Turbulence Characteristics In The Urban Surface Layer

In the recently years, the distribution of buildings, soil type and vegetation coverage has changed so much in the urban region with the growing of urbanization rate. The complex geometric structure and inhomogeneous distribution characteristics of the land surface roughness elements make the research of surface-atmosphere interaction in the urban area become a hot and difficulty topic in atmospheric boundary layer field. In this dissertation, the micrometeorology data of three urban sites in Nanjing Municipal Party School, Baiyun Campus of Changzhou University and Suzhou Municipal Environmental Monitoring Center Station was used to analyze the turbulence characteristic in the urban surface layer. Some interesting results were found as follows:The analysis of dimensionless turbulence statistics at the three different levels shows that:Constant -flux hypothesis isn’t valid in the urban roughness sublayer. The dimensionless velocity standard deviation can be satisfactorily described by the local similarity theory. The dimensionless velocity gradient can also be satisfactorily described by the local similarity theory. The coefficients of the flux-gradient relationship for momentum at the highest level are closest to that of the classical similarity function. In the stable condition, the relationship between the critical Richardson number and the coefficient is Ric= Prt/βm. The value of Ric in the urban roughness sublayer is larger than the classical value 0.2 and decreases to 0.2 with the level. This phenomenon can be explained by the fact that the production of the mechanical turbulence is stronger. Linear extrapolation of the coefficient can get the approximate height of the roughness layer:zr≈3zh.The analysis of the drag coefficient shows that:(1) The relationship between u/U and z/zh at six levels has good correspondence with the empirical equation proposed by Roth. (2) The calculation of the aerodynamic roughness length zOu uses two levels’data in Nanjing Municipal Party School with two different similarity relationships i.e. the classic similarity theory and the similarity relationship. Both of the results show that zOa is largest in the near neutral condition and decreases with thermodynamic stability (?)1 exponentially. The parameterization scheme of u/U/((?)1) modified by the substitute near neutral aerodynamic roughness length z0 with zOu((?)1) correspond with the measurement result well. Moreover, the method using the similarity relationship shows a closer correspondence with the measurement result.The analysis of turbulence spectra at the two levels of Nanjing Municipal Party School shows that:(1) In near neutral condition, the two levels’turbulence kinetic energy density is higher than that of homogeneous grass land and the result of low level is higher that of high level. Turbulence at high level has better local isotropy than that at low level. The production of the turbulence is stronger in urban roughness sublayer than that in homogeneous grass land and is strongest near urban canopy where turbulence has largest deviation from local isotropy. 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 an individual element has prominent effect on turbulence in urban roughness sublayer. (2) In near neutral condition, turbulence kinetic energy dissipation rate φε at the two levels are higher than result over the homogeneous grass land. As to the two levels’results,φε is higher at low level. In unstable condition, the growing rate of φε with stability is quicker than result over the homogeneous grass land. In stable condition, the growing rate of φε with stability is similar with the result over the homogeneous grass land.