Countergradient heat flux observations during the evening transition period
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 de...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2014-09-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | http://www.atmos-chem-phys.net/14/9077/2014/acp-14-9077-2014.pdf |
| _version_ | 1819037007969517568 |
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| author | E. Blay-Carreras E. R. Pardyjak D. Pino D. C. Alexander F. Lohou M. Lothon |
| author_facet | E. Blay-Carreras E. R. Pardyjak D. Pino D. C. Alexander F. Lohou M. Lothon |
| author_sort | E. Blay-Carreras |
| collection | DOAJ |
| description | 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.
<br><br>
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. |
| first_indexed | 2024-12-21T08:14:35Z |
| format | Article |
| id | doaj.art-60c92d8e8baf49e4b87acc891250898f |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-21T08:14:35Z |
| publishDate | 2014-09-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-60c92d8e8baf49e4b87acc891250898f2022-12-21T19:10:36ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242014-09-0114179077908510.5194/acp-14-9077-2014Countergradient heat flux observations during the evening transition periodE. Blay-Carreras0E. R. Pardyjak1D. Pino2D. C. Alexander3F. Lohou4M. Lothon5Department of Applied Physics, Universitat Politècnica de Catalunya, BarcelonaTech, Barcelona, SpainDepartment of Mechanical Engineering, University of Utah, Salt Lake City, UT, USADepartment of Applied Physics, Universitat Politècnica de Catalunya, BarcelonaTech, Barcelona, SpainDepartment of Mechanical Engineering, University of Utah, Salt Lake City, UT, USAUniversité de Toulouse – CNRS 5560, Laboratoire d'Aérologie, Toulouse, FranceUniversité de Toulouse – CNRS 5560, Laboratoire d'Aérologie, Toulouse, FranceGradient-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. <br><br> 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.http://www.atmos-chem-phys.net/14/9077/2014/acp-14-9077-2014.pdf |
| spellingShingle | E. Blay-Carreras E. R. Pardyjak D. Pino D. C. Alexander F. Lohou M. Lothon Countergradient heat flux observations during the evening transition period Atmospheric Chemistry and Physics |
| title | Countergradient heat flux observations during the evening transition period |
| title_full | Countergradient heat flux observations during the evening transition period |
| title_fullStr | Countergradient heat flux observations during the evening transition period |
| title_full_unstemmed | Countergradient heat flux observations during the evening transition period |
| title_short | Countergradient heat flux observations during the evening transition period |
| title_sort | countergradient heat flux observations during the evening transition period |
| url | http://www.atmos-chem-phys.net/14/9077/2014/acp-14-9077-2014.pdf |
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