Turbulence kinetic energy budget during the afternoon transition – Part 2: A simple TKE model
A simple model for turbulence kinetic energy (TKE) and the TKE budget is presented for sheared convective atmospheric conditions based on observations from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. It is based on an idealized mixed-layer approximation and a...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2016-07-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/8873/2016/acp-16-8873-2016.pdf |
| _version_ | 1818130270747885568 |
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| author | E. Nilsson E. Nilsson M. Lothon F. Lohou E. Pardyjak O. Hartogensis C. Darbieu |
| author_facet | E. Nilsson E. Nilsson M. Lothon F. Lohou E. Pardyjak O. Hartogensis C. Darbieu |
| author_sort | E. Nilsson |
| collection | DOAJ |
| description | A simple model for turbulence kinetic energy (TKE) and the TKE
budget is presented for sheared convective atmospheric conditions
based on observations from the Boundary Layer Late Afternoon and
Sunset Turbulence (BLLAST) field campaign. It is based on an
idealized mixed-layer approximation and a simplified near-surface
TKE budget. In this model, the TKE is dependent on four budget terms
(turbulent dissipation rate, buoyancy production, shear production
and vertical transport of TKE) and only requires measurements of
three available inputs (near-surface buoyancy flux, boundary layer
depth and wind speed at one height in the surface layer) to predict vertical profiles of TKE and TKE budget terms.<br><br>This simple model is shown to reproduce some of the observed
variations between the different studied days in terms of
near-surface TKE and its decay during the afternoon transition
reasonably well. It is subsequently used to systematically study the
effects of buoyancy and shear on TKE evolution using idealized
constant and time-varying winds during the afternoon
transition. From this, we conclude that many different TKE decay
rates are possible under time-varying winds and that generalizing
the decay with simple scaling laws for near-surface TKE of the form
<i>t</i><sup><i>α</i></sup> may be questionable.<br><br>The model's errors result from the exclusion of processes such as
elevated shear production and horizontal advection. The model also
produces an overly rapid decay of shear production with
height. However, the most influential budget terms governing
near-surface TKE in the observed sheared convective boundary layers
are included, while only second-order factors are
neglected. Comparison between modeled and averaged observed
estimates of dissipation rate illustrates that the overall behavior
of the model is often quite reasonable. Therefore, we use the model
to discuss the low-turbulence conditions that form first in the
upper parts of the boundary layer during the afternoon transition
and are only apparent later near the surface. This occurs as
a consequence of the continuous decrease in near-surface buoyancy
flux during the afternoon transition. This region of weak afternoon
turbulence is hypothesized to be a “pre-residual layer”, which is
important in determining the onset conditions for the weak sporadic
turbulence that occur in the residual layer once near-surface
stratification has become stable. |
| first_indexed | 2024-12-11T08:02:23Z |
| format | Article |
| id | doaj.art-e7cb92416612462cbd8202086902ce9c |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-11T08:02:23Z |
| publishDate | 2016-07-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-e7cb92416612462cbd8202086902ce9c2022-12-22T01:15:05ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242016-07-01168873889810.5194/acp-16-8873-2016Turbulence kinetic energy budget during the afternoon transition – Part 2: A simple TKE modelE. Nilsson0E. Nilsson1M. Lothon2F. Lohou3E. Pardyjak4O. Hartogensis5C. Darbieu6Laboratoire d'Aerologie, University of Toulouse, CNRS, Toulouse, FranceDepartment of Earth Sciences, Uppsala University, Uppsala, SwedenLaboratoire d'Aerologie, University of Toulouse, CNRS, Toulouse, FranceLaboratoire d'Aerologie, University of Toulouse, CNRS, Toulouse, FranceDepartment of Mechanical Engineering, Utah University, Salt Lake City, UT, USAMeteorology and Air Quality Section, Wageningen University, Wageningen, the NetherlandsLaboratoire d'Aerologie, University of Toulouse, CNRS, Toulouse, FranceA simple model for turbulence kinetic energy (TKE) and the TKE budget is presented for sheared convective atmospheric conditions based on observations from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. It is based on an idealized mixed-layer approximation and a simplified near-surface TKE budget. In this model, the TKE is dependent on four budget terms (turbulent dissipation rate, buoyancy production, shear production and vertical transport of TKE) and only requires measurements of three available inputs (near-surface buoyancy flux, boundary layer depth and wind speed at one height in the surface layer) to predict vertical profiles of TKE and TKE budget terms.<br><br>This simple model is shown to reproduce some of the observed variations between the different studied days in terms of near-surface TKE and its decay during the afternoon transition reasonably well. It is subsequently used to systematically study the effects of buoyancy and shear on TKE evolution using idealized constant and time-varying winds during the afternoon transition. From this, we conclude that many different TKE decay rates are possible under time-varying winds and that generalizing the decay with simple scaling laws for near-surface TKE of the form <i>t</i><sup><i>α</i></sup> may be questionable.<br><br>The model's errors result from the exclusion of processes such as elevated shear production and horizontal advection. The model also produces an overly rapid decay of shear production with height. However, the most influential budget terms governing near-surface TKE in the observed sheared convective boundary layers are included, while only second-order factors are neglected. Comparison between modeled and averaged observed estimates of dissipation rate illustrates that the overall behavior of the model is often quite reasonable. Therefore, we use the model to discuss the low-turbulence conditions that form first in the upper parts of the boundary layer during the afternoon transition and are only apparent later near the surface. This occurs as a consequence of the continuous decrease in near-surface buoyancy flux during the afternoon transition. This region of weak afternoon turbulence is hypothesized to be a “pre-residual layer”, which is important in determining the onset conditions for the weak sporadic turbulence that occur in the residual layer once near-surface stratification has become stable.https://www.atmos-chem-phys.net/16/8873/2016/acp-16-8873-2016.pdf |
| spellingShingle | E. Nilsson E. Nilsson M. Lothon F. Lohou E. Pardyjak O. Hartogensis C. Darbieu Turbulence kinetic energy budget during the afternoon transition – Part 2: A simple TKE model Atmospheric Chemistry and Physics |
| title | Turbulence kinetic energy budget during the afternoon
transition – Part 2: A simple TKE model |
| title_full | Turbulence kinetic energy budget during the afternoon
transition – Part 2: A simple TKE model |
| title_fullStr | Turbulence kinetic energy budget during the afternoon
transition – Part 2: A simple TKE model |
| title_full_unstemmed | Turbulence kinetic energy budget during the afternoon
transition – Part 2: A simple TKE model |
| title_short | Turbulence kinetic energy budget during the afternoon
transition – Part 2: A simple TKE model |
| title_sort | turbulence kinetic energy budget during the afternoon transition part 2 a simple tke model |
| url | https://www.atmos-chem-phys.net/16/8873/2016/acp-16-8873-2016.pdf |
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