14,965 research outputs found
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Doppler W-band polarization diversity space-borne radar simulator for wind studies
CloudSat observations are used in combination with collocated European Centre for Medium-Range
Weather Forecasts (ECMWF) reanalysis to simulate spaceborne W-band Doppler observations from slant-looking
radars. The simulator also includes cross-polarization effects
which are relevant if the Doppler velocities are derived from
polarization diversity pulse pair correlation. A specific conically scanning radar configuration (WIVERN), recently proposed to the ESA-Earth Explorer 10 call that aims to provide
global in-cloud winds for data assimilation, is analysed in
detail in this study.
One hundred granules of CloudSat data are exploited to investigate the impact on Doppler velocity estimates from three
specific effects: (1) non-uniform beam filling, (2) wind shear
and (3) crosstalk between orthogonal polarization channels
induced by hydrometeors and surface targets. Errors associated with non-uniform beam filling constitute the most important source of error and can account for almost 1 m sâ1
standard deviation, but this can be reduced effectively to less
than 0.5 m sâ1 by adopting corrections based on estimates
of vertical reflectivity gradients. Wind-shear-induced errors
are generally much smaller (⌠0.2 m sâ1
). A methodology
for correcting these errors has been developed based on estimates of the vertical wind shear and the reflectivity gradient. Low signal-to-noise ratios lead to higher random errors
(especially in winds) and therefore the correction (particularly the one related to the wind-shear-induced error) is less
effective at low signal-to-noise ratio. Both errors can be underestimated in our model because the CloudSat data do not
fully sample the spatial variability of the reflectivity fields,
whereas the ECMWF reanalysis may have smoother velocity fields than in reality (e.g. they underestimate vertical wind
shear).
The simulator allows for quantification of the average
number of accurate measurements that could be gathered by
the Doppler radar for each polar orbit, which is strongly impacted by the selection of the polarization diversity H â V
pulse separation, Thv. For WIVERN a selection close to 20 ”s
(with a corresponding folding velocity equal to 40 m sâ1
)
seems to achieve the right balance between maximizing the
number of accurate wind measurements (exceeding 10 % of
the time at any particular level in the mid-troposphere) and
minimizing aliasing effects in the presence of high winds.
The study lays the foundation for future studies towards
a thorough assessment of the performance of polar orbiting
wide-swath W-band Doppler radars on a global scale. The
next generation of scanning cloud radar systems and reanalyses with improved resolution will enable a full capture of the
spatial variability of the cloud reflectivity and the in-cloud
wind fields, thus refining the results of this study
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Implementation of polarization diversity pulse-pair technique using airborne W-band radar
This work describes the implementation of polarization diversity on the National Research Council Canada W-band Doppler radar and presents the first-ever airborne Doppler measurements derived via polarization diversity pulse-pair processing. The polarization diversity pulse-pair measurements are interleaved with standard pulse-pair measurements with staggered pulse repetition frequency, this allows a better understanding of the strengths and drawbacks of polarization diversity, a methodology that has been recently proposed for wind-focused Doppler radar space missions. Polarization diversity has the clear advantage of making possible Doppler observations of very fast decorrelating media (as expected when deploying Doppler radars on fast-moving satellites) and of widening the Nyquist interval, thus enabling the observation of very high Doppler velocities (up to more than 100âmâsâ1 in the present work). Crosstalk between the two polarizations, mainly caused by depolarization at backscattering, deteriorated the quality of the observations by introducing ghost echoes in the power signals and by increasing the noise level in the Doppler measurements. In the different cases analyzed during the field campaigns, the regions affected by crosstalk were generally associated with highly depolarized surface returns and depolarization of backscatter from hydrometeors located at short ranges from the aircraft. The variance of the Doppler velocity estimates can be well predicted from theory and were also estimated directly from the observed correlation between the H-polarized and V-polarized successive pulses. The study represents a key milestone towards the implementation of polarization diversity in Doppler space-borne radars
Determination of the muon Yukawa coupling at high energy e+e- linear colliders
The perspectives for the observation of the rare decay H -> mumu decay and
the determination of the muon Yukawa coupling at a TeV-class and at a multi-TeV
e+e- linear colliders are discussed. The signal for the decay can be obtained
at 0.8 TeV and a first estimate of the coupling derived. A linear collider
operating at 3 TeV, with high luminosity, is able to improve the accuracy on
this couplings to 4% to 11% for 120 GeV < M_H < 150 GeV.Comment: 3 pages, 2 figures, to appear on the proceedings of the Snowmass 2001
Summer Study, Snowmass CO, June-July 200
Sterile neutrinos as subdominant warm dark matter
In light of recent findings which seem to disfavor a scenario with (warm)
dark matter entirely constituted of sterile neutrinos produced via the
Dodelson-Widrow (DW) mechanism, we investigate the constraints attainable for
this mechanism by relaxing the usual hypothesis that the relic neutrino
abundance must necessarily account for all of the dark matter. We first study
how to reinterpret the limits attainable from X-ray non-detection and
Lyman-alpha forest measurements in the case that sterile neutrinos constitute
only a fraction fs of the total amount of dark matter. Then, assuming that
sterile neutrinos are generated in the early universe solely through the DW
mechanism, we show how the X-ray and Lyman-alpha results jointly constrain the
mass-mixing parameters governing their production. Furthermore, we show how the
same data allow us to set a robust upper limit fs < 0.7 at the 2 sigma level,
rejecting the case of dominant dark matter (fs = 1) at the ~ 3 sigma level.Comment: Minor changes; added references; version accepted for publication in
Phys. Rev.
B2 and G2 Toda systems on compact surfaces: a variational approach
We consider the B2 and G2 Toda systems on compact surfaces. We attack the
problem using variational techniques. We get existence and multiplicity of
solutions under a topological assumption on the surface and some generic
conditions on the parameters. We also extend some of the results to the case of
general systems.Comment: 28 pages, accepted on Journal of Mathematical Physic
Light neutralino dark matter in MSSM
Three dark matter direct detection experiments, DAMA, COGENT and CRESST, have
reported a possible signal of WIMP interaction corresponding to very light
particles, close to the edge of the XENON 100 and CDMS sensitivity. Imposing
the latest constraints from colliders, flavour physics, electroweak precision
tests and direct and indirect dark matter searches, we show that viable MSSM
scenarios with a light neutralino, in agreement with all the present data, are
feasible. An analysis of the characteristics of the resulting scenarios will be
presented.Comment: 6 pages, 2 figures, contribution to the ICHEP 2012 Conference
proceedings, 4-11 July 2012, Melbourne, Australi
Study of e+e- -> H+H- at a 800 GeV Linear Collider
The production and decay of heavy charged Higgs bosons at a 800 GeV e+e-
linear collider have been studied. The analysis of the H+H- -> tb tb, expected
to be dominant in the MSSM, and H+H- -> W+h0 W-h0 decay modes leading to the
same final state consisting of two W bosons and four b quarks, provides with a
determination of the boson mass to 1 GeV and of the production cross section
with 10% accuracy for 500 fb-1 of data.Comment: 4 pages, 1 figure, to appear in the Proceedings of the 5th Linear
Collider Workshop Fermilab, October 200
Implications of LHC Higgs and SUSY searches for MSSM
The implications of the LHC SUSY searches as well as the discovery of a new
bosonic state compatible with the lightest Higgs boson will be discussed in the
context of constrained and general MSSM scenarios. Exploring the MSSM through
the Higgs sector is an alternative and complementary path to direct searches,
and tight constraints on the MSSM parameter space can be obtained.Comment: 7 pages, 5 figures, contribution to the ICHEP 2012 Conference
proceedings, 4-11 July 2012, Melbourne, Australi
Hybrid Pixel Detector Development for the Linear Collider Vertex Tracker
In order to fully exploit the physics potential of the future high energy
e+e- linear collider, a Vertex Tracker able to provide particle track
extrapolation with very high resolution is needed. Hybrid Si pixel sensors are
an attractive technology due to their fast read-out capabilities and radiation
hardness. A novel pixel detector layout with interleaved cells has been
developed to improve the single point resolution. Results of the
characterisation of the first processed prototypes by electrostatic
measurements and charge collection studies are discussed.Comment: 5 pages, 1 figure, to appear in the Proceedings of the 9th Int.
Workshop on Vertex Detectors, Lake Michigan MI (USA), September~200
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