Structures and Characteristics of the Atmospheric Boundary Layer over Beijing Area in Autumn

Using air sounding data from 3 stations in the western urban and suburban districts of Beijing in September and October, 2000, the authors study the diurnal variation of the atmosphere boundary layer height,and analyze the vertical structures and characteristics of temperatures, wind directions and wind speeds in the boundary layers. Differences of the variables between urban and suburban districts are compared. The results show that, in the autumn of Beijing, the maximum atmosphere boundary-layer height is about 1?km in daytime, and the nocturnal stable boundary layer height is between 200 and 400?m. For clear days, daytime temperatures in urban area are higher than those in suburban districts, indicating the effect of urban thermal-island. However, temperature difference at night is not obvious. A lower boundary layer, caused by strong temperature inversion over it, leads to weak vertical exchange between the boundary layer and upper layers. It thus favors the formation of fogs and haze. According to the vertical wind profiles, three types of nocturnal flows below 400?m could be identified, they are (i) weak winds in the whole layer; (ii) lower-level jet; and (iii) with maximum of wind speed near surface. All these flows are closely related to different thermal stratifications.