Investigation of the flow within urban canopies during the DAPPLE campaign

We present measurements of the mixing and transport in streets made during the DAPPLE campaign in London. The observations were made during a four-week period in Spring 2003 using an array of sonic anemometers deployed within streets of varying geometries and around an intersection. A reference wind speed and direction was measured at the top of a nearby tall building.

The geometry of the buildings surrounding the streets was complicated. Nevertheless the measurements show that the main features of the flow within the streets are channelling and a street canyon vortex: the same features as found in idealized 2-D street canyons. Moreover, the broad range of reference wind directions experienced in the campaign allows a simple novel picture of the flow within the street canyons based on a vector decomposition of the reference wind. Thus the component of the reference wind along the street gives the direction of the advection (channelling) at the street level, whilst the component of the reference wind perpendicular to the street drives the street canyon vortex. The results demonstrate that this simple picture is robust enough to occur in streets of non- ideal geometry. The measurements taken at the street intersection show that the wind direction can flip between different routes out of the intersection. This finding suggests that intersections are a potent mechanism for dispersion.