A robust and parsimonious regional disaggregation method for deriving hourly rainfall intensities for the UK

International audienceA regional rainfall disaggregation method from daily to hourly intensities is presented for the entire UK, which was developed for use with regionalised hydrological and water quality models. The approach is based on the inter-dependence of the hourly rainfall intensities during a rainfall event. The analysis of 23 229 days with at least 15 mm of precipitation from 238 weather stations throughout the UK allowed regional parameters for climatically homogeneous regions of the UK to be derived for each season. The method reproduces well the main statistical characteristics of the data (mean, minimum and maximum intensity and standard deviation). The method is fully operational, computationally efficient and can be applied to any location throughout the UK

Anisotropic excitation spectrum of a dipolar quantum Bose gas

We measure the excitation spectrum of a dipolar Chromium Bose Einstein Condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy which originates from the dipole-dipole interactions between the atoms. We compare our results with the Bogoliubov theory based on the local density approximation, and, at large excitation wavelengths, with numerical simulations of the time dependent Gross-Pitaevskii equation. Our results show an anisotropy of the speed of soundComment: 3 figure

O18O and C18O observations of rho Oph A

Observations of the (N_J=1_1-1_0) ground state transition of O_2 with the Odin satellite resulted in a about 5 sigma detection toward the dense core rho Oph A. At the frequency of the line, 119 GHz, the Odin telescope has a beam width of 10', larger than the size of the dense core, so that the precise nature of the emitting source and its exact location and extent are unknown. The current investigation is intended to remedy this. Telluric absorption makes ground based O_2 observations essentially impossible and observations had to be done from space. mm-wave telescopes on space platforms were necessarily small, which resulted in large, several arcminutes wide, beam patterns. Although the Earth's atmosphere is entirely opaque to low-lying O_2 transitions, it allows ground based observations of the much rarer O18O in favourable conditions and at much higher angular resolution with larger telescopes. In addition, rho Oph A exhibits both multiple radial velocity systems and considerable velocity gradients. Extensive mapping of the region in the proxy C18O (J=3-2) line can be expected to help identify the O_2 source on the basis of its line shape and Doppler velocity. Line opacities were determined from observations of optically thin 13C18O (J=3-2) at selected positions. During several observing periods, two C18O intensity maxima in rho Oph A were searched for in the 16O18O (2_1-0_1) line at 234 GHz with the 12m APEX telescope. Our observations resulted in an upper limit on the integrated O18O intensity of < 0.01 K km/s (3 sigma) into the 26.5" beam. We conclude that the source of observed O_2 emission is most likely confined to the central regions of the rho Oph A cloud. In this limited area, implied O_2 abundances could thus be higher than previously reported, by up to two orders of magnitude.Comment: 7 pages, 6 figures (5 colour), Astronomy & Astrophysic

Control of dipolar relaxation in external fields

We study dipolar relaxation in both ultra-cold thermal and Bose-condensed chromium atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from 3D to 2D. Although dipolar relaxation generally increases as a function of a static magnetic field intensity, we find a range of non-zero magnetic field intensities where dipolar relaxation is strongly reduced. We use this resonant reduction to accurately determine the S=6 scattering length of chromium atoms: $a_6 = 103 \pm 4 a_0$. We compare this new measurement to another new determination of $a_6$, which we perform by analysing the precise spectroscopy of a Feshbach resonance in d-wave collisions, yielding $a_6 = 102.5 \pm 0.4 a_0$. These two measurements provide by far the most precise determination of $a_6$ to date. We then show that, although dipolar interactions are long-range interactions, dipolar relaxation only involves the incoming partial wave $l=0$ for large enough magnetic field intensities, which has interesting consequences on the stability of dipolar Fermi gases. We then study ultra-cold chromium gases in a 1D optical lattice resulting in a collection of independent 2D gases. We show that dipolar relaxation is modified when the atoms collide in reduced dimensionality at low magnetic field intensities, and that the corresponding dipolar relaxation rate parameter is reduced by a factor up to 7 compared to the 3D case. Finally, we study dipolar relaxation in presence of radio-frequency (rf) oscillating magnetic fields, and we show that both the output channel energy and the transition amplitude can be controlled by means of rf frequency and Rabi frequency.Comment: 25 pages, 17 figure

New analytical solution for the study of hydraulic interaction between Alpine tunnels and groundwater

La solution analytique dite du « puits artésien » développée par Jacob et Lohman [1952] est appliquée au contexte des tunnels profonds en haute montagne pour modéliser la décroissance en fonction du temps du débit des venues d’eau. La comparaison de la solution avec un modèle numérique démontre sa validité durant les premiers instants après le percement du tunnel. Ensuite, les conditions aux limites supposées dans la résolution du cas analytique – milieu infini – ne sont plus conformes à la réalité. Les simulations numériques effectuées montrent que la durée de validité de la solution de Jacob et Lohman est notamment inversement proportionnelle à la diffusivité hydraulique du massif. Les résultats de ces simulations permettent de scinder la décroissance du débit des venues d’eau en deux phases distinctes : la première phase correspond à la période de validité de la formule de Jacob et Lohman. Elle est caractérisée par l’extension d’un cône de rabattement autour de l’ouvrage après percement. Durant la seconde phase, le cône de rabattement atteint les limites physiques de l’aquifère (latérales et supérieure) et les débits décroissent plus rapidement suite à la décroissance des potentiels hydrauliques aux limites. Sous réserve du peu de données disponibles, ce type de comportement a été mis en évidence dans les Alpes aux tunnels du Simplon et du Mont-Blanc. Dans la seconde partie de l’article, un abaque paramétrique est présenté, permettant d’évaluer, après le percement d’un tunnel, le temps nécessaire pour qu’un rabattement donné soit atteint en un point défini du massif. Bien que négligeant les effets de limites mis en évidence dans la première partie de l’article, cet abaque permet néanmoins de mettre en évidence la grande inertie, à cette échelle d’observation, des aquifères en massifs montagneux. Il peut être appliqué à la détermination du temps nécessaire pour que l’effet hydraulique du tunnel soit mesurable en surface et dès lors étendu au dimensionnement des campagnes de surveillance des sources lors du percement d’un ouvrage souterrain. Cet abaque démontre l’influence de la géométrie du tunnel (rayon, profondeur) et des caractéristiques hydrauliques de l’aquifère (diffusivité, hauteur de la nappe) sur la durée nécessaire des campagnes de surveillance. Celles-ci devraient être ajustées au cas par cas en fonction du contexte de l’ouvrage souterrain

Radial orbit instability: review and perspectives

This paper presents elements about the radial orbit instability, which occurs in spherical self-gravitating systems with a strong anisotropy in the radial velocity direction. It contains an overview on the history of radial orbit instability. We also present the symplectic method we use to explore stability of equilibrium states, directly related to the dissipation induced instability mechanism well known in theoretical mechanics and plasma physics.Comment: 10 pages, submitted to Transport Theory and Statistical Physics, proceedings of Vlasovia 2009 International Conference. Corrected for typos, redaction, and references adde

The Radius of Metric Subregularity

There is a basic paradigm, called here the radius of well-posedness, which quantifies the "distance" from a given well-posed problem to the set of ill-posed problems of the same kind. In variational analysis, well-posedness is often understood as a regularity property, which is usually employed to measure the effect of perturbations and approximations of a problem on its solutions. In this paper we focus on evaluating the radius of the property of metric subregularity which, in contrast to its siblings, metric regularity, strong regularity and strong subregularity, exhibits a more complicated behavior under various perturbations. We consider three kinds of perturbations: by Lipschitz continuous functions, by semismooth functions, and by smooth functions, obtaining different expressions/bounds for the radius of subregularity, which involve generalized derivatives of set-valued mappings. We also obtain different expressions when using either Frobenius or Euclidean norm to measure the radius. As an application, we evaluate the radius of subregularity of a general constraint system. Examples illustrate the theoretical findings.Comment: 20 page

Cerebrospinal Fluid Cortisol and Dehydroepiandrosterone Sulfate, Alzheimer's Disease Pathology, and Cognitive Decline.

Elevated cortisol levels have been reported in Alzheimer's disease (AD) and may accelerate the development of brain pathology and cognitive decline. Dehydroepiandrosterone sulfate (DHEAS) has anti-glucocorticoid effects and it may be involved in the AD pathophysiology. To investigate associations of cerebrospinal fluid (CSF) cortisol and DHEAS levels with (1) cognitive performance at baseline; (2) CSF biomarkers of amyloid pathology (as assessed by CSF Aβ levels), neuronal injury (as assessed by CSF tau), and tau hyperphosphorylation (as assessed by CSF p-tau); (3) regional brain volumes; and (4) clinical disease progression. Individuals between 49 and 88 years (n = 145) with mild cognitive impairment or dementia or with normal cognition were included. Clinical scores, AD biomarkers, brain MRI volumetry along with CSF cortisol and DHEAS were obtained at baseline. Cognitive and functional performance was re-assessed at 18 and 36 months from baseline. We also assessed the following covariates: apolipoprotein E (APOE) genotype, BMI, and education. We used linear regression and mixed models to address associations of interest. Higher CSF cortisol was associated with poorer global cognitive performance and higher disease severity at baseline. Cortisol and cortisol/DHEAS ratio were positively associated with tau and p-tau CSF levels, and negatively associated with the amygdala and insula volumes at baseline. Higher CSF cortisol predicted more pronounced cognitive decline and clinical disease progression over 36 months. Higher CSF DHEAS predicted more pronounced disease progression over 36 months. Increased cortisol in the CNS is associated with tau pathology and neurodegeneration, and with decreased insula and amygdala volume. Both CSF cortisol and DHEAS levels predict faster clinical disease progression. These results have implications for the identification of patients at risk of rapid decline as well as for the development of interventions targeting both neurodegeneration and clinical manifestations of AD