Roughness sublayer over vegetation canopy: A wind tunnel study

• Published in: Agricultural and forest meteorology 2022-04, Vol.316, p.108880, Article 108880
• Main Authors: Mo, Ziwei, Liu, Chun-Ho, Chow, Hei-Lim, Lam, Ming-Kei, Lok, Yiu-Him, Ma, See-Wah, Wong, Fu-Lam, Yip, Pik-Yan
• Format: Article
• Language: English
• Subjects: Drag coefficient Cd; Frequency spectrum; Inertial sublayer (ISL); Quadrant-hole analysis; Roughness sublayer (RSL) and wind tunnel experiment
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2022.108880

•Turbulence statistics over reduced-scale models of idealized forest canopies is sampled.•Improved analytical solution to roughness-sublayer (RSL) wind speeds is proposed.•RSL transport is dominated by high-speed downdrafts and low-speed updrafts.•RSL sweep Q4 contributes more to momentum flux than does Q2 at the canopy level.•Surface roughness has notable influence on the motion scales of ejection Q2 but not Q4. Atmospheric flows in the inertial sublayer (ISL) and the roughness sublayer (RSL) are complicated by vegetation canopies. They subsequently modify the exchange of momentum and tracer fluxes between the plants and the overlying atmospheric surface layer (ASL). In this study, the flows over idealized vegetation canopies are modeled by wind tunnel experiments. Using fast-response, constant-temperature (CT) hot-wire anemometry (HWA), mean winds and turbulence are measured to test the sensitivity of the dynamics to the configurations of tree models. The drag coefficient measured is in the range of 4.0 × 10-3 ≤ Cd ≤ 7.6 × 10−3. The dimensionless profiles of mean and fluctuating velocities agree well with previous results obtained by wind tunnel experiments and large-eddy simulation (LES) over various vegetation canopies. Incorporating a correction function, a new analytical solution to the mean-wind-speed profile, which is applicable to both RSL and ISL continuously, is developed that is verified favorably by the current wind tunnel data. One of the key parameters, the RSL constant, is found in the range of 2.1 ≤ μ ≤ 2.6 that is larger than its counterpart over urban canopies. Unlike ISL, the skewness and kurtosis of flows show that RSL momentum transport is mainly governed by rare, high-speed, downdrafts (u’ > 0 and w’