Representing the effects of building height variability on urban canopy flow

Abstract

We conducted large‐eddy simulations over 98 urban arrays with varying building densities and height distributions. Compared to uniform‐height urban arrays, the influence of height variability on urban flow is pronounced and acts differently in two idealized urban configurations: the low buildings induce higher wind speed and stronger turbulence over staggered arrays but act inversely over aligned building configurations. The flow motions around tall buildings generate strong dispersive fluxes, which are sometimes of similar magnitude to the turbulent momentum flux and responsible for a persistent isolated roughness flow pattern in the upper canopy regardless of the urban density. Tall buildings further contribute disproportionately to the form drag of the urban surface, reaching up to 3.9 times the form drag induced by buildings of height equal to the average building height, in dense layouts. The flow inflection points ‐ i.e., the largest wind speed gradient that defines the aerodynamic interface between the urban canopy flow and the surface layer flow above, are found to be displaced to the maximum building height if less than 25% of buildings are below the mean building height. These findings provide critical insight for the development of urban canopy models, where the impacts of height variability on flow are often linked to the vertical variation in urban density alone. To address this deficiency, we provide a case study that considers the drag amplification due to the impact of vertical urban structures in the urban canopy model, enabling high‐resolution regional climate models to reproduce urban air flows better.This article is protected by copyright. All rights reserved.

A one-dimensional urban flow model with an eddy-diffusivity mass-flux (EDMF) scheme and refined turbulent transport (MLUCM v3.0)

“…Generally, C µ increases with density over sparse and medium layouts, which indicates the overweighting trend of viscous dissipation of TKE over the turbulent transport that is consistent with the transition of the flow regime (Oke et al, 2017). Meanwhile, denser layouts amplify the impact of building height variability, leading to a higher standard deviation among binned scenarios, which is consistent with Lu et al (2023a). Turbulent viscosity for momentum lumping the dispersive flux C µ,K m,T +D shows a good agreement with that from Nazarian et al ( 2020) except for the peaking at higher density.…”

“…Simulations are conducted in the Parallelized Large-Eddy Simulation Model (PALM, version r4554) (Maronga et al, 2020) with λ p ranging from 0.0625 to 0.64, representing urban densities seen in global cities. The numerical setup follows (Lu et al, 2023a;Nazarian et al, 2020;Lu et al, 2023b), which has been validated against direct numerical simulation (DNS) (Coceal et al, 2007) and wind tunnel experiments (Brown et al, 2001) but will be listed here for completeness.…”

“…The detailed design of building arrays can be found in Lu et al (2023a) and is briefly explained here: while maintaining the same volume of roughness elements (i.e., same mean building height, H mean = 16 m), we consider three height distributions, i.e., H std ∈ [0 m, 2.8 m, 5.6 m], which result in different maximum building height H max . Additionally, we consider three types of urban arrays: (a) continuous distribution of building height with a relatively low H max ; (b) clustered configuration with two clusters (three buildings) of low and tall building blocks; and (c) high-rise configuration, featuring one tall and a cluster (three) of low building blocks.…”

“…The eddy diffusivity of scalars (K c ) and momentum (K m ) maintains a similar shape as a result of the relatively simpler mechanism in their production, destruction, and transport, which justifies the simplification (K c ∼ K m ) that is consistent with Li and Bou-Zeid (2019). However, the eddy diffusivity of TKE (K k ) demonstrates a greater complexity with some negative values at the ground and canopy interface that marks the counter-gradient transport of TKE indicating two vital local shear production regions: (a) the canopy-top shear zone is relatively wellstudied and is induced by clearings of roughness elements (Giometto et al, 2016) and the (b) near-ground shear zone that is unique in medium-dense layouts (λ p ≤ 0.25) where flow reversal appears and counteracts with the pressure gradient, inducing a significant shear source of TKE (Lu et al, 2023a). This lack of consideration of the near-ground flow features partially explains the underestimation of local TKE seen in Nazarian et al (2020) and can be mitigated by imposing a height-dependent model constant (e.g., Glazunov et al, 2021), but is beyond the scope of this study that aims to refine the turbulent transport characterization within the whole canopy.…”

“…Based on the aforementioned fundamental deficiencies embedded in the multi-layer model, a better performance of UCMs on more complex flows requires a finer and more physically based characterization of dispersive fluxes. In this research, based on a recently developed urban flow dataset (Lu et al, 2023a), we aim to assess and improve the prediction of the TKE and momentum in the urban canopy through a more accurate parameterization of turbulent transport in the MLUCM (Nazarian et al, 2020) in two ways:…”