Profiles of turbulence statistics in and above an urban street canyon

Results from an extensive urban climate study are presented. Eighteen months of continuous measurements of mean variables and a total of 131 runs of turbulence measurements at various heights within and above an urban street canyon are analysed. Scaled profiles of mean wind speed and also of the velocity variances are found to be strongly stability dependent. In the street canyon the air is consistently warmer than the air in the upper part of the roughness sublayer and well mixed with respect to temperature. On average, the roughness sublayer is near-neutrally stratified at night and unstable during the day. The profiles of the scaled velocity variances exhibit a strong dependence on stability. The vertical component, in particular, decreases with height in near-neutral stratification while increasing under strongly unstable conditions. Although the velocity spectra at a mid-roughness sublayer height have a similar shape as the well established non-urban surface layer spectra, with a − slope at the high-frequency end, no inertial subrange is observed, since the ratio of horizontal to vertical spectral densities does not approach the required value of Furthermore, the peaks of the spectra of both the horizontal velocity and temperature occur at much higher frequencies than expected for standard surface layer conditions. Closer to the roof level and inside the street canyon, energy is shifted to higher frequencies and the turbulence spectra become flatter.