Turbulent flow modification in the atmospheric surface layer over a dense city

Winds in the atmospheric surface layer (ASL) over distinctive urban morphology are investigated by building-resolved large-eddy simulation (LES). The exponential law is applied to urban canopy layers (UCLs) unprecedentedly to parameterize vertical profiles of mean-wind-speed u¯_z and examine the influence of morphological factors. The skewness of streamwise velocity S_u is peaked at the zero-plane displacement d (drag center) where flows decelerate mostly. The dynamics and intermittency in roughness sublayers (RSLs) are further contrasted. It helps determine the critical strength of the organized structures (ejection, Q2 and sweep Q4) in their contributions to the average momentum transport (i.e., 3<u”w”> to 5<u”w”>). Two key factors of the local-scale dynamics are revealed - building heterogeneity and upstream giant wakes that could amplify turbulence kinetic energy (TKE) and energetic intermittent Q4 by different mechanisms. The former is conductive for large-eddy generation that promotes vertical fluctuating velocity w”, stimulating intermittent, energetic Q2 and Q4. The latter, whose footprints are identified by the two-point correlation of streamwise velocity R_uu with specific size and inclination, facilitates intermittent, fast streamwise fluctuating velocity u”, forming vigorous Q4. Nevertheless, excessive planar density λ_p (≈ 0.7) is detrimental to both transport processes. These findings contribute to the theoretical and empirical wall models of large-scale roughness that help urban planners and policymakers to improve air quality.