A Momentum Exchange Model for the Surface Layer over Bare-Soil and Canopy-Covered Surfaces

Abstract

A multilayered turbulent transport model for the surface layer based on collisions between atmospheric layers is developed. The model can represent the average and perturbed components of turbulence relatively well, for both bare-soil and canopy-covered surfaces. The number of collisions increases linearly with height for a bare-soil surface, and decreases as the leaf-area density increases within the canopy layer. The intermittent nature of turbulent motion is reproduced in the model, even though the model is one-dimensional. The number of collisions increasing with height may cause the intermittent nature of the turbulence and may also relate to the gust motions for canopy flow; such gusts transfer large momentum downward intermittently and cause a secondary maximum in wind velocity in the lower layer within the canopy. In other words, the model can include the large-eddy effect by considering the different number of collisions among layers. Turbulence data measured with an ultrasonic anemometer at 10 Hz in the field were used as model input; plausible results were obtained. The model can represent the overall characteristics of turbulence in the surface layer.