A barycenter-based approach for the multi-model ensembling of subseasonal forecasts

Ensemble forecasts and their combination are explored from the perspective of a probability space. Manipulating ensemble forecasts as discrete probability distributions, multi-model ensembles (MMEs) are reformulated as barycenters of these distributions. Barycenters are defined with respect to a given distance. The barycenter with respect to the L2-distance is shown to be equivalent to the pooling method. Then, the barycenter-based approach is extended to a different distance with interesting properties in the distribution space: the Wasserstein distance. Another interesting feature of the barycenter approach is the possibility to give different weights to the ensembles and so to naturally build weighted MME. As a proof of concept, the L2- and the Wasserstein-barycenters are applied to combine two models from the S2S database, namely the European Centre Medium-Range Weather Forecasts (ECMWF) and the National Centers for Environmental Prediction (NCEP) models. The performance of the two (weighted-) MMEs are evaluated for the prediction of weekly 2m-temperature over Europe for seven winters. The weights given to the models in the barycenters are optimized with respect to two metrics, the CRPS and the proportion of skilful forecasts. These weights have an important impact on the skill of the two barycenter-based MMEs. Although the ECMWF model has an overall better performance than NCEP, the barycenter-ensembles are generally able to outperform both. However, the best MME method, but also the weights, are dependent on the metric. These results constitute a promising first implementation of this methodology before moving to combination of more models.Comment: 24 pages, 9 figure

Optimal transport for the multi-model combination of sub-seasonal ensemble forecasts

International audienceCombining ensemble forecasts from several models has been shown to improve the skill of S2S predictions. One of the most used method for such combination is the “pooled ensemble” method, i.e. the concatenation of the ensemble members from the different models. The members of the new multi-model ensemble can simply have the same weights or be given different weights based on the skills of the models. If one sees the ensemble forecasts as discrete probability distributions, then the “pooled ensemble” is their (weighted-)barycenter with respect to the L2 distance.Here, we investigate whether a different metric when computing the barycenter may help improve the skill of S2S predictions. We consider in this work a second barycenter with respect to the Wasserstein distance. This distance is defined as the cost of the optimal transport between two distributions and has interesting properties in the distribution space, such as the possibility to preserve the temporal consistency of the ensemble members.We compare the L2 and Wasserstein barycenters for the combination of two models from the S2S database, namely ECMWF and NCEP. Their performances are evaluated for the weekly 2m-temperature over seven winters in Europe (land) in terms of different scores. The weights of the models in the barycenters are estimated from the data using grid search with cross-validation. We show that the estimation of these weights is critical as it greatly impacts the score of the barycenters. Although the NCEP ensemble generally has poorer skills than the ECMWF one, the barycenter ensembles are able to improve on both single-model ensembles (although not for all scores). At the end, the best ensemble depends on the score and on the location. These results constitute a promising first step before implementing this methodology with more than two ensembles, and ensembles having less contrasting skills

Application de l'immunofluorescence a la detection de Phoma exigua var. foveata, agent de la gangrene de la pomme de terre

National audienceAn antiserum against Phoma exigua var. foveata was prepared using pycniospores of the fungus produced in pure culture. The indirect fluorescence antibody technique of NAIRN (1964) was used. The serum had an estimated titre of 1/4000, by immunofluorescence, and was specific for the spores of the variety foveata ; it could be used to differentiate between the varieties P. exigua var. exigua and P. exigua var. foveata, the latter variety being mainly responsible for potato gangrene. No immunofluorescence reaction was noted with other species of Phoma. A comparative study was carried out with the two varieties using immunofluorescence and the classical identification techniques of isolation on agar media after wound inoculation of tubers. A good correlation was obtained between the two identification techniques. Eight samples were positive in I.F. and seven with the isolation technique. Three to five spores were observed in each field of the microscope at magnification 500. Therefore the application of I.F. for the identification of this fungus appears to be useful and feasible.Un antisérum a été obtenu à partir des pycniospores de Phoma exigua var. foveata provenant d’une souche pure cultivée in vitro ; son titre, estimé en immunofluorescence, est de 1/4000e. La technique d’immunofluorescence (I.F.) indirecte de NAIRN (1964) a été utilisée. Le sérum réagit spécifiquement avec la variété foveata et seulement avec les spores de cette variété. Il ne révèle ni le mycélium de cette variété, ni la forme voisine, P. exigua var exigua; il permet donc de différencier les deux formes lorsqu’il s’agit de spores. De même, aucune réaction n’a été observée en I.F. avec d’autres espèces appartenant ou non au genre Phoma (tabl. 1). La recherche de la variété foveata, principal agent responsable de la gangrène de la pomme de terre a été réalisée sur tubercules de façon simultanée par I.F. et par la technique classique d’isolement sur milieu gélosé après blessure des tubercules ; les deux méthodes donnent des résultats très voisins puisque 8 lots sont positifs en I.F. contre 7 par la technique d’isolement. Chaque préparation provenant d’un lot infecté présentait en moyenne 3 à 5 spores par champ du microscope au grandissement 500. L’application de l’I.F à la détection de ce champignon sur pomme de terre paraît donc envisageable

Application de l'immunofluorescence à la détection de Phoma exigua var. foveata, agent de la gangrène de la pomme de terre

International audienc

Evidence of the impact of Substitutions in Ferrihydrite on Rare Earth Element Adsorption

International audienceThe use of Rare Earth Elements (REE) in technology and other sectors has rapidly increased in the last few decades, and more REE are mined every year. This has led to concerns about their impact on the environment, and their designation as an emerging pollutant.One of the factors controlling the fate of REE in the environment is adsorption on metal hydroxides. In nature, ferrihydrite (Fh), amorphous iron hydroxide with a high adsorption capacity, is ubiquitous and known to commonly contain metal substitutions. Though REE adsorption on Fh has been studied for its pure form [1-3], there is no study on the influence of substitutions on this adsorption.For this purpose, a series of ferrihydrites were synthesized with varying contents (0 to 100%) of aluminium (Al, omnipresent and easily substituted into iron hydroxides), and gallium (Ga, chemical analogue Al) as substitutes. Total REE and pure cerium (Ce) adsorption experiments were then performed.The obtained REE patterns for pure end members (i.e. Fh, AlO(OH), and GaO(OH)) are all different. While the pure ferrihydrite is superimposable to the ones from previous studies [1-3], the aluminium and gallium substitution modify the pattern shape. Though gallium does not strongly affect the pattern shape, for the aluminium substitution there is a strong increase in Heavy REE enrichment with increasing aluminium content. This makes the pattern more closely resemble that of amorphous AlO(OH).Furthermore, cerium adsorbed on the substituted ferrihydrite series exhibit variable Ce L3-edge XANES spectra. While Ce(III) and Ce(IV) are both present, the Ce(III)/Ce(IV) ratio varies depending on the substituting metal, increasing with the aluminium substitution. These results contradict the usually assumed importance of Fe(III) redox reactivity as the main driver of Ce(III) oxidation, as oxidation is enhanced with higher redox-inert Al(III) content.With these results we show how strong the commonly occurring Al-substitution can affect the reactivity of ferrihydrite towards REE in nature.References: Bau, M. (1999). Geochimica et Cosmochimica Acta, 63(1), 67-77. Ohta, A., & Kawabe, I. (2000). Geochemical Journal, 34(6), 439-454. Quinn, K. A., Byrne, R. H., & Schijf, J. (2006). Marine Chemistry, 99(1-4), 128-150

Evidence of the impact of Substitutions in Ferrihydrite on Rare Earth Element Adsorption

International audienceThe use of Rare Earth Elements (REE) in technology and other sectors has rapidly increased in the last few decades, and more REE are mined every year. This has led to concerns about their impact on the environment, and their designation as an emerging pollutant.One of the factors controlling the fate of REE in the environment is adsorption on metal hydroxides. In nature, ferrihydrite (Fh), amorphous iron hydroxide with a high adsorption capacity, is ubiquitous and known to commonly contain metal substitutions. Though REE adsorption on Fh has been studied for its pure form [1-3], there is no study on the influence of substitutions on this adsorption.For this purpose, a series of ferrihydrites were synthesized with varying contents (0 to 100%) of aluminium (Al, omnipresent and easily substituted into iron hydroxides), and gallium (Ga, chemical analogue Al) as substitutes. Total REE and pure cerium (Ce) adsorption experiments were then performed.The obtained REE patterns for pure end members (i.e. Fh, AlO(OH), and GaO(OH)) are all different. While the pure ferrihydrite is superimposable to the ones from previous studies [1-3], the aluminium and gallium substitution modify the pattern shape. Though gallium does not strongly affect the pattern shape, for the aluminium substitution there is a strong increase in Heavy REE enrichment with increasing aluminium content. This makes the pattern more closely resemble that of amorphous AlO(OH).Furthermore, cerium adsorbed on the substituted ferrihydrite series exhibit variable Ce L3-edge XANES spectra. While Ce(III) and Ce(IV) are both present, the Ce(III)/Ce(IV) ratio varies depending on the substituting metal, increasing with the aluminium substitution. These results contradict the usually assumed importance of Fe(III) redox reactivity as the main driver of Ce(III) oxidation, as oxidation is enhanced with higher redox-inert Al(III) content.With these results we show how strong the commonly occurring Al-substitution can affect the reactivity of ferrihydrite towards REE in nature.References: Bau, M. (1999). Geochimica et Cosmochimica Acta, 63(1), 67-77. Ohta, A., & Kawabe, I. (2000). Geochemical Journal, 34(6), 439-454. Quinn, K. A., Byrne, R. H., & Schijf, J. (2006). Marine Chemistry, 99(1-4), 128-150

Evidence of the Impact of Substitutions in Goethite on Rare Earth Element Adsorption

Since the 1980's the uses, and thus mining, of Rare EarthElements (REE) have grown exponentially. This has led toquestions about the impact of REE on the environment, and REEbeing designated emerging pollutants. Currently, knowledgeabout the behaviour of REE in the environment is stillincomplete. For example, there are few studies on REEadsorption on iron (Fe) (oxy)hydroxides, which are consideredkey carriers of metal pollution, and on the other factorscontrolling this adsorption [1-4]. Knowing that in natural(oxy)hydroxides, Fe is frequently substituted by other elementsin the environment in which they form, we were interested in theeffect of these substitutions on the adsorption capacity of(oxy)hydroxides towards REE.This study focusses on aluminium (Al, common in naturalenvironments and easily substituted into Fe-oxyhydroxides) andgallium (Ga, chemical analogue to Al) substitutions in goethite(Goe), representative of commonly found Fe-(oxy)hydroxideswhose well-defined structure allows for modelling of sorptionand surface interactions. For this purpose, experiments of REEwere conducted on pure and substituted goethites (Goe) withvarying Al and Ga content.A characterisation of these goethites, shows that thesubstitution does not affect the shape, size, and specific surfacearea significantly. Nevertheless, the REE adsorption patterns areaffected by substitutions: 5% substitution decreases the sorptioncapacity of the goethite, and at 10% substitution the preferredadsorption of the mid and heavy REE diminishes, resulting in a'flatter' pattern. Meanwhile, the Neodymium (Nd) L3-edgeEXAFS experiments show no modification of its binding modeto the Goe surfaces, forming corner sharing bidentate-binuclearcomplexes regardless of the substitution rate. However, TEMobservations and Cerium (Ce) L 3-edge XANES experimentsdemonstrate that the Ga substitution favours a higher Ce(IV)O2adsorption, while the Al substitution favours Ce(III) adsorptioncompared to the pure Goe.These results reveal the large impact of substitutions on thereactivity of goethite towards REE