Numerical modelling of the wind over forests: roughness versus canopy drag

Parameterizing the effect of vertically-distributed vegetation through an effective roughness (z_0,eff) – whereby momentum loss through a three-dimensional foliage volume is represented as momentum loss over an area at one vertical level – can facilitate the use of forest data in flow models, to any level of detail, and simultaneously reduce computational cost. Results of numerical experiments and comparison with observations show that a modelling approach based on z_0,eff can estimate wind speed and turbulence levels over forested areas, at heights of interest for wind energy applications (∼ 60 m and higher), but only above flat terrain. Caution must be exercised in the application of such a model to zones of forest edges. Advanced flow models capable of incorporating local (distributed) drag forces are recommended for complex terrain covered by forest.