Momentum Transfer within Canopies

Abstract

To understand the basic characteristics of the observed S-shaped wind profile and the exponential flux profile within forest canopies, three hypotheses are postulated. The relationship between these fundamental profiles is well established by combining the postulated hypotheses with momentum equations. Robust agreements between theoretical predictions and observations indicate that the nature of momentum transfer within canopies can be well understood by combining the postulated hypotheses and momentum equations. The exponential Reynolds stress profiles were successfully predicted by the leaf area index (LAI) profile alone. The characteristics of the S-shaped wind profile were theoretically explained by the plant morphology and local drag coefficient distribution. Predictions of maximum drag coefficient were located around the maximum leaf area level for most forest canopies but lower than the maximum leaf area level for a corn canopy. A universal relationship of the Reynolds stress between the top and bottom of the canopy is predicted for all canopies. This universal relationship can be used to understand what percentage of the Reynolds stress at the top of canopy is absorbed by the whole canopy layer from the observed LAI values alone. All of these predictions are consistent with the conclusions from dimensional analysis and satisfy the continuity requirement of Reynolds stress, mean wind speed, and local drag coefficient at the top of canopy.