Energy balance and boundary layer interactions over urban areas

We present a coupled boundary layer/energy balance model, which accounts for the effects of the urban area on radiation, surface heat flux and ground heat flux. The model shows graphically the two-way coupling involved between the urban surface and the boundary layer above. The boundary layer depth and advection within the boundary layer across the rural to urban transition, together with effects of the urban surface on the energy balance are key controls on the urban-rural differences and the structure of the urban boundary layer. We show how this coupled interaction provides understanding of some of the key differences between the urban and rural boundary layers, such as the phase differences between the surface heat flux and thence differences in surface air temperature between urban and rural areas (ie the magnitude and vertical structure of the urban heat island). Finally, the role of advection is shown to be critical in producing a positive heat flux into the nocturnal urban boundary layer and thence a well mixed nocturnal urban boundary layer. We conclude that proper interpretation of observations of the urban boundary layer require that full attention be paid to the surface energy balance and advection.