Modelling the diurnal cycle of urban microclimates using large-eddy simulation
File(s)
Author(s)
Owens, Sam
Type
Thesis or dissertation
Abstract
This thesis involves upgrading uDALES, a large-eddy simulation (LES) model with a surface energy balance (SEB) scheme, and using it to study microscale urban boundary layer (UBL) processes over a diurnal cycle. The upgraded code (v2.0) has two main new features: an improved parallelisation that substantially increases the scalability, and a high-fidelity surface treatment that can capture geometries that are not aligned with the Cartesian computational grid. uDALES v2.0 is validated against other models and
experiments, and demonstrated to offer better accuracy than previous versions.
Subsequently, uDALES v2.0 is used to investigate the microclimate in a realistic masterplan in a hot, arid climate. Analysis of the bulk SEB shows that while behaviour is primarily driven by radiation, thermal inertia is important as well. Moreover, the flow behaviour in the wake of the buildings is shown to be more important than the surface temperature in determining the local air temperature. This case is compared with a set of simulations that each represent one hour of the day and in which the SEB reaches a steady
state. The lack of thermal inertia in the steady-state simulations results in systematic differences in the SEB and surface temperature over the diurnal cycle.
Finally, uDALES v2.0 is coupled to a mesoscale model (MetUM) for a diurnal cycle of the UBL in London, considering both idealised and realistic geometrical configurations. Comparing the models, the daytime convective boundary layer is similar overall, though
there are differences in the near-surface wind speed, and the nocturnal boundary layer generally shows less good agreement. The bulk SEB is predicted similarly, whereas the surface temperature exhibits a consistent discrepancy. The SEB for the various uDALES
geometries are closer to each other than to MetUM. The models exhibit fair agreement with observations near the surface, and poorer agreement at higher altitude.
experiments, and demonstrated to offer better accuracy than previous versions.
Subsequently, uDALES v2.0 is used to investigate the microclimate in a realistic masterplan in a hot, arid climate. Analysis of the bulk SEB shows that while behaviour is primarily driven by radiation, thermal inertia is important as well. Moreover, the flow behaviour in the wake of the buildings is shown to be more important than the surface temperature in determining the local air temperature. This case is compared with a set of simulations that each represent one hour of the day and in which the SEB reaches a steady
state. The lack of thermal inertia in the steady-state simulations results in systematic differences in the SEB and surface temperature over the diurnal cycle.
Finally, uDALES v2.0 is coupled to a mesoscale model (MetUM) for a diurnal cycle of the UBL in London, considering both idealised and realistic geometrical configurations. Comparing the models, the daytime convective boundary layer is similar overall, though
there are differences in the near-surface wind speed, and the nocturnal boundary layer generally shows less good agreement. The bulk SEB is predicted similarly, whereas the surface temperature exhibits a consistent discrepancy. The SEB for the various uDALES
geometries are closer to each other than to MetUM. The models exhibit fair agreement with observations near the surface, and poorer agreement at higher altitude.
Version
Open Access
Date Issued
2024-12-07
Date Awarded
2025-07-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
van Reeuwijk, Maarten
Publisher Department
Department of Civil and Environmental Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)
Rights Embargo Date
2026-06-30