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

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 H_ave, 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 S_u > 0 and S_w < 0) and frequent, decelerating, bulky updraft (S_u < 0 and S_w > 0). Besides, quadrant analysis shows that RSL ejection Q2 (u" < 0 and w" > 0) occurs more frequently (but contributes less to momentum flux <u"w">) than does sweep Q4 (u" > 0 and w" < 0). Conditional sampling further demonstrates that RSL (ISL) consists of majority large-scale Q4 (Q2) and small-scale Q2 (Q4). Comparing both the occurrence T_i,_η of and contribution S_i,_η from Q2 and Q4 contrasts their roles in the transport process. The motion scale η (around 4 times of the average momentum flux) differentiates the dominance between Q2 and Q4 over a real, dense city, improving the fundamental understanding.