AN ANALYTICAL ONE-DIMENSIONAL MODEL OF MOMENTUM-TRANSFER BY VEGETATION OF ARBITRARY STRUCTURE

An analytical one-dimensional model of momentum transfer by vegetation with variable foliage distribution, sheltering and drag coefficient i s developed. The model relies on a simple parameterization of the rati o of the above-canopy friction velocity, u, to the wind speed at the top of the canopy, u(h), to predict vegetation roughness length (z(0)) and displacement height (d) as functions of canopy height (h) and dra g area index. Model predictions of d/h and z(0)/h compare very favorab ly with observed values. A model sensitivity analysis suggests that sh eltering effects for momentum transfer tend to make canopies with non- uniform foliage distribution resemble canopies with more uniform folia ge distribution and that any influence wind speed has on d/h and z(0)/ h is more likely to be related to the influence that wind speed may ha ve on u/u(h) rather than the influence wind speed may have on the fol iage drag coefficient. Model results indicate that z(0)/h and d/h are sensitive to uncertainties in the numerical values of the model parame ters, foliage density and distribution, sheltering effects and variati ons in drag coefficient within the canopy. In addition z(0)/h is also shown to be sensitive to the presence or absence of the roughness subl ayer. Given the simplicity of the model it is suggested that it may be of use for land surface parameterizations in large scale models.