Abstract:With multi-source observations and ERA5 reanalysis data, this paper systematically analyzes an extreme torrential rain event that occurred in North China in late July 2025, and reveals the diversity in the initiation, organization and propagation mechanisms of mesoscale convective systems during three days with extreme rainfall (24, 25 and 27 July) under a similar large-scale warm-sector background. Besides, the synoptic causes for the extreme torrential rain are investigated. The results show that this extreme rainfall event was characterized by significant extremity with heavy rainfall intensity and large accumulated precipitation amount. The multiple extreme precipitation centers within the southern warm-sector rainband posed big challenges to the forecasting operation. On 24 July, the convection initiated at the foothills, and the extreme rainfall was dominated by a “train effect” from backward-propagating and band-shaped convection, which resulted in the most widespread rainfall area spanning mountains, foothills and plains. On 25 July, the convection originated in mountains, and the extreme rainfall was associated with a quasi-stationary system, leading to the localized precipitation. On July 27, the convection mostly initiated in plains and intensified after moving to the foothills. The extreme rainfall was related to the “train effect” from line-shaped convections, and the heavy precipitation occurred mainly in mountainous areas. On 24 July, the synoptic scale forcing was most significant. The strong and deep low-level jet (LLJ), coupled with orographic lifting and sustained moisture transport, played a key role in the initiation and back-propagation of the convection. The convergent frontal zone formed by the outflow of the cold pool had a very important role in organizing the rainfall system and its expansion into plains. On 25 July, under the control of the subtropical high, a weaker and shallower LLJ restricted the downstream propagation of the convective system, making orographic lifting the dominant mechanism for the triggering and enhancement of heavy precipitation. This was accompanied by a mesocyclone-like structure that produced the heaviest local hourly rainfall. On 27 July, under the weakest synoptic scale forcing, the convection was triggered by boundary layer easterly airflow and low-level weak convergence line. The mesoscale front area formed by the cold pool in mountainous areas and the warm ridge in plains provided favorable conditions for backward propagation. Moreover, the convergence and lifting of the southwest airflow in front of the terrain built powerful dynamic conditions for the significant intensification of the convection moving from plains into foothills.