Daniela Cava

Spectral Maxima In A Perturbed Stable Boundary Layer

Wind velocity data have been collected on Nansen Ice Sheet, Antarctica, close to the base of a steeply sloping glacier along which frequently flow katabatic winds. The aim of this study is to investigate how turbulent energy and momentum flux are perturbed by the flow interaction with topography and by the strong mechanical mixing produced by downslope flows. Spectral and cospectral analyses, performed on the wind velocity components, provide evidence that such a perturbation, at any stability, is restricted to frequencies lower than the inertial subrange. Longitudinal spectra display an energy increment, due to turbulence generated by topography and by mechanical forcing related to the katabatic wind structure. The energy, supplied by the topographic forcing, displaces the turbulent energy maximum toward lower frequencies. In near-neutral stratification the spectral maximum occurs at a reduced frequency, which seems to be consistent with the height of the steepest part of the slope, and seems to shift toward higher frequencies as a linear ,function of the local stability parameter,Ll. The parameterisation of the orographic perturbation by means of a similarity relationship allows us to scale u spectra in the same way as over uniform terrain. The scaled, perturbed spectra collapse onto a unique curve in the mid-frequency as well in the inertial subrange, while maxima are grouped in a cluster. Lateral and vertical velocity spectra exhibit shapes independent of stability, suggesting a topographic perturbation that is predominantly over stability effects.

Structure Functions In A Wall-Turbulent Shear Flow

Wavelet and quadrant analyses were applied to turbulent velocity data in order to investigate the transition from the anisotropy of energy-containing eddies to the isotropy of the inertial subrange scales. The quadrant analysis of the wavelet coefficients of longitudinal and vertical velocity components allows the evaluation of the velocity structure functions and the momentum cospectrum as a function of the separation distance and of the quadrants. In an isotropic condition the contribution both of ejections and sweeps (even quadrants), and both of reflections and deflections (odd quadrants), has to be equal. The analysis has shown that in neutrally stratified conditions the transition to isotropy occurs in a frequency range (0.2 < r/z < 3) usually referred to as internal to the inertial subrange (r is separation distance, z is height). In the transition region, as in the isotropic region, the velocity structure functions still agree with the 1941 and 1962 Kolmogorov theories; but on the other hand the structure functions of the even and odd quadrants are fitted by power laws of different slopes in the transition region. The proposed analysis allows the investigation within the transition region of the different dynamical structure in the energy transfer from the energy-containing scales to the isotropic scales.

Stationarity, Homogeneity, and Ergodicity in Canopy Turbulence

One of the defining syndromes of turbulence is nonlinear stochasticity. This view of turbulence motivated the development of statistical mechanics theories that have served to connect the basic Navier-Stokes (NS) equations of motion to the statistical results of numerous field experiments. In general, the proper averaging operator for stochastic processes is ensemble averaging. Given the transient nature of flow boundary conditions in natural systems, field experiments are typically unable to capture a suitable ensemble, in a strict sense. Instead, field experiments typically focus on time averaged statistics. Stationarity and ergodicity are two central concepts (required conditions) used to link field measurements and the NS equations or field measurements to “boundary conditions” at the land-atmosphere interface. In this Chapter, we present an elementary review of these two concepts for the atmospheric surface layer (ASL) and canopy sublayer (CSL) and proceed to show why the stable CSL tends to violate both conditions. A weaker form of these two conditions may be applicable to CSL flows that are only moderately stably stratified. Practical implications for night time CO2 flux corrections are also discussed.

Generalized logarithmic scaling for high-order moments of the longitudinal velocity component explained by the random sweeping decorrelation hypothesis

Expressions for the logarithmic variations of the normalized turbulent longitudinal velocity (u2p¯+)1/p with normalized distance z/δ from a boundary for high-order (p) moments in the intermediate region of wall bounded flows characterized by thickness δ are derived. The ansatz that (u2p¯+)1/p variation in ln(z/δ) originates from a compound effect of random sweeping and -1 power-law scaling in the longitudinal velocity spectrum Eu(k) is discussed, where k is the wavenumber. Using velocity time series sampled above a uniform ice sheet, an Eu(k) ∼ k−1 scaling is confirmed for kz 1. The data were then used to analyze assumptions required for the utility of the random sweeping decorrelation (RSD) hypothesis connecting the k−1 power-law with log-scaling in (u2p¯+)1/p. It has been found out that while the RSD hypothesis is operationally applicable to scales associated with attached eddies bounded by kz 1, significant interactions among high-order turbulent velocity and velocity incremen...

Monitoring the micrometeorology of a coastal site next to a thermal power plant from the surface to 140 m

AbstractA 140-m micrometeorological tower has been operating since August 2016 at 4 km from the coastline and 250 m from a thermal power plant that releases heat from its 20-m stacks in southeaster...

Turbulence Characteristics Of A Stable BoundaryLayer Over Sloping Terrain

During the austral summer 1993-94 turbulence data have been taken in a coastal region of Antarctica over a gently sloping surface. The objective of this study is to determine how the turbulent structure of the stable boundary layer is perturbed by slope and waves. Spectral analysis of wind velocity components evidences the production of energy, prevalently in the mid frequency subrange, by local orographic forcing. The power contribute of waves is highly variable as single or multiple waves of different amplitudes are observed. A criterion, based on the shear stress correlation coefficient, to distinguish turbulent flows from the ones affected also by waves activity is suggested. This procedure should give an estimate of the relative entity of wavy and turbulent contributes to velocity standard deviations.