Links Between Weather Phenomena and Characteristics of Refractivity Measured by Precipitation Radar

Refractivity depends on meteorological parameters such as temperature and water vapour pressure and can be measured using a weather radar. A realistic atmospheric simulation from the Meso-NH numerical model is used in order to describe and establish the relation between refractivity and the dynamic and thermodynamic phenomena responsible for the development and propagation of convection. These investigations lead to discussion of the complementarity between the refractivity and the convective available potential energy. The relation observed between the refractivity signal and the meteorological parameters calls the refractivity measurement into question, since it is based on phase differentiation with time and space and can be degraded by phase aliasing problems. These aliasing problems increase with the radar frequency (perceptible in the S-band, serious in the C-band, and more serious in the X-band) and also with the integration range and sampling time. Thus, a statistical approach permits us to simulate the possibility of measuring the refractivity with operational radar during convective events. A typical case in the south-east region of France is selected to simulate measurements by radar (S-band, C-band, X-band) in convective systems, in order to evaluate the measurement feasibility, particularly in terms of phase ambiguity, related to temporal and spatial sampling, of a future implementation of the refractivity measurement over the French operational radar network. This numerical statistical approach is completed with a similar study using in-situ measurements performed at the Trappes station. The seasonal and diurnal dependencies of aliasing are investigated, leading to clarification of the impact of the turbulent fluxes on the refractivity measurement.     

Modelling the spatio-temporal repartition of right-truncated data: an application to avalanche runout altitudes in Hautes-Savoie

In this paper, we propose a novel approach for generating avalanche hazard maps based on the spatial dependence of avalanche runout altitudes. The right-truncated data are described with a Bayesian hierarchical model in which the spatio-temporal process is assumed to be the sum of independent spatial and temporal terms. Topography is roughly taken into account according to valley altitude and path exposition, and the spatial dependence is modelled with a Matérn covariance function. An application is performed to the Haute-Savoie region, French Alps. A spatial dependence in runout altitudes is identified, and an effective range of about 10 km is inferred. The temporal trend extracted highlights the increase of avalanche runout altitudes from 1955, attributed to both anthropogenic factors and climate warming. In a cross validation scheme, spatial predictions are provided on undocumented paths using kriging equations. All in all, although our model is unable to take into account small topographic features, it is a first-ever approach that produces very encouraging results. It could be enhanced in future work by incorporating a numerical physically-based code into the modelling.     

Fractionation change of hydrogen isotopes in trees due to atmospheric pollutants

Atmospheric pollution effects on hydrogen fractionation in trees are demonstrated for the first time in this study. The δ²H ring-cellulose series obtained for black spruce trees at a site near a SO₂-emitting smelter display short-term fluctuations superimposed on a first order −11‰ shift coincident with the onset of smelter operations. The isotopic depletion in trees exposed to various levels of SO₂ diminish with increasing distance relative to the location of the smelter, and it is not detected at the most distant selected stand, 116 km downwind from the point source. Both the spatial and temporal isotopic trends likely result from the combined effects of leaf transpiration, biochemical processes and water uptake by the root system. The spatial and temporal trends of δ²H values are the reverse of the δ¹³C trends previously obtained for the same tree ring series. These coupled isotopic fractionations underline an important response of trees to pollution stresses likely generated by ambient atmospheric SO₂ concentrations. The degradation of global air quality during the last 150 yr likely generated large scale modifications in the way terrestrial vegetation grows. In this respect, hydrogen dendrogeochemistry combined with other tracers such as C isotope ratios constitutes a new tool to evaluate the past behavior of forest ecosystems in terms of C uptake and acclimation to various types of atmospheric pollution.     

Changes in structural and functional diversity of nematode communities during a spring phytoplankton bloom in the southern North Sea

The response of nematode communities to the sedimentation of a spring phytoplankton bloom in a sandy, well-oxygenated sediment at a single station (station 330) in the Southern North Sea was investigated monthly from early March to July 1999. Both structural (nematode density, diversity, vertical distribution and community composition) and functional (feeding type distributions, number of species within feeding groups) characteristics showed considerable changes shortly after the arrival of fresh organic material at the sediment surface. The general increase in numerical densities and diversity was related to changes within the groups of selective deposit-feeding and epistrate-feeding nematodes. It is hypothesised that sedimentation and subsequent remineralisation of fresh organic matter during the spring phytoplankton bloom result in an increase of suitable food items (both living and dead). This, combined with the availability of oxygen, creates conditions in which many nematode species can co-exist.     

Bayesian stochastic modelling for avalanche predetermination: from a general system framework to return period computations

Stochastic models are recent but unavoidable tools for snow avalanche hazard mapping that can be described in a general system framework. For the computation of design return periods, magnitude and frequency have to be evaluated. The magnitude model consists of a set of physical equations for avalanche propagation associated with a statistical formalism adapted to the input–output data structure. The friction law includes at least one latent friction coefficient. The Bayesian paradigm and the associated simulation techniques assist considerably in performing the inference and taking estimation errors into account for prediction. Starting from the general case, simplifying hypotheses allows computing the predictive distribution of high return periods on a case-study. Only release and runout altitudes are considered so that the model can use the French database. An inversible propagation model makes it possible to work with the latent friction coefficient as if it is observed. Prior knowledge is borrowed from an avalanche path with similar topographical characteristics. Justifications for the working hypotheses and further developments are discussed. In particular, the whole approach is positioned with respect to both deterministic and stochastic hydrology.     

Retrospective multivariate Bayesian change-point analysis: A simultaneous single change in the mean of several hydrological sequences

Although hydrological time series for different sites in a given region are usually correlated and that climate changes should have a regional impact on water resources, very little has appeared in the literature about multivariate change-point analysis. This paper generalizes the univariate Bayesian approach for the detection of a single shift in the mean level to study a change in the mean-vector of a sequence of multivariate normal vectors. Two different problems are considered: the first one is the estimation of the unknown regional change-point under the hypothesis that a shift occurred, while the second one is the overall assessment of change versus no change. This method is illustrated by an application to streamflow data series for six rivers situated in the Northern Québec Labrador region.     

A spatio-temporal modelling framework for assessing the fluctuations of avalanche occurrence resulting from climate change: application to 60 years of data in the northern French Alps

Based on a previous township-scale model, a spatio-temporal framework is proposed to study the fluctuations of avalanche occurrence possibly resulting from climate change. The regional annual component is isolated from the total variability using a two-factor nonlinear analysis of variance. Moreover, relying on a Conditional AutoRegressive sub-model for the spatial effects, the structured time trend is distinguished from the random noise with different time series sub-models including autocorrelative, periodic and change-point models. The hierarchical structure obtained takes into account the uncertainty related to the estimation of the annual component for the quantification of the time trend. Bayesian inference is performed using Monte Carlo simulations. This allows a comparison of the different time series models and the prediction of future activity in an explicit unsteady context. Application to the northern French Alps illustrates the information provided by the model’s different components, mainly the spatial and temporal terms as well as the spatio-temporal fluctuation of the relative risk. For instance, it shows no strong modifications in mean avalanche activity or in the number of winters of low or high activity over the last 60 years. This suggests that climate change has recently had little impact on the avalanching rhythm in this region. However, significant temporal patterns are highlighted: a complex combination of abrupt changes and pseudo-periodic cycles of approximately 15 years. For anticipating the future response of snow avalanches to climate change, correlating them with fluctuations of the constraining climatic factors is now necessary.     

3D geologic framework models for regional hydrogeology and land-use management: a case study from a Quaternary basin of southwestern Quebec, Canada

During a regional hydrogeologic survey in the St. Lawrence Lowlands, Canada, a computer-based 3D Geologic Framework Model (GFM) was constructed to obtain a consistent representation of this typical Quaternary glaciated basin over a 1,400 km² area. Such a detailed stratigraphic reconstruction was needed because the Quaternary sediments control the recharge to the underlying regional fractured rock aquifer and also because buried granular aquifers are partly connected to the regional system. The objectives of this geomodeling effort are 1) to improve understanding of subsurface conditions above the regional aquifer and; 2) to provide a common stratigraphic framework for hydrogeologic applications. The method draws on knowledge-driven discrete modeling using gOcad, as well as standardization and quality control procedures to maximize the use of a multisource database. The resulting model represents the bedrock topography and the complex stratigraphic architecture of overlying sediments. The regional till aquitard, the marine clay aquiclude and the buried granular aquifers have been modeled with unprecedented details thus providing a well-constrained 3D hydrostratigraphic framework. The recharge zones of the rock aquifer represent about 35% of the study area. Buried granular aquifers are directly connected to the regional aquifer system over about 10% of the area. The model allows several applications such as assessing aquifer vulnerability and areal groundwater recharge rates; improving the GFM inter-operability with groundwater modeling systems would be the next logical step.

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Resumen Se construyó un Modelo del Marco Geológico (GFM) 3D basado en computadora durante un levantamiento hidrogeológico regional en las Tierras Bajas de St. Lawrence, Canada para obtener una representación consistente de esta cuenca glaciar Cuaternaria típica en un área de 1,400 km². Se necesitó tal grado de reconstrucción estratigráfica debido a que los sedimentos Cuaternarios controlan la recarga del acuífero rocoso fracturado regional subyacente y también porque los acuíferos granulares enterrados están parcialmente conectados con el sistema regional. Los objetivos de este esfuerzo de modelizado geológico son: 1) mejorar el entendimiento de las condiciones subsuperficiales por encima del acuífero regional y; 2) aportar un marco estratigráfico común para aplicaciones hidrogeológicas. El método se basa en el conocimiento de modelizado discreto utilizando gOcad, así como también en estandarización y procedimientos de control de calidad para maximizar el uso de bancos de datos de fuentes múltiples. El modelo obtenido representa la topografía del macizo rocoso y la arquitectura de la compleja estratigrafía de los sedimentos sobreyacentes. El acuitardo regional de tillita, el acuicludo arcilloso marino y los acuíferos granulares enterrados se han modelizado con detalles sin precedentes aportando de este modo un marco hidroestratigráfico 3D bien definido. Las zonas de recarga del acuífero rocoso representan casi el 35% del área de estudio. Los acuíferos granulares enterrados están conectados directamente al sistema acuífero regional sobre aproximadamente el 10% del área. El modelo permite varias aplicaciones tal como evaluar la vulnerabilidad de acuíferos y ritmos de recarga areales de agua subterránea; el paso lógico siguiente sería mejorar la inter-operabilidad del GFM con los sistemas de modelizado de agua subterránea.

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Résumé Dans le cadre dune étude dhydrogéologie régionale dans les basses terres du Saint-Laurent (Canada), un Modèle surfacique géologique en 3D (MSG) a été développé pour obtenir une représentation cohérente par ordinateur de ce bassin quaternaire sur une superficie de plus de 1,400 km². Une telle reconstitution stratigraphique était nécessaire en raison du contrôle quexercent les sédiments quaternaires sur la recharge de laquifère régional fracturé sous-jacent et aussi parce que les aquifères granulaires enfouis sont partiellement connectés au système régional. Les objectifs de cet effort de géomodélisation sont: 1) daméliorer la compréhension des conditions de sous-surface au-dessus de laquifère régional et; 2) de fournir un cadre stratigraphique commun à des fins de caractérisation hydrogéologique. La méthode repose sur lintégration de linterprétation géologique dans la modélisation discrète effectuée à laide du logiciel gOcad, ainsi que sur des procédures duniformisation et de contrôle de la qualité des données afin doptimiser lutilisation dune base de données multisources. Le MSG représente la topographie du roc et larchitecture stratigraphique des sédiments sus-jacents. Laquitard régional (till), laquiclude dargile marine et les aquifères granulaires ont été modélisés à un niveau de détail sans précédent, fournissant ainsi un cadre hydrostratigraphique solidement établi. Les zones de recharge de laquifère rocheux représentent environ 35% de la zone détude. Les aquifères granulaires enfouis sont connectés directement au système aquifère régional sur au moins 10% du territoire. Le MSG permet de multiples usages tels lévaluation de la vulnérabilité des aquifères et lestimation de la distribution de la recharge; la prochaine étape logique serait lamélioration de linteropérabilité avec les systèmes de modélisation numérique de lécoulement.

     

Ability of a dual polarized X-band radar to estimate rainfall

The aim of this study is to assess rainfall estimates by a dual polarized X-band radar. This study was part of the European project FRAMEA (Flood forecasting using Radar in Alpine and Mediterranean Areas). Two radars were set up near the small town of Collobrières in South Eastern France. The first radar was a dual polarized X-band radar (Hydrix^®) associated with a ZPHI^® algorithm while the second one was an S-band radar (Météo France). We compared radar rainfall data with measurements obtained by two rain gauge networks (Météo France and Cemagref). During the experiments from February 2006 to June 2007, four significant rainfall events occurred. The accuracy of the rain rate obtained with both S-band and X-band radars decreased significantly beyond 60 km, in particular for the X-band radar. At closer ranges, such as 30–60 km from the radars, the X-band and the S-band radar retrievals showed similar performance with Nash criteria around 0.80 for the X-band radar and 0.75 for the S-band radar. Furthermore, the X-band radar did not require calibration on rainfall records, which tends to make it a useful method to assess rainfall in areas without a rain gauge network.     

A hierarchical generalized linear model with variable selection: studying the response of a representative fish assemblage for large European rivers in a multi-pressure context

Global change calls for an understanding of how temperature and flow regimes influence aquatic ecosystems. Fish assemblages are a major component of river ecosystems and are thought to exhibit more integrative informative responses than single species to environmental variations, whether rare and sudden or gradual and continuous. The use of long-term datasets is thus of primary importance, allied to statistical modeling. For each of three previously identified species clusters, we performed Bayesian variable selection and inference within a hierarchical log Poisson Generalized Linear Model using a spike and slab normal prior to pinpoint which subset of environmental variables is of importance for each fish assemblage. Fish counts from electrofishing experiments are known to provide overdispersed data and, not surprisingly, the contribution of recorded environmental effects is found to be weak compared with those of other intra-assemblage sources of variation. The posterior distribution of the regression parameters is in coherence with what was expected from biological knowledge of the three species clusters. In particular, thermophilic species tend to benefit from warmer waters, whereas the recruitment of cold water species decreases due to global warming effects. Our study provides an example of the advantages of hierarchical modeling for quantifying interspecies ecological effects and selecting common environmental variables of importance.