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Volume 16, issue 14 | Copyright

Special issue: The Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST)...

Atmos. Chem. Phys., 16, 8873-8898, 2016
https://doi.org/10.5194/acp-16-8873-2016
© Author(s) 2016. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 19 Jul 2016

Research article | 19 Jul 2016

Turbulence kinetic energy budget during the afternoon transition – Part 2: A simple TKE model

Erik Nilsson1,2, Marie Lothon1, Fabienne Lohou1, Eric Pardyjak3, Oscar Hartogensis4, and Clara Darbieu1 Erik Nilsson et al.
  • 1Laboratoire d'Aerologie, University of Toulouse, CNRS, Toulouse, France
  • 2Department of Earth Sciences, Uppsala University, Uppsala, Sweden
  • 3Department of Mechanical Engineering, Utah University, Salt Lake City, UT, USA
  • 4Meteorology and Air Quality Section, Wageningen University, Wageningen, the Netherlands

Abstract. 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.

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 tα may be questionable.

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.

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A new simple model for turbulence kinetic energy (TKE) and its budget is presented for the sheared convective atmospheric boundary layer. It is used to study effects of buoyancy and shear on TKE evolution during the afternoon transition, especially near the surface. We also find a region of weak turbulence during unstable afternoon conditions below the inversion top, which we refer to as a "pre-residual layer".
A new simple model for turbulence kinetic energy (TKE) and its budget is presented for the...
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