147 research outputs found
Analysis of 2 meter temperature and relative humidity
This note documents the recent installation of the analysis of the near surface parameters into the HIRLAM system. It is intended to explain why this analysis is primarely needed for the new surface parameterization that is being developed, although a version able to run within the old surface parameterization context has been prepared also. A procedure for the vertical reduction of the first guess fields to the station height has been introduced, and is specially useful in complex terrain. Since the method of analysis is Optimum Interpolation, a re-evaluation of the statistics of the first guess errors was necessary. First guess error covariances have been obtained for a selection of stations in Spain and in Scandinavia for winter and summer, in order to study the possible dependence on season and latitude. The effect of the orography as source of anisotropy, both because to the coastline and the inland topography, is included in the formulation of a new structure function
Perturbations in Hybrid Loop Quantum Cosmology: Continuum Limit in Fourier Space
We analyze the passage to a continuum limit of the mode spectrum of
primordial perturbations around flat cosmological spacetimes in hybrid Loop
Quantum Cosmology, showing that this limit can be reached even if one starts by
considering a finite fiducial cell as spatial slice. We focus our attention on
regimes in which the background cosmology follows the effective dynamics of
Loop Quantum Cosmology, although we comment on extensions of our arguments
beyond this regime, as well as to some formalisms other than the hybrid
approach. Whereas the perturbed system can be described in an invariant way
under changes of the fiducial volume using the standard variables of the
improved prescription for Loop Quantum Cosmology, we show that the desired
continuum limit can be established by means of scaling transformations of the
physical volume when this volume grows unboundedly. These transformations lead
to a model with a continuum of modes and independent of any scale of reference
for the physical volume. For the sake of comparison, we also consider an
alternative road to the continuum in Fourier space that has been employed in
geometrodynamics and is based on the use of scaling transformations of the
fiducial volume, together with variables that are independent of them.Comment: 13 page
On the evolution of the volume in Loop Quantum Cosmology
The dynamics of the expectation value of the volume is one of the key
ingredients behind the replacement of the Big Bang singularity by a bounce in
Loop Quantum Cosmology. As such, it is of great importance that this quantity
is mathematically well-defined in the space of physical states of the theory. A
number of caveats have been raised about such a definition entering in conflict
with the quantum evolution of states, motivated by the situation found in
quantum geometrodynamics. We show that there are ways around these caveats, all
of which are related to the existence of quantization prescriptions leading to
a nondegenerate curvature operator in Loop Quantum Cosmology. Furthermore, we
explicitly check that there exist families of physical states for which the
expectation value of the volume is actually finite under evolution. These
include the Gaussian states traditionally used in investigations of the quantum
bounce. Interestingly, the properties of the curvature operator that allow for
such a good behavior of the volume are only possible thanks to the discreteness
of the geometry characteristic of the loop quantization procedure.Comment: 13 page
Usage of satellite data in the HIRLAM new snow depth analysis
Comunicación presentada en: 4th SRNWP/HIRLAM workshop on surface processes and assimilation of surface variables celebrado en Norrköping, Suecia, del 15 al 17 de septiembre de 2004.New Optimum Interpolation Snow Depth Analysis has been recently developed for HIRLAM. Now, SSMI derived NESDIS snow cover weekly product and MODIS TERRA daily 0.05º snow cover data have been tested as information supply for this analysis. Some conclusions are drawn from them. Additionally, we tested
the differences found in the snow cover field depending on the spatial resolution of the experiments
Usage of SYNOP 10 meters wind observations in HIRLAM
It is not easy to deal with 10 meters wind observations oven land in NWP models. This is due mainly to the difficulty of the models to represent the real orography and physiography and the high degree of dependence of near surface wind observations with both, physiography and orography. Other source of errors in the analyzed 10 meters wind field is the usage of observations which are not representative of the neighbouring terrain. Here, we describe the method we used to, subjectively, identify good and bad wind stations, the way to introduce SYNOP 10 meters wind observations in HIRLAM using whitelists and the analysis of the experiments and case studies carried out
Fermions in Hybrid Loop Quantum Cosmology
This work pioneers the quantization of primordial fermion perturbations in
hybrid Loop Quantum Cosmology (LQC). We consider a Dirac field coupled to a
spatially flat, homogeneous, and isotropic cosmology, sourced by a scalar
inflaton, and treat the Dirac field as a perturbation. We describe the
inhomogeneities of this field in terms of creation and annihilation variables,
chosen to admit a unitary evolution if the Dirac fermion were treated as a test
field. Considering instead the full system, we truncate its action at quadratic
perturbative order and construct a canonical formulation. In particular this
implies that, in the global Hamiltonian constraint of the model, the
contribution of the homogeneous sector is corrected with a quadratic
perturbative term. We then adopt the hybrid LQC approach to quantize the full
model, combining the loop representation of the homogeneous geometry with the
Fock quantization of the inhomogeneities. We assume a Born-Oppenheimer ansatz
for physical states and show how to obtain a Schr\"odinger equation for the
quantum evolution of the perturbations, where the role of time is played by the
homogeneous inflaton. We prove that the resulting quantum evolution of the
Dirac field is indeed unitary, despite the fact that the underlying homogeneous
geometry has been quantized as well. Remarkably, in such evolution, the fermion
field couples to an infinite sequence of quantum moments of the homogeneous
geometry. Moreover, the evolved Fock vacuum of our fermion perturbations is
shown to be an exact solution of the Schr\"odinger equation. Finally, we
discuss in detail the quantum backreaction that the fermion field introduces in
the global Hamiltonian constraint. For completeness, our quantum study includes
since the beginning (gauge-invariant) scalar and tensor perturbations, that
were studied in previous works.Comment: 29 pages. It matches published versio
Modified FRW cosmologies arising from states of the hybrid quantum Gowdy model
We construct approximate solutions of the hybrid quantum Gowdy cosmology with
three-torus topology, linear polarization, and local rotational symmetry, in
the presence of a massless scalar field. More specifically, we determine some
families of states for which the complicated inhomogeneous and anisotropic
Hamiltonian constraint operator of the Gowdy model is approximated by a much
simpler one. Our quantum states follow the dynamics governed by this simpler
constraint, while being at the same time also approximate solutions of the full
Gowdy model. This is so thanks to the quantum correlations that the considered
states present between the isotropic and anisotropic sectors of the model.
Remarkably, this simpler constraint can be regarded as that of a flat
Friedmann-Robertson-Walker universe filled with different kinds of perfect
fluids and geometrically corrected by homogeneous and isotropic curvature-like
terms. Therefore, our quantum states, which are intrinsically inhomogeneous,
admit approximate homogeneous and isotropic effective descriptions similar to
those considered in modified theories of gravity.Comment: Version accepted for publication in PR
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