Precipitation forecasting through an analog sorting technique: a comparative study

This study aims at comparing two quantitative precipitation forecasting techniques based on the meteorological analogy concept. Method A considers first a selection of analogous situations at synoptic scale. Second a subset of the most similar situations in terms of hygrometry is extracted. Method B extends method A by two innovative ways, which are restricting the search for analogues with temperature information instead of the common season criterion, and exploiting the information about vertical motion considering vertical velocity. Forecasts are evaluated in a perfect prognosis context and in operational conditions as well, by mean of verification measures (Continuous Ranked Probability Skill Score and scores computed from contingency tables). Results of the case study in France show that: (1) there is an increase in forecast skill when temperature and vertical velocity are included in the procedure, (2) it is possible to anticipate rainfall events up to one week ahead and (3) the introduction of new variables such as vertical velocity may be useless beyond few days ahead if the forecast of the weather model is not reliable

La prévision des précipitations par recherche d'analogues : état de l'art et perspectives

La prévision des précipitations par analogie, adaptée des sorties de modèles numériques de prévision, s'est améliorée ces dix dernières années et est actuellement implémentée opérationnellement dans diverses infrastructures françaises. Le premier objectif de cette étude est de dresser l'état de l'art de cette approche. L'application de cette méthode nécessite une base de données contenant les champs des variables qui permettent de caractériser les situations météorologiques passées. Etant donné qu'il existe deux archives de génération différente (réanalyses ERA-40 et NCEP/NCAR), la sensibilité de cette méthode de prévision au choix de l'archive a été étudiée. Les résultats révèlent une faible sensibilité, même si de manière générale les performances sont légèrement supérieures avec l'archive ERA-40, notamment pour les événements pluvieux extrêmes. Enfin, des perspectives d'amélioration de la méthode susceptibles d'être exploitées à court terme sont évoquées. / Precipitation forecasting based on an adaptation of model outputs through an analog sorting technique has been improved for around ten years. The method runs operationally in several French institutions. First, this short paper presents the state of the art of this approach and the more recent developments. Second, a sensitivity analysis to the choice of the database from which the variables that characterise the past meteorological situations are extracted is performed. Two available archives are tested (ERA-40 and NCEP/NCAR re-analyses). The results show that despite the performances obtained with the ERA-40 database are slightly better, especially for heavy rainfall events, the sensitivity is weak. Finally, further ways for improvement that could be investigated are suggested

Etude de la performance et de la sensibilité à l'archive de réanalyses d'un système de prévision de crue basé sur une prévision probabiliste des précipitations

International audienceFlood forecasting systems commonly use a single precipitation forecast. Hence the main well-known source of uncertainties on discharges forecast is not taken into account. This problem could be overcome by using a probabilistic precipitation forecasting method. Several probabilistic methods allow to forecast precipitation amounts in terms of probability. One of them uses the principle of analogy between meteorological situations. The method applied in this study has been developed by Obled et al. (2002). It requires (i) a precipitation amounts archive and (ii) a predictors archive that characterizes the meteorological situation. However, this method strongly depends on the input data, that is why the use of different databases for predictors could be of interest to quantify the model dependence. The aim of this presentation is to assess the forecasting system sensitivity to two different archives on a case study in France. This study focuses on the Saone river basin (30 000 km²), for which a precipitation archive has been built. Two available re-analysis archives are exploited: the 45-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and the first National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis. These archives are considered with the same spatial resolution (2,5°). The variables introduced in the precipitation forecast method are extracted from these databases and used as input data. In this work, the AM runs in a context of perfect prognosis, that is to say that the evaluation is based on situations observed in the past. A comparison of the method performance using the two re-analysis is carried out, based on statistics computed over the whole period: the mean Continuous Ranked Probability Score (CRPS) and the Receiver Operating Characteristics (ROC) diagram. The CRPS score enables to check the accuracy and the sharpness of the probabilistic predictions. The ROC diagram is built for different rainfall thresholds and for each re-analysis, to assess the efficiency of the methods for a particular user. The results show that: in average, there is a weak sensitivity to the choice of the meteorological archive; nevertheless, a more detailed analysis shows that ERA-40 database is more suited to predict extreme events than NCEP database. Finally for illustration, a lumped rainfall-runoff model is forced by the precipitation forecasts to predict daily discharges at a station of the Saone river. As the forecasting system is probabilistic, several possible discharges are computed at any lead-time and compared to deterministic discharge forecasts. The results show that the flood event is anticipated several days ahead and the beginning of the event is predicted four days in advance. Lastly, six days before the flood peak, the evolution of the forecasted discharge is rather well reproduced, even if the maximum discharge is underestimated. This is a direct consequence of the underestimation of the rainfall amounts predicted for the previous days.La prévision des crues est généralement faite à partir d'une unique prévision des pluies. Les incertitudes dans la prévision des débits ne sont donc pas prises en compte. Pour palier ce problème, une méthode de prévision probabiliste des pluies paraît plus adaptée. Il existe plusieurs méthodes probabilistes, dont celle qui est basée sur le principe d'analogie entre situations météorologiques. La méthode appliquée dans cette étude a été développée par Bontron et Obled (2003). Elle utilise (i) une archive de cumuls pluviométriques et (ii) une archive de prédicteurs, c'est-à-dire des variables d'analogie qui permettent de caractériser une situation météorologique. Cependant, la méthode dépend des données d'entrée, telles que les variables d'analogie. Etant donné qu'il existe deux archives disponibles, cette présentation vise à évaluer la sensibilité de la méthode de prévision des pluies à l'archive utilisée. Dans cette étude, la méthode est appliquée au bassin de la Saône (30 000 km²), pour lequel une archive de cumuls pluviométriques a été construite. Deux archives de réanalyses météorologiques suffisamment longues sont disponibles à la communauté scientifique à résolution 2,5° : les réanalyses « 45-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis » (ERA-40) et les réanalyses « first National Centers for Environmental Prediction/National Center for Atmospheric Research » (NCEP/NCAR). Les variables d'analogie utilisées dans la méthode de prévision des pluies sont ainsi extraites de ces archives. Dans ce papier, la méthode de prévision par analogie est appliquée dans un contexte de prévision parfaite, c'est-à-dire que les situations météorologiques pour lesquelles on émet une prévision des pluies ont été observées dans le passé. La sensibilité de la méthode à l'archive de réanalyses est étudiée en utilisant deux scores d'évaluation de performances : le Continuous Ranked Probability Score (CRPS) et le diagramme Receiver Operating Characteristics (ROC). Le CRPS permet de quantifier la justesse et la finesse de la distribution probabiliste des pluies, tandis que le diagramme ROC permet d'évaluer la qualité de la méthode par rapport à un utilisateur particulier (au travers de seuils de pluie). Les résultats montrent qu'en moyenne, la sensibilité de la méthode au choix de l'archive est faible, même si l'ERA-40 est plus appropriée pour la prévision d'événements extrêmes. Enfin, à titre d'exemple, un cas de crue sur la Saône est étudié en entrant les prévisions de pluie dans un modèle de transformation pluie-débit. Etant donné que la prévision de pluie est probabiliste, plusieurs débits sont prévus en sortie. Ces scénarios de débit sont alors comparés au débit observé. Les résultats montrent que l'on anticipe les événements pluvieux plusieurs jours à l'avance, et que le début de l'épisode de crue est anticipé quatre jours à l'avance. Enfin, six jours avant la pointe de crue, l'évolution du débit prévue est respectée, même si le débit de pointe est sous-estimé, conséquence directe de la sous-estimation des pluies fournies par la méthode des analogues

Towards Real-Time Daily PQPF by an Analog Sorting Approach. Application to Flash Flood Catchments

International audienceHeavy rainfall events are rather common in Southern France and result frequently in devastating flash floods. Thus, an appropriate anticipation of future rainfall is required for early flood warning, at least 12 to 24 hours in advance, as well as for alerting operational services, at least 2 or 3 days ahead. Precipitation forecasts are generally provided by numerical weather prediction models (NWP) and their associated uncertainty is generally estimated through an ensemble approach. Precipitation forecasts have also to be adapted to hydrological scales. This study describes an alternative approach to commonly used Limited Area Models. Probabilistic Quantitative Precipitation Forecasts (PQPFs) are provided through an analog sorting technique, which directly links synoptic scale NWP output to catchment scale rainfall probability distributions. A first issue concerns our latest developments to implement a daily version of this technique into operational conditions. It is shown that the obtained PQPFs depend on the meteorological forecasts used for selecting analogous days and that the methodology has to be re-optimized when changing the source of synoptic forecasts, because of the NWP output uncertainties. Secondly, an evaluation of the PQPFs demonstrates that the analog technique performs well for early warning of heavy rainfall events and provides useful information as potential input of a hydrological ensemble prediction system. However, it is shown that the obtained daily rainfall distributions can be unreliable. A statistical correction of the observed bias is proposed as a function of the no rain frequency values, leading to a significant improvement in PQPF's sharpness

Variables météorologiques utilisées dans une méthode d'analogues : comparaison entre les réanalyses ERA-40 et NCEP/NCAR

International audienceThis presentation gives a brief overview of the exploratory analysis of meteorological variables extracted from two archives and used in a medium-term quantitative precipitation forecasts system. In this study, we consider the two following reanalyses: the 45-Year European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and the first National Centers for Environmental prediction/National Center for Atmospheric Research (NCEP/NCAR) re-analysis. This study focuses on the four variables introduced in the precipitation forecasting method developed by Bontron (2004): the 1000 and 500 hPa geopotential heights, the 850 hPa relative humidity and the total column of water. The study area covers most of Europe and a part of the northern Atlantic Ocean. The results show that fields of geopotential height and precipitable water are often closed, whereas strong discrepancies are observed for the relative humidity at 850 hPa level. A more detailed study of these differences allowed to detect unrealistic values of humidity in the ERA-40 re-analysis. A few values are lower than 0 % and several values are higher than 100 %, with a significant proportion of positive anomalies. For each anomaly in the ERA-40 archive, the corresponding value in the NCEP/NCAR archive is compared. However, no systematic error between the reanalyses has been detected, that inhibits a possible correction of unrealistic values in the ERA-40 using the concomitant values extracted from the NCEP/NCAR reanalysis. Moreover the relative humidity is calculated by the atmospheric model from the specific humidity. This variable may be recalculated to verify that anomalies do not derive from the model calculation. Indeed, unrealistic values are also detected in specific humidity. Finally, to identify the probable source of this issue, data from both archives are finally compared to radio-sounding observations on the stations that are closest to area grid points. The main conclusion is that at the present time it is not possible to avoid such unrealistic values of 850 hPa relative humidity in the ERA-40 re-analysis. The ECMWF will soon provide a new re-analysis, which could hopefully be free of such anomaly

Daily quantitative precipitation forecasts based on the analogue method: Improvements and application to a French large river basin

International audienceThis paper presents some improvements of a probabilistic quantitative precipitation forecasting method based on analogues, formerly developed on small basins located in South-Eastern France. The scope is extended to large scale basins mainly influenced by frontal systems, considering a case study area related to the Saône river, a large basin in eastern France. For a given target situation, this method consists in searching for the most similar situations observed in a historical meteorological archive. Precipitation amounts observed during analogous situations are then collected to derive an empirical predictive distribution function, i.e. the probabilistic estimation of the precipitation amount expected for the target day. The former version of this forecasting method (Bontron, 2004) has been improved by introducing two innovative variables: temperature, that allows taking seasonal effects into account and vertical velocity, which enables a better characterization of the vertical atmospheric motion. The new algorithm is first applied in a perfect prognosis context (target situations come from a meteorological reanalysis) and then in an operational forecasting context (target situations come from weather forecasts) for a three years period. Results show that this approach yields useful forecasts, with a lower false alarm rate and improved performances from the present day D to day D+2

Optimising predictor domains for spatially coherent precipitation downscaling

Statistical downscaling is widely used to overcome the scale gap between predictors from numerical weather prediction models or global circulation models and predictands like local precipitation, required for example for medium-term operational forecasts or climate change impact studies. The predictors are considered over a given spatial domain which is rarely optimised with respect to the target predictand location. In this study, an extended version of the growing rectangular domain algorithm is proposed to provide an ensemble of near-optimum predictor domains for a statistical downscaling method. This algorithm is applied to find five-member ensembles of near-optimum geopotential predictor domains for an analogue downscaling method for 608 individual target zones covering France. Results first show that very similar downscaling performances based on the continuous ranked probability score (CRPS) can be achieved by different predictor domains for any specific target zone, demonstrating the need for considering alternative domains in this context of high equifinality. A second result is the large diversity of optimised predictor domains over the country that questions the commonly made hypothesis of a common predictor domain for large areas. The domain centres are mainly distributed following the geographical location of the target location, but there are apparent differences between the windward and the lee side of mountain ridges. Moreover, domains for target zones located in southeastern France are centred more east and south than the ones for target locations on the same longitude. The size of the optimised domains tends to be larger in the southeastern part of the country, while domains with a very small meridional extent can be found in an east–west band around 47° N. Sensitivity experiments finally show that results are rather insensitive to the starting point of the optimisation algorithm except for zones located in the transition area north of this east–west band. Results also appear generally robust with respect to the archive length considered for the analogue method, except for zones with high interannual variability like in the Cévennes area. This study paves the way for defining regions with homogeneous geopotential predictor domains for precipitation downscaling over France, and therefore de facto ensuring the spatial coherence required for hydrological applications

Elevage bovin : prospective 2020

Dans la continuité des travaux menés par le groupe de la DATAR (intitulé prospective 2015 : agriculture et territoires) et suite à une sollicitation de la Commission nationale d'amélioration génétique, un groupe de réflexion s'est penché sur l'évolution potentielle, à l'horizon 2020, des grands équilibres du secteur européen de l'élevage bovin (lait et viande). La démarche poursuivie comporte deux phases : la première a trait à l'élaboration "à dire d'experts" de plusieurs hypothèses d'évolution de la demande dans les secteurs de la viande bovine et du lait (consommation interne de l'Union européenne et échanges avec les pays tiers), la seconde propose une articulation entre ces hypothèses et quatre scénarios de politique agricole, contrastés selon la nature de l'intervention publique (régulation de l'offre, prise en compte de la multifonctionnalité) et le mode de gestion interne des filières (prix des produits payés aux producteurs, signes de qualité, etc.). Les différentes hypothèses qui en découlent sont testées sur la situation de référence 2000 et permettent ainsi une première évaluation de leurs implications. Cette analyse met en évidence le rôle déterminant du secteur laitier dans les équilibres futurs du marché de la viande bovine

CASSIOPEE. D. Verificateur de regles de dessin en symbolique NMOS

SIGLECNRS-CDST / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc