Vertical Variation of the Effects of Atmospheric Stability on Turbulence Statistics Within the Roughness Sublayer Over Real Urban Canopy

摘要

Eddy correlation measurements have been carried out at six different heights above the urban canopy in three cities of China, that is, Nanjing, Changzhou, and Suzhou. Observations at these heights are used to evaluate the dependence of normalized standard deviations of velocity components σ_i/u_(*l) (i = u, v, w) and temperature σT_v =T_(v*l) on local atmospheric stability ζ (=(z - z_d)/Λ), where u_(*l) is the local friction velocity, T_(v*l) is the local temperature scale, z_d is the displacement height, and Λ is the local Obukhov length. It is found that the universal functions a_i(1 + b_i| ζ| )~(1/3) and bT_v |ζ|~(-1/3), in which a_i, b_i, and bT_v are empirical coefficients, are suitable for describing the behaviors of σ_i/u_(*l) and σT_v =T_(v*l) , respectively. Under neutral conditions (|ζ| < 0.05), the value of σi/u*l (approximately equal to ai) decreases with height. On the other hand, bi increases with height under both unstable (ζ < - 0.05) and stable (ζ > 0.05) conditions, suggesting that the thermal effect of stratification on the standard deviations of velocity components increases with height in the urban roughness sublayer. However, the behavior of σT_v =T_(v*l) is different from that of σ_i/u_(*l). bT_v remains constant with height under unstable conditions but increases with height under stable conditions. Vertical variations of these coefficients in the universal functions are consistent with the concept of local similarity, suggesting that the turbulence behavior in the urban roughness sublayer is different from that over the flat terrain.