北京城市下垫面动力学参数的计算与分析

Estimation and analysis of aerodynamic parameters of Beijing

采用北京325 m铁塔2008—2012年的单层超声观测资料,基于莫宁-奥布霍夫相似理论(Monin-Obukhov similarity theory)和前人提出的最小误差分析方法,计算了铁塔周边下垫面的零平面位移高度和动力粗糙度长度。结果表明,由于铁塔位于北京市区,其周边下垫面呈现极其复杂的非均匀性,所以对应铁塔周边不同的扇区,零平面位移高度和动力粗糙度长度各有不同。平均而言,在2008—2012年间,铁塔周边下垫面的零平面位移高度为34.4 m,动力粗糙度长度为1.16 m。此外,综合前人的计算结果发现,铁塔周边的零平面位移高度和动力粗糙度长度在2001年之前呈显著增加的趋势,而在2001年以后并未增长,这一现象与铁塔周边的城市化进程相对应。

With the accelerated pace of urbanization in Beijing,the expansion of urban scale outside the Third Ring Road in twentieth Century has been extremely rapid.That which has been suburban non-uniform underlying sur-face for the past80years has now developed into rough complex urban areas under the underlying surface,and corresponding increases in urban buildings and population density have also resulted in a significant increase in the surface dynamics of urban land.Displacement height(<i)and surface roughness length(z0)can be used to characterize the physical properties of the surface,and their size can directly affect the exchange between the ground and atmosphere fluxes,which in turn affects the physical and chemical conditions of the entire atmosphere.However,for urban underlying surfaces,it is often difficult to determine d and z0precisely because of their significant non-uniform surfaces(e.g.different shapes of houses and trees of different heights).Martano(2000)proposed a method for calculating d.and z0based on monolayer observations.This method simplifies the mathematical problem of solving two-dimensional variables into one-dimensional variables by using the least squares method.Gao and Bian(2004)used this method and calculated the surrounding d and z0using the observa-tion data of the Beijing325m meteorological tower in2001,and obtained d=36.0m,z0=1.6m.However,these two values were obtained more than a decade ago,thus it was necessary to reassess the two values due to the de-velopment of the area.As a follow-up study of Gao and Bian(2004),the present paper also uses Martano^meth-od to calculate the d and z0around the tower by using the observation data of the325m meteorological tower in Beijing from January2008to March2012.The core purpose of this paper is to obtain the displacement height d and surface roughness length z0around the tower,which can then be applied to the atmospheric numerical model,and be used to minimize the error caused by the inaccuracy of these two parameters.The study results show that,due to the heterogeneity of the underlying surface,both displacement height d and surface roughness length z〇vary with the wind directions.From an average perspective,d=34.4m and zQ=l.16m are obtained as the repre-sentative values of the site for the years2008-2012.By comparing the results obtained in this study with those from previous studies,it is shown that the zero-plane displacement d and aerodynamic roughness length z0at this site had increased before2001,but have not since.Such a phenomenon is associated with the urbanization of the tower’s surrounding areas.