THE INFLUENCE OF CANOPY STRUCTURE AND DENSITY UPON THE MIXING LENGTH WITHIN AND ABOVE VEGETATION

Taking into account the intermittent gust motion within a canopy layer and the influence of both the canopy elements and the underlying ground surface on the turbulent motion within the layer, a new mixing-length model was developed to simulate the exchange of momentum, sensible heat and water vapor between the atmosphere and the vegetated surfaces. It was found that traditional models overestimate the mixing length when the canopy density is neither dense nor sparse, since these models assume that the mixing length is limited by either the canopy elements or the height from the underlying ground surface. According to the present model, for the case of a vertically uniform canopy, the mixing length within the canopy layer is approximately equal to kz (κ: von Karman constant, z: height) near the underlying ground surface. It remains approximately constant far enough from the ground surface, decreasing gradually as the canopy density increases. The validity of the model was determined by comparing model results with observed data. © 1990, Meteorological Society of Japan.