16,379 research outputs found
Fronts and frontogenesis as revealed by high time resolution data
Upper air sounding are used to examine a cold front of average intensity. Vertical cross sections of potential temperature and wind, and horizontal analyses were compared and adjusted for consistency. These analyses were then used to study the evolution of the front, found to consist of a complex system of fronts occurring at all levels of the troposphere. Low level fronts were strongest at the surface and rapidly weakened with height. Fronts in the midddle troposphere were much more intense. The warm air ahead of the fronts was nearly barotropic, while the cold air behind was baroclinic through deep layers. A deep mixed layer was observed to grow in this cold air
The distribution of baroclinity within the atmosphere
A three dimensional numerical interpolation scheme which resolves frontal gradients with fidelity was developed. The scheme is applied to the study of atmospheric upper baroclinic zones
The uniqueness of the invariant polarisation-tensor field for spin-1 particles in storage rings
We argue that the invariant tensor field introduced in [1] is unique under
the condition that the invariant spin field is unique, and thereby complete
that part of the discussion in that paper.Comment: 8 page
From linear to non-linear scales: analytical and numerical predictions for the weak lensing convergence
Weak lensing convergence can be used directly to map and probe the dark mass
distribution in the universe. Building on earlier studies, we recall how the
statistics of the convergence field are related to the statistics of the
underlying mass distribution, in particular to the many-body density
correlations. We describe two model-independent approximations which provide
two simple methods to compute the probability distribution function, pdf, of
the convergence. We apply one of these to the case where the density field can
be described by a log-normal pdf. Next, we discuss two hierarchical models for
the high-order correlations which allow one to perform exact calculations and
evaluate the previous approximations in such specific cases. Finally, we apply
these methods to a very simple model for the evolution of the density field
from linear to highly non-linear scales. Comparisons with the results obtained
from numerical simulations, obtained from a number of different realizations,
show excellent agreement with our theoretical predictions. We have probed
various angular scales in the numerical work and considered sources at 14
different redshifts in each of two different cosmological scenarios, an open
cosmology and a flat cosmology with non-zero cosmological constant. Our
simulation technique employs computations of the full 3-d shear matrices along
the line of sight from the source redshift to the observer and is complementary
to more popular ray-tracing algorithms. Our results therefore provide a
valuable cross-check for such complementary simulation techniques, as well as
for our simple analytical model, from the linear to the highly non-linear
regime.Comment: 20 pages, final version published in MNRA
Methodological reflections on the evaluation of the implementation and adoption of national electronic health record systems
Copyright @ 2012, International Journal of Integrated Care (IJIC). This work is licensed under a (http://creativecommons.org/licenses/by/3.0) Creative Commons Attribution 3.0 Unported License.Introduction/purpose of presentation: Far-reaching policy commitments to information technology-centered transformations of healthcare systems have now been made in many countries. There is as yet little empirical evidence to justify such decisions, hence the need for rigorous independent evaluation of current implementation efforts. Such evaluations however pose a number of important challenges. This presentation has been designed as a part of a Panel based on our experience of evaluating the National Health Service’s (NHS) implementation of electronic health records (EHR) systems in hospitals throughout England. We discuss the methodological challenges encountered in planning and undertaking an evaluation of a program of this scale and reflect on why and how we adapted our evaluation approach—both conceptually and methodologically—in response to these challenges. Study design/population studied: Critical reflections on a multi-disciplinary and multi-facet independent evaluation of a national program to implement electronic health record systems into 12 ‘early wave’ NHS hospitals in England. Findings: Our initial plan was to employ a mixed methods longitudinal ‘before-during-after’ study design. We however found this unsustainable in the light of fluxes in policy, contractual issues and over-optimistic schedules for EHR deployments. More importantly, this research design failed adequately to address the core of multi-faceted evolving EHRs as understood by key stakeholders and as worked out in their distinct work settings. Thus conventional outcomes-centric evaluations may not easily scale-up when evaluating transformational programs and may indeed prove misleading. New assumptions concerning the implementation process of EHR need to be developed that recognize the constantly changing milieu of policy, product, projects and professions that are inherent to such national implementations. The approaches we subsequently developed substitute the positivist view that EHR initiatives are self-evident and self-contained interventions, which are amenable to traditional quantitative evaluations, to one that focuses on how they are understood by various stakeholders and made to work in specific contexts. These assumptions recast the role of evaluation towards an approach that explores and interprets processes of socio-technical change that surround EHR implementation and adoption as seen by multiple stakeholders. Conclusions and policy implications: There is likely to be an increase in politically-driven national programs of reform of healthcare based on information and communication technologies. Programs on such a scale are inherently complex with extended temporalities and extensive and dynamic sets of stakeholders. They are, in short, different and pose new evaluation challenges that previously formulated evaluation methods for health information systems cannot easily address. This calls for methodological innovation amongst research teams and their supporting bodies. We argue that evaluation of such system-wide transformation programs are likely to demand both breadth and depth of experience within a multidisciplinary research team, constant questioning of what is and what can be evaluated and how, and a particular way of working that emphasizes continuous dialogue and reflexivity. Making this transition is essential to enable evaluations that can usefully inform policy-making. Health policy experts urgently need to reassess the evaluation strategies they employ as they come to address national policies for system-wide transformation based on new electronic health infrastructures
Optimal Axes of Siberian Snakes for Polarized Proton Acceleration
Accelerating polarized proton beams and storing them for many turns can lead
to a loss of polarization when accelerating through energies where a spin
rotation frequency is in resonance with orbit oscillation frequencies.
First-order resonance effects can be avoided by installing Siberian Snakes in
the ring, devices which rotate the spin by 180 degrees around the snake axis
while not changing the beam's orbit significantly. For large rings, several
Siberian Snakes are required.
Here a criterion will be derived that allows to find an optimal choice of the
snake axes. Rings with super-period four are analyzed in detail, and the HERA
proton ring is used as an example for approximate four-fold symmetry. The
proposed arrangement of Siberian Snakes matches their effects so that all
spin-orbit coupling integrals vanish at all energies and therefore there is no
first-order spin-orbit coupling at all for this choice, which I call snakes
matching. It will be shown that in general at least eight Siberian Snakes are
needed and that there are exactly four possibilities to arrange their axes.
When the betatron phase advance between snakes is chosen suitably, four
Siberian Snakes can be sufficient.
To show that favorable choice of snakes have been found, polarized protons
are tracked for part of HERA-p's acceleration cycle which shows that
polarization is preserved best for the here proposed arrangement of Siberian
Snakes.Comment: 14 pages, 16 figure
Quasiperiodic spin-orbit motion and spin tunes in storage rings
We present an in-depth analysis of the concept of spin precession frequency
for integrable orbital motion in storage rings. Spin motion on the periodic
closed orbit of a storage ring can be analyzed in terms of the Floquet theorem
for equations of motion with periodic parameters and a spin precession
frequency emerges in a Floquet exponent as an additional frequency of the
system. To define a spin precession frequency on nonperiodic synchro-betatron
orbits we exploit the important concept of quasiperiodicity. This allows a
generalization of the Floquet theorem so that a spin precession frequency can
be defined in this case too. This frequency appears in a Floquet-like exponent
as an additional frequency in the system in analogy with the case of motion on
the closed orbit. These circumstances lead naturally to the definition of the
uniform precession rate and a definition of spin tune. A spin tune is a uniform
precession rate obtained when certain conditions are fulfilled. Having defined
spin tune we define spin-orbit resonance on synchro--betatron orbits and
examine its consequences. We give conditions for the existence of uniform
precession rates and spin tunes (e.g. where small divisors are controlled by
applying a Diophantine condition) and illustrate the various aspects of our
description with several examples. The formalism also suggests the use of
spectral analysis to ``measure'' spin tune during computer simulations of spin
motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio
Exact clesed form of the return probability on the Bethe lattice
An exact closed form solution for the return probability of a random walk on
the Bethe lattice is given. The long-time asymptotic form confirms a previously
known expression. It is however shown that this exact result reduces to the
proper expression when the Bethe lattice degenerates on a line, unlike the
asymptotic result which is singular. This is shown to be an artefact of the
asymptotic expansion. The density of states is also calculated.Comment: 7 pages, RevTex 3.0, 2 figures available upon request from
[email protected], to be published in J.Phys.A Let
Systems comparison of direct and relay link data return modes for advanced planetary missions
Advanced planetary missions using direct and relay link data return mode
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