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| Titel |
Flow tilt angles near forest edges – Part 1: Sonic anemometry |
| VerfasserIn |
E. Dellwik, J. Mann, K. S. Larsen |
| 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 ; 7, no. 5 ; Nr. 7, no. 5 (2010-05-26), S.1745-1757 |
| Datensatznummer |
250004781
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| Publikation (Nr.) |
 copernicus.org/bg-7-1745-2010.pdf |
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| Zusammenfassung |
An analysis of flow tilt angles from a fetch-limited beech forest site with
clearings is presented in the context of vertical advection of carbon
dioxide. Flow angles and vertical velocities from two sonic anemometers by
different manufacturers were analyzed. Instead of using rotations, where
zero-flow angles were assumed for neutral flow, the data was interpreted in
relation to upstream and downstream forest edges.
Uncertainties caused by flow distortion, vertical misalignment and limited
sampling time (statistical uncertainty) were evaluated and found to be highly
significant. Since the attack angle distribution of the wind on the sonic
anemometer is a function of atmospheric stratification, an instrumental error
caused by imperfect flow distortion correction is also a function of the
atmospheric stratification. In addition, it is discussed that the sonic
anemometers have temperature dependent off-sets. These features of the
investigated sonic anemometers make them unsuitable for measuring vertical
velocities over highly turbulent forested terrain. By comparing the sonic
anemometer results to that of a conically scanning Doppler lidar (Dellwik et
al., 2010b), sonic anemometer accuracy for measuring mean flow tilt angles
was estimated to between 2° and 3°. Use of planar fit
algorithms, where the mean vertical velocity is calculated as the difference
between the neutral and non-neutral flow, does not solve this problem of low
accuracy and is not recommended.
Because of the large uncertainties caused by flow distortion and vertical
alignment, it was only possible to a limited extent to relate sonic
anemometer flow tilt angles to upwind forest edges, but the results by the
lidar indicated that an internal boundary layer affect flow tilt angles at
21m above the forest. This is in accordance with earlier studies at the site.
Since the mean flow tilt angles do not follow the terrain, an estimate of the
vertical advection term for near-neutral conditions was calculated using
profile measurements of carbon dioxide. The estimated advection term is
large, but it is not recommended to include it in the surface carbon balance,
unless all terms in the carbon dioxide conservation equation can be precisely
estimated. |
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