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 a n 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 t he air in the upper part of the roughness sublayer and well mixed with respect to temperature. On average, the roughness sublayer is near-ne utrally stratified at night and unstable during the day. The profiles of the sealed velocity variances exhibit a strong dependence on stabil ity. The vertical component, in particular, decreases with height in n ear-neutral stratification while increasing under strongly unstable co nditions. Although the velocity spectra at a mid-roughness sublayer he ight have a similar shape as the well established non-urban surface la yer spectra, with a -2/3 slope at the high-frequency end, no inertial subrange is observed, since the ratio of horizontal to vertical spectr al densities does not approach the required value of 4/3. Furthermore, the peaks of the spectra of both the horizontal velocity and temperat ure occur at much higher frequencies than expected for standard surfac e layer conditions. Closer to the roof level and inside the street can yon, energy is shifted to higher frequencies and the turbulence spectr a become flatter.