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| Titel |
Low-level jets and above-canopy drainage as causes of turbulent exchange in the nocturnal boundary layer |
| VerfasserIn |
T. S. El-Madany, H. F. Duarte, D. J. Durden, B. Paas, M. J. Deventer, J.-Y. Juang, M. Y. Leclerc, O. Klemm |
| 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 ; 11, no. 16 ; Nr. 11, no. 16 (2014-08-27), S.4507-4519 |
| Datensatznummer |
250117563
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| Publikation (Nr.) |
 copernicus.org/bg-11-4507-2014.pdf |
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| Zusammenfassung |
Sodar (SOund Detection And Ranging), eddy-covariance, and tower profile
measurements of wind speed and carbon dioxide were performed during 17
consecutive nights in complex terrain in northern Taiwan. The scope of the
study was to identify the causes for intermittent turbulence events and to
analyze their importance in nocturnal atmosphere–biosphere exchange as
quantified with eddy-covariance measurements. If intermittency occurs
frequently at a measurement site, then this process needs to be quantified
in order to achieve reliable values for ecosystem characteristics such as
net ecosystem exchange or net primary production.
Fourteen events of intermittent turbulence were identified and classified
into above-canopy drainage flows (ACDFs) and low-level jets (LLJs) according to
the height of the wind speed maximum. Intermittent turbulence periods lasted
between 30 and 110 min. Towards the end of LLJ or ACDF events, positive
vertical wind velocities and, in some cases, upslope flows occurred,
counteracting the general flow regime at nighttime. The observations suggest
that the LLJs and ACDFs penetrate deep into the cold air pool in the valley,
where they experience strong buoyancy due to density differences, resulting
in either upslope flows or upward vertical winds.
Turbulence was found to be stronger and better developed during LLJs and
ACDFs, with eddy-covariance data presenting higher quality. This was
particularly indicated by spectral analysis of the vertical wind velocity
and the steady-state test for the time series of the vertical wind velocity
in combination with the horizontal wind component, the temperature, and
carbon dioxide.
Significantly higher fluxes of sensible heat, latent heat, and shear stress
occurred during these periods. During LLJs and ACDFs, fluxes of sensible heat,
latent heat, and CO2 were mostly one-directional. For example,
exclusively negative sensible heat fluxes occurred while intermittent
turbulence was present. Latent heat fluxes were mostly positive during LLJs
and ACDFs, with a median value of 34 W m−2, while outside these periods
the median was 2 W m−2. In conclusion, intermittent turbulence periods
exhibit a strong impact on nocturnal energy and mass fluxes. |
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