| Popis: |
Zero plane displacement length (d0) and roughness length for momentum (z0m), describe the aerodynamic characteristics of a land surface and govern its fluxes. Usually, d0 and z0m are assumed to be constants representing the physical characteristics of the surface, such as the canopy height or leaf area index. Prior evidence in the literature and our examination of OzFlux and Ameriflux sites data show that d0 and z0m variation occurs at sites with vegetated canopies, but is almost absent in a short-canopy grassland, and cannot be adequately explained by variations in wind or friction velocity. We hypothesize that in sites with vegetated canopies, variation in d0 and z0m arises from the canopy's response to the wind. The hypothesis was developed by matching four analytical canopy velocity models to a logarithmic above-canopy velocity profile at canopy height. This generated d0 and z0m in terms of 2 non-dimensional terms, the canopy momentum absorption capacity (a parameter) and a (measurable) Peclet number. We tested the velocity profiles from Ozflux and Ameriflux sites. These tests produced estimates of canopy momentum absorption capacity which varied consistently with phenological changes in the canopies. Conversely, simple linear regression of d0 and z0m against wind speed and friction velocity generated fitting parameters that did not exhibit physically interpretable variations. The canopy velocity models were able better to much of the variations in d0 and z0m. This technique can be applied at any eddy covariance site where velocity is measured at the canopy height. |
