The Experts below are selected from a list of 96 Experts worldwide ranked by ideXlab platform
Gabriel G Katul - One of the best experts on this subject based on the ideXlab platform.
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foliage shedding in deciduous forests lifts up long distance seed dispersal by wind
Proceedings of the National Academy of Sciences of the United States of America, 2005Co-Authors: Ran Nathan, Gabriel G KatulAbstract:Seed terminal velocity and release height are recognized as key biotic determinants of long-distance dispersal (LDD) of seeds by wind. Yet, potential determinants at the ecosystem level, such as seasonal dynamics in foliage density characterizing many deciduous forests, have received much less attention. We integrated detailed field observations and experiments with a mechanistic wind dispersal model to assess how seasonal variation in foliage density, estimated by leaf-area index (LAI), affects LDD in deciduous forests. We found that the model, previously shown to accurately predict seed dispersal by wind, also reliably describes the effects of LAI variation on wind statistics for a wide range of canopy types. Sparser canopies are characterized by more organized vertical Eddy Motion that promotes LDD by uplifting seeds to higher elevations where winds are stronger. Yet, sparser canopies are also characterized by reduced mean windspeed aloft. We showed that former effect more than compensates for the latter, i.e., conditions of low LAI are favorable for LDD. This may account for the tendency of many temperate tree species to restrict seed release to either early spring or late fall, when LAI is relatively low. Sensitivity analysis reveals that the typical seasonal variation in LAI can be more important to LDD of seeds by wind than the natural variation in seed terminal velocity. Because our model accurately describes the effects of LAI variation for distinctly different sites, species, and life forms, we suggest that its results reflect a general association between LDD and foliage density dynamics.
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Quantifying Organization of Atmospheric Turbulent Eddy Motion Using Nonlinear Time Series Analysis
Boundary-Layer Meteorology, 2003Co-Authors: Karen H. Wesson, Gabriel G Katul, Mario B. S. SiqueiraAbstract:Using three methods from nonlinear dynamics, we contrast the level of organization inthe vertical wind velocity (w) time series collected in the atmospheric surface layer(ASL) and the canopy sublayer (CSL) for a wide range of atmospheric stability (ξ)conditions. The nonlinear methods applied include a modified Shannon entropy, waveletthresholding, and mutual information content. Time series measurements collected overa pine forest, a hardwood forest, a grass-covered forest clearing, and a bare soil, desertsurface were used for this purpose. The results from applying all three nonlinear timeseries measures suggest that w in the CSL is more organized than that in the ASL, and that as the flows in both layers evolve from near-neutral to near-convective conditions, the level of organization increases. Furthermore, we found that the degree of organization in w associated with changes in ξ is more significant than the transition from CSL to ASL.
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identification of low dimensional energy containing flux transporting Eddy Motion in the atmospheric surface layer using wavelet thresholding methods
Journal of the Atmospheric Sciences, 1998Co-Authors: Gabriel G Katul, Brani VidakovicAbstract:The partitioning of turbulent perturbations into a ‘‘low-dimensional’’ active part responsible for much of the turbulent energy and fluxes and a ‘‘high-dimensional’’ passive part that contributes little to turbulent energy and transport dynamics is investigated using atmospheric surface-layer (ASL) measurements. It is shown that such a partitioning scheme can be achieved by transforming the ASL measurements into a domain that concentrates the low-dimensional part into few coefficients and thus permits a global threshold of the remaining coefficients. In this transformation‐thresholding approach, Fourier rank reduction and orthonormal wavelet and wavelet packet methods are considered. The efficiencies of these three thresholding methods to extract the events responsible for much of the heat and momentum turbulent fluxes are compared for a wide range of atmospheric stability conditions. The intercomparisons are performed in four ways: (i) compression ratios, (ii) energy conservation, (iii) turbulent flux conservation, and (iv) finescale filtering via departures from Kolmogorov’s K41 power laws. For orthonormal wavelet and wavelet packets analysis, wavelet functions with varying time‐frequency localization properties are also considered. The study showed that wavelet and wavelet packet Lorentz thresholding can achieve high compression ratios (98%) with minimal loss in energy (3% loss) and fluxes (4%). However, these compression ratios and energy and flux conservation measures are comparable to the linear Fourier rank reduction method if a Lorentz threshold function is applied to the latter. Finally, it is demonstrated that orthonormal wavelet and wavelet packets thresholding are insensitive to the analyzing wavelet.
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turbulent Eddy Motion at the forest atmosphere interface
Journal of Geophysical Research, 1997Co-Authors: Gabriel G Katul, Cheng-i Hsieh, Greg Kuhn, David S Ellsworth, Dalin NieAbstract:Ejection and sweep Eddy Motions in the atmospheric surface layer (ASL) are widely accepted as being responsible for much of land surface evaporation, sensible heat flux, and momentum flux; however, less is known about this type of Eddy Motion within the canopy sublayer (CSL) of forested systems. The present study analyzed the ejection-sweep properties at the canopy-atmosphere interface of a 13 m tall, uniformly aged southern loblolly pine stand and a 33 m tall, unevenly aged hardwood stand using velocity and scalar (temperature, water vapor, and carbon dioxide) fluctuation measurements at the canopy-atmosphere interface. It was found that the measured sweeps and ejections time fractions for scalars and momentum are comparable and are in good agreement with other laboratory and field experiments. This investigation demonstrates that the third-order cumulant expansion method (CEM) reproduces the measured relative flux contribution of ejections and sweeps (ΔS 0 ) and the difference between sweep and ejection time fractions for both momentum and scalars at the canopy-atmosphere interface in contrast to findings from a previous ASL experiment. A linkage between ΔS 0 and the scalar flux budget is derived and tested via the third-order CEM at the canopy-atmosphere interface for the pine and the hardwood stands. It is shown that ΔS 0 can be related to the dimensionless scalar flux transport term whose gradient is central to the scalar variance budget. Also, the derived relationship is independent of canopy roughness or scalar sources and sinks. Hence this investigation establishes an analytical linkage between second-order closure models, the ejection-sweep cycle, and third-order CEM at the canopy-atmosphere interface. Dissimilarity between the ejection-sweep cycle for scalar and momentum transport is considered via conditional probability distributions at both forest stands. In contrast to a laboratory heat dispersion experiment, it is shown that while the ejection-sweep cycles for scalar and momentum transport are intimately linked, they are not identical. Therefore the results from momentum ejection-sweeps investigations cannot be extrapolated to scalar transport. Comparisons with other laboratory experiments are also discussed, especially in relation to the scalar ejection and sweep time fractions.
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The "inactive" Eddy Motion and the large-scale turbulent pressure fluctuations in the dynamic sublayer
Journal of the Atmospheric Sciences, 1996Co-Authors: Gabriel G Katul, Marc B Parlange, John D. Albertson, Cheng-i Hsieh, Paul S. Conklin, John Sigmon, Ken R. KnoerrAbstract:Abstract The statistical structure of the turbulent pressure fluctuations was measured in the dynamic sublayer of a large grass-covered forest clearing by a free air static pressure probe and modeled using Townsend's hypothesis. Townsend's hypothesis states that the Eddy Motion in the equilibrium layer can be decomposed into an active component, which is only a function of the ground shear stress and height, and an inactive component, which is produced by turbulence in the outer region. It is demonstrated that the inactive Eddy Motion contributes significantly to the pressure and longitudinal velocity power spectra for wavenumbers much smaller than that corresponding to the height above the ground surface. Because of the importance of this inactive Eddy Motion contribution, it was possible to derive and validate a scaling law for the pressure power spectrum at low wavenumbers. The root-mean-square pressure was derived from the ground shear stress using simplifications to the Poisson equation that relate t...
Brani Vidakovic - One of the best experts on this subject based on the ideXlab platform.
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identification of low dimensional energy containing flux transporting Eddy Motion in the atmospheric surface layer using wavelet thresholding methods
Journal of the Atmospheric Sciences, 1998Co-Authors: Gabriel G Katul, Brani VidakovicAbstract:The partitioning of turbulent perturbations into a ‘‘low-dimensional’’ active part responsible for much of the turbulent energy and fluxes and a ‘‘high-dimensional’’ passive part that contributes little to turbulent energy and transport dynamics is investigated using atmospheric surface-layer (ASL) measurements. It is shown that such a partitioning scheme can be achieved by transforming the ASL measurements into a domain that concentrates the low-dimensional part into few coefficients and thus permits a global threshold of the remaining coefficients. In this transformation‐thresholding approach, Fourier rank reduction and orthonormal wavelet and wavelet packet methods are considered. The efficiencies of these three thresholding methods to extract the events responsible for much of the heat and momentum turbulent fluxes are compared for a wide range of atmospheric stability conditions. The intercomparisons are performed in four ways: (i) compression ratios, (ii) energy conservation, (iii) turbulent flux conservation, and (iv) finescale filtering via departures from Kolmogorov’s K41 power laws. For orthonormal wavelet and wavelet packets analysis, wavelet functions with varying time‐frequency localization properties are also considered. The study showed that wavelet and wavelet packet Lorentz thresholding can achieve high compression ratios (98%) with minimal loss in energy (3% loss) and fluxes (4%). However, these compression ratios and energy and flux conservation measures are comparable to the linear Fourier rank reduction method if a Lorentz threshold function is applied to the latter. Finally, it is demonstrated that orthonormal wavelet and wavelet packets thresholding are insensitive to the analyzing wavelet.
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The partitioning of attached and detached Eddy Motion in the atmospheric surface layer using Lorentz wavelet filtering
Boundary-Layer Meteorology, 1996Co-Authors: Gabriel Katul, Brani VidakovicAbstract:Townsend's attached Eddy hypothesis states that the turbulent structure in the constant stress layer can be decomposed into attached and detached Eddy Motion. This paper proposes and tests a methodology for separating the attached and detached Eddy Motion from time series measurements of velocity and temperature. The proposed methodology is based on the time-frequency localization and filtering capabilities of the orthonormal wavelet transforms. Using a relative entropy statistical measure, the optimal wavelet basis is identified first. The turbulence time series measurements are then transformed into the wavelet domain where the contribution of specific events in the time-frequency domain is identified. The filtering scheme utilizes a recently constructed Lorentz thresholding methodology that successfully eliminates all wavelet coefficients associated with the detached Eddy Motion. While this filtering scheme lacks the compression efficiency of the classical Donoho and Johnstone's universal thresholding model, it conserves the higher-order statistics and important turbulence interactions related to the Reynolds stresses. Following the filtering scheme, the attached Eddy Motion time series is re-constructed by an inverse wavelet transform of the non-zero wavelet coefficients. The proposed partitioning methodology for attached and detached Eddy Motion is tested using 56 Hz triaxial sonic anemometer velocity and temperature measurements above a uniform dry lake bed in Owens valley, California, for a wide range of atmospheric stability conditions. Validation that the wavelet filtered time series represents the attached Eddy Motion is also discussed in the context of conservation of turbulence energy and surface fluxes.
Mario B. S. Siqueira - One of the best experts on this subject based on the ideXlab platform.
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Quantifying Organization of Atmospheric Turbulent Eddy Motion Using Nonlinear Time Series Analysis
Boundary-Layer Meteorology, 2003Co-Authors: Karen H. Wesson, Gabriel G Katul, Mario B. S. SiqueiraAbstract:Using three methods from nonlinear dynamics, we contrast the level of organization inthe vertical wind velocity (w) time series collected in the atmospheric surface layer(ASL) and the canopy sublayer (CSL) for a wide range of atmospheric stability (ξ)conditions. The nonlinear methods applied include a modified Shannon entropy, waveletthresholding, and mutual information content. Time series measurements collected overa pine forest, a hardwood forest, a grass-covered forest clearing, and a bare soil, desertsurface were used for this purpose. The results from applying all three nonlinear timeseries measures suggest that w in the CSL is more organized than that in the ASL, and that as the flows in both layers evolve from near-neutral to near-convective conditions, the level of organization increases. Furthermore, we found that the degree of organization in w associated with changes in ξ is more significant than the transition from CSL to ASL.
Chia R. Chu - One of the best experts on this subject based on the ideXlab platform.
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LOCAL ISOTROPY AND ANISOTROPY IN THE SHEARED AND HEATED ATMOSPHERIC SURFACE LAYER
Boundary-Layer Meteorology, 1995Co-Authors: Gabriel G Katul, Marc B Parlange, John D. Albertson, Chia R. ChuAbstract:Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of Eddy-Motion anisotropy within the inertial subrange. Rather than simply test the adequacy of locally isotropic relations, we investigated directly the sources of anisotropy. These sources, in a daytime desert-like climate, include: (1) direct interaction between the large-scale and small-scale Eddy Motion, and (2) thermal effects on the small-scale Eddy Motion. In order to explore these two anisotropy sources, we developed statistical measures that are sensitive to such interactions. It was found that the large-scale/small-scale interaction was significant in the inertial subrange up to 3 decades below the production scale, thus reducing the validity of the local isotropy assumption. The anisotropy generated by thermal effects was also significant and comparable in magnitude to the former anisotropy source. However, this thermal anisotropy was opposite in sign and tended to counteract the anisotropy generated by the large-scale/smallscale interaction. The thermal anisotropy was attributed to organized ramp-like patterns in the temperature measurements. The impact of this anisotropy cancellation on the dynamics of inertial subrange Eddy Motion was also considered. For that purpose, the Kolmogorov-Obukhov structure function equation, as derived from the Navier-Stokes equations for locally isotropic turbulence, was employed. The Kolmogorov-Obukhov structure function equation in conjunction with Obukhov's constant skewness closure hypothesis reproduced the measured second- and third-order structure functions. Obukhov's constant skewness closure scheme, which is also based on the local isotropy assumption, was verified and was found to be in good agreement with the measurements. The accepted 0.4 constant skewness value derived from grid turbulence experiments overestimated our measurements. A suggested 0.26 constant skewness value, which we derived from Kolmogorov's constant, was found to be adequate.
Marc B Parlange - One of the best experts on this subject based on the ideXlab platform.
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The "inactive" Eddy Motion and the large-scale turbulent pressure fluctuations in the dynamic sublayer
Journal of the Atmospheric Sciences, 1996Co-Authors: Gabriel G Katul, Marc B Parlange, John D. Albertson, Cheng-i Hsieh, Paul S. Conklin, John Sigmon, Ken R. KnoerrAbstract:Abstract The statistical structure of the turbulent pressure fluctuations was measured in the dynamic sublayer of a large grass-covered forest clearing by a free air static pressure probe and modeled using Townsend's hypothesis. Townsend's hypothesis states that the Eddy Motion in the equilibrium layer can be decomposed into an active component, which is only a function of the ground shear stress and height, and an inactive component, which is produced by turbulence in the outer region. It is demonstrated that the inactive Eddy Motion contributes significantly to the pressure and longitudinal velocity power spectra for wavenumbers much smaller than that corresponding to the height above the ground surface. Because of the importance of this inactive Eddy Motion contribution, it was possible to derive and validate a scaling law for the pressure power spectrum at low wavenumbers. The root-mean-square pressure was derived from the ground shear stress using simplifications to the Poisson equation that relate t...
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LOCAL ISOTROPY AND ANISOTROPY IN THE SHEARED AND HEATED ATMOSPHERIC SURFACE LAYER
Boundary-Layer Meteorology, 1995Co-Authors: Gabriel G Katul, Marc B Parlange, John D. Albertson, Chia R. ChuAbstract:Longitudinal velocity and temperature measurements above a uniform dry lakebed were used to investigate sources of Eddy-Motion anisotropy within the inertial subrange. Rather than simply test the adequacy of locally isotropic relations, we investigated directly the sources of anisotropy. These sources, in a daytime desert-like climate, include: (1) direct interaction between the large-scale and small-scale Eddy Motion, and (2) thermal effects on the small-scale Eddy Motion. In order to explore these two anisotropy sources, we developed statistical measures that are sensitive to such interactions. It was found that the large-scale/small-scale interaction was significant in the inertial subrange up to 3 decades below the production scale, thus reducing the validity of the local isotropy assumption. The anisotropy generated by thermal effects was also significant and comparable in magnitude to the former anisotropy source. However, this thermal anisotropy was opposite in sign and tended to counteract the anisotropy generated by the large-scale/smallscale interaction. The thermal anisotropy was attributed to organized ramp-like patterns in the temperature measurements. The impact of this anisotropy cancellation on the dynamics of inertial subrange Eddy Motion was also considered. For that purpose, the Kolmogorov-Obukhov structure function equation, as derived from the Navier-Stokes equations for locally isotropic turbulence, was employed. The Kolmogorov-Obukhov structure function equation in conjunction with Obukhov's constant skewness closure hypothesis reproduced the measured second- and third-order structure functions. Obukhov's constant skewness closure scheme, which is also based on the local isotropy assumption, was verified and was found to be in good agreement with the measurements. The accepted 0.4 constant skewness value derived from grid turbulence experiments overestimated our measurements. A suggested 0.26 constant skewness value, which we derived from Kolmogorov's constant, was found to be adequate.
