Aerodynamic roughness of urban areas derived from wind observations

This study contributes to the sparse literature on anemometrically determined roughness parameters in cities. Data were collected using both slow and fast response anemometry in suburban areas of Chicago, Los Angeles, Miami and Vancouver. In all cases the instruments were mounted on tall towers, data were sorted by stability condition, and zero-plane displacement (zd) was taken into account. Results indicate the most reliable slow response estimates of surface roughness are based on the standard deviation of the wind speed obtained from observations at one level. For residential areas, winter roughness values (leaf-off) are 80–90% of summer (leaf-on) values. Direct comparison of fast and slow response methods at one site give very similar results. However, when compared to estimates using morphometric methods at a wider range of sites, the fast response methods tend to give larger roughness length values. A temperature variance method to determine zd from fast response sensors is found to be useful at only one of the four sites. There is no clear best choice of anemometric method to determine roughness parameters. There is a need for more high quality field observations, especially using fast response sensors in urban settings.