The relationship between diurnal variations of precipitation and boundary layer winds over the Dabie Mountains during May—July

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.