84,878 research outputs found
Incorporating geostrophic wind information for improved space-time short-term wind speed forecasting
Accurate short-term wind speed forecasting is needed for the rapid
development and efficient operation of wind energy resources. This is, however,
a very challenging problem. Although on the large scale, the wind speed is
related to atmospheric pressure, temperature, and other meteorological
variables, no improvement in forecasting accuracy was found by incorporating
air pressure and temperature directly into an advanced space-time statistical
forecasting model, the trigonometric direction diurnal (TDD) model. This paper
proposes to incorporate the geostrophic wind as a new predictor in the TDD
model. The geostrophic wind captures the physical relationship between wind and
pressure through the observed approximate balance between the pressure gradient
force and the Coriolis acceleration due to the Earth's rotation. Based on our
numerical experiments with data from West Texas, our new method produces more
accurate forecasts than does the TDD model using air pressure and temperature
for 1- to 6-hour-ahead forecasts based on three different evaluation criteria.
Furthermore, forecasting errors can be further reduced by using moving average
hourly wind speeds to fit the diurnal pattern. For example, our new method
obtains between 13.9% and 22.4% overall mean absolute error reduction relative
to persistence in 2-hour-ahead forecasts, and between 5.3% and 8.2% reduction
relative to the best previous space-time methods in this setting.Comment: Published in at http://dx.doi.org/10.1214/14-AOAS756 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Numerical simulations of negative-index refraction in wedge-shaped metamaterials
A wedge-shaped structure made of split-ring resonators (SRR) and wires is
numerically simulated to evaluate its refraction behavior. Four frequency
bands, namely, the stop band, left-handed band, ultralow-index band, and
positive-index band, are distinguished according to the refracted field
distributions. Negative phase velocity inside the wedge is demonstrated in the
left-handed band and the Snell's law is conformed in terms of its refraction
behaviors in different frequency bands. Our results confirmed that negative
index of refraction indeed exists in such a composite metamaterial and also
provided a convincing support to the results of previous Snell's law
experiments.Comment: 18 pages, 6 figure
Theory of time-resolved spectral function in high-temperature superconductors with bosonic modes
We develop a three-temperature model to simulate the time dependence of
electron and phonon temperatures in high-temperature superconductors displaying
strong anistropic electron-phonon coupling. This model not only takes the
tight-binding band structure into account, but also is valid in superconducting
state. Based on this model, we calculate the time-resolved spectral function
via the double-time Green's functions. We find that the dip-hump structure
evolves with the time delay. More interestingly, new phononic structures are
obtained when the phonons are excited by a laser field. This signature may
serve as a direct evidence for electron-vibration mode coupling.Comment: 5 pages, 3 figure
Community detection in multiplex networks using locally adaptive random walks
Multiplex networks, a special type of multilayer networks, are increasingly
applied in many domains ranging from social media analytics to biology. A
common task in these applications concerns the detection of community
structures. Many existing algorithms for community detection in multiplexes
attempt to detect communities which are shared by all layers. In this article
we propose a community detection algorithm, LART (Locally Adaptive Random
Transitions), for the detection of communities that are shared by either some
or all the layers in the multiplex. The algorithm is based on a random walk on
the multiplex, and the transition probabilities defining the random walk are
allowed to depend on the local topological similarity between layers at any
given node so as to facilitate the exploration of communities across layers.
Based on this random walk, a node dissimilarity measure is derived and nodes
are clustered based on this distance in a hierarchical fashion. We present
experimental results using networks simulated under various scenarios to
showcase the performance of LART in comparison to related community detection
algorithms
One-side forward-backward asymmetry at the LHC
Forward-backward asymmetry is an essential observable to study
the nature of coupling in the standard model and physics beyond the standard
model, as shown at LEP and Tevatron. As a proton-proton collider, the LHC does
not have the preferred direction contrary to her counterpart, namely, LEP and
Tevatron. Therefore is not applicable at the LHC. However for the
proton the momentum of valence quark is usually larger than that of the sea
quark. Utilizing this feature we have defined a so-called one-side
forward-backward asymmetry for the top quark pair production at
LHC in the previous work. In this paper we extend our studies to the charged
leptons and bottom quarks as the final states. Our numerical results show that
at the LHC can be utilized to study the nature of the couplings
once enough events are collected.Comment: 19 pages, 7 figures, 1 table, published versio
Variable - temperature scanning optical and force microscope
The implementation of a scanning microscope capable of working in confocal,
atomic force and apertureless near field configurations is presented. The
microscope is designed to operate in the temperature range 4 - 300 K, using
conventional helium flow cryostats. In AFM mode, the distance between the
sample and an etched tungsten tip is controlled by a self - sensing
piezoelectric tuning fork. The vertical position of both the AFM head and
microscope objective can be accurately controlled using piezoelectric coarse
approach motors. The scanning is performed using a compact XYZ stage, while the
AFM and optical head are kept fixed, allowing scanning probe and optical
measurements to be acquired simultaneously and in concert. The free optical
axis of the microscope enables both reflection and transmission experiments to
be performed.Comment: 24 pages, 9 figures, submitted to the journal "Review of Scientific
Instruments
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