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  • Author or Editor: M. Y. Leclerc x
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B. J. Turner
and
M. Y. Leclerc

Abstract

The development and evaluation of a technique to detect and retrieve coherent structures embedded in a record of atmospheric surface-layer temperature fluctuations is described. This new detection scheme, based on a local wavelet transform, is used to conditionally sample the organized structures from the more random background turbulence. This method provides a much needed alternative to the commonly used but not so objective variable interval time averaging. When conditional sampling using the wavelet transform is applied, the series is decomposed into two signals, one with high wavelet coefficients and one with low coefficients, corresponding to the strong events and to random turbulence of the flow, respectively. A test of the method shows it to be successful in detecting and retrieving localized structures in a record of temperature fluctuations.

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T. V. Prabha
,
M. Y. Leclerc
, and
D. Baldocchi

Abstract

Flux footprints for neutral shear-driven canopy flows are evaluated using large-eddy simulation (LES) and a Lagrangian stochastic (LS) model. The Lagrangian stochastic model is driven by flow statistics derived from the large-eddy simulation. LES results suggest that both surface and elevated sources inside the canopy contribute equally to the cumulative flux from an upwind distance of 4 times the canopy height. LES flux footprints are more contracted than those obtained using the Lagrangian stochastic model. This is attributed to an enhanced vertical diffusion and reduced horizontal diffusion. The ejection and sweep contributions to momentum exchange in the Lagrangian stochastic model are weaker than those in the large-eddy simulation. Ejections of low-momentum air dominate at all levels in the canopy modeled by the LES. In contrast, high-momentum sweep events are dominant within the LES canopy and low-momentum ejection events are dominant above the canopy. Dispersion parameters for the first- and second-order statistics of concentration from both LES and LS for three line sources representing the canopy crown, midcanopy, and surface sources are also investigated. Lagrangian model results are sensitive to the choice of the time scale. A time scale based on the dissipation rate agrees well with the LS and LES plume heights of surface source. However, flux footprints from LS are closer to those from the LES, while an intermediate time scale (0.15z/σw ) was used inside the canopy.

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R. D. Taylor
,
M. Y. Leclerc
, and
G. S. Stiles

Abstract

Modern meteorological experiments are capable of generating such large quantities of data that the processing of the results can be prohibitively expensive on a mainframe or far too time-consuming on most reasonably priced desktop systems. This paper describes a cost-effective alternative based upon the transputer, a powerful 32-bit microprocessor designed specifically for developing multiprocessor systems. A single processing node, consisting of one transputer and one megabyte of RAM, can be assembled from fewer than a dozen chips for around $1200, or purchased commercially for about $1500. The system described here consists of three such nodes hosted by a PC/AT clone and calculates the space-time correlation functions of atmospheric turbulence data about 10% faster than a VAX 8650; the system can easily be expanded to tens of nodes with a nearly proportionate increase in computational power. This multiprocessor system is as easy to program as a single processor system.

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M. Y. Leclerc
,
R. H. Shaw
,
G. Den Hartog
, and
H. H. Neumann

Abstract

This paper shows that the inclusion of thermal effects is necessary to correctly interpret the physical processes involved in the generation or suppression of Reynolds stress and turbulent kinetic energy inside a form canopy. In both of thew budgets, thermal effects are largest in the upper third of the canopy where the foliage is densest and the radiation load highest. The magnitude of the buoyant production term in both these budgets increases almost linearly with instability in the upper region of the canopy. The onset of stability exerts a strong influence on the behavior of the shear production in both the budgets of Reynolds stress and turbulent kinetic energy. In strong thermal stratification, the shear production term becomes a sink of Reynolds stress and turbulent kinetic energy in the lower half of the canopy.

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D. Finn
,
B. Lamb
,
M. Y. Leclerc
,
S. Lovejoy
,
S. Pecknold
, and
D. Schertzer

Abstract

A codimension multifractal methodology was used to analyze and to model scalar concentration fluctuations within sulfur hexafluoride tracer gas plumes from a line source in atmospheric surface-layer flows. Correspondence was exhibited between the double trace moments parameters α and C 1 of the codimension methodology and the experimentally measured plume concentration characteristics of peak-to-mean ratio and concentration fluctuation intensity. Data series were generated using an extremal Levy, stochastic multifractal model, with the experimental α and C 1 as inputs. Uncertainties in experimentally determined plume characteristic values overlapped the uncertainties in model-simulated values. The utility of the procedure includes 1) characterizing the state of scalar turbulent mixing, 2) helping to evaluate and to model hazardous plume concentrations, and 3) being able to estimate the probability of realizing extreme events at timescales of observation beyond or at magnitudes in excess of those present in the actual observations.

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