Characteristics and Evolutions of the Atmospheric River in the “23.7” Extreme Rainstorm Event in Hebei Province of China in July 2023

Abstract An extreme rainstorm occurred in Hebei Province, China, from 29 July to 2 August 2023. In this study, the rainfall observation data and reanalysis data are used to diagnose the evolution characteristics of the atmospheric river (AR) in this rainstorm case. The water vapor (WV) sources and transport paths are also analyzed by using the Lagrangian trajectory tracking model (viz., the Flexible Particle dispersion model) and the Weather Research and Forecasting (WRF) Model. The results indicate that the low-latitude AR exists throughout this rainstorm process, and it is mainly located in the Arabian Sea, the Bay of Bengal, and the South China Sea. This AR event in Hebei is the result of the circulations of Typhoon Doksuri and Typhoon Khanun transporting WV to Hebei. The AR develops most vigorously below 500 hPa and is often accompanied by a strong low-level jet. The convergence in the left front of its exit region, along with the divergence south of the upper-level westerly jet entrance, enhances the dynamic conditions that sustain the low-level shear line. Combined with the topographic uplifting effect from the Taihang and Yanshan Mountains, the coupling of upper-level jet and low-level jet provides a good circulation background for the ascending motion over the precipitation area. The WV source tracking shows that the target air parcels that substantially contribute to the heavy rainfall in Hebei come from Typhoons Doksuri and Khanun, which greatly strengthen the AR intensity and prolong the AR path. The sensitivity experiments based on the WRF Model further verify the extension of the AR by Typhoons Doksuri and Khanun, as well as the fact that the AR is blocked by the Taihang and Yanshan Mountains while moving northward, forcing the WV to uplift and thus triggering the extreme rainstorm.