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| Titel |
Momentum and scalar transport within a vegetation canopy following atmospheric stability and seasonal canopy changes: the CHATS experiment |
| VerfasserIn |
S. Dupont, E. G. Patton |
| 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 ; 12, no. 13 ; Nr. 12, no. 13 (2012-07-12), S.5913-5935 |
| Datensatznummer |
250011306
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| Publikation (Nr.) |
 copernicus.org/acp-12-5913-2012.pdf |
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| Zusammenfassung |
Momentum and scalar (heat and water vapor) transfer between a walnut
canopy and the overlying atmosphere are investigated for two seasonal
periods (before and after leaf-out), and for five thermal stability
regimes (free and forced convection, near-neutral condition,
transition to stable, and stable). Quadrant and octant analyses of
momentum and scalar fluxes followed by space-time autocorrelations of
observations from the Canopy Horizontal Array Turbulence Study's
(CHATS) thirty meter tower help characterize the motions exchanging
momentum, heat, and moisture between the canopy layers and aloft.
During sufficiently windy conditions, i.e. in forced convection,
near-neutral and transition to stable regimes, momentum and scalars
are generally transported by sweep and ejection motions associated
with the well-known canopy-top "shear-driven" coherent eddy
structures. During extreme stability conditions (both unstable and
stable), the role of these "shear-driven" structures in
transporting scalars decreases, inducing notable dissimilarity between
momentum and scalar transport.
In unstable conditions, "shear-driven" coherent structures are
progressively replaced by "buo-yantly-driven" structures, known
as thermal plumes; which appear very efficient at transporting
scalars, especially upward thermal plumes above the canopy. Within
the canopy, downward thermal plumes become more efficient at
transporting scalars than upward thermal plumes if scalar sources
are located in the upper canopy. We explain these features by
suggesting that: (i) downward plumes within the canopy correspond
to large downward plumes coming from above, and (ii) upward plumes
within the canopy are local small plumes induced by canopy heat
sources where passive scalars are first injected if there sources
are at the same location as heat sources. Above the canopy, these
small upward thermal plumes aggregate to form larger scale upward
thermal plumes. Furthermore, scalar quantities carried by downward
plumes are not modified when penetrating the canopy and crossing
upper scalar sources. Consequently, scalars appear to be preferentially
injected into upward thermal plumes as opposed to in downward thermal
plumes.
In stable conditions, intermittent downward and upward motions
probably related to elevated shear layers are responsible for
canopy-top heat and water vapor transport through the initiation
of turbulent instabilities, but this transport remains small. During
the foliated period, lower-canopy heat and water vapor transport
occurs through thermal plumes associated with a subcanopy unstable
layer. |
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