On Zero-plane Displacement and Roughness Length in the Wind Velocity Profile Equation over a Corn Canopy

Abstract

In order to check TAKEDA's theory (equation (2)), observations of wind velocity profiles were made over the sorgo canopy in August 1967. Furthermore, to test the theory another set of observations were made over the corn canopy in Tanashi, Tokyo, in July and August 1968. The author wants to know if the theory can be applied to all wind velocity profiles over different kinds of plant canopies at different stages of growth and development.The wind velocity profiles were measured with five small cup-anemometers. The plant height (h), the leaf area index (L.A.I.) the mean leaf area density (M.L.A.D.=L.A.I./h) and the rate of heading were measured, too.Five results were obtained from these observations as follows:I. An opposite relationship between the zero-plane displacement (d) and the roughness length (z0) was found, in which a decrease in d and an increase in z0 with wind velocity were shown (see Fig. 3).II. A good correspondence was found between d versus u* (friction velocity) and M.L.A.D. with growth and development of the corn canopy (see Fig. 3).III. A good correspondence was found between z0 vs. u* and the heading of corn plants with growth and development of the corn canopy (see Fig. 3).IV. It was found that the values d and z0 moved with the particular characteristic on the rough surface diagram introduced by TAKEDA (see Fig. 4).V. It was found that the variation of H (effective plant height), obtained from equation (2), in which α is taken to be 0.087, vs, u* decreased slightly with the increase of friction velocity on 24.25 July, that H was constant, even if u* increase, on 1-3 August and that the variation of H vs. u* increased slightly with the increase of u*, furthermore the plotting points scattered under the condition of u*>80cm/sec on 5-9 August (see Fig. 5).Finally it was concluded that the theory was applicable even to the corn canopy taking structures of canopy on 24.25 July and on 1-3 August into account, and that, on the contrary, observed results on 5-9 August disagreed with the theory probably due to the oscillatory movements of plants with long periods.