Large-Eddy Simulation of Microscale Urban Surface Energy Balances over a Diurnal Cycle

A methodology is presented for studying the diurnal cycle of the surface energy balance of individual urban facets. An important consideration is the accurate representation of the urban boundary layer (UBL) due to the influence on turbulent surface-atmosphere exchanges. To this end, the Met Office Unified Model (MetUM) was run over London for a 36-hour interval within a sequence of days with clear skies and low wind. During this period, it can be seen from MetUM output and observations that the UBL follows a characteristic, approximately periodic evolution. This data was one-way coupled with uDALES, a building-resolving urban atmospheric large-eddy simulation framework equipped with a three-dimensional surface energy balance model for each facet. This coupling is necessary because the processes responsible for the characteristic evolution of the UBL are not captured by uDALES.

When using large-eddy simulation, it is necessary to generate a turbulent inflow boundary condition. A precursor simulation was used for this purpose, and time-height profile assimilation was used to force the flow towards area-averaged MetUM data. The profiles were adjusted so that they are fully periodic in time, which makes it possible to simulate multiple days using a single day of MetUM data. The target simulation can then be used to study the surface energy balance and turbulent exchanges whilst not affecting the evolution of the UBL as defined by the inflow boundary condition. A staggered cube geometry was used for both the precursor and target simulations using urban surface properties representative of central London.

The forcing method is found to reproduce the mean flow well. In order to check the turbulence is also reasonable, the shear stress and heat flux profiles for the uDALES precursor simulation are compared against that of MetUM. The models are found to agree fairly well during the day, with greater discrepancy at night. The models are also compared against observations, with mixed results. Finally, to demonstrate the analysis that this methodology enables, surface energy balances are presented for the uDALES target simulation and compared to that of MetUM in light of the different approaches taken by the two models.