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)over bar>>/U-infinity 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 ) 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,T-eta of and contribution S-i,S-eta from Q2 and Q4 contrasts their roles in the transport process. The motion scale eta (around 4 times of the average momentum flux) differentiates the dominance between Q2 and Q4 over a real, dense city, improving the fundamental understanding. (250 words)

Automated summary

This study investigates the dynamics and organized turbulence structure in the atmospheric boundary layer (ABL) over a real urban area in Hong Kong using large-eddy simulation (LES). The results show that the enhanced mixing induced by the real urban morphology leads to faster and more uniform flows in the roughness sublayer (RSL) than predicted by the conventional logarithmic law-of-the-wall. Higher-order moments reveal different turbulence characteristics in the RSLs and inertial sublayer (ISLs). Quadrant analysis shows that RSL ejection occurs more frequently than sweep, although it contributes less to momentum flux. Conditional sampling demonstrates the composition of large-scale and small-scale events in the RSLs and ISLs. By comparing the occurrence and contribution of different events, this study improves our fundamental understanding of transport processes in a real, dense city.