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

A "eld experiment was conducted to study the mean and turbulent characteristics of air#ow within, and above, a street under neutral strati"cation. Pro"les of the mean wind and turbulent statistics were obtained and compared with #ow over smoother terrain. Our results modify the conventional picture of a persistent re-circulation within the street with small-scale turbulent #uctuations about this mean (Oke, 1987). Instead, the mean re-circulation within the street was found to be much weaker than the unsteady turbulent #uctuations. Hence, the mean #ow 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 "nding is likely to be important for dispersion in urban areas. ( 2000 Elsevier Science Ltd. All rights reserved.