Near-neutral Atmospheric Surface Layer Research Articles

A Theoretical and Experimental Investigation of Energy-Containing Scales in the Dynamic Sublayer of Boundary-Layer Flows

The existence of universal power laws at low wavenumbers (K) in the energy spectrum (Eu) of the turbulent longitudinal velocity (u) is examined theoretically and experimentally for the near-neutral atmospheric surface layer. Newly derived power-law solutions to Tchen's approximate integral spectral budget equation are tested for strong- and weak-interaction cases between the mean flow and turbulent vorticity fields. To verify whether these solutions reproduce the measured Eu at low wavenumbers, velocity measurements were collected in the dynamic sublayer of the atmosphere at three sites and in the inner region of a laboratory open channel. The atmospheric surface layer measurements were carried out using triaxial sonic anemometers over tall corn, short grass, and smooth desert-like sandy soil. The open channel measurements were performed using a two-dimensional boundary-layer probe above a smooth stainless steel bed. Comparisons between the proposed analytical solution for Eu, the dimensional analysis by Kader and Yaglom, and the measured Haar wavelet Eu spectra are presented. It is shown that when strong interaction between the mean flow and turbulent vorticity field occurs, wavelet spectra measurements, predictions by the analytical solution, and predictions by the dimensional analysis of Kader-Yaglom (KY) are all in good agreement and confirm the existence of a -1 power law in Eu(= Cuuu2* K-1, where Cuu is a constant and u* is the friction velocity). The normalized upper wavenumber limit of the -1 power law (Kz = 1, where z is the height above the zero-plane displacement) is estimated using two separate approaches and compared to the open channel and atmospheric surface-layer measurements. It is demonstrated that the measured upper wavenumber limit is consistent with Tchen's budget but not with the KY assumptions. The constraints as to whether the mean flow and turbulent vorticity strongly interact are considered using a proposed analysis by Panchev. It is demonstrated that the arguments by Panchev cannot be consistent with surface-layer turbulence. Using dimensional analysis and Heisenberg's turbulent viscosity model, new constraints are proposed. The new constraints agree with the open channel and atmospheric surface-layer measurements, Townsend's ‘inactive eddy motion hypothesis’, and the Perry et al. analysis.

Organized Turbulence Structures in the Near-Neutral Atmospheric Surface Layer

The Tiederman method, which was originally designed for unambiguous detection of “bursts” in the buffer layer of smooth surface laboratory boundary-layer flow, is shown to work equally well in the neutral atmospheric surface layer. It enables estimation of characteristic timescales of significant structures responsible for the momentum transport: mean duration of and mean interval between bursts (upward transport of momentum deficit) and mean duration and interval between sweeps (downward transport of momentum excess). Data representing near neutral conditions from three field experiments over flat and homogeneous areas but with very different roughness characteristics are used in the analysis. For two low vegetation sites (i.e., the measuring heights are very much larger than the roughness sublayer height), it is found that all above-mentioned timescales are constant for any height and identical at the two sites. Scaling the results with the friction velocity indicates the corresponding scaling height to be proportional to the scaling height of the neutral boundary layer. The third site is a forest site, and the measurements were taken in the roughness sublayer. Here canopy inflection scaling is found to apply. The structures identified with the Tiederman method are shown to be responsible for more than 90% of the momentum flux, most of which is accomplished by a mean flow component within the structures. Results are discussed in light of direct numerical simulation results for fully turbulent but low Reynolds number flow over a dynamically smooth surface and large-eddy simulation results for a high Reynolds number case, and these show encouraging consistencies for the logarithmic layer. Turbulence production at the surface is, however, fundamentally different in the rough atmospheric case compared to the smooth case and is presumably governed by canopy wind profile inflection instability.

Recurrence statistics of concentration fluctuations in plumes within a near-neutral atmospheric surface layer

This study examines the statistical properties of the concentration derivative, χ′, for a dispersing plume in a near-neutrally stratified atmospheric surface layer. Towards this goal, the probability density function (pdf) of χ′, and the conditional pdf of χ′ given a fixed concentration level, χ, have been measured. These pdfs are found to be modeled well by a generalizedq-Gaussian (gqG) distribution with intermittency exponent,q, equal to 0.3 and 3/4, respectively. These results highlight the strong intermittency effect (patchiness) of the small-scale concentration eddy structures in the plume. The distribution of time intervals between successive high peaks in the squared derivative process, x′2, is found to be well approximated by a power-law distribution, implying that occurrences of these high peaks are much more clustered than would be predicted by a Poisson or shot-noise process. The results are used to improve models for the joint pdf of χ and χ′, and for the expected number of upcrossings per unit time interval of a fixed concentration level that have been proposed by Kristensenet al. (1989). The predictions of the improved models are in accord with observations, and suggest that the intercorrelation between χ and χ′ must be explicitly incorporated if good estimates of the upcrossing intensity are to be obtained.