Statistical analysis of the organized turbulence structure in the inertial and roughness sublayers over real urban area by building-resolved large-eddy simulation

Abstract Dynamics and organized turbulence structure in the atmospheric boundary layer (ABL) over a real urban area, Mong Kok neighborhood in Kowloon Peninsula, Hong Kong, is investigated using the large-eddy simulation (LES) to enrich the transport mechanism. The urban morphology is explicitly resolved and the LES results are validated by wind tunnel measurements. Wind speed u ¯ >/U∞ profiles manifest the inflections and zero-plane displacement d for 2 to 3 times the average building height Have, signifying higher blockage (stronger shear) than idealized geometries. The enhanced mixing induced by real urban morphology leads to faster, more uniform flows in the roughness sublayer (RSL) than those extrapolated from the inertial sublayer (ISL) by the conventional logarithmic law-of-the-wall (log-law). Higher-order moments signify that RSLs and ISLs feature, respectively, tiny, accelerating, spurlike downdraft (skewness Su > 0 and Sw   0). Besides, quadrant analysis shows that RSL ejection Q2 (u’’   0) occurs more frequently (but contributes less to momentum flux ) than does sweep Q4 (u’’ > 0 and w’’