Application of a Large-Eddy Simulation Database to Optimisation of First-Order Closures for Neutral and Stably Stratified Boundary Layers |
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Authors: | Igor N Esau Øyvind Byrkjedal |
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Institution: | (1) G.C. Rieber Climate Institute, Nansen Environmental and Remote Sensing Centre, Thormohlensgt, 47, Bergen, 5006, Norway;(2) Bjerknes Centre for Climate Research, Bergen, Norway;(3) Geophysical Institute, University of Bergen, Bergen, Norway |
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Abstract: | Large-eddy simulation (LES) is a well-established numerical technique, resolving the most energetic turbulent fluctuations
in the planetary boundary layer. By averaging these fluctuations, high-quality profiles of mean quantities and turbulence
statistics can be obtained in experiments with well-defined initial and boundary conditions. Hence, LES data can be beneficial
for assessment and optimisation of turbulence closure schemes. A database of 80 LES runs (DATABASE64) for neutral and stably
stratified planetary boundary layers (PBLs) is applied in this study to optimize first-order turbulence closure (FOC). Approximations
for the mixing length scale and stability correction functions have been made to minimise a relative root-mean-square error
over the entire database. New stability functions have correct asymptotes describing regimes of strong and weak mixing found
in theoretical approaches, atmospheric observations and LES. The correct asymptotes exclude the need for a critical Richardson
number in the FOC formulation. Further, we analysed the FOC quality as functions of the integral PBL stability and the vertical
model resolution. We show that the FOC is never perfect because the turbulence in the upper half of the PBL is not generated
by the local vertical gradients. Accordingly, the parameterised and LES-based fluxes decorrelate in the upper PBL. With this
imperfection in mind, we show that there is no systematic quality deterioration of the FOC in the strongly stable PBL provided
that the vertical model resolution is better than 10 levels within the PBL. In agreement with previous studies, we found that
the quality improves slowly with the vertical resolution refinement, though it is generally wise not to overstretch the mesh
in the lowest 500 m of the atmosphere where the observed, simulated and theoretically predicted stably stratified PBL is mostly
located.
The submission to a special issue of the “Boundary-Layer Meteorology” devoted to the NATO advanced research workshop “Atmospheric Boundary Layers: Modelling and Applications for Environmental Security”. |
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Keywords: | Atmospheric turbulence First-order turbulence closure Large-eddy simulation Parameterization accuracy Stable boundary layer |
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