Drag Distribution in Idealized Heterogeneous Urban Environments
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Drag Distribution in Idealized Heterogeneous Urban Environments Birgit S. Sützl1
· Gabriel G. Rooney2
· Maarten van Reeuwijk1
Received: 22 April 2020 / Accepted: 27 August 2020 © The Author(s) 2020
Abstract Large-eddy simulations of nine idealized heterogeneous urban morphologies with identical building density and frontal area index are used to explore the impact of heterogeneity on urban airflow. The fractal-like urban morphologies were generated with a new open-source Urban Landscape Generator tool (doi:10.5281/zenodo.3747475). The vertical structure of mean flow and the dispersive vertical momentum transport within the roughness sublayer are shown to be strongly influenced by the building morphologies. The friction velocity and displacement height show high correlations with the maximum building height rather than the average height. Well-known roughness parametrizations of the logarithmic layer cannot adequately capture the large spread observed in the large-eddy simulation data. A generalized frontal area index f is introduced that characterizes the vertical distribution of the frontal area in the urban canopy. The vertically distributed stress profiles, which differ significantly per simulation, are shown to roughly collapse upon plotting them against f . The stress distribution representing urban drag can be fitted with a third degree polynomial. The results can be used for more detailed and robust representations of building effects in the development of urban canopy models. Keywords Drag parametrization · Heterogeneity · Large-eddy simulation · Urban canopy
1 Introduction Buildings modify the airflow and momentum exchange within cities, and therefore strongly affect local wind, temperature, humidity, and pollution. Because buildings cannot be explicitly resolved in numerical weather prediction (NWP), the impacts on airflow need to be parametrized. Developing parametrizations of urban terrain is a formidable challenge: the parameter space is very large and neither the atmosphere nor the urban surface are typically in a statistically steady state (Martilli 2007; Barlow 2014). Regional weather models typically represent buildings by ground-based surface-cover parameters such as building density λ p
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Birgit S. Sützl [email protected]
1
Department of Civil and Environmental Engineering, Imperial College London, London SW7 2AZ, UK
2
Met Office, FitzRoy Road, Exeter EX1 3PB, UK
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(building plan area per unit plan area) and frontal area index λ f (frontal area per unit plan area), and the vertical extent of buildings is often simply represented by an average building height z H , which may refer to the mean building height, or a (frontal-area-) weighted average height (Grimmond and Oke 1999). These geometric parameters are used in NWP to describe the aerodynamic effect of buildings, commonly in terms of a logarithmic wind profile with a displacement height z d and roughness length z 0 (e.g., Porson et al. 2010). However, there are large uncertainties associated with e
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