Principal length scales in second-order closure models for canopy turbulence

Triaxial sonic anemometer velocity measurements vertically arrayed at six l evels within and above a pine forest were used to examine the performance o f two second-order closure models put forth by Wilson and Shaw and by Wilso n. Based on these measurements, it was demonstrated that Wilson's model rep roduced the longitudinal velocity standard deviation sigma(u) better than d id Wilson and Shaw's model. However, Wilson and Shaw's model reproduced the measured mean velocity tit) near the forest-atmosphere interface better th an Wilson's model did. The primary mechanisms responsible for discrepancies between modeled and measured [u] and sigma(u) profiles were investigated. The conceptual formulations of these two closure models differ in the chara cteristic length scales and timescales used in the closure parameterization s of the mean turbulent kinetic energy dissipation rate term, the pressure- strain rate term, and the flux-transport term. These characteristic length scales were computed and compared with measured integral length scales insi de the canopy. A discussion on how these length scales compare with the mix ing layer analogy also is presented.