A first-order closure model for the wind flow within and above vegetation canopies

A first-order closure model that has the general utility for predicting the wind flow within and above vegetation canopies is presented. Parameterization schemes that took into account the influence of large turbulent eddies were developed for the Reynolds stress and the mixing length in the model. The results predicted by the model were compared with measured data for wind speeds within and above six types of vegetation canopy, including agricultural crops, deciduous and coniferous forests, and a rubber tree plantation during fully leafed, partially leafed and leafless periods. The predicted results agreed well with the measured data; the root-mean-square errors in the predicted wind speeds (non-dimensionalized by the friction velocity above the canopy) were about 0.2 or less for all of the canopies. The secondary wind maxima that occurred in the lower canopies were also correctly predicted. The influence of foliage density on the wind profiles within and above a vegetation canopy was successfully simulated by the model for a rubber tree plantation during fully leafed, partially leafed and leafless periods. The bulk momentum transfer coefficients (CM) and the values of λ (which are defined by z0=λ(h−d), where z0 is the roughness and d is the zero-displacement height of the canopy) for the vegetation canopies were also studied, and the relationships CMh=0.0618 exp(0.792CF) and λ=0.209 exp(0.414CF) were determined; here, CMh is the bulk momentum transfer coefficient at the canopy top; CF=Cd PAI zmax/h, where Cd is the effective drag coefficient of the canopy, PAI is the plant area index and zmax is the height at which the plant area density is maximum. The values of λ ranged from 0.22 to 0.32 for the canopies studied.