Relationships between the Zero-plane Displacement and the Roughness Length over Sorghum and Alfalfa Canopies.

Abstract

The effect of wind speed on the aerodynamical parameters, namely the zero-plane displacement d and roughness length z0, and the relationships between d and z0 is discussed. Experiments conducted in sorghum and alfalfa fields reveal that d decreases and z0 increases with increasing wind speed or friction velocity. The relations between d and z0 obtained in these experiments are not well expressed by previous equations that were presented during the last several decades. Thus, new equations representing the relationships between d and z0, and a newly designed concept for eddy viscosity within the canopy are proposed and experimentally tested in sorghum, alfalfa, and tanukimame fields. The test results demonstrate that calculations from present equations coincide with the observed results. In the tanukimame field, however, the zero-plane displacement and roughness length do not vary with wind speed when the leaf area index LAI is large (LAI=5.6). On this point, it is considered that the dependence of the zero-plane displacement and roughness length on wind speed are influenced by the leaf area index and the drag coefficient for an individual leaf.