Analyse et amélioration d'un indice pluviométrique mensuel régional pour les grandes plaines du sud des États-Unis

L'indice pluviométrique mensuel proposé par le National Climatic Data Center (NCDC) est égal à la moyenne arithmétique des précipitations observées à certaines stations de la division climatique. Les différents problèmes d'homogénéité des données prises en compte par le NCDC pour le calcul de l'indice sont quantifiés pour la région climatique centrale de l'Oklahoma. Une amélioration de la méthode de calcul est proposée. Le calcul de ce nouvel indice utilise un nombre fixe de stations et fait appel à une méthode d'estimation des données manquantes. L'estimation des valeurs manquantes permet de disposer d'un jeu de données complet, ce qui augmente la représentativité de l'indice. Les moyennes mensuelles des valeurs absolues des différences entre l'indice NCDC et l'indice proposé sont comprises entre 6 % (mai) et 13 % (août) des précipitations moyennes et entre 9 % (novembre) et 24 % (août) des écart-types mensuels. Ces valeurs démontrent que les problèmes liés à la méthode de calcul d'un indice pluviométrique mensuel régional utilisée par le NCDC peuvent être importants. Les résultats de recherches sur la variabilité temporelle des précipitations utilisant un indice pluviométrique régional devraient être interprétés en connaissance de ces différences.The NCDC monthly precipitation index is computed as a simple average of the monthly precipitation at several stations within a climate division. The influences of the discontinuities in records used by NCDC are quantified for the central climate division of Oklahoma. An improvement of the calculation method is proposed. The number of stations used is fixed and the missing monthly data values are filled. The monthly averages of the absolute values of the differences between the two indices vary from 6 % (May) to 13 % (August) of the mean monthly precipitation and from 9 % (November) to 24 % (August) of their mean temporal variations. These values demonstrate that the discontinuities in station records of the NCDC divisional precipitation index can be relevant and that research results on regional precipitation variability should be interpreted with consideration of the approximation errors introduced

Variations spatiales et temporelles des précipitations des neuf division climatiques de l'Oklahoma et implications pour l'utilisation locale de l'indice régional

L'objectif de cette étude est de quantifier l'importance des variations spatiales des précipitations mensuelles par rapport à leurs variations temporelles régionales à l'échelle des divisions climatiques de l'Oklahoma. Les variations des précipitations à l'intérieur d'une division climatique sont supposées être constituées par trois composantes : les variations spatiales systématiques, les variations temporelles moyennes de la division climatique et les variations aléatoires. Les variations spatiales systématiques sont définies par le gradient des précipitations moyennes sur l'intégralité de la période. Les variations temporelles moyennes de la division climatique sont représentées par les variations temporelles de la moyenne spatiale des précipitations observées aux stations de la division, cette moyenne étant appelée indice régional. Les variations aléatoires sont estimées avec les différences entre les valeurs centrées réduites des précipitations observées aux stations et celles de l'indice régional. Cette étude montre que les amplitudes des variations aléatoires des précipitations mensuelles des neuf divisions climatiques de l'Oklahoma sont significatives par rapport aux variations temporelles régionales. La quantification de l'amplitude des variations aléatoires est importante pour l'utilisation des prévisions régionales des précipitations, car elle permet de déterminer les plages de variation des précipitations locales autour de l'indice régional et donc l'augmentation des risques pris par les utilisateurs des prévisions régionales pour des applications locales.The objective of this study is to quantify the magnitude of the spatial variations of the monthly precipitation and relate them to the regional temporal variations of the monthly precipitation at the spatial scale of the climate division of Oklahoma. The precipitation variations within a climate division are assumed to consist of three components: the systematic spatial variations, the mean temporal variations of the climate division, and random variations. The systematic spatial variations are defined with the long-term precipitation gradient. The mean temporal variations of the climate division are represented by the temporal variations of the spatial average of the precipitation observed at the stations included in the climate division; this average is called divisional precipitation. The random variations are estimated with the differences between the standardized values of station and divisional precipitation. This study shows that the magnitude of the random variations of the monthly precipitation of the nine climate divisions of Oklahoma is significant compared to the regional temporal variations. The quantification of the magnitude of the random variations is critical for the use of regional precipitation forecasts, because it allows one to define ranges of local precipitation around the divisional precipitation, and then to quantify the increase of the risk taken by local users of the regional precipitation forecasts

Quantum Corrections to the Reissner-Nordstr\"{o}m and Kerr-Newman Metrics

We use effective field theory techniques to examine the quantum corrections to the gravitational metrics of charged particles, with and without spin. In momentum space the masslessness of the photon implies the presence of nonanalytic pieces $\sim \sqrt{-q^2},q^2\log -q^2$ etc. in the form factors of the energy-momentum tensor. We show how the former reproduces the classical non-linear terms of the Reissner-Nordstr\"{o}m and Kerr-Newman metrics while the latter can be interpreted as quantum corrections to these metrics, of order $G\alpha\hbar/mr^3$Comment: 16 page latex file with two figure

Energy Density in Expanding Universes as Seen by Unruh's Detector

We consider the response of an Unruh detector to scalar fields in an expanding space-time. When combining transition elements of the scalar field Hamiltonian with the interaction operator of detector and field, one finds at second order in time-dependent perturbation theory a transition amplitude, which actually dominates in the ultraviolet over the first order contribution. In particular, the detector response faithfully reproduces the particle number implied by the stress-energy of a minimally coupled scalar field, which is inversely proportional to the energy of a scalar mode. This finding disagrees with the contention that in de Sitter space, the response of the detector drops exponentially with particle energy and therefore indicates a thermal spectrum.Comment: 15 pages, 1 figur

Constraining the Natural MSSM through tunneling to color-breaking vacua at zero and non-zero temperature

We re-evaluate the constraints on the parameter space of the minimal supersymmetric standard model from tunneling to charge- and/or color-breaking minima, taking into account thermal corrections. We pay particular attention to the region known as the Natural MSSM, where the masses of the scalar partners of the top quarks are within an order of magnitude or so of the electroweak scale. These constraints arise from the interaction between these scalar tops and the Higgs fields, which allows the possibility of parameter points having deep charge- and color-breaking true vacua. In addition to requiring that our electro-weak-symmetry-breaking, yet QCD- and electromagnetism-preserving vacuum has a sufficiently long lifetime at zero temperature, also demanding stability against thermal tunneling further restricts the allowed parameter space.Comment: 7 pages, 2 figures, software available from http://vevacious.hepforge.org/ - version 2 matches that accepted for publication in Phys. Lett.

Superradiance in stars: non-equilibrium approach to damping of fields in stellar media

Superradiance in black holes is well-understood but a general treatment for superradiance in stars has until now been lacking. This is surprising given the ease with which we can observe isolated neutron stars and the array of signatures which would result from stellar superradiance. In this work, we present the first systematic pipeline for computing superradiance rates in rotating stars. Our method can be used with any Lagrangian describing the interaction between the superradiant field and the constituents of the star. Our scheme falls into two parts: firstly we show how field theory at finite density can be used to express the absorption of long wavelength modes into the star in terms of microphsyical scattering processes. This allows us to derive a damped equation of motion for the bosonic field. We then feed this into an effective theory for long wavelengths (the so-called worldline formalism) to describe the amplification of superradiant modes of arbitrary multipole moment for a rapidly rotating star. Our method places stellar superradiance on a firm theoretical footing and allows the calculation of the superradiance rate arising from any interaction between a bosonic field and stellar matter

Accelerated detectors in Dirac vacuum: the effects of horizon fluctuations

We consider an Unruh-DeWitt detector interacting with a massless Dirac field. Assuming that the detector is moving along an hyperbolic trajectory, we modeled the effects of fluctuations in the event horizon using a Dirac equation with random coefficients. First, we develop the perturbation theory for the fermionic field in a random media. Further we evaluate corrections due to the randomness in the response function associated to different model detectors.Comment: 19 pages, 1 figur

Impact of surface defects on LaNiO3 perovskite electrocatalysts for the oxygen evolution reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO3 perovskite electrocatalysts. Hydrogen reduction of 700¿°C calcined LaNiO3 induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO3; the former exhibit a low onset overpotential of 380 mV at 10 mA¿cm-2 and a small Tafel slope of 70.8 mV¿dec-1. Oxygen vacancy formation is accompanied by mixed Ni2+/Ni3+ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO3 in accordance with the enhanced OER activity that is observed.Peer ReviewedPostprint (author's final draft