Abstract:
In this study, based on the Hailing Island wind profile radar observation data, precipitation data of Automatic Weather Station and ERA5 reanalysis data during the first rainy season of 2014-2016 in South China, we investigate the structural characteristics, diurnal variation characteristics and formation mechanism of low-level jets (LLJs) over the west coast of Guangdong Province. The effects of low-level boundary layer jets with different intensities on the temporal and spatial distribution of orographic precipitation in three key regions of Guangdong are also discussed. Results show that: (1) Based on the classification criteria of LLJs proposed in this study, the cumulative occurrence probability of LLJs is 21.2%. Most LLJs fall into the class 1-3 category, and class 4 is less common. 77.1% of the speed of LLJs are less than 14 m?s-1, 84.7% of the direction of LLJs are southwest. Most of the vertical shear of wind speed below the maximum wind speed of LLJs are between (5~25) ×10-3s-1. The height of the maximum wind speed of LLJs show a bimodal structure, and most of LLJs appear in the boundary layer within 1 km. (2) The synoptic-system-related LLJs (SLLJ) show a nocturnal peak, while the boundary layer jets (BLJ) show a double peak at midnight to the early morning and afternoon. The inertial oscillations triggered by the local-scale land-sea breeze circulation is responsible for nocturnal peak of BLJ. The pressure difference between the low latitude Asian continental low pressure and the Northwest Pacific Ocean high pressure near the surface, plays a key role in the formation of BLJs with different intensities. The development of continental low pressure during daytime is the main reason for the formation of afternoon peak of strong BLJ. (3) The influencing mechanism of BLJs on the distribution and intensity of orographic precipitation in Guangdong Province is complex. The orographic precipitation on the windward slope of large scale mountains dominates the inland of northern central Guangdong and coastal areas of eastern Guangdong. The stronger the BLJs, the stronger the orographic precipitation. The locations of the heavy orographic precipitation along the east coast of Guangdong are stable, while the inland orographic precipitation center in northern central Guangdong moves northwest with the intensification of BLJ. In the coastal area of west Guangdong, the windward slopes and the downwind slopes of the the medium and small-scale mountains, the lee-side wake convergence area of the terrain can produce obvious orographic precipitation. The heavy orographic precipitation must occurs under the background of suitable low-level inflow wind speed. (4) Under the influence of different intensity BLJ, the orographic precipitation in the inland regions of central and northern Guangdong shows a bimodal structure in the afternoon and in the morning. While the precipitation intensity of the afternoon peak is about twice that of the morning peak, the precipitation intensity of the bimodal peak increased with the strengthening of BLJ. As the intensity of BLJ increases, the diurnal bimodal structure of precipitation evolves into trimodal structure in the eastern coastal area of Guangdong Province. Under the influence of weak or strong jet stream, the precipitation in the west coast of Guangdong shows bimodal or trimodal structure, while under the influence of class 3 BLJ, it shows a single-peak structure at noon.