Turbulent Prandtl Number in Stably Stratified Atmospheric Boundary Layer: Intercomparison between LES and SHEBA DataEzau, Igor and Grachev, Andrey (2007) Turbulent Prandtl Number in Stably Stratified Atmospheric Boundary Layer: Intercomparison between LES and SHEBA Data. e-WindEng, 2007 (006). pp. 01-17. ISSN 1901-9181 This is the latest version of this eprint. Full text available as:
Official URL: http://ejournal.windeng.net AbstractTurbulence-resolving modelling technique, widely known as large-eddy simulation (LES), becomes a popular tool to investigate environmental turbulence and to aggregate the impact of the turbulent mixing on larger, meteorologically important scales. Although the LES are proved to be useful, the technique still needs careful validation against available data sets. In the research of the stably stratified layers, one of the key questions is relative mixing efficiency for heat, mass and momentum. Turbulence mixing efficiency could be measured by the turbulent Prandtl number, Pr. It is defined as a ratio of the turbulent eddy viscosity to the turbulent temperature diffusivity. Increase of Pr with increasing stability, measured through the gradient Richardson number, is observed both in the atmospheric field experiments (e.g. SHEBA data) and the turbulence resolving LES data. At the same time, Pr is observed to decrease with decay of the turbulence intensity, measured through the flux Richardson number and the non-dimensional height. This fact indicates more significant role of the turbulent heat conductivity in the stably stratified boundary layers, possibly due to momentum loss through irradiation of internal gravity waves, which partially offset the general suppression of the turbulent exchange by strong static stability. One practically important consequence of the more intensive momentum mixing than it would be under constant Pr is that the surface and the atmosphere always remain dynamically coupled. This coupling prevents the surface cooling to values (-80o C or so) characterizing the surface radiative equilibrium, thus significantly warming the polar climates. Direct observational studies of the mixing efficiency are difficult and generally committed only within a shallow surface layer. Laboratory studies are rarely reach strong stabilities. In these circumstances, homogeneous, high quality LES data covering the entire boundary layer are potentially of great significance to theoretical turbulence research as well as parameterization design. In this paper, we show for the first time that the observed SHEBA and numerical LES data are in good agreement across large span of static stabilities. Despite this fact, important discrepancies remain. The discrepancies have been observed for very weakly and very strongly stratified cases where correspondingly observations or numerical simulations are difficult to achieve.
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