Coupling between air flow in streets and the well-developed boundary layer aloft

A field experiment was conducted to study the mean and turbulent characteristics of air Row within, and above, a street under neutral stratification Profiles of the mean wind and turbulent statistics were obtained and compared with flow over smoother terrain. Our results modify the conventional picture of a persistent re-circulation within the street with small-scale turbulent fluctuations about this mean (Oke, 1987). Instead, the mean re-circulation within the street was found to be much weaker than the unsteady turbulent fluctuations. Hence, the mean Row is merely a residual of an unsteady turbulent re-circulation. The re-circulation in the street is coupled to the wind aloft through a shear layer that develops at the roof-level. The shear layer is unstable, through Kelvin-Helmholtz instability, which leads to intermittency in the re-circulation in the street, and thence to ventilation of the air in the street. This finding is likely to be important for dispersion in urban areas. (C) 2000 Elsevier Science Ltd. All rights reserved.