Abstract:
Surface and atmospheric conditions in urban environments are strongly influenced by the diurnal cycle of solar radiation. This talk presents the application of uDALES, a building-resolving large-eddy simulation model with a two-way coupled surface energy balance (SEB) scheme, to two case studies investigating urban airflow and heat transfer over a diurnal cycle.
The first case is based on a realistic masterplan consisting of a set of buildings by the coast in a hot, arid climate. Motivated by how the built environment influences thermal comfort in such conditions, the aim is to understand the development of the urban microclimate in terms of the surface and atmospheric conditions. A key finding is that the flow behaviour in the wake of the buildings is more important than local surface conditions in determining the air temperature. In addition, this case is compared with a set of simulations, each representing one hour of the day, with the SEB allowed to reach a steady state, in order to investigate the impact of thermal inertia on the microclimate.
In the second case, uDALES is coupled to output from a mesoscale model (the Met Office Unified Model; MetUM) for a diurnal cycle of the entire urban boundary layer (UBL) in central London. The aim is to investigate the effect of surface morphology on the development of the UBL and SEB, using both idealised and realistic buildings, comparing with MetUM and observations. The daytime convective boundary layer is broadly similar for the two models, however there are differences in the near-surface wind speed and the nocturnal boundary layer. Furthermore, there is a consistent discrepancy in the bulk surface temperature, though several key SEB quantities are predicted similarly. In general, the uDALES morphological configurations exhibit greater similarity with each other than with MetUM, with implications for parameterising microscale processes in mesoscale models.