Surface roughness parameters, namely the roughness length and displacementheight, are an integral input used to model surface fluxes. However, mostmodels assume these parameters to be a fixed property of plant functionaltype and disregard the governing structural heterogeneity and dynamics. Inthis study, we use large-eddy simulations to explore, in silico, the effects ofcanopy-structure characteristics on surface roughness parameters. Weperformed a virtual experiment to test the sensitivity of resolved surfaceroughness to four axes of canopy structure: (1) leaf area index, (2) thevertical profile of leaf density, (3) canopy height, and (4) canopy gapfraction. We found roughness parameters to be highly variable, but uncoveredpositive relationships between displacement height and maximum canopyheight, aerodynamic canopy height and maximum canopy height and leaf areaindex, and eddy-penetration depth and gap fraction. We also found negativerelationships between aerodynamic canopy height and gap fraction, as well asbetween eddy-penetration depth and maximum canopy height and leaf areaindex. We generalized our model results into a virtual"biometric"parameterization that relates roughness length and displacement height tocanopy height, leaf area index, and gap fraction. Using a decade of wind andcanopy-structure observations in a site in Michigan, we tested theeffectiveness of our model-driven biometric parameterization approach inpredicting the friction velocity over heterogeneous and disturbed canopies.We compared the accuracy of these predictions with the friction-velocitypredictions obtained from the common simple approximation related to canopyheight, the values calculated with large-eddy simulations of the explicitcanopy structure as measured by airborne and ground-based lidar, two otherparameterization approaches that utilize varying canopy-structure inputs,and the annual and decadal means of the surface roughness parameters at thesite from meteorological observations. We found that the classicalrepresentation of constant roughness parameters (in space and time) as afraction of canopy height performed relatively well. Nonetheless, of theapproaches we tested, most of the empirical approaches that incorporateseasonal and interannual variation of roughness length and displacementheight as a function of the dynamics of canopy structure produced moreprecise and less biased estimates for friction velocity than models withtemporally invariable parameters.
展开▼
