Urban areas affect regional-scale weather processes by altering the momentum, heat and moisture content of the atmospheric boundary layer. The boundary layer above in turn drives the street-level circulations within urban areas, which again affect mixing into the boundary layer. These coupled interactions are not currently represented within regional-scale numerical weather prediction (NWP) models, which usually represent urban areas as regions of enhanced surface roughness only. What these NWP models are missing is a representation of the weather modification effects of urban areas.

With this application in mind an urban canopy model has been developed to represent the effects of drag and heating that urban areas exert both on the boundary layer above and the canopy layer below. Using this model it is shown that mean wind speeds and turbulent stresses within the canopy layer adjust on a lengthscale Lc which depends on the geometry and layout of the buildings comprising the urban area. Above the canopy the boundary layer evolves on a much longer lengthscale. Similar methods are used to quantify the effect of heated buildings as well as stability of the upstream boundary layer on winds and temperatures.

The model hence shows how the boundary layer structure develops in response to an urban area. The results have implications for how the impact of the urban area relates to its size, which is important in siting measurement campaigns.