WIND OF AN AIR LAYER CHARACTERIZED BY VEGETATIONS

Abstract

The purpose of the present paper is to clarify the behaviour of the roughness length zo and the zero-plane displacement d against friction velocity and to explain the vertical wind profile within a canopy using introduced theoretical relationships between physical quantities. Measurements were carried out at the heat and water balances observation field in the Environmental Research Center of the University of Tsukuba, Ibaraki, Japan. The field surface was covered by pasture grass (rye grass, Secale Cereale) with a height of 0.46m. The observations under the near neutral condition show that mean values of zo and d increase with the growth of plant height h. The relations are expressed by z0=0.065 h and d=0.401 h respectively. The value of z0/(h-d)=0.109 is smaller than those found by many other canopies, for instance, 0.44 over wheat field, 0.26 over needle-leaned tree, 0.24 over bean field and 0.12 over corn field (Hayashi and Kotoda, 1980). It is noteworthy that the pasture canopy contributes less effectively to aerodynamical roughness than the other canopies. Tall, flexible vegetations are more deformed with the increase of wind speed. Therefore, z0 and d change in accordance with wind speed at a fully rough surface. Then z0 increases slightly with the increase of the friction velocity u* as follows (Fig. 6): z0=0.044 u*+0.018 On the other hand d diminishes with the increase of friction velocity as follows (Fig. 7): d=-0.361 u*+0.288 This is attributed to the phenomena that the wind penetrates more into the canopy as the speed increases and that the aerodynamical surface, a level of no downward momentum flux, becomes lower. Under such conditions, tall vegetations are exposed more to the wind, and the canopy changes its aerodynamical roughness. For the flat surface, the friction velocity u* is in direct proportion to the wind speed uh. But for the pasture field, the friction velocity settles down at a value of u*=0.370m/s and deviates from the linear relation between u and u* in the range of uh_??_2.0m/s. The main cause for the deviation of the value of u* is considered to be the difference in the shape of the canopy under such windy conditions (Fig. 8). The relation between the momentum diffusivity KM and the friction velocity is expressed essentially by two straight lines as follows (Fig. 9): KM=0.093 u* (u* 0.3m/s) We can also recognize that the drag coefficient is independent of the friction velocity and the mean value of the drag coefficient CD is 3.07×10-2 at z=0.46 m, although the deviation is not small (Fig. 10). For the purpose of verification of the theoretical relationships between the physical quantities within the canopy, a numerical solution of the wind profile is done and compared with the results of the field observation. Under near neutral condition the wind profile over the pasture canopy is represented by a logarithmic law. But in the canopy layer, the following relations are given: τ=ρKMdu/dz KM=βζ0muh(1-ra) where ζ0 is the normalized roughness length, i.e. ζ0=z0/h, /h, τ the shearing stress, p the air density, Cd the drag coefficient of canopy element, a the leaf area density, and α, β and m are nondimensional constants, and r is the newly defined luxuriant length.