Observation And Simulation Of Boundary Layer Low-level Jet In Beijing Urban Zone

Boundary layer low-level jet plays an important role in the transportation of heat,water vapor and pollutants,and is closely related to strong convective weather such as heavy rain.Based on the data of sounding observed in 2016-2017 in Beijing,this paper selects four typical months of 1,4,7,and 10 to analyze the seasonal characteristics of the boundary layer low-level jet.Using the data of June,July and August to analyze the difference of summer jets and day and night characteristics,combined with the encryption data of sounding observed at Baoian,Chaoyang and Daxing sites on August 27-September 2,2016,the daily variation characteristics of the low-level jet is analyzed.The results show:(1)Boundary layer low-level jet has the higher frequency in spring,autumn and winter,and lowest in summer.The jet strength is strongest in winter and the weakest in summer.The jet wind in the winter is mostly northwesterly,and the southwesterly wind in summer.The velocity of the jet mostly distributed in the range of 6-18 m/s.The overall jet axis height distribution exhibits a bimodal structure,the height of the autumn and winter is much higher than that of spring and summer.The LLJ in the summer has obvious diurnal variation characteristics.The daytime frequency is low and the intensity is small.The frequency is high at night and the intensity is high.The wind speed profile is closely related to the vertical distribution characteristics of temperature and wind direction.When the jet occurs,there is more hanging inversion,and there is a good correlation between the hanging inversion and the minimum wind speed.When the jet occurs,the wind direction changes frequently.The height of the wind direction change has a good correlation with the height of the minimum wind speed.(2)There are significant diurnal variations in the urban boundary layer low-level jet of Beijing urban zone.The boundary layer low-level jet in the city and suburbs has a large difference.The frequency of jet in the city is lower than suburbs,the intensity in urban is smaller,and the jet height in urban is higher.During the development of the boundary layer low-level jet in the nighttime,the jet wind vector rotates clockwise,and the inertial oscillation is an important factor affecting the formation of the boundary layer low-level jet.(3)WRF could simulate the boundary layer low-level jet phenomenon in Beijing well.The wind speed distribution in different heights in Beijing shows jet sample.The area where the jet occurs is mostly located in the plain area beside the mountain.Combined with the vertical characteristics of wind speed components,Combined with the vertical characteristics of the wind speed component,it is considered that the formation of the boundary layer low-level jet in Beijing is significantly affected by the terrain.(4)The terrain significantly affects the formation of the jet in Beijing.The ocean is conducive to the development and maintenance of the boundary layer low-level jet in Beijing.The urban underlying surface raises the height of the boundary layer low-level and reduces the intensity of the jet.The difference of terrain,land-sea thermal has a greater impact on the jet current than the urban underlying surface,and the terrain factor is the main influencing factors.