Evolution of Fine Structure of a Meso-β-scale Convection System Responsible for Extreme Rainfall over North China

Extreme rainfall (ER) produced by convective storms moving from mountains to plain areas in North China has long been a great challenge in short-term forecasting, since the production of rainfall is closely related to the fine structure of convection. This type of ER event struck northern Beijing on 26 July 2025, causing widespread community concern. Based on multi-source observations and convection-permitting simulation, this study attempts to reveal the thermodynamic and microphysical structures in the meso-β-scale convection system (MCS-k) that produced the ER. Observational facts show that the weakening of cold outflow together with the southeast wind in the warm sector hindered the movement of the MCS-k after travelling downhill, and the more abundant water vapor and stronger convective instability in plain areas led to its back-building development. The content of liquid water significantly increased, and more large raindrops were produced by coalescence, indicating more intense warm-rain processes. Simulation experiments show that direct contribution of the warm-rain processes dominated the production of ER, since they provided 77.8% of the increase in sedimentation of rainwater. Although the ice-phase processes were intensified, most of the snow and graupel were transported out of the convection and did not directly contribute to the production of ER. Meanwhile, the latent heating of ice-phase processes played an important role. The sensitivity experiment excluding the latent heat from the production of snow and graupel showed significant weakening of the ascending motion and production of rainwater in the MCS-k, which decreased the 6-h rainfall amount in northern Beijing by 44%.