北京市秋季大气边界层结构与特征分析

利用2000年9-10月北京市西部城、郊3个站点的探空资料,分析该地区秋季边界层高度的平均变化规律,探讨边界层内温度、风向、风速的廓线性质和结构特征,了解其时空变化和差异.结果表明,北京市秋季白天边界层最大高度平均约1 km,夜间稳定层高度在200～400 m之间.晴朗天气里,白天城区边界层温度高于郊区,反映出城市热岛的作用,夜间热岛现象不明显.较低的边界层高度和其顶部经常维持的强稳定层(强逆位温层)阻断上下层流动的关联,有利于近地面大气维持静稳状态和该季节雾、霾天气的出现.夜间约400 m以下气层的流动大体分为3种形态,即整层小风型、低空急流型和贴地风速极大型.温度层结与夜间流动结构有密切对应关系.

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.