Simple first-order turbulent transfer model to predict canopy momentum, heat, and moisture regimes

Various models of turbulence and turbulent transfer within and above canopies exist, ranging from simple first-order closure schemes up to more complex third-order closures. Recently, Pinard and Wilson (JAM 40: 1762-1768, 2001) suggested that little advantage is gained practically, if not theoretically, when applying second- and third-order closure schemes to real canopies given the uncertainties with which input parameters are known. They presented a first-order model that predicts turbulent diffusivity from turbulent kinetic energy, which is modelled from a simple but realistic budget equation, and a length scale determined from reasonable choices for "inner" and "outer" scales. However, their model is incapable of modelling counter-gradient transfers often measured in canopies associated with large scale transfers. Li et al. (BLM 33: 77-83, 1985) presented a simple first-order model that can describe counter-gradient momentum transfer but their specification of turbulent diffusivity is highly unrealistic within a canopy, their large scale transfer term is highly sensitive to input parameters, and they did not consider heat and moisture regimes. This paper presents a model that combines the best attributes of both these works and calibrates and tests it against field measurements in forest canopies.