4,737 research outputs found
Topographical scattering of waves: a spectral approach
The topographical scattering of gravity waves is investigated using a
spectral energy balance equation that accounts for first order wave-bottom
Bragg scattering. This model represents the bottom topography and surface waves
with spectra, and evaluates a Bragg scattering source term that is
theoretically valid for small bottom and surface slopes and slowly varying
spectral properties. The robustness of the model is tested for a variety of
topographies uniform along one horizontal dimension including nearly
sinusoidal, linear ramp and step profiles. Results are compared with
reflections computed using an accurate method that applies integral matching
along vertical boundaries of a series of steps. For small bottom amplitudes,
the source term representation yields accurate reflection estimates even for a
localized scatterer. This result is proved for small bottom amplitudes
relative to the mean water depth . Wave reflection by small amplitude bottom
topography thus depends primarily on the bottom elevation variance at the Bragg
resonance scales, and is insensitive to the detailed shape of the bottom
profile. Relative errors in the energy reflection coefficient are found to be
typically .Comment: Second revision for Journal of Waterways Ports and Coastal
Engineerin
Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors
We present a comparative study of ultrafast photo-conversion dynamics in
tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical
pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is
activated and temperature-independent respectively, demonstrating dominant
singlet-triplet exciton fission. In Pc films (as well as C-doped films)
this decay channel is suppressed by electron trapping. These results
demonstrate the central role of crystallinity and purity in photogeneration
processes and will constrain the design of future photovoltaic devices.Comment:
Evaluating Sensitivities of Economic Factors through Coupled Economics-ALMANAC Model System
Using crop models to simulate crop growth and productivity at a regional scale is a complex process designed to represent the observed impact of individual farmer decision-making on the agricultural landscape. Typically, during agricultural simulation efforts, the planting acreages have largely been based on a set of predetermined, static scenarios. In this study, we developed a system to dynamically enhance the Agricultural Land Management Alternative with Numerical Assessment Criteria (ALMANAC) crop simulation model through a two-way linkage with an economics land-use model. This coupled model approach integrated farmersâ land-use choices based on relative economic returns and produced dynamic land-use probabilities for ALMANAC simulations through a feedback loop. The coupled model approach was intercompared with static crop modeling through a historic acreage approach, and comparable accuracies were found from both modeling efforts for the 2014 growing season. Furthermore, as a proof-of-concept effort, the method was applied to evaluate the impact of two scenarios on crop simulations: major crops (maize, soybean, and wheat) intensification through price increases (e.g., market change) and incentivized grassland conservation (e.g., policy change). The results of this sensitivity study suggest that the coupled system has the capability to integrate economic factors into traditional crop simulation, allowing for insight into the impacts of changes in markets and policies on agricultural landscapes and crop yields
Incorporating Wildlife Connectivity into Forest Plan Revision Under the United States Forest Service\u27s 2012 Planning Rule
The United States Forest Service promulgated new planning regulations under the National Forest Management Act in 2012 (i.e., the Planning Rule). These new regulations include the first requirements in U.S. public land management history for National Forests to evaluate, protect, and/or restore ecological connectivity as they revise their land management plans. Data and resource limitations make single-species, functional connectivity analyses for the myriad species that occur within the 78 million ha the Forest Service manages implausible. We describe an approach that relies on freely available data and generic species, virtual species whose profile consists of ecological requirements designed to reflect the needs of a group of real species, to address the new Planning Rule requirements. We present high-resolution connectivity estimates for 10 different generic species across a 379,000 ha study area centered on the Custer Gallatin National Forest (CGNF) in Montana and South Dakota under two different movement models. We identify locations important for connectivity for multiple species and characterize the role of the CGNF for regional connectivity. Our results informed the Plan Revision process on the CGNF and could be readily exported to other National Forests currently or planning to revise their land management plans under the new Planning Rule
The Hamiltonian of the V Spin System from first-principles Density-Functional Calculations
We report first-principles all-electron density-functional based studies of
the electronic structure, magnetic ordering and anisotropy for the V
molecular magnet. From these calculations, we determine a Heisenberg
Hamiltonian with four antiferromagnetic and one {\em ferromagnetic} coupling.
We perform direct diagonalization to determine the temperature dependence of
the susceptibility. This Hamiltonian reproduces the experimentally observed
spin =1/2 ground state and low-lying =3/2 excited state. A small
anisotropy term is necessary to account for the temperature independent part of
the magnetization curve.Comment: 4 pages in RevTeX format + 2 ps-figures, accepted by PRL Feb. 2001
(previous version was an older version of the paper
Reasoning About a Service-oriented Programming Paradigm
This paper is about a new way for programming distributed applications: the
service-oriented one. It is a concept paper based upon our experience in
developing a theory and a language for programming services. Both the
theoretical formalization and the language interpreter showed us the evidence
that a new programming paradigm exists. In this paper we illustrate the basic
features it is characterized by
Long-Term Evidence for Fire as an Ecohydrologic Threshold-Reversal Mechanism on Woodland-Encroached Sagebrush Shrublands
Encroachment of sagebrush (Artemisia spp.) shrublands by pinyon (Pinus spp.) and juniper (Juniperus spp.) conifers (woodland encroachment) induces a shift from bioticâcontrolled resource retention to abioticâdriven loss of soil resources. This shift is driven by a coarsening of the vegetation structure with increasing dominance of site resources by trees. Competition between the encroaching trees and understory vegetation for limited soil and water resources facilitates extensive bare intercanopy area between trees and concomitant increases in runâoff and erosion that, over time, propagate persistence of the shrublandâtoâwoodland conversion. We evaluated whether tree removal by burning can decrease lateâsuccession woodland ecohydrologic resilience by increasing vegetation and ground cover over a 9âyear period after fire and whether the soil erosion feedback on lateâsuccession woodlands is reversible by burning. To address these questions, we employed a suite of vegetation and soil measurements and rainfall simulation and concentrated overland flow experiments across multiple plot scales on unburned and burned areas at two sagebrush sites in the later stages of woodland succession. Prior to burning, tree cover was approximately 28% at the sites, and more than 70% of the area at the sites was intercanopy with depauperate understory vegetation and extensive bare ground (52â60% bare soil and rock). Burning initially increased bare ground across fine (\u3c1 m2) to patch (tens of metres) scales, resulting in enhanced sediment availability at the fine scale, sustained high runâoff and erosion within degraded intercanopies, amplified runâoff and erosion from tree canopy areas, and amplified sediment delivery across fine to patch scales. However, fireâinduced increases in grass cover over nine growing seasons improved infiltration, limited runâoff and sediment delivery from the fine scale, and reduced intercanopy runâoff and erosion at the patch scale. These changes reflect a switch in vegetation structure, triggered by burning and subsequent vegetation reâestablishment, and a shift to biotic control on runâoff and erosion across spatial scales. The responses and persistence over the 9âyear period postfire at the two sites demonstrate that fire can decrease woodland ecohydrologic resilience by altering plant community physiognomy and thereby can reverse the soil erosion feedback on sagebrush shrublands in the later stages of woodland encroachment
Effect of hydrogen on ground state structures of small silicon clusters
We present results for ground state structures of small SiH (2 \leq
\emph{n} \leq 10) clusters using the Car-Parrinello molecular dynamics. In
particular, we focus on how the addition of a hydrogen atom affects the ground
state geometry, total energy and the first excited electronic level gap of an
Si cluster. We discuss the nature of bonding of hydrogen in these
clusters. We find that hydrogen bonds with two silicon atoms only in SiH,
SiH and SiH clusters, while in other clusters (i.e. SiH,
SiH, SiH, SiH, SiH and SiH) hydrogen is bonded
to only one silicon atom. Also in the case of a compact and closed silicon
cluster hydrogen bonds to the cluster from outside. We find that the first
excited electronic level gap of Si and SiH fluctuates as a function
of size and this may provide a first principles basis for the short-range
potential fluctuations in hydrogenated amorphous silicon. Our results show that
the addition of a single hydrogen can cause large changes in the electronic
structure of a silicon cluster, though the geometry is not much affected. Our
calculation of the lowest energy fragmentation products of SiH clusters
shows that hydrogen is easily removed from SiH clusters.Comment: one latex file named script.tex including table and figure caption.
Six postscript figure files. figure_1a.ps and figure_1b.ps are files
representing Fig. 1 in the main tex
Electrostatic model of atomic ordering in complex perovskite alloys
We present a simple ionic model which successfully reproduces the various
types of compositional long-range order observed in a large class of complex
insulating perovskite alloys. The model assumes that the driving mechanism
responsible for the ordering is simply the electrostatic interaction between
the different ionic species. A possible new explanation for the anomalous
long-range order observed in some Pb relaxor alloys, involving the proposed
existence of a small amount of Pb^4+ on the B sublattice, is suggested by an
analysis of the model.Comment: 4 pages, two-column style with 1 postscript figure embedded. Uses
REVTEX and epsf macros. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/index.html#lb_orde
Soft Phonon Anomalies in the Relaxor Ferroelectric Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3
Neutron inelastic scattering measurements of the polar TO phonon mode
dispersion in the cubic relaxor Pb(Zn_1/3Nb_2/3)_0.92Ti_0.08O_3 at 500K reveal
anomalous behavior in which the optic branch appears to drop precipitously into
the acoustic branch at a finite value of the momentum transfer q=0.2 inverse
Angstroms, measured from the zone center. We speculate this behavior is the
result of nanometer-sized polar regions in the crystal.Comment: 4 pages, 4 figure
- âŠ