220 research outputs found
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Some challenges of middle atmosphere data assimilation
The assimilation of measurements from the stratosphere and mesosphere is becoming increasingly common as the lids of weather prediction and climate models rise into the mesosphere and thermosphere. However, the dynamics of the middle atmosphere pose specific challenges to the assimilation of measurements from this region. Forecast-error variances can be very large in the mesosphere and this can render assimilation schemes very sensitive to the details of the specification of forecast error correlations. An example is shown where observations in the stratosphere are able to produce increments in the mesosphere. Such sensitivity of the assimilation scheme to misspecification of covariances can also amplify any existing biases in measurements or forecasts. Since both models and measurements of the middle atmosphere are known to have biases, the separation of these sources of bias remains a issue. Finally, well-known deficiencies of assimilation schemes, such as the production of imbalanced states or the assumption of zero bias, are proposed explanations for the inaccurate transport resulting from assimilated winds. The inability of assimilated winds to accurately transport constituents in the middle atmosphere remains a fundamental issue limiting the use of assimilated products for applications involving longer time-scales
Coherent \pi^0 threshold production from the deuteron at Q^2 = 0.1 GeV^2/c^2
First data on coherent threshold \pi^0 electroproduction from the deuteron
taken by the A1 Collaboration at the Mainz Microtron MAMI are presented. At a
four-momentum transfer of q^2=-0.1 GeV^2/c^2 the full solid angle was covered
up to a center-of-mass energy of 4 MeV above threshold. By means of a
Rosenbluth separation the longitudinal threshold s wave multipole and an upper
limit for the transverse threshold s wave multipole could be extracted and
compared to predictions of Heavy Baryon Chiral Perturbation Theory.Comment: 7 pages, 7 figures, latex2
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Nonlinear bias correction for satellite data assimilation using Taylor series polynomials
Output from a high-resolution ensemble data assimilation system is used to assess the ability of an innovative nonlinear bias correction (BC) method that uses a Taylor series polynomial expansion of the observation-minus background departures to remove linear and nonlinear conditional biases from all-sky satellite infrared brightness temperatures. Univariate and multivariate experiments were performed in which the satellite zenith angle and variables sensitive to clouds and water vapor were used as the BC predictors. The results showed that even though the bias of the entire observation departure distribution is equal to zero regardless of the order of the Taylor series expansion, there are often large conditional biases that vary as a nonlinear function of the BC predictor. The linear 1st order term had the largest impact on the entire distribution as measured by reductions in variance; however, large conditional biases often remained in the distribution when plotted as a function of the predictor. These conditional biases were typically reduced to near zero when the nonlinear 2nd and 3rd order terms were used. The univariate results showed that variables sensitive to the cloud top height are effective BC predictors especially when higher order Taylor series terms are used. Comparison of the statistics for clear-sky and cloudy-sky observations revealed that nonlinear departures are more important for cloudy-sky observations as signified by the much larger impact of the 2nd and 3rd order terms on the conditional biases. Together, these results indicate that the nonlinear BC method is able to effectively remove the bias from all-sky infrared observation departures
Measurement of the recoil polarization in the p (\vec e, e' \vec p) pi^0 reaction at the \Delta(1232) resonance
The recoil proton polarization has been measured in the p (\vec e,e'\vec p)
pi^0 reaction in parallel kinematics around W = 1232 MeV, Q^2 = 0.121 (GeV/c)^2
and epsilon = 0.718 using the polarized c.w. electron beam of the Mainz
Microtron. Due to the spin precession in a magnetic spectrometer, all three
proton polarization components P_x/P_e = (-11.4 \pm 1.3 \pm 1.4) %, P_y =
(-43.1 \pm 1.3 \pm 2.2) %, and P_z/P_e = (56.2 \pm 1.5 \pm 2.6) % could be
measured simultaneously. The Coulomb quadrupole to magnetic dipole ratio CMR =
(-6.4\pm 0.7_{stat}\pm 0.8_{syst}) % was determined from P_x in the framework
of the Mainz Unitary Isobar Model. The consistency among the reduced
polarizations and the extraction of the ratio of longitudinal to transverse
response is discussed.Comment: 5 pages LaTeX, 1 table, 2 eps figure
Self energies of the pion and the delta isobar from the ^3He(e,e'pi^+)^3H reaction
In a kinematically complete experiment at the Mainz microtron MAMI, pion
angular distributions of the He(e,e'H reaction have been measured
in the excitation region of the resonance to determine the
longitudinal (), transverse (), and the interference part of the
differential cross section. The data are described only after introducing
self-energy modifications of the pion and -isobar propagators. Using
Chiral Perturbation Theory (ChPT) to extrapolate the pion self energy as
inferred from the measurement on the mass shell, we deduce a reduction of the
mass of MeV/c in the
neutron-rich nuclear medium at a density of fm. Our data are consistent with the self energy
determined from measurements of photoproduction from He and heavier
nuclei.Comment: Elsart, 12 pages and 4 figures, Correspondent: Professor Dr. Dr. h.c.
mult. Achim Richter, [email protected], submitted to Phys. Rev.
Let
Polarization transfer in the HeH reaction
Polarization transfer in the 4He(e,e'p)3H reaction at a Q^2 of 0.4 (GeV/c)^2
was measured at the Mainz Microtron MAMI. The ratio of the transverse to the
longitudinal polarization components of the ejected protons was compared with
the same ratio for elastic ep scattering. The results are consistent with a
recent fully relativistic calculation which includes a predicted medium
modification of the proton form factor based on a quark-meson coupling model.Comment: 5 pages, Latex, 2 postscript figures, submitted to Physics Letters
The first determination of Generalized Polarizabilities of the proton by a Virtual Compton Scattering experiment
Absolute differential cross sections for the reaction (e+p -> e+p+gamma) have
been measured at a four-momentum transfer with virtuality Q^2=0.33 GeV^2 and
polarization \epsilon = 0.62 in the range 33.6 to 111.5 MeV/c for the momentum
of the outgoing photon in the photon-proton center of mass frame. The
experiment has been performed with the high resolution spectrometers at the
Mainz Microtron MAMI. From the photon angular distributions, two structure
functions which are a linear combination of the generalized polarizabilities
have been determined for the first time.Comment: 4 pages, 3 figure
Variational analysis of drifter positions and model outputs for the reconstruction of surface currents in the central Adriatic during fall 2002
Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 113 (2008): C04004, doi:10.1029/2007JC004148.In this paper we present an application of a variational method for the reconstruction of the velocity field in a coastal flow in the central Adriatic Sea, using in situ data from surface drifters and outputs from the ROMS circulation model. The variational approach, previously developed and tested for mesoscale open ocean flows, has been improved and adapted to account for inhomogeneities on boundary current dynamics over complex bathymetry and coastline and for weak Lagrangian persistence in coastal flows. The velocity reconstruction is performed using nine drifter trajectories over 45 d, and a hierarchy of indirect tests is introduced to evaluate the results as the real ocean state is not known. For internal consistency and impact of the analysis, three diagnostics characterizing the particle prediction and transport, in terms of residence times in various zones and export rates from the boundary current toward the interior, show that the reconstruction is quite effective. A qualitative comparison with sea color data from the MODIS satellite images shows that the reconstruction significantly improves the description of the boundary current with respect to the ROMS model first guess, capturing its main features and its exchanges with the interior when sampled by the drifters.Four of the authors are supported by the
Office of Naval Research, V.T. and A.G. under grants N00014-05-1-0094
and N00014-05-1-0095, P.M.P. under grant N00014-03-1-0291, and S.C.
under grant N00014-05-1-0730. CNR-ISMAR
activity was partially supported by P.O.R. ‘‘CAINO’’ (Regione Puglia),
VECTOR (Italian MIUR) project, and ECOOP (EU project)
Oracle-based optimization applied to climate model calibration
In this paper, we show how oracle-based optimization can be effectively used for the calibration of an intermediate complexity climate model. In a fully developed example, we estimate the 12 principal parameters of the C-GOLDSTEIN climate model by using an oracle- based optimization tool, Proximal-ACCPM. The oracle is a procedure that finds, for each query point, a value for the goodness-of-fit function and an evaluation of its gradient. The difficulty in the model calibration problem stems from the need to undertake costly calculations for each simulation and also from the fact that the error function used to assess the goodness-of-fit is not convex. The method converges to a Fbest fit_ estimate over 10 times faster than a comparable test using the ensemble Kalman filter. The approach is simple to implement and potentially useful in calibrating computationally demanding models based on temporal integration (simulation), for which functional derivative information is not readily available
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Corporate social responsibility: The good, the bad and the ugly
In this article I critically analyze contemporary discourses of corporate social responsibility and related discourses of sustainability and corporate citizenship. I argue that despite their emancipatory rhetoric, discourses of corporate citizenship, social responsibility and sustainability are defined by narrow business interests and serve to curtail interests of external stakeholders. I provide an alternate perspective, one that views discourses of corporate citizenship, corporate social responsibility, and sustainability as ideological movements that are intended to legitimize and consolidate the power of large corporations. I also problematize the popular notion of organizational 'stakeholders'. I argue that stakeholder theory of the firm represents a form of stakeholder colonialism that serves to regulate the behavior of stakeholders. I conclude by discussing implications for critical management studies
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