Abstract:The boundary layer low-level jet can enhance the exchange of matter and energy, and is closely related to precipitation, air pollution and other issues. Studying the boundary layer low-level jet helps to improve the forecasting ability of air pollution and extreme weather. During the study of the boundary layer low-level jet in Beijing by using the Doppler wind lidar data from March 2018 to February 2019, a special jet was found. By using the numerical simulation method, this paper analyzes the formation mechanism of this phenomenon. The results are as follows. The jet intensity is about 6 m·s-1, appearing mainly from 02:00 BT to 09:00 BT. The thickness of the jet is only about 500 m, and the height of the jet axis is only 200-300 m, which is obviously lower than that of the classical boundary layer low-level jet. The vertical wind shear is obvious. The dominant wind inside the jet is northeast wind, while above the jet is southwest wind. In the wind direction conversion area, a weak wind speed zone is formed, wrapping the low-level jet, so it is called the weak wind speed zone boundary layer low-level jet (WBLLJ). Terrain forcing is the root cause for the formation of WBLLJ. Blocked by Taihang Mountains and Yanshan Mountains, under the joint action of night topographic cold discharge, a shallow northeast wind control zone, which is 130 km long, 10 km wide, and 600 m high， is formed in the plain area near the mountain. This wind zone is the location of the WBLLJ, and can also explain the cause of the low-level jet height. A large amount of cold air brought by the mountain wind at night wedges into the bottom of the plain and forms obvious topographic inversion. Under the action of the inversion layer, the atmospheric turbulent motion weakens rapidly, and the upper air flow is decoupled from the ground forming a low-level jet. With the thickening and southward expansion of the plain cold pool, the low-level jet continues to develop southward and upward. The low-level jet gradually weakens and dissipates with the dissipation of the inversion layer after sunrise. The WBLLJ plays an important role in the diffusion of atmospheric bottom-layer pollutants and the evolution of urban heat island in Beijing.