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| Titel |
Countergradient heat flux observations during the evening transition period |
| VerfasserIn |
E. Blay-Carreras, E. R. Pardyjak, D. Pino, D. C. Alexander, F. Lohou, M. Lothon |
| 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. 17 ; Nr. 14, no. 17 (2014-09-03), S.9077-9085 |
| Datensatznummer |
250119001
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| Publikation (Nr.) |
 copernicus.org/acp-14-9077-2014.pdf |
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| Zusammenfassung |
Gradient-based turbulence models generally assume that the buoyancy flux
ceases to introduce heat into the surface layer of the atmospheric boundary
layer in temporal consonance with the gradient of the local virtual potential
temperature. Here, we hypothesize that during the evening transition a delay
exists between the instant when the buoyancy flux goes to zero and the time
when the local gradient of the virtual potential temperature indicates a sign
change. This phenomenon is studied using a range of data collected over
several intensive observational periods (IOPs) during the Boundary Layer Late
Afternoon and Sunset Turbulence field campaign conducted in Lannemezan,
France. The focus is mainly on the lower part of the surface layer using a
tower instrumented with high-speed temperature and velocity sensors.
The results from this work confirm and quantify a flux-gradient delay.
Specifically, the observed values of the delay are ~ 30–80 min.
The existence of the delay and its duration can be explained
by considering the convective timescale and the competition of forces
associated with the classical Rayleigh–Bénard problem. This combined
theory predicts that the last eddy formed while the sensible heat flux
changes sign during the evening transition should produce a delay. It appears
that this last eddy is decelerated through the action of turbulent momentum
and thermal diffusivities, and that the delay is related to the convective
turnover timescale. Observations indicate that as horizontal shear becomes
more important, the delay time apparently increases to values greater than
the convective turnover timescale. |
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