The relationship between the extreme heavy precipitation event in Zhengzhou, China during the summer of 2021 and zonal baroclinic Rossby wave packets

From July 17 to 22, 2021, Zhengzhou City and its surrounding areas in Henan Province, China, experienced the strongest precipitation event since the inception of modern meteorological records in the country. Using daily precipitation data from China Meteorological Administration stations spanning 1951–2024 and daily gridded reanalysis data from ERA5 over 1979–2024, we investigated the relationship between zonal baroclinic Rossby wave packets (RWPs) and this extreme heavy precipitation event in Zhengzhou. The results demonstrated that during the extreme precipitation event in Zhengzhou, no noticeable cyclone but cyclonic curvature of streamlines was observed at 850 hPa, while a “moisture tongue” provided rich water vapor necessary for the intense rainfall. The activities of the zonally baroclinic RWPs were closely linked to the onset and cessation of the heavy precipitation. Throughout the period of the extreme precipitation event, there consistently exhibited more pronounced daily activities of Rossby waves with wavenumbers 5–7 in the Eastern Hemisphere as compared to those in the Western Hemisphere. The RWPs influencing the extreme event originated in the upper atmosphere from Western Europe and propagated to the Zhengzhou area over a period of approximately five days. The zonal baroclinic RWPs at 300 hPa contributed to negative vorticity over Zhengzhou, facilitating the development and maintenance of extreme precipitation events there. These findings provide valuable insights into the mechanisms behind extreme heavy precipitation and offer useful clues to forecasting such events.