Some insights into typical urban canyon airflows

Previous studies have suggested that the key characteristics of wind flows within an urban canyon can be determined from the relative geometry of the canyon; in particular from the height to width ratio of the canyon, and, to a lesser extent, from the length to height ratio. Further, it is often assumed that the flow at mid-canyon is essentially two-dimensional and little influenced by along-canyon flow. Analysis of data collected from a field program indicates that within canyon flow is more complex than this and that the channelling effect of the canyon must be included in any description of the flow. It is also shown that the relative geometry of the canyon is insufficient on its own to determine the fundamental nature of the flow. In the field program, based on a relatively long canyon, vortex-based “skimming flow” was observed. On the basis of previous wind tunnel and numerical studies, such a flow regime is not expected for this relatively low canyon. Further analysis of the wind data confirms previous work of Nakamura and Oke (1988, Atmospheric Environment 22, 2691–2700) concerning the relationship between above-roof wind direction and canyon-floor wind direction. An explanation of this relationship in terms of the channelling effect of the canyon is provided.