Secondary Wind Speed Maxima Inside Plant Canopies

It is noted that wind profiles measured in forest and crop canopies normally contain a secondary maximum or a region of very small shear beneath the level of greatest foliage density. One-dimensional models utilizing a turbulent transport coefficient cannot predict a reversed velocity gradient and, as a result, profile analyses normally imply coefficients that are unrealistic or nonsensical. Examination of the equation for the local rate of change of Reynolds stress u′w′ shows that the velocity gradient can reverse in sign if the divergence of the turbulent transport of stress is of opposite sign and exceeds in magnitude the pressure-velocity gradient correlation. Direct measurements of the turbulent transport of u′w′ in corn (Zea mays L.) indicate that its value is considerably larger than in the air layers above and show that stress is transported downward from the upper parts of the vegetation. A one-dimensional model of canopy flow which solves the equations for momentum, Reynolds stress and the three components of turbulent kinetic energy, without relating the stress to the mean velocity gradient, predicts a weak secondary maximum in the wind profile for a corn canopy.