Influence of stable atmospheric thermal stratification on urban street-canyon reaeration

To assist validation of numerical models of urban pollution dispersion, the boundary layer (BL) developing above a model town and the dispersion of street-level pollution on the surrounding buildings in typical street-canyon geometries, have been investigated in a wind tunnel under both neutral and stable thermal stratification conditions. Different street aspect ratios and roof configurations were used. The results indicate that stable stratification results in pollution 'capping' in the urban street canyons. The effect depends on the shape of the building roofs, with slanted roofs somewhat compensating for this 'capping' effect. As a rule, wider street configurations are more sensitive to oncoming thermal stratification than narrower canyons, irrespective of roof shape.