Analysis and research on the impact of terrain on the "23.7" extremely heavy rainstorm

From 08:00 on July 29 to 08:00 on August 2, 2023, under the influence of typhoon "Dussuri", an extremely heavy rainstorm process occurs in Hebei and Beijing. The precipitation in some areas of the windward slope of Taihang Mountains exceeds 250mm, and in some areas it exceeds 500mm. The distribution of heavy precipitation is basically consistent with the terrain of the windward slope. Using the 6-minute radar retrieved wind field network data developed by the CMA (China meteorological administration) Meteorological Observation Center for analysis, it is found that from 13:00 on July 29th to 20:00 on August 1st, a southeast-oriented ultra-low-level jet greater than 12 m/s was maintained in the 925-hPa field over Hebei and Beijing. The angle between the jet and the Taihang Mountains is almost 90°, and at the same time, a 850-hPa typhoon trough stays on the windward slope for a long time, resulting in stable and less movement of heavy precipitation echoes. This series of factors together led to the occurrence of the extremely heavy rainstorm process. Using the ERA5 hourly reanalysis data as the initial field and based on the WRF4.5 model, a sensitivity test is conducted on this process using three-layer bidirectional nesting (grid spacing of 9km, 3km, and 1km, respectively). The experiment reduces the Yanshan and Taihang Mountains to half of their original heights and 50 meters, respectively (equivalent to the altitude of Beijing). The experimental results indicate that: (1) Precipitation impact: Due to the easterly winds brought by typhoons, the eastern side of Taihang Mountains is on the windward slope, which has a significant impact on precipitation. When the height of Taihang Mountains decreases, the precipitation intensity significantly weakens; When the terrain height drops to 50m, the precipitation location is biased to the west compared to the actual situation. (2) The experiment showed that the blocking effect of Taihang Mountains formed mesoscale low vortex and convergence line on the windward slope. When the height of Taihang Mountains drops to half of its original height or only 50 meters, the mesoscale low vortex and convergence line move westward to Shaanxi Province. (3) The vertical profile analysis along the east-west direction of Taihang Mountains shows strong upward movement in the windward slope area, with positive vorticity in the lower level and negative vorticity in the upper level. When the height of Taihang Mountains decreases, the upward movement significantly weakens, and the positive and negative vorticity weakens until it disappears, indicating that the dynamic effect of terrain has a significant impact on precipitation processes. (4) The Yanshan Mountains are oriented east-west, and parallel to the environmental winds. Therefore, when its height decreases, its impact on physical quantities such as precipitation, wind field, vertical velocity, and vorticity is relatively small.

Key words: terrain, "23.7" extremely heavy rainstorm, analysis