The surface energy balance and boundary layer over urban street canyons

A model is developed for the energy balance of an urban area, represented as a sequence of two-dimensional street canyons. The model incorporates a novel formulation for the sensible-heat flux, that has previously been validated against wind tunnel models, and a formulation for radiation that includes multiple reflections and shadowing. This energy balance model is coupled to a model for the atmospheric boundary layer. Results are analysed to establish how the physical processes combine to produce the observed features of urban climate, and to establish the roles of building form and fabric on the urban modification to climate. Over a diurnal cycle there are morning and evening transition periods when the net flux of radiation is largely balanced by the flux of heat into the surface. The urban surface has a large surface area in contact with the air, and hence a large active heat capacity, and so the urban area needs to absorb a larger amount of heat than a rural area to change the surface temperature. The morning and evening transitions are therefore prolonged over urban areas, delaying the onset of convective or stable boundary layers after sunrise and sunset. The model shows that the energy balance of the roof behaves very differently from the combined energy balance of the street canyon system of walls and street. The sensible-heat flux from the street canyon into the boundary layer is increased by the increased surface area, but is decreased by the buildings reducing the local flow speeds. The net result is that, for the two-dimensional geometry investigated here, the sensible-heat flux from the canyon is not strongly sensitive to canyon geometry. The sensible-heat flux from the roof is larger than from the street, and so the total sensible-heat flux into the boundary layer, and hence also the air temperature, is strongly dependent on the fraction of plan area occupied by roofs. The radiation budget of the street canyon, which largely drives the temperatures of the canyon surfaces, is significantly changed by the limited sky view and multiple reflections caused by the local building form. The canyon surface temperatures thus depend strongly on local building morphology. Finally, two mechanisms are suggested for how urban areas might maintain a positive sensible-heat flux at night. Firstly, if the roof material has much lower heat capacity than the street canyon surfaces, then the roof can cool the boundary-layer air faster than the street canyon surfaces cool, leading to a positive heat flux out of the street canyon. Secondly, advection decouples the boundary layer from the local surface energy balance. In this way, cool air, perhaps from a rural area, advected on to an urban surface can lead to a positive sensible-heat flux which then tends to neutralize any stable stratification in the boundary layer.