Roughness sublayer over vegetation canopy: A wind tunnel study
2022
Mo, Ziwei | Liu, Chun-Ho | Chow, Hei-Lim | Lam, Ming-Kei | Lok, Yiu-Him | Ma, See-Wah | Wong, Fu-Lam | Yip, Pik-Yan
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’ < 0; sweep Q4) and massive, low-speed, updrafts (u’ < 0 and w’ > 0; ejection Q2). This finding concurs the dominant events unveiled by quadrant-hole analysis. The results contrast the dissimilarity between ISL and RSL that improve our understandings of their dynamics over vegetation canopies.
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