The Critical Role of the Upper-Level Synoptic Disturbance on the China Henan “21.7” Extreme Precipitation Event

Abstract

On 18–20 July 2021, Henan Province in China experienced a historically rare extreme precipitation event, termed as the “21.7” event. Its synoptic environment was characterized by a large amount of moisture supply by binary typhoons located over the ocean and a potential vorticity intrusion in the upper level. The present study examines the importance of the latter by conducting WRF model experiments. A qualitatively similar rainfall amount to observation is obtained when the zonal wavenumber 7 and larger is kept above 300 hPa in the initial and lateral boundary conditions. When only the large-scale disturbances with wavenumbers 2–4 are kept, the precipitation is greatly reduced. This result indicates that the upper-level synoptic-scale disturbance, which leads to the development of potential vorticity anomaly and its downward intrusion, has likely played a critical role in the development of this event along with a large amount of moisture transport in the low level.