In order to fully understand the exchange of heat, momentum, and moisture between the urban surface and the overlying atmosphere, it is necessary to study the evolution of the urban boundary layer and surface energy balance on the timescale of the dominant forcing – the diurnal cycle of solar radiation. In my talk, I will discuss the use of large-eddy simulation for this purpose, with a focus on achieving a realistic atmospheric state. This work involves using observations from sites in London, and data from the numerical weather prediction model used by the Met Office. The aim is to be able to carry out multi-day simulations examining the influence of urban form on the microclimate of cities.