Statistical Characteristics of Turbulence in Urban Roughness Sublayer

Eddy-correlation observations were carried out at different heights above the urban canopy in three cities, Nanjing, Changzhou and Suzhou, China. At the site in Nanjing, the two observational heights are 33.5m and 48.5m above ground, and the relative heights are 1.6 Z_h and 2.4 Z_h, where Z_h is the mean building height. At the site in Suzhou, the three observational heights are 20 m, 23 m and 32 m above ground, and the relative heights are 1.2 Z_h, 1.4 Z_h and 1.9 Z_h respectively. The single level observation is at the site in Changzhou, which is at the height of 35m above ground, and its relative height is 3.4 Z_h. The six heights are in the urban roughness sublayer (Zou et al., 2015).

At each height the normalized standard deviations of velocity components (ó_i/u_*,i=u,v,w) and temperature (ó_T/T_*) are analyzed according to ζ and respectively, where ζ is the stability parameter (ζ=(z-zd)/L, zd is the zero-plane displacement height, L is Obukhov length).The results show that the statistics of ó_u/u_*, ó_v/u_*, ó_w/u_* and ó_T/T_* are in good agreement with the local similarity theory. However, the coefficients C1_i ,C2_i and C3_i all vary with height. Under the near neutral conditions(-0.05<ζ<0.05) C1_u, C1_v and C1_w decrease with height, implying that the mechanical effects of canopy on the velocity standard deviations decrease with height in the urban roughness sublayer. Under both stable (ζ>0.05) and unstable (ζ<-0.05) conditions, C2_u, C2_v and C2_w increase with height, suggesting that the thermal effects on the velocity standard deviations increase with height in the urban roughness sublayer.

For the normalized temperature standard deviations, under the near neutral conditions (-0.05<ζ<0.05) the exponent in the power-law relationship is between -2/3 and -1 rather than -1/3. Under both stable (ζ>0.05) and unstable (ζ<-0.05) conditions, the -1/3 power-law relationship is obeyed well. However, the coefficient decreases slightly with height under unstable conditions (ζ<-0.05) but it increases significantly with height under stable conditions (ζ>0.05).