which were notably affected by the terrain features. Located over Henan Province, the high-pressure ridge in the upper troposphere, the low-pressure trough in the middle troposphere, the horizontal shear line and the mesoscale convective vortex in the lower troposphere were the key weather systems that caused the extreme hourly precipitation in Zhengzhou. The high-level jet and sub-jet in the northeast of the South Asia high (SAH) caused the rapid development of the upper-tropospheric short-wave trough over Shaanxi Province through the strong cold advection. The baroclinic development of the shortwave trough made the warm advection in its front significantly enhanced, which led to the rapid development of the high-pressure ridge in the upper troposphere over Henan Province. This maintained the strong high-level divergence, which provided a very favorable background circulation condition for the occurrence of extreme hourly precipitation in Zhengzhou. The ULJ and sub-jet in the northeast of the SAH showed significant horizontal unevenness. Among them, the wind speed enhancement trend and the strength of the cold advection both reach maximum values at the bifurcation point of the ULJ (Region 1). This provides the most favorable conditions for the rapid intensification of the upper tropospheric shortwave trough over Shaanxi. The kinetic energy budget showed that the horizontal kinetic energy transport by the westerly wind in the northeast of the SAH acted as the most favorable factor for the development/maintenance of the ULJ and sub-jet in this region, with the kinetic energy mainly come from the ULJ area on the northern edge of the SAH. In this stage, the pressure gradient force mainly did negative work, which was the most unfavorable factor for the development/maintenance of the upper jet and sub jet.