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Analysis of Turbulence Structure in the Surface Layer with a Modified Similarity Formulation for Near Neutral Conditions

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  • 1 Department of Meteorology, Uppsala University, Uppsala, Sweden
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Abstract

Data from a recent detailed surface layer experiment are critically examined in terms of the turbulent kinetic energy budget and the other second order moment budgets formed by the three velocity components and temperature. In moderately unstable and slightly stable conditions nondimensional terms of all the moment budgets studied agree reasonably well with results reported from the Kansas study (after application of a flow distortion correction). In the near-neutral range, where the present experiment contains a large amount of data, results deviate significantly from previous studies in general and, in particular, for ideal, zero-pressure gradient turbulent boundary layers. Several moments, such as u2W, v2w and W2 are not, as expected, constant in the surface layer, but vary logarithmically with height, making instead their non dimensional vertical gradients constant. Some moments scale with the roughness length and others with a length scale containing the large-scale pressure gradient or, with an alternative interpretation the height of the neutral PBL. Evidence is presented that these apparent anomalies are due to so called “inactive” turbulence. From the present analysis, and from some previously published atmospheric studies, it is concluded that the values for the various nondimensional gradients may be universally valid, thus suggesting a modified similarity formulation for the neutral atmospheric surface layer.

Abstract

Data from a recent detailed surface layer experiment are critically examined in terms of the turbulent kinetic energy budget and the other second order moment budgets formed by the three velocity components and temperature. In moderately unstable and slightly stable conditions nondimensional terms of all the moment budgets studied agree reasonably well with results reported from the Kansas study (after application of a flow distortion correction). In the near-neutral range, where the present experiment contains a large amount of data, results deviate significantly from previous studies in general and, in particular, for ideal, zero-pressure gradient turbulent boundary layers. Several moments, such as u2W, v2w and W2 are not, as expected, constant in the surface layer, but vary logarithmically with height, making instead their non dimensional vertical gradients constant. Some moments scale with the roughness length and others with a length scale containing the large-scale pressure gradient or, with an alternative interpretation the height of the neutral PBL. Evidence is presented that these apparent anomalies are due to so called “inactive” turbulence. From the present analysis, and from some previously published atmospheric studies, it is concluded that the values for the various nondimensional gradients may be universally valid, thus suggesting a modified similarity formulation for the neutral atmospheric surface layer.

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