Based on ERA5 reanalysis and observation data from auto-weather stations, radars and lighting locators, the characteristics and connection between boundary layer winds and precipitation around the Dabie mountains (DBM) is studied by analyzing the characteristics of dual-peak precipitation in the afternoon and early morning in the middle and lower reaches of the Yangtze river basin from May to July, and the diurnal variations of Boundary Layer Jet (BLJ) frequency and convective environmental factors. It is confirmed that under the influence of radiative heating in the afternoon, the DBM area is dominated by convergent winds towards the mountain top, and precipitation and convection begin to strengthen significantly from noon to afternoon. Divergent winds flowing to mountain tops on both sides are dominated in the valley between the Dabie mountain and the Huang mountain (DHV). In the daytime, the southwesterly jet stream in DHV occurs less frequently, and precipitation is weaker than that on the top of the DBM. After 00:00 BT at night, the BLJ in the middle and lower reaches of the Yangtze river strengthens significantly, and from 04:00 BT to 05:00 BT in the morning, accelerated airflows from the Jianghan plain and the Poyang lake plain converge in the entrance area of the DHV. Meanwhile, the diurnal southwesterly wind perturbation in the DHV reaches its largest value in coincidence with the maximum occurrence frequency of the BLJ. At the same time, due to the nocturnal radiative cooling at mountain top, downslope wind perturbation is also close to its maximum value in surrounding areas of the Dabie mountains and Huang mountains, which results in cyclonic wind shear between the downslope winds from the DBM top and the accelerated BLJ in the DHV during late night to early morning. Positive vorticity forms at the adjacent district between the southern edge of the DBM and DHV and converges with water vapor flux. This region is also at the top of large nighttime convective available potential energy region with high water vapor flux, which is conducive to the triggering and bursting of convection, resulting in diurnal rainfall peak in early morning in the DHV area.

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.