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Claudia Angelini, Applicazioni della Matematica, CNR, Naples, Italy (2001):The work focused on the conceptual framework for modeling the inertial
subrange by the Kolmogorov cascade phenomena, which is nowadays the subject of significant reinterpretation. It has been argued that the effects of boundary conditions influence large-scale motion
and direct interaction between large and small scales is possible by means other than passing sequentially through the full cascade. Using longitudinal (u) and vertical (w) velocity and temperature (T) time series measurements
collected in the atmospheric surface layer (ASL), we evaluate whether the inertial subrange multifractral function (f(D)) of all three flow variables is influenced by atmospheric stability, which is a bulk measure of the effect of boundary conditions on large scale flow properties for ASL turbulence. This study is the first to demonstrate that stability significantly influences f(D) for all three flow variables. Here, statistical significance is evaluated using a novel wavelet-based Functional Analysis of Variance (FANOVA) approach that explicitly considers different classes of stability, the flow variable type, and possible interactions between stability and the three flow variables.
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Daniela Cava, CNR - Istituto per lo Studio dell'Inquinamento Atmosferico e l'Agrometeorologia, Lecce (2002): The work focused on quantifying the
role of organized motion in scalar transport within the canopy sublayer under stable atmospheric stability conditions. In particular, the type of organized motion considered includes gravity waves, inverted ramps, as well
as other low frequncy motions. We found that ramps are the most efficient eddy motion in transporting CO2 from within the canopy into the atmosphere. While gravity waves, with time scales of 40-60 s impact the CO2 concentration spectrum, their mass transporting capabilities are negligible. Finally, this work distinguishes itself from previous efforts by examining the role of clouds on CO2 flushing of the canopy volume. We found that passage of clouds, in many
occassions, introduces perturbations that re-vitalize the ramp structure previously damped by stability.
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Davide Poggi, Politecnico di Torino, Torino (2002): The work focused on
analyzing detailed spatial and temporal measurements of the turbulent flow field in an open channel with bottom roughness made of regularly spaced vertical rods, meant to simulate dense and sparse canopies of identical height. The behavior of key turbulent characteristics and the quadrant analysis show that the type of flow changes with the rod density shifting from an atypical boundary layer at low densities to a highly-disturbed mixing layer at very high densities. These results are the first to suggest that the "canonical" structure of canopy turbulence is a superposition of a perturbed mixing layer and a rough-wall boundary layer, with high foliage density defining one end-member (mixing layer) and low foliage density (boundary layer) defining the other. We also evaluate the consequences of such foliage density variation on the sweep-ejection sequence controlling the momentum exchange between the canopy and the surface layer above. The implications of such a perturbation on scalar transport models, such as turbulent diffusivities controlling the exchange rates of water vapor and carbon dioxide in natural ecosystems, are also investigated.
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Merel Soons, Department of Biology, Utrecht (2002): The work focused on the mechanisms that determine seed dispersal distances by wind using model calculations and field measurements collected in several grassland ecosystems in the Netherlands. Two mechanistic models are used in these studies: - one neglects the effects of vertical velocity excursion by turbulence while the other explicitly considers such excursions via a coupled Eulerian-Lagrangian model. Both models are compared to seed dispesral measurements conducted within these grassland ecosystem. The comparisons between the two model calculations and the measurements demonstrated that long distance dispersal is intimately linked to vertical velocity excursions produced by turbulence. Given that the turbulent dispersal model reproduced well the seed dispersal distances permitted us to conduct a detailed sensitivity analysis on the key variables controlling dispersal. Our analysis suggests that seed release height and vegetation height are critical variables determining seed dispersal potential; once dispersing, wind speed is the most important factor. |
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Ran Nathan, Department of Life Sciences, Ben Gurion University (2002):The work focused on developing mechanistic models that couple seed release and aerodynamics with turbulent transport processes to provide accurate probabilistic description of long-dispersal distance (LDD) of seeds by wind. The proposed model was tested with vertical distribution of dispersed seeds of five tree species observed along a 45-m high tower in an Eastern US deciduous forest. Simulations show that uplifting above the forest canopy is necessary and sufficient for LDD; hence, they provide the means to define LDD quantitatively rather than arbitrarily. Seed uplifting probability thus sets an upper bound on the probability of long-distance colonization. Uplifted yellow poplar seeds are on average lighter than seeds at the forest floor, but also include the heaviest seeds. Because uplifting probabilities are appreciable (as much as 1-5%), and tree seed crops are commonly massive, some LDD events will establish individuals that can critically affect plant dynamics at large scales.
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Brani Vidakovic, School of Industrial and Systems Engineering, Georgia Institute of Technology (2002)
The work focused on investigating local and global intermittency buildup in the inertial subrange. Global scaling exponents and other statistical
properties were derived using non-decimated wavelet transforms NDWT and critically sampled orthonormal wavelet transforms OWT. These statistical measures were contrasted
to similar statistical measures derived by applying NDWT and OWT to an ensemble of fractional Brownian motion (fBm) time
series with Hurst exponent of 1/3. Such comparisons permit direct assessment as to whether discrepancies in observed intermittency corrections are artifacts of wavelet transformations or
limitations in sample size. This study demonstrated that both NDWT and OWT were able to resolve intermittency-based departures from global power laws observed in higher-order structure functions
of turbulence time series. Particularly, global power laws inferred from OWT and NDWT were consistent with new intermittency correction results derived from the dynamics of the fourth order
structure functions. In terms of local exponents, we found that the application of NDWT to fBm time series resulted in a wide empirical frequency distribution of local scaling exponents ( a). The latter finding clearly
demonstrates that previous and present a determined by wavelet analysis cannot be used as evidence for multifractality in turbulence. We also demonstrated that the classical local regression
estimation of a is theoretically impaired by heteroscedascity when the local scale is finite. While the spread in a does not reflect any multifractal signatures, the modes of the local a frequency
distribution support findings from global exponent analysis. We found that the modes of the local a distribution are well reproduced by global intermittency models for u and by K41 for the fBm.
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Tomo Kumagai, University Forest in Miyazaki, Kyushu University, Shiiba-son, Miyazaki, Japan (2003): The work focused on the hydrologic budget of the Southeastern Asian tropical rain forests under current and projected climate conditions. These forests are among the most important biomes in
terms of annual productivity and water cycling. How their hydrologic budgets are altered by projected shifts in precipitation is examined using a combination of field
measurements, global climate model (GCM) simulation output, and a simplified hydrologic model. The simplified hydrologic model is developed with its primary forcing
term being rainfall statistics. A main novelty in this analysis is that the effects of increased (or decreased) precipitation on increased (or decreased) cloud cover and hence
evapotranspiration is explicitly considered. The model is validated against field measurements conducted in a tropical rain forest in Sarawak, Malaysia. It is demonstrated
that the model reproduces the probability density function of soil moisture content (s), transpiration (Tr ), interception (Ic ), and leakage loss (Q). On the basis of this model and
projected shifts in precipitation statistics by GCM the probability distribution of Ic , Q and, to a lesser extent, s varied appreciably at seasonal timescales. The probability distribution
of Tr was least impacted by projected shifts in precipitation.
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