Journal of Agricultural Meteorology

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The author experimentally studied the dependence of aerodynamic characteristics on the surface roughness of various canopies to verify a new equation proposed in a previous paper (Maki, 1975). The relation is expressed as follows:
lnh-d/z₀=(κκ²/c)h-d/z₀ (1),
where h is mean plant height, d zero-plane displacement, z₀ aerodynamic roughness length, κ von Karman's constant and c an empirical constant obtained by the author.
Changes of aerodynamic characteristics of a teosinte canopy with growth were investigated by use of the data of wind velocity profiles above the canopy.
The magnitude of z₀ increased with friction velocity (u_*) and the reverse was the case for d of a teosinte canopy and glass rod model canopies.
The relation between d and z₀ obtained from equation (1) with a value of c, 0.47, was found to be in good agreement with experimental values of d and z_o on various plant canopies.
The d and z₀ values of artificial roughness elements increased generally as the surface area density (SAD) of the elements increased. The values of z₀ obtained for a diagonal arrangement type (D) were larger than those for a square arrangement type (S). The d value increased appreciably with SAD in the case of D type, but for S type it was almost constant in the observed region of SAD.
The ratio of the effective plant height (H_e) to the mean plant height (h) increased with increment of the total surface area density (TSAD) asymptotically to 1.0 at the highest range of TSAD. The equation that relates d and z₀ to the height of roughness elements was found to be valid for a surface of them with SAD of 0.05cm²/cm³ or above. In order for the equation to be valid for a sparsely planted surface, an effective plant height which decreases with SAD should be introduced.
Equation (11) was deduced to take into account the stability correction on the relation as indicated by equation (1). This equation could be applied to cases under moderately stable and unstable conditions, because equation (3) is valid only for these conditions. However, it is yet to be checked in observational studies.

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The data of wind measurements are analyzed to yield the information of characteristics of turbulence in the rice. An exponential function is presented to express the wind profiles observed in the rice canopy. Calculations are carried out to make clear the vertical profiles of momentum flux, mixing length and turbulent transfer coefficient within the rice canopy. It is shown that the magnitude of the extinction coefficients of the respective profiles are somewhat smaller than that in a maize canopy with relatively lax leaves.
The power spectra of both horizontal and vertical components of wind are calculated above and within the rice canopy. The spectral curves of the vertical wind component are used to obtain the turbulent transfer coefficient.

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