城市近地层动量通量-梯度关系分析

Flux-gradient relationships for momentum in the urban surface layer

利用南京市委党校和常州大学白云校区的湍流和风速梯度连续观测资料,分析了处于粗糙子层中三个不同高度的动量通量-梯度关系φm(ζ),并与适用于均匀平坦下垫面的经典M-O相似关系进行了比较.结果表明:城市粗糙子层中动量通量不满足常值通量层假设,但无量纲风速梯度满足局地相似关系;在不稳定情况下,三层φm(ζ)的拟合结果均与经典相似函数相差很小;在稳定情况下,三层φm(ζ)的拟合关系仍为线性函数,系数βm均小于经典相似关系中的系数,φm(ζ)的拟合结果随高度增加趋近经典相似关系.临界理查森数Ric与βm关系为Ric=Prt/βm,粗糙子层的Ric大于均匀平坦下垫面上的值,说明越接近冠层顶机械湍流的生成率越大.

Continuous measurements of the momentum fluxes and wind gradients were carried out at two urban sites in Nanjing Municipal Party School and Baiyu Campus of Changzhou University respectively. The data at three heights in the urban roughness sublayer were used to analyze the momentum flux-gradient relationship φm （ξ） in the framework of local similarity,where ξ is the local stability parameter. The forms of φm （ξ）at different levels are obtained by a least square fit to the measurement data,and compared to the classical similarity function. The results show that the momentum flux varies with height and the constant-flux assumption in Monin-Obukhov Similarity theory is not valid in the urban roughness sublayer,but the dimensionless velocity gradient can be satisfactorily described by the local similarity theory. Under the unstable conditions,although the formulae of φm （ξ）at the three levels are all very close to the classical similarity function, the coefficient rm in φm （ξ）increases with height and approaches to the value of 15.0 in the classical similarity function. Under the stable conditions, the coefficient βm in φm （ξ）at each level is smaller than that in the classical similarity function. The coefficient also increases with height and approaches to the value of 4.7 in the classical similarity function. The samecharacteristics of the momentum flux-gradient relationships under both unstable and stable conditions suggest that the turbulent flow in the urban roughness sublayer can be described by the classical similarity theory when the height reaches the top of roughness sublayer. The relationship between the critical Richardson number Ric and the coefficient βm is Ric = Prc βm. The value of Ric in the urban roughness sublayer is larger than the classical value 0.2. This phenomenon indicates that the production of mechanical turbulence is stronger in the urban roughness sublayer.