ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Study on a Boundary Low-Level Jet phenomenon in Beijing
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Abstract:
The boundary layer low-level jet plays an important role in the exchange of material and energy, and is closely related to precipitation, air pollution and other issues. Studying the boundary layer low-level jet helps to solve related problems and improve the quality of human life. During the study of the boundary layer low-level jet in Beijing by using the Doppler ladar data from March 2018 to February 2019, a special jet was found. This paper uses numerical simulation method to study the formation mechanism of this phenomenon. The results are as followed. The jet intensity is about 6 m·s-1, mainly from 02:00 to 09:00. 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 direction inside the jet is northeaster wind, while above the jet is southwest wind. The wind direction conversion area is also a weak wind speed area, which wraps the low-level jet, so it is called the weak wind speed zone boundary low-level jet (WBLLJ). Terrain forcing is the root cause of the formation of WBLLJ. Blocked by Taihang Mountains and Yanshan Mountains, under the joint action of night topographic cold discharge, a shallow northeaster wind control zone is formed in the plain area near the mountain, with 130 km length, 10 km width, and 600 m high. 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 will wedge into the bottom of the plain and form 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 to form 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 pollutants and the evolution of urban heat island in Beijing.
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