| Popis: |
Many models that describe the energy balance of plant canopies are unsuitable for use as components of larger-scale climate models because their computational and data input requirements are too large. Models based on ‘higher-order-closure’ and Lagrangian simulations of particle trajectories provide examples. More practical models are usually based on diffusion theory, even though turbulent transport within canopies is not a diffusive process. Recently, Raupach (Q. J. R. Meteorol. Soc., 115, 609–632, 1989) presented an efficient analytical model based on Lagrangian principles. In Raupach's theory, the profile of scalar concentration within a canopy is described as the sum of a 'far-field' contribution, predicted using diffusion theory, and a ‘near-field’ contribution, for which the diffusion equation is inappropriate. The latter contribution is referred to as ‘non-diffusive’ transport. This approach still requires a detailed description of canopy turbulence and the distribution of sources. In this paper, Raupach's theory is used to develop a model for a ‘typical’ two-layer canopy consisting of an overstorey and a ground layer. An expression for the average ‘near-field’ contribution within the upper storey is derived by integrating Raupach's equations. This average contribution is computed for a range of ‘typical’ scalar source profiles and turbulence profiles in the overstorey. It is shown that the near-field contribution is rather insensitive to the description of the canopy turbulence and it is not unduly sensitive to the assumed source distribution, particularly as its total contribution is rather small. The average ‘near-field’ contribution to the scalar profile in the overstorey can be represented by a ‘near-field resistance’, rn, which can be added to a traditional resistance model. The modified model can then predict counter-gradient fluxes in appropriate cases. Because the near-field resistance is small compared with the ‘far-field’ resistances of the model, the results show that non-diffusive exchange processes play a minor role in determining transport of scalars between the atmosphere and dense plant canopies. |
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