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| Titel |
Large-eddy simulations of surface roughness parameter sensitivity to canopy-structure characteristics |
| VerfasserIn |
K. D. Maurer, G. Bohrer, W. T. Kenny, V. Y. Ivanov |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 8 ; Nr. 12, no. 8 (2015-04-30), S.2533-2548 |
| Datensatznummer |
250117917
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| Publikation (Nr.) |
 copernicus.org/bg-12-2533-2015.pdf |
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| Zusammenfassung |
| Surface roughness parameters, namely the roughness length and displacement
height, are an integral input used to model surface fluxes. However, most
models assume these parameters to be a fixed property of plant functional
type and disregard the governing structural heterogeneity and dynamics. In
this study, we use large-eddy simulations to explore, in silico, the effects of
canopy-structure characteristics on surface roughness parameters. We
performed a virtual experiment to test the sensitivity of resolved surface
roughness to four axes of canopy structure: (1) leaf area index, (2) the
vertical profile of leaf density, (3) canopy height, and (4) canopy gap
fraction. We found roughness parameters to be highly variable, but uncovered
positive relationships between displacement height and maximum canopy
height, aerodynamic canopy height and maximum canopy height and leaf area
index, and eddy-penetration depth and gap fraction. We also found negative
relationships between aerodynamic canopy height and gap fraction, as well as
between eddy-penetration depth and maximum canopy height and leaf area
index. We generalized our model results into a virtual
"biometric"
parameterization that relates roughness length and displacement height to
canopy height, leaf area index, and gap fraction. Using a decade of wind and
canopy-structure observations in a site in Michigan, we tested the
effectiveness of our model-driven biometric parameterization approach in
predicting the friction velocity over heterogeneous and disturbed canopies.
We compared the accuracy of these predictions with the friction-velocity
predictions obtained from the common simple approximation related to canopy
height, the values calculated with large-eddy simulations of the explicit
canopy structure as measured by airborne and ground-based lidar, two other
parameterization approaches that utilize varying canopy-structure inputs,
and the annual and decadal means of the surface roughness parameters at the
site from meteorological observations. We found that the classical
representation of constant roughness parameters (in space and time) as a
fraction of canopy height performed relatively well. Nonetheless, of the
approaches we tested, most of the empirical approaches that incorporate
seasonal and interannual variation of roughness length and displacement
height as a function of the dynamics of canopy structure produced more
precise and less biased estimates for friction velocity than models with
temporally invariable parameters. |
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