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Titel |
Vertical and horizontal transport of energy and matter by coherent motions in a tall spruce canopy |
VerfasserIn |
A. Serafimovich, L. Siebicke, T. Foken |
Konferenz |
EGU General Assembly 2009
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Medientyp |
Artikel
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Sprache |
Englisch
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Digitales Dokument |
PDF |
Erschienen |
In: GRA - Volume 11 (2009) |
Datensatznummer |
250021356
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Zusammenfassung |
In a forested ecosystem low frequency coherent events contribute significantly to the budgets
of momentum, heat and matter. In the frame of EGER (ExchanGE processes in mountainous
Regions) project the contribution of coherent structures to the vertical and horizontal
transfer of energy and matter in a tall spruce canopy was investigated. Two measuring
campaigns were carried out in North-Eastern Bavaria at the Waldstein site in the
Fichtelgebirge mountains. Observations of coherent structures were obtained by a vertical
profile of sonic anemometers equipped with fast CO2 and H2O analyzers covering
all parts of the forest up to the lower part of the roughness sub layer. In addition
five small masts were set up in the trunk space of the forest and equipped with
sonic anemometers, humidity and temperature sensors as well as CO2 analyzers.
Combination of measurements done in vertical and horizontal directions allows us to
investigate coherent structures, their temporal scales, their role in flux transport
and vertical coupling between the subcanopy, canopy and air above the canopy
level.
To extract coherent structures from the turbulent time series, the technique based on the
wavelet transform has been used. Conditional sampling analysis showed a domination of
coherent structure signatures in vertical wind measurements with probable temporal scales in
the order of 10 s to 30 s. The mean temporal scale of coherent structures detected in the
trunk space of the forest was 30 - 40 s. The number of coherent structures detected at the
slim and tall tower was found to be 40% less than the number of coherent structures detected
at the heavy main tower. In contrast to the slim tower the main tower is more massive
and was equipped with more instruments resulting for additional generation of
turbulence.
The Reynolds-averaged flux and flux contribution of coherent structures were derived using a
triple decomposition for the detected and conditionally averaged time series, when
coherent structures were present. The analysis shows dominant momentum and
sensible heat transport by coherent structures in the canopy space. Carbon dioxide and
latent heat transport by coherent structures increases with height within the canopy
and reaches a maximum at the upper canopy level. The flux contribution of the
ejection phase decreases with increasing height within the canopy and becomes
dominant above the canopy level. The flux fraction transported during the downward
directed sweep phase increases with height within the canopy and becomes the
dominating exchange process at the upper canopy level. Close to the ground surface
in the subcanopy space, ejection and sweep phase contribute equally to the flux
transport.
The determined exchange regimes indicate consistent decoupling between trunk space,
canopy, air above the canopy during evening, night and morning hours. Entire coupling
between all canopy levels and trunk space of the forest was observed around noon. |
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