An investigation of higher-order closure models for a forested canopy

Simultaneous triaxial sonic anemometer velocity measurements vertically arr ayed at six levels within and above a uniform pine forest were used to exam ine two parameterization schemes for the triple-velocity correlation tensor employed in higher-order closure models. These parameterizations are the g radient-diffusion approximation typically used in second-order closure mode ls, and the full budget for the triple-velocity correlation tensor typicall y employed in third-order closure models. Both second- and third-order clos ure models failed to reproduce the measured profiles of the triple-velocity correlation within and above the canopy. However, the Reynolds stress tens or profiles (including velocity variances) deviated greatly from the measur ements only within the lower levels of the canopy. It is shown that the Rey nolds stresses are most sensitive to the parameterization of the triple-vel ocity correlation in these lower canopy regions where local turbulent produ ction is negligible and turbulence is mainly sustained by the flux transpor t term. The failure of the third-order closure model to reproduce the measu red third moments in the upper layers of the canopy-top contradicts conclus ions from a previous study over shorter vegetation but agrees with another study for a deciduous forest. Whether the third-order closure model failure is due to the zero-fourth-cumulant closure approximation is therefore cons idered. Comparisons between measured and predicted quadruple velocity corre lations suggest that the zero-fourth-cumulant approximation is valid close to the canopy-atmosphere in agreement with recent experiments.