Flow field measurements in the proximity of an urban intersection in London, UK

Observations were made using a roof-top automatic weather station and four ultrasonic anemometers deployed in the proximity of an intersection in London, UK, during the 4-week DAPPLE project field campaign in spring 2003. At the intersection, the measurements show that the wind direction can switch between the different streets, suggesting that intersections are potent mechanisms for dispersion. Despite the complexity of the building geometry in the vicinity of the intersection, measurements in the adjoining streets indicate that the main large-scale features are along-street channelling and an across-street recirculating vortex, similar to those observed in idealised two-dimensional street canyons. Analysis over a relatively broad range of roof-top wind directions demonstrates that flow within the streets is the vector sum of a channelling and a recirculation vortex. Furthermore, channelling depends linearly on the along-street component of the roof-top reference wind, whilst the cross-street recirculation vortex depends linearly on the component of the roof-top reference wind perpendicular to the street. The results demonstrate that these simple ideas are robust enough to occur in streets of non-ideal geometry and are established a short distance from an intersection.