Preliminary Analysis of Fine-Scale Characteristics and Formation Cause of Beijing "25·7" Extreme Rainstorm

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 data from Automatic Weather Stations in the Beijing-Tianjin-Hebei region, multi-band networked radars, Beijing Doppler weather radar, ERA5 reanalysis, Beijing RISE 100-metre resolution gridded analysed wind field, and Beijing RMAPS- NOW 1km resolution gridded data, this paper analyses the characteristics of rainfall stages, fine-scale convective features, and the causes of extreme heavy rainfall. The results showing: (1) The '25.7' heavy rain event was characterised by a long duration, large cumulative rainfall, concentrated heavy precipitation in the northern mountainous areas often occurring at night, local and extreme characteristics, and severe disaster impacts. The rainfall in Beijing in late July this year was 203.8% higher than the average for the same period in previous years, ranking fifth in history. (2) This torrential rain occurred in late July against a relatively stable large-scale circulation background, with the Subtropical-High （SH） pressure significantly located westward and northward and stronger than usual. Tropical systems were active on the southern side of the SH, and the easterly winds on the northern side of the typhoon strengthened, facilitating the transport of water vapour inland. The southerly moisture flux on the Beijing-Tianjin-Hebei Plain showed significant moisture flux convergence at the front, with precipitable water exceeding 70 mm and low-level warmth anomalies, providing a high-temperature, high-humidity, and high-CAPE environment for the extreme rainfall. (3) The rainfall in Beijing lasted for 147 hours, with four stages corresponding to the periphery of the SH, a significant northward shift, a slight southward retreat, and the influence of the northern upper-air trough and cold air. The first and fourth stages had widespread heavy rain, but the overall intensity was not strong; the second and third stages had rainfall that was weak in many areas but strong in spots, with locally extreme cumulative rainfall, and high disaster potential. On the 26th, heavy rain mainly occurred at altitudes of 200–600 m, whereas on the 27th, heavy rain mostly occurred below 300 m. (4) On the nights of the 26th and 27th, the windward slopes in the northern mountainous areas experienced near-surface convergence of southerly and south-easterly winds, and were located at the exit region of a locally enhanced low-level jet (LLJ). This resulted in strong convergence and vertical upward motion, favourable for the continuous triggering of backbuilding convection near the foothills, creating a training effect. The strongest convergence at the LLJ exit caused extreme rainfall. The convective latent heat release in the upper cloud significantly enhanced the vertical upward motion to over 10 m, providing a positive feedback for the occurrence of extreme rainfall.