Novel Geometric Parameters for Assessing Flow Over Realistic Versus Idealized Urban Arrays

Urban heterogeneity, such as the variation of street layouts, building shapes, and building heights, cannot be fully represented by density parameters commonly used in idealized urban environmental analyses. To address this shortcoming and better model flow fields over complex urban neighborhoods, we propose two novel descriptive geometric parameters, alignedness and building facet entropy, which quantify the connectivity of inter-building spaces along the prevailing wind direction and the variation of building facet orientations, respectively. We then conducted large eddy simulations over 101 urban layouts, including realistic urban configurations with uniform building height as well as idealized building arrays with variable heights, and evaluated the resulting bulk flow properties. Urban canopy flow over realistic neighborhoods resembles staggered building arrays for low urban densities but becomes similar to aligned configurations beyond lambda(p) similar to 0.25 where the realistic flow is less sensitive to changes in density. We further show that compared to traditional density parameters (such as plan and frontal area densities), the mean alignedness, a measure of connectivity of flow paths in street canyons, better predicts canopy-averaged flow properties. Furthermore, for realistic urban flow, the dispersive momentum flux shows a clear increasing trend with building density, and a decreasing trend with alignedness, which is in contrast with idealized cases that exhibit no clear trend. This distinct behavior further highlights the necessity of evaluating flow over realistic urban layouts for flow parameterization. This study provides an improved method of describing urban layouts for flow characterization that can be applied in neighborhood-scale urban canopy parameterization. Plain Language Summary We run 101 flow simulations over realistic and idealized urban neighborhoods to evaluate the degree of dissimilarity by introducing realism in urban geometry. The realistic urban neighborhoods are prepared by OSM2LES while the idealized building arrays are constructed in a conventional way with cube-shaped buildings evenly arranged in "aligned" and "staggered" form. In contrast to the previous approach employing "staggered" building arrays to calibrate model constants, we find the bulk flow properties (such as wind speed and turbulence level) over realistic layouts become insensitive to the density growth and behave similarly to "aligned" urban arrays over medium urban packing density. The spatial variability of realistic urban flow as indicated by dispersive momentum flux shows an increasing trend over the density growth, unlike those on idealized urban arrays that do not have a clear trend. To characterize the distinctive behavior of realistic urban flow, two types of geometric parameters were introduced. The "alignedness" that focuses on the uninterrupted urban streets performs better than packing densities for being inclusive to all types of urban layouts and an agreeable tolerance to building height variability.