A numerical investigation of the momentum balance of a Spruce forest.
Abstract
The bulk aerodynamic characteristics of a Picea sitchensis forest near Aberdeen, and hence the relation between the shearing stress on the top of the canopy ( tau h) and wind speed, were obtained from 66 mean hourly wind profiles measured above the canopy (height = h). From 43 wind profiles measured within the canopy, the parameter alpha (defining the shape of the profile in an equation already developed, see FA 33, 3651) was calculated for various wind speeds (u) and levels (z). These data, together with drag coefficients for Spruce shoots, were used to analyse the momentum balance of the forest and calculate tau (z), the shear stress at any level, for six values of uh. A shelter factor (p) was applied to the tau (z) values so that the momentum balance SIGMA tau (z) = tau h was satisfied. Momentum exchange coefficients (KM(z)) were estimated from the corrected tau (z) values and the slope of the wind profiles within the canopy. The ratio KM(z)/KM(h) was approximately constant, at a value around unity, in the upper levels of the canopy, a result which conforms with the pattern previously shown for a bean crop.