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| Titel |
Observations of the scale-dependent turbulence and evaluation of the flux–gradient relationship for sensible heat for a closed Douglas-fir canopy in very weak wind conditions |
| VerfasserIn |
D. Vickers, C. K. Thomas |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 14, no. 18 ; Nr. 14, no. 18 (2014-09-16), S.9665-9676 |
| Datensatznummer |
250119034
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| Publikation (Nr.) |
 copernicus.org/acp-14-9665-2014.pdf |
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| Zusammenfassung |
Observations of the scale-dependent turbulent fluxes, variances, and the bulk transfer parameterization for sensible heat
above, within, and beneath a tall closed Douglas-fir canopy in very
weak winds are examined. The daytime sub-canopy vertical velocity
spectra exhibit a double-peak structure with peaks at timescales of
0.8 s and 51.2 s. A double-peak structure is also
observed in the daytime sub-canopy heat flux co-spectra. The daytime
momentum flux co-spectra in the upper bole space and in the sub-canopy are
characterized by a relatively large cross-wind component, likely due
to the extremely light and variable winds, such that the definition
of a mean wind direction, and subsequent partitioning of the
momentum flux into along- and cross-wind components, has little
physical meaning. Positive values of both momentum flux components
in the sub-canopy contribute to upward transfer of momentum,
consistent with the observed sub-canopy secondary wind speed maximum. For the smallest resolved scales in the canopy
at nighttime, we find increasing
vertical velocity variance with decreasing timescale, consistent
with very small eddies possibly generated by wake shedding from the
canopy elements that transport momentum, but not heat. Unusually large values of the velocity aspect ratio within the
canopy were observed, consistent with enhanced suppression of the horizontal wind
components compared to the vertical by the very dense canopy.
The flux–gradient approach for sensible heat flux is found to be
valid for the sub-canopy and above-canopy layers when considered
separately in spite of the very small fluxes on the order of a few W m−2 in the sub-canopy. However, single-source approaches that ignore the canopy fail because they make the heat flux appear to be counter-gradient
when in fact it is aligned with the local temperature gradient in
both the sub-canopy and above-canopy layers. While sub-canopy Stanton numbers agreed well with values typically reported in the literature, our estimates for the above-canopy Stanton number were much larger, which likely leads to underestimated modeled sensible heat
fluxes above dark warm closed canopies. |
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