城市粗糙子层湍流能谱特征分析

Characteristics of turbulence spectra in the urban roughness layer

对南京市委党校两个不同高度层湍流速度谱特性分析表明:(1)近中性情况下,两层湍流动能密度均大于平坦草地上结果,并且低层湍流动能密度大于高层,高层的湍流比低层的湍流更接近局地各向同性,说明城市粗糙子层的湍流生成率要高于草地上的结果,并且越靠近冠层顶部,湍流的生成率越高,湍流越偏离局地各向同性.两层都有约20%的能谱出现了谱隙,湍流次尺度的平均量级与粗糙元的平均尺度比较接近,城市粗糙子层中粗糙元效应比较明显.(2)两层湍流动能耗散率随稳定度变化,中性情况下,粗糙子层中两层湍流动能耗散率均大于平坦草原上的结果,并且低层的值要高于高层;不稳定情况下,粗糙子层中湍流动能耗散率随不稳定度增加的增长率要高于草地上结果,稳定情况下,两者相差不大.

The field observations for turbulence characteristics in the urban roughness sublayer were conducted on the meteorological tower in Nanjing Municipal Party School.The analysis of power spectra of wind speed at the two levels shows that：（1）In near neutral conditions,the two levels’turbulence kinetic energy density is larger than that near homogeneous grass land,and the value at the lower level is larger than that at the higher level.The turbulence at the lower level is more anisotropic than that at the higher level.The production of the turbulence is stronger over the urban canopy than that over the homogeneous grass land,and the value at the lower level is also larger than that at the higher level.About 20% of the spectra has a spectra gap.The mean scale of the secondary spectra peaks is similar with the scale of the roughness elements,which means that the individual element has prominent effect on turbulence in urban roughness sublayer.（2 ）In near neutral conditions,turbulence kinetic energy dissipation rate at the two levels are higher than the result over the homogeneous grass land.As to the two levels’results,turbulence kinetic energy dissipation rate is higher at low level.In unstable conditions,the turbulence kinetic energy dissipation rate grows faster with increasing instability than the result observed over the homogeneous grass land.In stable con-ditions,the growing rates of turbulence kinetic energy dissipation rate with increasing stability over the two kind surfaces are similar.