Observational analysis of Mount Taihang’s Orographic Effects on the “23.7” Extreme Precipitation Event in North China

Based on high-resolution multi-source observational data, the topographic effects of the Taihang Mountains on intensity, centers and convective development characteristics of the extreme precipitation struck North China was analyzed. This long-lasting, widely impacting extreme precipitation event with high-intensity was primarily caused by the northward movement of the remnant circulation of Super Typhoon Doksuri, with the maximum cumulative precipitation up to 1003.4 mm. And the extreme rainfall area and the recording-broken daily precipitation were distributed on the upwind slope on the eastern side of Taihang Mountain. The peak of the cumulative precipitation occurred at the altitude of about 400 m on the upwind slope, and centers of accumulated precipitation above 600 mm were located on the inner corner of open-valley terrain composed of mesoscale mountains on the eastern side of Taihang Mountain. Due to the blocking effect of the northern continental high, the Western Pacific Subtropical High and the topography of the Taihang Mountains and Shanxi Plateau, the residual vortex of Doksuri moved slowly since July 29. The Beijing-Tianjin-Hebei region has been dominated by convergence induced by the inverted-trough on the north side of the vortex and the topographical blocking, forming synoptic uplifting. Low-level easterly jet formed between the typhoon residual vortex and the high pressure barrier. As a result, water vapor flux climbed from the boundary layer over the plain to the upslope of the Taihang Mountains. The convergence center of water vapor flux and the upward movement occurred near 950-925hPa on the mid-slope, which was conducive to the formation of precipitation peak on the mid-slope. The fine terrain of the Taihang Mountains took significant impact on the occurrence and development of the Mesoscale Convective System (MCS). On the night of July 29, the MCS approaching to the upwind slope was significantly enhanced, and under the northeast wind background in front of the mountain, a convergence center was formed on the inner side of the mesoscale trumpet shaped terrain, leading to sustained heavy precipitation. Because of the boundary organization of the cold pool on the east boundary of MCS, the initiation and development of the new convection led to short-term rainstorm in the piedmont plain. On the night of July 30, under the organization of the southerly boundary jet and the topographic vortex circulation, the locally developed meso-β scale linear MCS caused an extreme rainfall intensity exceeding 100 . This study deepens the scientific understanding of the mechanism of extreme precipitation in North China and helps to provide a reference for future forecasting decisions.