ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Preliminary Study on Fine-Scale Characteristics and Formation Mechanisms of the
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Abstract:
Abstract:From 23 to 29 July 2025 (referred to as "25.7"), extraordinary heavy rainfall occurred in Beijing and other North China areas. Based on multi-source observational data, ERA5 reanalysis, RISE, and RMAPS-NOW high-resolution grid analysis data, this paper analyses the characteristics of rainfall stages, the fine features and causes of extreme rainstorm convection. The results showing: (1) The "25.7" heavy rainfall occurred in late July under a stable circulation background where the Subtropical-High （SH） pressure was significantly shifted westward and northward and stronger than usual, with active tropical systems on its southern side. The four stages of rainfall corresponded to the periphery of the SH, the northward lift of the SH, the slight southward retreat of the SH, and the influence of an upper-level trough and cold air.The lower atmosphere had warm anomalies, with a positive 925hPa water vapour flux anomaly and a significantly enhanced water vapour convergence area in northern and eastern Beijing, creating conditions of high temperature, high humidity and high CAPE. (2) This heavy rain is characterised by its long duration, large cumulative rainfall, nocturnal occurrence, localised and extremity. Extreme heavy rain occurred in the second phase (night of the 26th) and the third phase (night of the 27th), concentrated in the northern mountainous areas, featuring a "weak overall but strong locally" pattern, accompanied by extreme hourly heavy rainfall. (3) The radar echoes in the second stage of precipitation showed obvious backward triggering and "training effect" characteristics, The second phase of precipitation echoes is characterised by rearward convection triggering and the "training effect", lasting 4-5 hours, with heavy rain occurring at altitudes of 200-600 m; while the third phase is characterised by continuous generation and dissipation of convection within banded echoes, lasting more than ten hours, with heavy rain mainly below 300 m altitude. (4) A conceptual model of the "25.7" extreme rainfall has been established. The exit area of the meso-scale Low Level Jet, the convergence of southerly and southeasterly winds in the the boundary layer , as well as the horn-shaped terrain and windward slopes of the mountains, resulted in local deep-level convergence and uplift, providing the favourable dynamic and topographic conditions for extreme rainfall in the second and third stages. In addition, the release of latent heat in the middle and upper layers of the atmosphere had a positive feedback effect on the intensification of rainfall. However, the extreme rainfall conditions between the second and three stages were also different: the former involved the warm region heavy rainfall at the edge of the SH, where the southern boundary-layer jet exit area formed a line of convergence along the northern shallow mountains, which was conducive to the triggering and stagnation of backward convection, and the extreme rainfall was caused by the "training effect"; the third stage was influenced by a weak cold air and was related to dynamic convergence on the southern side of a shear line and a meso-scale low vortex.
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Project Supported:
Open Project of the State Key Laboratory of Disaster Weather Science and Technology (2025KFA02), General Program of the National Natural Science Foundation of China (42375020), Youth Innovation Team of
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