Numerical study on aerodynamic roughness of forest

Leaf area index(LAI) of forest is the sum of vertical projection area of plant leaves per unit area. Stand density refers to the number of trees per unit area. These two parameters are important indexes to characterize the wind resistance effect of forest underlying surface. This paper presents a theoretical model of the flow field above a forest canopy layer and the variation of forest aerodynamic roughness (Z₀), zero-plane displacement (d), and friction speed (U _*) with stand density and leaf area index (LAI) were investigated. The results show that stand density does not affect the variation of Z₀, d, or u_* with LAI. The aerodynamic roughness first increases and then decreases with increasing LAI, and the ratio of roughness to forest height Z₀/h is below 0.16. The value of d increases to a maximum with increasing of LAI and then remains stable. Moreover, the maximum value increases with increasing stand density. The maximum d/h ratios corresponding to stand densities of 400 ha⁻¹, 1000 ha⁻¹ and 1600 ha⁻¹ were 0.68, 0.88, and 0.93, respectively. Friction speed decreased with increasing LAI, then tends to become stable. The minimum friction speed decreases with increasing stand density and the minimum values of u_* corresponding to stand densities of 400 ha⁻¹, 1000 ha⁻¹, and 1600 ha⁻¹ were 0.65, 0.48, and 0.44, respectively. At the minimum friction velocity, the corresponding LAI was found to increase with increasing stand density.