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 flow 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 flow 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.