Fine characteristics of the July 2023 extreme rainfall in North China and associated synoptic weather patterns

From 29 July to 1 August 2023, Beijing-Tianjin-Hebei region was struck by extremely heavy rain, which resulted in severe flooding, huge economic losses and considerable casualties. Based on minute-interval precipitation observations, wind profiler radar observations, raindrop spectrum observations, and the ERA5 reanalysis product, detailed characteristics and synoptic weather causes of the July 2023 extreme rainstorm in North China are analyzed and revealed in this paper. The results show that: (1) The precipitation process presented a significant extremity and regional differences. In the mountainous areas, precipitation was relatively stable and persistent, and the raindrop spectrum was similar to that of marine type, with high rain droplet number concentration and small droplet diameter. Over the eastern plain areas dominated by a lower troposphere southeasterly jet stream, however, active mesoscale convective rain bands appeared, and the raindrop spectrum was close to that of continental type with short-term and highly convective rainfall. (2) Under the rare stable weather pattern of "high in the north and low in the south" and "low in the west and high in the east" in 500 hPa geopotential height, a stable inverted trough formed between the topographic barrier flow on the north side of the remnant vortex of typhoon Doksuri and the low-level southeasterly jet stream, leading to continuous convergence and updrafts. Water vapor transport from both the South China Sea and the north of typhoon Khanun over the East China Sea converged in the upper reaches of North China, sustaining the extreme rainstorm. (3) In the phased rainfall development process, the typhoon remnant vortex, the inverted trough, the low-level southeasterly jet, and warm shear lines modulated the organization and development of the mesoscale convective systems and affected the rainfall location and intensity. On 31 July, a mesoscale easterly jet core exceeding 22 m/s occurred in the lower boundary layer and significantly enhanced convective precipitation, which reached 100 mm/h in the west of Beijing. Nonetheless, more refined attributes of various weather systems and the role of diurnal variation as well as predictability of such an extreme rainstorm need to be further studied.