Observational verification of urban surface roughness parameters derived from morphological models

This study focuses on the verification of the aerodynamic roughness length and zero-plane displacement in an urban area estimated from morphological models. Anemometrical observations were held in downtown area of Nanjing in summer and winter respectively. Data were collected using both slow and fast response anemometry, and the instruments were mounted on a tall tower. Based on the logarithmic wind profile, the dimensionless friction velocities (normalized by the mean wind speed at an upper level) can be calculated from the estimated roughness lengths and zero-plane displacements, and compared to the measurement data. Although the friction velocity was observed just above the top of the canopy layer, while the mean wind speed was observed at a relatively low level that is generally thought to be in the roughness sublayer, the analysis indicates that the fluctuations of the dimensionless friction velocity were mainly caused by the heterogeneity of the urban surface, which was represented by different roughness lengths and different zero-plane displacements. The results imply that the similarity relation for the mean wind profile is still valid in the roughness sublayer once the heterogeneity of the urban surface is taken into account.