Water Vapor Transport Patterns and Dominant Causal Factors of the "23·7" Extreme Rainstorm in Hebei Province

From July 29 to August 2, 2023, an extreme rainstorm event (dubbed "23·7") struck North China, China, triggering extensive flooding and secondary hazards. To investigate the water vapor transport characteristics and key driving mechanisms of this event, this study employed a multi-data approach, including observations from surface meteorological stations, ERA5 reanalysis data, S-band Doppler radar data, and simulations from the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The key findings are as follows: (1) At the lower troposphere (850 hPa), intense water vapor fluxes originating from the Western Pacific and the Bay of Bengal converged over North China, forming a prominent water vapor convergence center that supplied sufficient moisture for the rainstorm. (2) At the upper troposphere (250 hPa), the southwesterly flow peripherally associated with the subtropical high intensified water vapor divergence. This upper-level divergence, in conjunction with the lower-level convergence zone, enhanced vertical water vapor transport and extended the duration of heavy rainfall. (3) Radar reflectivity profile analysis revealed that the cloud top height of convective systems exceeded 18 km during the rainstorm, indicating that strong vertical ascending motion was critical for sustaining extreme precipitation. (4) HYSPLIT trajectory simulations confirmed that the primary water vapor sources were the Western Pacific, the South China Sea, and the Bay of Bengal. Collectively, this study advances the understanding of extreme precipitation formation mechanisms in North China and provides scientific insights to improve numerical weather prediction models and strengthen early warning systems for extreme weather events.