The traditional view of cities is of hot dry areas compared to their rural surroundings. This view has developed partly because there is little water available for moisture fluxes on buildings or road surfaces. However, there are large areas of urban environments which contain vegetation and hence have access to the store of moisture in the soil. Recent observational experiments have shown that the moisture fluxes can be a significant term in the energy balance within suburban areas. So how important is this vegetation within the cities on the urban heat island, the atmospheric structure of the boundary layer, or on any mesoscale circulations?

The high resolution mesoscale version of the Met Office Non-Hydrostatic Unified Model has been used to make an initial assessment of these fundamental questions. This model has a tile or mosaic scheme for calculating its surface exchange. This means that the surface exchange is calculated for up to 9 different types of surface and the fluxes are then averaged, using blending height techniques, to give grid box values. One of the surface types is taken to represent the buildings and roads within urban areas and is explicitly represented using the canopy approach which has been shown tonrepresent many of the observed urban phenomenon. By varying the fractions of build up and vegetative areas within an urban environment, it is possible to assess the influence that vegetation has on the magnitude of the urban heat island. It is also possible to investigate the impact of vegetation on any mesoscale circulation generated by the presence of an urban area and the resulting atmospheric boundary layer structures.