ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Differences in Convective Evolution Characteristics and Environmental Conditions of the “25·7” Extreme Warm-Sector Rainstorm over North China
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Abstract:
Based on multi-source observations and ERA5 reanalysis data, this study systematically analyzed an extreme rainfall event in North China in late July 2025. It revealed the diversity in the initiation, organization, and propagation mechanisms of Mesoscale Convective Systems (MCSs) during three extreme rainfall days (July 24, 25, and 27) under a similar large-scale warm sector background, and investigated their synoptic causes.The results indicate that: (1) The event was characterized by significant extremity with high rainfall intensity and large accumulation. The multiple extreme precipitation centers within the southern warm-sector rainband posed the main forecast challenge. (2) The MCS evolution varied significantly by day. On July 24, convection initiated at the foothills, and the extreme rainfall was dominated by a "training effect" from backward-propagating and band-shaped convection, resulting in the most widespread rainfall area spanning mountains, foothills, and plains. On July 25, convection originated in the mountains, and the extreme rainfall was associated with a quasi-stationary system, leading to the most localized precipitation. On July 27, convection mostly initiated in the plains and intensified after moving to the foothills, with extreme rainfall linked to a "training effect" from line-shaped convection.(3) The primary cause for this diversity lies in the varied interactions between the environmental field and complex terrain. On July 24, the strongest synoptic-scale forcing, characterized by a strong and deep Low-Level Jet (LLJ) coupled with orographic lift and sustained moisture transport, played a key role in convective initiation and back-propagation. On July 25, under the control of the subtropical high, a weaker and shallower LLJ limited downstream propagation, making orographic lift the dominant mechanism for triggering and enhancement, which was accompanied by a mesocyclone-like structure that produced the strongest local hourly rainfall. On July 27, under the weakest synoptic-scale forcing, convection was triggered by boundary layer easterlies and a weak low-level convergence line, while a mesoscale front and orographic lifting of southwesterly flow facilitated its intensification and maintenance.
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