Urban areas consist of complex road networks incorporating street canyons, intersections and side streets. Previous field and laboratory studies have indicated that flow and turbulence inside cities is complex and urban canyon intersections are particularly interesting, as they are thought to be local “hot spots” for a range of pollutants. They are challenging to model and sparse information exists on the flow, turbulence and dispersion patterns associated with them.

This paper presents results from turbulent wind measurements within and above a busy intersection between two street canyons (Marylebone Road and Gloucester Place) in Westminster London (NW1) and forms part of the DAPPLE (“Dispersion of Air Pollution and Penetration into the Local Environment”) 2007 field campaign. Data was collected using sonic anemometers operating at 10Hz at various heights and positions around the intersection: on the rooftop of an adjacent building, on two lampposts on opposite sites of the intersection at two heights, at a single height on a pavement lamppost within Marylebone Road (40 m east of the intersection), and at two heights on a tower on Gloucester Place (30 m south of the intersection). Data analysis and the variation of in-canyon flow and turbulence parameters with above roof wind direction is discussed.

Both flow-channelling and recirculation were identified within the canyon and intersection. Oblique flows across the intersection caused both flow channelling from the adjoining streets and some evidence of reversed flow. For oblique flows across the intersection, the site geometry, merging of channelled flow from the two canyons and flow disturbance by vehicular traffic appears to increase the flow complexity and turbulence intensity. Asymmetric features were identified within the intersection with flow acceleration and flow direction variability occurring for some background wind sectors but not the opposite ones. The least variability for each sector range occurs for the highest magnitude of the above roof wind speeds. Traffic-induced flow disturbance may also help explain the observed flow acceleration, flow direction variability and enhanced turbulence for oblique flows and low wind conditions.