On the Wind Profile above Plant Canopies and its Aerodynamic Properties

Abstract

To make clear the relationships between wind profile parameters and aerodynamic properties of the plant canopy, an analysis of wind profiles above canopy was carried out.
The results are summarized as follows:
(1) The equations of wind profile parameters ζ₀(=z_o/H) and δ(=d/H) were obtained in terms of u_H/u_* and C_D as follows:
ζ₀=(β₀/ku_H/u_*)^1/(1-m)exp[-k/(1-m)-u_H/u_*]=(β₀/k√C_D)^1/(1-m)exp[-k/(1-m)√C_D],
and
δ=1-(β₀/ku_H/u_*)^1/(1-m)exp[-km/(1-m)u_H/u_*]=1-(β0/k√C_D)^1/(1-m)exp[-km/(1-m)√C_D],
where z_o is roughness length, d zero-plane displacement, H height of canopy top, u_H wind velocity at the height of H, u_* friction velocity, C_D total drag coefficient of the canopy, ζ₀ the relative roughness length, and δ the relative zero-plane displacement. β0 and m are empirical constants.
(2) The empirical constants (β₀ and m) may be determined by the equation,
λ₀=β₀ζ^m₀=-k²(1-δ)/ln(1-δ/ζ₀) The values of β₀ and m estimated for several canopies are shown in Table 1. The main cause of differences in β₀ and m may be ascribed to the changing of aerodynamic structure of canopies.
(3) The relation among z_o, d and H was derived as:
lnH-d/z₀=k²/β₀H(H-d)/(z₀/H)^m.
The above relation seems to be applied for various types of plant canopies.
(4) The dependences of ζ₀(=z_o/H) and δ(=d/H) for Japanese larch canopies (Allen, 1968), maize crop (Maki, 1975a, 1976), and pasture grass (Kotoda, 1979) upon the total drag coefficient C_D were fairly well approximated by Eq. (14) and Eq. (15), respectively.