Abstract:Using refined hourly meteorological observations, radar data, and other multi-source observation data, along with ERA5 reanalysis data, this paper analyzes the characteristics and causes of the ultra-long-duration regional extreme torrential rain event that occurred in Beijing from 23 to 29 July 2025. The results indicate that this event was the longest-lasting heavy precipitation event in the past decade, and the number of stations with daily precipitation ≥50 mm and the daily maximum precipitation during the event ranked the second after the extreme torrential rain event on 30 July 2023. This event also exhibited significant nocturnal rainfall characteristics, with the maximum hourly rainfall intensity concentrated in the period from midnight to early morning hours. The areas of heavy precipitation were primarily distributed over the plains in front of the Yanshan Mountains and on the windward slopes. This torrential rain event was mainly influenced by the subtropical high system, low-latitude tropical weather systems, and mid-to-high-latitude westerly troughs. The analysis of the geopotential height anomaly field from July 23 to 28 reveals that the subtropical high was positioned more northward than usual. The tropical low-pressure system overlapped with the abnormally northward-shifted peripheral airflow of the subtropical high, forming a sustained southerly airflow that was transported northward to the Beijing Region, which provided abundant moisture for the extreme precipitation. The moisture content was significantly higher than the climatological average, with areas of high specific humidity overlapping with southerly wind anomalies. Coupled with the orographic uplift effect of the Yanshan Mountains, these factors significantly enhanced precipitation intensity. A prolonged high-humidity environment was maintained below 500 hPa throughout this event, with the average specific humidity at 925 hPa (14.3 g·kg-1), 850 hPa (12.2 g·kg-1), and 700 hPa (7.4 g·kg-1) exceeding the average values recorded in torrential rain events over the past decade (based on statistics from the Beijing Sounding Station). Under the daytime conditions of high temperature and humidity, unstable energy was accumulated. At night, the enhanced low-level jet, combined with terrain lifting, contributed to the pronounced nocturnal rainfall characteristics of this torrential rain event. The major impact systems of the extreme torrential rain over the night of 26 July could not be identified with the conventional data including ERA5 reanalysis data, but the radar imagery clearly reveals that the main impact systems of the extreme torrential rain were the low-level convergence line and the small-scale low-level jet.