244 research outputs found
Explicit representation and parametrised impacts of under ice shelf seas in the z∗ coordinate ocean model NEMO 3.6
Ice-shelf-ocean interactions are a major source of freshwater on the Antarctic continental shelf and have a strong impact on ocean properties, ocean circulation and sea ice. However, climate models based on the ocean-sea ice model NEMO (Nucleus for European Modelling of the Ocean) currently do not include these interactions in any detail. The capability of explicitly simulating the circulation beneath ice shelves is introduced in the non-linear free surface model NEMO. Its implementation into the NEMO framework and its assessment in an idealised and realistic circum-Antarctic configuration is described in this study. Compared with the current prescription of ice shelf melting (i.e. at the surface), inclusion of open sub-ice-shelf cavities leads to a decrease in sea ice thickness along the coast, a weakening of the ocean stratification on the shelf, a decrease in salinity of high-salinity shelf water on the Ross and Weddell sea shelves and an increase in the strength of the gyres that circulate within the over-deepened basins on the West Antarctic continental shelf. Mimicking the overturning circulation under the ice shelves by introducing a prescribed meltwater flux over the depth range of the ice shelf base, rather than at the surface, is also assessed. It yields similar improvements in the simulated ocean properties and circulation over the Antarctic continental shelf to those from the explicit ice shelf cavity representation. With the ice shelf cavities opened, the widely used "three equation" ice shelf melting formulation, which enables an interactive computation of melting, is tested. Comparison with observational estimates of ice shelf melting indicates realistic results for most ice shelves. However, melting rates for the Amery, Getz and George VI ice shelves are considerably overestimated
Nonperturbative calculation of the anomalous magnetic moment in the Yukawa model
Within the covariant formulation of light-front dynamics, we calculate the
state vector of a fermion coupled to identical scalar bosons (the Yukawa
model). The state vector is decomposed in Fock sectors and we consider the
first three ones: a single fermion, a fermion coupled to one boson, and a
fermion coupled to two bosons. This last three-body sector generates nontrivial
and nonperturbative contributions to the state vector, and these contributions
are calculated with no approximations. The divergences of the amplitudes are
regularized using Pauli-Villars fermion and boson fields. Physical observables
can be unambiguously deduced using a systematic renormalization scheme we
developed. This renormalization scheme is a necessary condition in order to
avoid uncancelled divergences when Fock space is truncated. As an example, we
present preliminary numerical results for the anomalous magnetic moment of a
fermion in the Yukawa model.Comment: 7 pages, 7 figures. Contribution to the proceedings of the Workshop:
Light-Cone 2008, "Relativistic Nuclear and Particle Physics", Mulhouse,
France, July 7-11, 2008. To be published in the online journal "Proceedings
of Science" - Po
The pion wave function in covariant light-front dynamics
The structure of the pion wave function in the relativistic constituent quark
model is investigated in the explicitly covariant formulation of light-front
dynamics. We calculate the two relativistic components of the pion wave
function in a simple one-gluon exchange model and investigate various physical
observables: decay constant, charge radius, electromagnetic and transition form
factors. We discuss the influence of the full relativistic structure of the
pion wave function for an overall good description of all these observables,
including both low and high momentum scales.Comment: 12 pages, 10 figure
Nonperturbative renormalization in light-front dynamics and applications
We present a general framework to calculate the properties of relativistic
compound systems from the knowledge of an elementary Hamiltonian. Our framework
provides a well-controlled nonperturbative calculational scheme which can be
systematically improved. The state vector of a physical system is calculated in
light-front dynamics. From the general properties of this form of dynamics, the
state vector can be further decomposed in well-defined Fock components. In
order to control the convergence of this expansion, we advocate the use of the
covariant formulation of light-front dynamics. In this formulation, the state
vector is projected on an arbitrary light-front plane \omega \cd x=0 defined
by a light-like four-vector . This enables us to control any violation
of rotational invariance due to the truncation of the Fock expansion. We then
present a general nonperturbative renormalization scheme in order to avoid
field-theoretical divergences which may remain uncancelled due to this
truncation. This general framework has been applied to a large variety of
models. As a starting point, we consider QED for the two-body Fock space
truncation and calculate the anomalous magnetic moment of the electron. We show
that it coincides, in this approximation, with the well-known Schwinger term.
Then we investigate the properties of a purely scalar system in the three-body
approximation, where we highlight the role of antiparticle degrees of freedom.
As a non-trivial example of our framework, we calculate the structure of a
physical fermion in the Yukawa model, for the three-body Fock space truncation
(but still without antifermion contributions). We finally show why our approach
is also well-suited to describe effective field theories like chiral
perturbation theory in the baryonic sector.Comment: 17 pages, 19 figures "Relativistic Description of Two- and Three-Body
Systems in Nuclear Physics", ECT*, October 19-23 200
Microscopic three-body force for asymmetric nuclear matter
Brueckner calculations including a microscopic three-body force have been
extended to isospin asymmetric nuclear matter. The effects of the three-body
force on the equation of state and on the single-particle properties of nuclear
matter are discussed with a view to possible applications in nuclear physics
and astrophysics. It is shown that, even in the presence of the three-body
force, the empirical parabolic law of the energy per nucleon vs isospin
asymmetry is fulfilled in the whole asymmetry range
up to high densities. The three-body force provides a strong
enhancement of symmetry energy increasing with the density in good agreement
with relativistic approaches. The Lane's assumption that proton and neutron
mean fields linearly vary vs the isospin parameter is violated at high density
in the presence of the three-body force. Instead the momentum dependence of the
mean fields is rather insensitive to three body force which brings about a
linear isospin deviation of the neutron and proton effective masses. The
isospin effects on multifragmentation events and collective flows in heavy-ion
collisions are briefly discussed along with the conditions for direct URCA
processes to occur in the neutron-star cooling.Comment: 11 pages, 7 figure
Ab initio nonperturbative calculation of physical observables in light-front dynamics. Application to the Yukawa model
We present a coherent and operational strategy to calculate, in a
nonperturbative way, physical observables in light-front dynamics. This
strategy is based on the decomposition of the state vector of any compound
system in Fock components, and on the covariant formulation of light-front
dynamics, together with the so-called Fock sector dependent renormalization
scheme. We apply our approach to the calculation of the electromagnetic form
factors of a fermion in the Yukawa model, in the nontrivial three-body Fock
space truncation, for rather large values of the coupling constant. We find
that, once the renormalization conditions are properly taken into account, the
form factors do not depend on the regularization scale, when the latter is much
larger than the physical masses. We then extend the Fock space by including
antifermion degrees of freedom.Comment: 22 pages, 16 figure
Taylor-Lagrange renormalization scheme, Pauli-Villars subtraction, and light-front dynamics
We show how the recently proposed Taylor-Lagrange renormalization scheme can
lead to extensions of singular distributions which are reminiscent of the
Pauli-Villars subtraction. However, at variance with the Pauli-Villars
regularization scheme, no infinite mass limit is performed in this scheme. As
an illustration of this mechanism, we consider the calculation of the
self-energy in second order perturbation theory in the Yukawa model, within the
covariant formulation of light-front dynamics. We show in particular how
rotational invariance is preserved in this scheme.Comment: 9 pages, 1 figure To be published in Physical Review
Investigating the consistency between proxy-based reconstructions and climate models using data assimilation: a mid-Holocene case study
The mid-Holocene (6 kyr BP; thousand years before present) is a key period to study the consistency between model results and proxy-based reconstruction data as it corresponds to a standard test for models and a reasonable number of proxy-based records is available. Taking advantage of this relatively large amount of information, we have compared a compilation of 50 air and sea surface temperature reconstructions with the results of three simulations performed with general circulation models and one carried out with LOVECLIM, a model of intermediate complexity. The conclusions derived from this analysis confirm that models and data agree on the large-scale spatial pattern but the models underestimate the magnitude of some observed changes and that large discrepancies are observed at the local scale. To further investigate the origin of those inconsistencies, we have constrained LOVECLIM to follow the signal recorded by the proxies selected in the compilation using a data-assimilation method based on a particle filter. In one simulation, all the 50 proxy-based records are used while in the other two only the continental or oceanic proxy-based records constrain the model results. As expected, data assimilation leads to improving the consistency between model results and the reconstructions. In particular, this is achieved in a robust way in all the experiments through a strengthening of the westerlies at midlatitude that warms up northern Europe. Furthermore, the comparison of the LOVECLIM simulations with and without data assimilation has also objectively identified 16 proxy-based paleoclimate records whose reconstructed signal is either incompatible with the signal recorded by some other proxy-based records or with model physics
Systematic renormalization scheme in light-front dynamics with Fock space truncation
Within the framework of the covariant formulation of light-front dynamics, we
develop a general non-perturbative renormalization scheme based on the Fock
decomposition of the state vector and its truncation. The counterterms and bare
parameters needed to renormalize the theory depend on the Fock sectors. We
present a general strategy in order to calculate these quantities, as well as
state vectors of physical systems, in a truncated Fock space. The explicit
dependence of our formalism on the orientation of the light front plane is
essential in order to analyze the structure of the counterterms. We apply our
formalism to the two-body (one fermion and one boson) truncation in the Yukawa
model and in QED, and to the three-body truncation in a scalar model. In QED,
we recover analytically, without any perturbative expansion, the
renormalization of the electric charge, according to the requirements of the
Ward identity.Comment: 32 pages, 14 figures, submitted in Phys. Rev.
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