Observational verification of urban surface roughness parameters derived from morphological models

Abstract

This study focuses on how to verify the aerodynamic roughness parameters in an urban area by using observational data collected in the roughness sub-layer. Anemometrical observations were made in the downtown area of Nanjing, China, in summer and winter. Data were collected using both slow and fast response anemometers mounted on a 36 m tower. The friction velocity was observed just above the top of the urban canopy layer, which is at the height of 19.7 m, while the mean wind speed was observed at a relatively low level that is generally thought to be in the roughness sub-layer, of which the top usually ranges from 2 to 5 times the height of the urban canopy layer. The results of the measurements show that the dimensionless friction velocities (normalized by the mean wind speed at an upper level) maintain approximately a constant (0.20) and the fluctuations are small (±0.02) in a period of 21 days. The results imply that the similarity relation for the mean wind profile is still valid and that the effect of stratification is negligibly small in the urban roughness sub-layer.Based on the measurement results, the logarithmic wind profile was applied to verify the aerodynamic roughness parameters empirically derived from four morphological models: the rule of thumb (Rt) method, the Bottema (Ba) method, the Macdonald (Ma) method and the Raupach (Ra) method. The results show that they are not very different from each other. The good performance of the Rt method may be due to the fact that the distribution of buildings in the study area is regular. The Ba and Ra methods are likely to be better since they can give reasonable estimates of roughness parameters. Evidence indicates that the Ma method underestimates the roughness length.

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