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 is developed. The model relies on a simple parameterization of the ratio 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 drag area index. Model predictions of d/h and z(0)/h compare very favorably with observed values. A model sensitivity analysis suggests that sheltering effects for momentum transfer tend to make canopies with non-uniform foliage distribution resemble canopies with more uniform foliage 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 have on u*/u(h) rather than the influence wind speed may have on the foliage drag coefficient. Model results indicate that z(0)/h and d/h are sensitive to uncertainties in the numerical values of the model parameters, foliage density and distribution, sheltering effects and variations 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 sublayer. Given the simplicity of the model it is suggested that it may be of use for land surface parameterizations in large scale models.