539 research outputs found
A robust method to identify cyclone tracks from gridded data
A system to derive tracks of barometric minima is presented. It is
deliberately using coarse input data in space (order of 2°×2°) and time (6-hourly to daily) as well as information from just one
geopotential level. It is argued that the results are, for one robust in the
sense of an assumption of the IMILAST Project that the use of as
simple as possible metrics should be strived for and for two tailored to the
input from reanalyses and GCMs. The methodology presented is a necessary
first step towards an automated storm track recognition scheme which will be
employed in a second paper to study the future development of atmospheric
dynamics in a changing climate. The process towards obtaining storm tracks is
two-fold. In its first step cyclone centers are being identified. The
performance of this step requires the existence of closed isolines, i.e., a
topology in which a grid-point is surrounded by neighbours which all exhibit
higher geopotential. The usage of this topology requirement as well as the
constraint of coarse data may lead, though, to limitations in identifying
centers in geopotential fields with shallow gradients that may occur in the
summer months; moreover, some centers may potentially be missed in case of a
configuration in which a small scale storm is located at the perimeter of a
deep and very large low (a kind of "dent in a crater wall"). The second step
of the process strings the identified cyclone centers together in a
meaningful way to form tracks. By way of several examples the capability to
identify known storm tracks is shown
STAT-IMM, a statistical approach to determine local and background contributions to PM 10 levels
Abstract. When studying concentrations of particulate matter with a size of 10 µm or below (PM 10 ), measured locally, it becomes evident that two main portions need to be quantified: The concentration produced by sources in the vicinity of the station and the long range transports. The traditional approaches include analyses of the components of PM 10 , comparisons upwind and downwind of a station, investigation of trajectories and complex chemical transport modelling. The development of an independent strategy which makes use of statistical methods, including regression and correlation analysis is a reasonable alternative. This method, presented here, does not apply the concept of PM 10 sources, but, rather, analyzes the relations between times series of PM 10 measurements and atmospheric properties. It is applied to identify the shares of the local portion and the large-scale background plus a stochastic portion that cannot be attributed to either of the two. Using regression analysis, a set of objectively chosen meteorological parameters is used to reconstruct the local PM 10 measurement series, defining the local portion. This weather-dependent part of the series is then removed and the residuum, which contains the large-scale PM 10 background and a stochastic portion is analyzed further with correlations. Results are shown for a three-year set of data which includes well over 250 PM 10 stations across Germany. The data is analyzed according to different stratifications, such as the PM 10 load and the wind direction as well as for the data set as a whole. In a further development of the method, a study of PM 10 transports across several border sections is shown
Dihadron Tomography of High-Energy Nuclear Collisions in NLO pQCD
Back-to-back dihadron spectra in high-energy heavy-ion collisions are studied
within the next-to-leading order (NLO) perturbative QCD parton model with jet
quenching incorporated via modified jet fragmentation functions due to
radiative parton energy loss in dense medium. The experimentally observed
appearance of back-to-back dihadrons at high is found to originate mainly
from jet pairs produced close and tangential to the surface of the dense
matter. However, a substantial fraction of observed high dihadrons also
comes from jets produced at the center of the medium after losing finite amount
of energy. Consequently, the suppression factor of such high- hadron pairs
is found to be more sensitive to the initial gluon density than the single
hadron spectra that are dominated by surface emission. A simultaneous
-fit to both the single and dihadron spectra can be achieved within a
narrow range of the energy loss parameters GeV/fm. Because
of the flattening of the initial jet production spectra, high dihadrons
at the LHC energy are found to be more robust as probes of the dense medium.Comment: 4 pages in revtex with 5 figures, final version in PRL The numerical
tables of the NLO single and dihadron spectra used in this manuscript can be
downloaded from ftp://www-nsdth.lbl.gov/pub/xnwang/dihadron
Photo-astrometric distances, extinctions, and astrophysical parameters for Gaia DR2 stars brighter than G = 18
Combining the precise parallaxes and optical photometry delivered by Gaia's
second data release (Gaia DR2) with the photometric catalogues of PanSTARRS-1,
2MASS, and AllWISE, we derive Bayesian stellar parameters, distances, and
extinctions for 265 million stars brighter than G=18. Because of the wide
wavelength range used, our results substantially improve the accuracy and
precision of previous extinction and effective temperature estimates. After
cleaning our results for both unreliable input and output data, we retain 137
million stars, for which we achieve a median precision of 5% in distance, 0.20
mag in V-band extinction, and 245 K in effective temperature for G<14,
degrading towards fainter magnitudes (12%, 0.20 mag, and 245 K at G=16; 16%,
0.23 mag, and 260 K at G=17, respectively). We find a very good agreement with
the asteroseismic surface gravities and distances of 7000 stars in the Kepler,
the K2-C3, and the K2-C6 fields, with stellar parameters from the APOGEE
survey, as well as with distances to star clusters. Our results are available
through the ADQL query interface of the Gaia mirror at the Leibniz-Institut
f\"{u}r Astrophysik Potsdam (gaia.aip.de) and as binary tables at data.aip.de.
As a first application, in this paper we provide distance- and
extinction-corrected colour-magnitude diagrams, extinction maps as a function
of distance, and extensive density maps, demonstrating the potential of our
value-added dataset for mapping the three-dimensional structure of our Galaxy.
In particular, we see a clear manifestation of the Galactic bar in the stellar
density distributions, an observation that can almost be considered a direct
imaging of the Galactic bar.Comment: 25 pages, 23 figures + appendix, accepted for publication in A&A.
Data (doi:10.17876/gaia/dr.2/51) are available through ADQL queries at
gaia.aip.d
Shear viscosity of hot scalar field theory in the real-time formalism
Within the closed time path formalism a general nonperturbative expression is
derived which resums through the Bethe-Salpter equation all leading order
contributions to the shear viscosity in hot scalar field theory. Using a
previously derived generalized fluctuation-dissipation theorem for nonlinear
response functions in the real-time formalism, it is shown that the
Bethe-Salpeter equation decouples in the so-called (r,a) basis. The general
result is applied to scalar field theory with pure lambda*phi**4 and mixed
g*phi**3+lambda*phi**4 interactions. In both cases our calculation confirms the
leading order expression for the shear viscosity previously obtained in the
imaginary time formalism.Comment: Expanded introduction and conclusions. Several references and a
footnote added. Fig.5 and its discussion in the text modified to avoid double
counting. Signs in Eqs. (45) and (53) correcte
Spallation Neutron Production by 0.8, 1.2 and 1.6 GeV Protons on various Targets
Spallation neutron production in proton induced reactions on Al, Fe, Zr, W,
Pb and Th targets at 1.2 GeV and on Fe and Pb at 0.8, and 1.6 GeV measured at
the SATURNE accelerator in Saclay is reported. The experimental
double-differential cross-sections are compared with calculations performed
with different intra-nuclear cascade models implemented in high energy
transport codes. The broad angular coverage also allowed the determination of
average neutron multiplicities above 2 MeV. Deficiencies in some of the models
commonly used for applications are pointed out.Comment: 20 pages, 32 figures, revised version, accepted fpr publication in
Phys. Rev.
Pathway to the Square Kilometre Array - The German White Paper -
The Square Kilometre Array (SKA) is the most ambitious radio telescope ever
planned. With a collecting area of about a square kilometre, the SKA will be
far superior in sensitivity and observing speed to all current radio
facilities. The scientific capability promised by the SKA and its technological
challenges provide an ideal base for interdisciplinary research, technology
transfer, and collaboration between universities, research centres and
industry. The SKA in the radio regime and the European Extreme Large Telescope
(E-ELT) in the optical band are on the roadmap of the European Strategy Forum
for Research Infrastructures (ESFRI) and have been recognised as the essential
facilities for European research in astronomy.
This "White Paper" outlines the German science and R&D interests in the SKA
project and will provide the basis for future funding applications to secure
German involvement in the Square Kilometre Array.Comment: Editors: H. R. Kl\"ockner, M. Kramer, H. Falcke, D.J. Schwarz, A.
Eckart, G. Kauffmann, A. Zensus; 150 pages (low resolution- and colour-scale
images), published in July 2012, language English (including a foreword and
an executive summary in German), the original file is available via the MPIfR
homepag
<i>Gaia</i> Data Release 1. Summary of the astrometric, photometric, and survey properties
Context. At about 1000 days after the launch of Gaia we present the first Gaia data release, Gaia DR1, consisting of astrometry and photometry for over 1 billion sources brighter than magnitude 20.7.
Aims. A summary of Gaia DR1 is presented along with illustrations of the scientific quality of the data, followed by a discussion of the limitations due to the preliminary nature of this release.
Methods. The raw data collected by Gaia during the first 14 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into an astrometric and photometric catalogue.
Results. Gaia DR1 consists of three components: a primary astrometric data set which contains the positions, parallaxes, and mean proper motions for about 2 million of the brightest stars in common with the HIPPARCOS and Tycho-2 catalogues – a realisation of the Tycho-Gaia Astrometric Solution (TGAS) – and a secondary astrometric data set containing the positions for an additional 1.1 billion sources. The second component is the photometric data set, consisting of mean G-band magnitudes for all sources. The G-band light curves and the characteristics of ∼3000 Cepheid and RR-Lyrae stars, observed at high cadence around the south ecliptic pole, form the third component. For the primary astrometric data set the typical uncertainty is about 0.3 mas for the positions and parallaxes, and about 1 mas yr−1 for the proper motions. A systematic component of ∼0.3 mas should be added to the parallax uncertainties. For the subset of ∼94 000 HIPPARCOS stars in the primary data set, the proper motions are much more precise at about 0.06 mas yr−1. For the secondary astrometric data set, the typical uncertainty of the positions is ∼10 mas. The median uncertainties on the mean G-band magnitudes range from the mmag level to ∼0.03 mag over the magnitude range 5 to 20.7.
Conclusions. Gaia DR1 is an important milestone ahead of the next Gaia data release, which will feature five-parameter astrometry for all sources. Extensive validation shows that Gaia DR1 represents a major advance in the mapping of the heavens and the availability of basic stellar data that underpin observational astrophysics. Nevertheless, the very preliminary nature of this first Gaia data release does lead to a number of important limitations to the data quality which should be carefully considered before drawing conclusions from the data
Cold Nuclear Matter Effects on Dijet Productions in Relativistic Heavy-ion Reactions at LHC
We investigate the cold nuclear matter(CNM) effects on dijet productions in
high-energy nuclear collisions at LHC with the next-to-leading order
perturbative QCD. The nuclear modifications for dijet angular distributions,
dijet invariant mass spectra, dijet transverse momentum spectra and dijet
momentum imbalance due to CNM effects are calculated by incorporating EPS, EKS,
HKN and DS param-etrization sets of parton distributions in nucleus . It is
found that dijet angular distributions and dijet momentum imbalance are
insensitive to the initial-state CNM effects and thus provide optimal tools to
study the final-state hot QGP effects such as jet quenching. On the other hand,
the invariant mass spectra and the transverse momentum spectra of dijet are
generally enhanced in a wide region of the invariant mass or transverse
momentum due to CNM effects with a feature opposite to the expected suppression
because of the final-state parton energy loss effect in the QGP. The difference
of EPS, EKS, HKN and DS parametrization sets of nuclear parton distribution
functions is appreciable for dijet invariant mass spectra and transverse
momentum spectra at p+Pb collisions, and becomes more pronounced for those at
Pb+Pb reactions.Comment: 10 pages, 11 figure
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