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 $${\Lambda }_f$$ Λ 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 $${\Lambda }_f$$ Λ 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.

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A recently developed, height-distributed urban drag parametrization is tested with the London Model, a sub-kilometre resolution version of the Met Office Unified Model over Greater London. The distributed-drag parametrization requires vertical morphology profiles in the form of height-distributed frontal-area functions, which capture the full extent and variability of building heights. London's morphology profiles are calculated and parametrized by an exponential distribution with the ratio of maximum to mean building height as the parameter. A case study evaluates the differences between the new distributed-drag scheme and the current London Model setup using the MORUSES urban land-surface model. The new drag parametrization shows increased horizontal spatial variability in total surface stress, identifying densely built-up areas, high-rise building clusters, parks, and the river. Effects on the wind speed in the lower levels include a lesser gradient and more heterogeneous wind profiles, extended wakes downwind of the city centre, and vertically growing perturbations that suggest the formation of internal boundary layers. The surface sensible heat fluxes are underpredicted, which is attributed to difficulties coupling the distributed momentum exchange with the surface-based heat exchange.

We study the compositional and configurational heterogeneity of Greater London at the city- and neighbourhood-scale using Geographic Information System (GIS) data. Urban morphometric indicators are calculated including plan-area indices and fractal dimensions of land cover, frontal area index of buildings, evenness, and contagion. To distinguish between city-scale heterogeneity and neighbourhood-scale heterogeneity, the study area of 720 km2 is divided into 1 [Formula: see text] 1 km2 neighbourhoods. City-scale heterogeneity is represented by categorisation of the neighbourhoods using a k-means clustering algorithm based on the morphometric indicators. This results in six neighbourhood types ranging from “greenspace” to “central business district”. Neighbourhood-scale heterogeneity is quantified using a hierarchical multi-scale analysis for each neighbourhood type. The analysis reveals the dominant length scales for land-cover and neighbourhood types and the resolutions with the most information gain. We analyse multi-scale anisotropy and show that small-scale features are homogeneous, and that anisotropy is present at larger length scales.

Abstract. Urban environments increasingly move to the fore of climate and air quality research due to their central role in the population’s health and well-being. Tools to model the local environmental conditions, urban morphology and interaction with the atmospheric boundary layer play an important role for sustainable urban planning and policy-making. uDALES is a high-resolution, building-resolving large-eddy simulation code for urban microclimate and air quality. uDALES solves a surface energy balance for each urban facet and models multi-refection shortwave radiation, longwave radiation, heat storage and conductance, as well as turbulent latent and sensible heat fluxes. Vegetated surfaces and their effect on outdoor temperatures and energy demand can be studied. Furthermore a scheme to simulate emissions and transport of aerosols and some reactive gas species is present. The energy balance has been tested against idealized cases and the particle dispersion against field measurements, yielding satisfying results. uDALES can be used to study the effect of specific new constructions and building measures on the local micro-climate; or to gain new insight about the general effect of urban morphology on local climate, ventilation and dispersion. uDALES is available online under GNU General Public License and remains under maintenance and development.