Steady-State Large-Eddy Simulations of Convective and Stable Urban Boundary Layers

Tom Grylls,Ivo Suter,Maarten van Reeuwijk

doi:10.1007/s10546-020-00508-x

Abstract

A comprehensive investigation is carried out to establish best practice guidelines for the modelling of statistically steady-state non-neutral urban boundary layers (UBL) using large-eddy simulation (LES). These steady-state simulations enable targeted studies under realistic non-neutral conditions without the complications associated with the inherently transient nature of the UBL. An extensive set of simulations of convective and stable conditions is carried out to determine which simplifications, volumetric forcings, and boundary conditions can be applied to replicate the mean and turbulent (variance and covariance) statistics of this intrinsically transient problem most faithfully. In addition, a new method is introduced in which a transient simulation can be ‘frozen’ into a steady state. It is found that non-neutral simulations have different requirements to their neutral counterparts. In convective conditions, capping the boundary-layer height h with the top of the modelled domain to h/5 and h/10 (which is common practice in neutral simulations) reduces the turbulent kinetic energy by as much as 61% and 44%, respectively. Consistent with the literature, we find that domain heights l_z ge 5 |L| are necessary to reproduce the convective-boundary-layer dynamics, where L is the Obukhov length. In stably stratified situations, the use of a uniform momentum forcing systematically underestimates the mechanical generation of turbulence over the urban canopy layer, and therefore leads to misrepresentations of both the inner- and outer-layer dynamics. The new ‘frozen-transient’ method that is able to maintain a prescribed flow state (including entrainment at the boundary-layer top) is shown to work well in both stable and convective conditions. Guidelines are provided for future studies of the capped and uncapped convective and stable UBL.

Highlights

As cities continue to grow and become more dense, it becomes increasingly important to understand the urban climate, in regard to the sustainability of urban life and global challenges such as urban air quality and the urban-heat-island effect
The boundary conditions, forcings and simplifications applied to produce these steady states are reviewed and guidelines are outlined for best practice in modelling the non-neutral steady-state urban boundary layer (UBL) in future studies
While thermal effects play an integral role in the urban environment, their incorporation into urban large-eddy simulation (LES) studies is complicated by corresponding changes in the state of the overlying UBL

Summary

Introduction

As cities continue to grow and become more dense, it becomes increasingly important to understand the urban climate, in regard to the sustainability of urban life and global challenges such as urban air quality and the urban-heat-island effect. The ability to model urban environments is integral to developing this understanding. The urban fabric and the anthropogenic processes occurring therein interact with the planetary boundary layer to produce a complex modelling problem over a large range of spatial and temporal scales. High-resolution modelling of the urban canopy layer (UCL) is imperative to resolving the unsteady and heterogeneous urban flow field, rendering large-eddy simulation (LES) a key modelling tool (Tominaga and Stathopoulos 2013; Barlow 2014; Lateb et al 2016). The production of statistical steady states is integral to satisfying both of these aims and allows for converged statistics to be obtained (directly with periodic boundary conditions and when used as driver simulations; see Tomas et al 2015)

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Conclusion