Combining short-range dispersion simulations with fine-scale meteorological ensembles: probabilistic indicators and evaluation during a 85Kr field campaign

Numerical atmospheric dispersion models (ADMs) are used for predicting the health and environmental consequences of nuclear accidents in order to anticipate countermeasures necessary to protect the populations. However, these simulations suffer from significant uncertainties, arising in particular from input data: weather conditions and source term. Meteorological ensembles are already used operationally to characterize uncertainties in weather predictions. Combined with dispersion models, these ensembles produce different scenarios of radionuclide dispersion, called “members”, representative of the variety of possible forecasts. In this study, the fine-scale operational weather ensemble AROME-EPS (Applications of Research to Operations at Mesoscale-Ensemble Prediction System) from Météo-France is coupled with the Gaussian puff model pX developed by the IRSN (French Institute for Radiation Protection and Nuclear Safety). The source term data are provided at 10 min resolution by the Orano La Hague reprocessing plant (RP) that regularly discharges 85Kr during the spent nuclear fuel reprocessing process. In addition, a continuous measurement campaign of 85Kr air concentration was recently conducted by the Laboratory of Radioecology in Cherbourg (LRC) of the IRSN, within 20 km of the RP in the North-Cotentin peninsula, and is used for model evaluation. This paper presents a probabilistic approach to study the meteorological uncertainties in dispersion simulations at local and medium distances (2–20 km). First, the quality of AROME-EPS forecasts is confirmed by comparison with observations from both Météo-France and the IRSN. Then, the probabilistic performance of the atmospheric dispersion simulations was evaluated by comparison to the 85Kr measurements carried out during a period of 2 months, using two probabilistic scores: relative operating characteristic (ROC) curves and Peirce skill score (PSS). The sensitivity of dispersion results to the method used for the calculation of atmospheric stability and associated Gaussian dispersion standard deviations is also discussed. A desirable feature for a model used in emergency response is the ability to correctly predict exceedance of a given value (for instance, a dose guide level). When using an ensemble of simulations, the “decision threshold” is the number of members predicting an event above which this event should be considered probable. In the case of the 16-member dispersion ensemble used here, the optimal decision threshold was found to be 3 members, above which the ensemble better predicts the observed peaks than the deterministic simulation. These results highlight the added value of ensemble forecasts compared to a single deterministic one and their potential interest in the decision process during crisis situations.</p

A New Assessment of the Predictability of Tropical Cyclone Tracks

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Représentation numérique et mathématique des structures météorologiques cohérentes d'échelle synoptique

The meteorological synoptic-scale phenomena may be explained by the nonlinear interactions between some finite-amplitude structures and their large-scale environment. This framework of study does not rely on the hardly justifiable hypotheses of the instability theories, but it requires to be able to represent in an objective manner the coherent structures, named like this by analogy with turbulent flows.The main purpose of this thesis is to build and to assess an algorithm of extraction of the synoptic-scale coherent structures. They are sought only in the field of potential vorticity, which is invertible in the dry atmosphere. The inversion yields the attribution of the other dynamical fields to the structure. The extraction relies on the projection of two-dimensional fields on a set of orthogonal wavelet bases. An optimal basis is associated with every structure. In order to avoid any redundancy between the structures, a technique of orthogonalization has been set. The comparison of the extraction with a non-objective method is made possible by a new definition of the temporal coherence, that relies on the independance of the evolution of the environment.The extraction has been applied to a large set of synoptic-scale phenomena: a high-frequency upper-level precursor, a jet-streak, and some upper-level fields associated with intense precipitating events in the Mediterranean. These studies help to show the dynamical consistency of the extracted structures, and to identify some possible applications of the algorithm. Its potential use for the short-term ensemble forecast of storms is also tackled.Les phénomènes météorologiques d'échelle synoptique peuvent s'expliquer par les interactions non linéaires entre structures d'amplitude finie, et leur environnement. Ce cadre d'étude, qui ne repose pas sur les hypothèses difficilement justifiables des théories d'instabilité, nécessite de pouvoir représenter de façon objective ces structures cohérentes, nommées ainsi par analogie avec les écoulements turbulents.L'objectif principal de cette thèse est de bâtir et d'évaluer un algorithme d'extraction des structures cohérentes d'échelle synoptique. Elles sont recherchées dans le seul champ de tourbillon potentiel, qui est inversible dans l'atmosphère sèche. L'attribution des autres champs dynamiques à la structure est ainsi rendue possible après inversion. L'extraction repose sur la projection des champs bidimensionnels sur un ensemble de bases d'ondelettes orthogonales. Une base optimale est associée à chaque structure ; pour éviter la redondance entre les structures, une technique d'orthogonalisation a été mise au point. La comparaison de l'extraction avec une méthode non objective est possible grâce à un nouveau critère de cohérence temporelle, qui repose sur l'indépendance de l'évolution de l'environnement.L'extraction est appliquée à plusieurs phénomènes d'échelle synoptique : un précurseur d'altitude haute-fréquence, un rapide de courant-jet et des champs d'altitude associés à des épisodes précipitants en Méditerranée. Ces études montrent la pertinence dynamique des structures extraites et dégagent des perspectives d'utilisation de l'algorithme. On aborde aussi sa possible application à la mise en place d'une prévision d'ensemble à courte échéance des tempêtes

Forecasting tropical cyclones using Aladin

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A cyclogenesis evolving into two distinct scenarios and its implications for short-term ensemble forecasting

Disponible à l'adresse : http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FMWR2955.1International audienceIn a nonlinear quasigeostrophic model with uniform potential vorticity, an idealized initial state sharing some features with atmospheric low-predictability situations is built. Inspired by previous work on idealized cyclogenesis, two different cyclogenesis scenarios are obtained as a result of a small change of the initial location of one structure. This behavior is interpreted by analyzing the baroclinic interaction between upper- and lower-level anomalies. The error growth mechanism is nonlinear; it does not depend on the linear stability properties of the jet, which are the same in both evolutions. The ability of ensemble forecasts to capture these two possible evolutions is then assessed given some realistic error bounds in the knowledge of the initial conditions. First, a reference statistical distribution of each of the evolutions is obtained by means of a large Monte Carlo ensemble. Smaller ensembles with size representative of what is available in current operational implementations are then built and compared to the Monte Carlo reference: several singular-vector-based ensembles, a small Monte Carlo ensemble, and a “coherent structure”-based ensemble. This new technique relies on a sampling of the errors on the precursors of the cyclogenesis: amplitude and position errors. In this context, the precursors are handled as coherent structures that may be amplified or moved within realistic error bounds. It is shown that the singular vector ensemble fails to reproduce the bimodal distribution of the variability if the ensemble is not initially constrained, whereas it is accessible at a relatively low cost to the new coherent structures initialization

Tropical cyclone track forecasts using a limited-area model: sensitivity to the lateral boundary conditions

International audienceThe link between tropical cyclone track forecasts by Aladin-Réunion limited-area model and by its coupling model Arpege is investigated in the South-West Indian Ocean. A significant impact of the coupling model on the LAM tr ack forecasts is shown after the 2-days term, although the sensitivity to initial conditions remains predominant in some case

Représentation numérique et mathématique des structures météorologiques cohérentes d'échelle synoptique

TOULOUSE3-BU Sciences (315552104) / SudocTOULOUSE3-SCD-Bib. electronique (315559904) / SudocTOULOUSE-Observ. Midi Pyréné (315552299) / SudocSudocFranceF

A wavelet representation of synoptic-scale coherent structures

disponible à l'adresse : http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2008JAS2618.1International audienceMidlatitude cyclogenesis as interpreted in the framework of either baroclinic development or potential vorticity thinking heavily relies on the concept of synoptic-scale anomaly. Given the existence of potential vorticity inversion and attribution, what is at stake to provide a mathematical definition for this concept is a complete finite-amplitude alternative to the linear-based theory of cyclogenesis. The existence of a reasonably objective way to represent anomalies in both real and idealized flows would not only help understanding cyclogenesis, it would also have many other applications for both theory and in practical forecasts. Inspired by the recent theory of wavelet representation of coherent structures in two-dimensional fluid mechanics, a wavelet representation of three-dimensional potential vorticity anomalies is built. This algorithm relies on the selection of the appropriate two-dimensional wavelet coefficients from the stationary wavelet transform in order to guarantee the critical translation-invariance property. The sensitivity of the algorithm to the position, size, and shape of the structures is assessed. The wavelet extraction is then applied to the upper-level precursor of a real-case storm of December 1999 and is compared to a basic monopolar extraction. Using potential vorticity inversion and forecasts with a primitive-equation model, it is found that both anomalies have similar implications on the development of the surface cyclone. However, the coherence in time of the extracted wavelet structure in the forecast and analysis sequence is more satisfactory than the extracted monopole: this suggests that the underlying mathematical description of an anomaly proposed here does, indeed, point toward the direction of an actual physical reality of the concept