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Numerical dispersion models are used operationally worldwide to mitigate the effect of volcanic ash on aviation. In order to improve the representation of the horizontal dispersion of ash plumes and of the 3D concentration of ash, a study was conducted using the MOCAGE model during the European Natural Airborne Disaster Information and Coordination System for Aviation (EUNADICS-AV) project. Source term modelling and assimilation of different data were investigated. A sensitivity study of source term formulation showed that a resolved source term, using the FPLUME plume rise model in MOCAGE, instead of a parameterised source term, induces a more realistic representation of the horizontal dispersion of the ash plume. The FPLUME simulation provides more concentrated and focused ash concentrations in the horizontal and the vertical dimensions than the other source term. The assimilation of Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol optical depth has an impact on the horizontal dispersion of the plume, but this effect is rather low and local compared to source term improvement. More promising results are obtained with the continuous assimilation of ground-based lidar profiles, which improves the vertical distribution of ash and helps in reaching realistic values of ash concentrations. Using this configuration, the effect of assimilation may last for several hours and it may propagate several hundred kilometres downstream of the lidar profiles.

High-quality volcanic ash forecasts are crucial to minimize the economic impact of volcanic hazards on air traffic. Decision-making is usually based on numerical dispersion modelling with only one model realization. Given the inherent uncertainty of such an approach, a multi-model multi-source term ensemble has been designed and evaluated for the Eyjafjallajökull eruption in May 2010. Its use for flight planning is discussed. Two multi-model ensembles were built: the first is based on the output of four dispersion models and their own implementation of ash ejection. All a priori model source terms were constrained by observational evidence of the volcanic ash cloud top as a function of time. The second ensemble is based on the same four dispersion models, which were run with three additional source terms: (i) a source term obtained from a model background constrained with satellite data (a posteriori source term), (ii) its lower-bound estimate and (iii) its upper-bound estimate. The a priori ensemble gives valuable information about the probability of ash dispersion during the early phase of the eruption, when observational evidence is limited. However, its evaluation with observational data reveals lower quality compared to the second ensemble. While the second ensemble ash column load and ash horizontal location compare well to satellite observations, 3D ash concentrations are negatively biased. This might be caused by the vertical distribution of ash, which is too much diluted in all model runs, probably due to defaults in the a posteriori source term and vertical transport and/or diffusion processes in all models. Relevant products for the air traffic management are horizontal maps of ash concentration quantiles (median, 75 %, 99 %) at a finely resolved flight level grid as well as cross sections. These maps enable cost-optimized consideration of volcanic hazards and could result in much fewer flight cancellations, reroutings and traffic flow congestions. In addition, they could be used for route optimization in the areas where ash does not pose a direct and urgent threat to aviation, including the aspect of aeroplane maintenance.

Nitrogen fertilization is the main source of amonia (NH 3 ) emissions in France. Amonia is a major atmospheric pollutant impacting environment and human health, therefore its monitoring and forecasting is of main importance. Our research deals with improving the spatial and temporal variability of ammonia emissions from agriculture, in particular by exploring how meteorological constraints influence agricultural practices of fertilization. Within this context we developped a simple model based on air temperature, precipitations, and agricultural practices to estimate possible dates of fertilizations. This approach allows to reduce fertilization temporal windows thus giving the perspective to update the NH 3 inventories used as input by Chemistry-Transport Models (CTM).

Stratosphere-to-troposphere transport (STT) is an important natural source of tropospheric ozone, which can occasionally influence ground-level ozone concentrations relevant for air quality. Here, we analyse and evaluate the Copernicus Atmosphere Monitoring Service (CAMS) global and regional forecast systems during a deep STT event over Europe for the time period from 4 to 9 January 2017. The predominant synoptic condition is described by a deep upper level trough over eastern and central Europe, favouring the formation of tropopause folding events along the jet stream axis and therefore the intrusion of stratospheric ozone into the troposphere. Both global and regional CAMS forecast products reproduce the “hook-shaped” streamer of ozone-rich and dry air in the middle troposphere depicted from the observed satellite images of water vapour. The CAMS global model successfully reproduces the folding of the tropopause at various European sites, such as Trapani (Italy), where a deep folding down to 550 hPa is seen. The stratospheric ozone intrusions into the troposphere observed by WOUDC ozonesonde and IAGOS aircraft measurements are satisfactorily forecasted up to 3 days in advance by the CAMS global model in terms of both temporal and vertical features of ozone. The fractional gross error (FGE) of CAMS ozone day 1 forecast between 300 and 500 hPa is 0.13 over Prague, while over Frankfurt it is 0.04 and 0.19, highlighting the contribution of data assimilation, which in most cases improves the model performance. Finally, the meteorological and chemical forcing of CAMS global forecast system in the CAMS regional forecast systems is found to be beneficial for predicting the enhanced ozone concentrations in the middle troposphere during a deep STT event.

Volcanic eruptions emit significant amounts of aerosols in the atmosphere and have strong impact on aviation security and air traffic. Our research deals with spatial and temporal improvements of volcanic ash aerosols within the chemistry-transport model MOCAGE [1], [2], the operational air-quality and fast-response model of Meteo-France. To better characterize the source term of volcanic eruptions, we implemented the plume rise model FPLUME [3] and compared it to the Mastin [4] empirical parametrization, which confirmed that uncertainties of the volcanic ash modelling are considerable and very sensitive to the source term. For this reason, we explore the potential of assimilating spaceborne, ground-based and in-situ observations. Assimilation of Aerosols Optical Depth (AOD) from MODIS instrument allowed to correct the extent and intensity of the plume of the 2010 Eyjafjallajokull eruption. Further experiments are ongoing to take advantage of LIDAR soundings from EARLINET network and CALIOP instrument, as well as in-situ airborne measurements.

The role of an active phase of the Madden–Julian Oscillation (MJO) on the evolution of a mesoscale convective systems (MCS) leading to a tropical depression is investigated in the South-West Indian Ocean during the Dynamics of the Madden–Julian Oscillation (DYNAMO) field experiment, with a numerical limited-area atmospheric model. A mesoscale vortex is followed in the low-troposphere from the initiation of the active MJO phase. It is shown that the interaction of the vortex with the Equatorial jet associated with the MJO plays an important role on the vortex development. As the vortex encounters the southern part of the low-level jet, it undergoes intensification that is explained by the barotropic conversion of kinetic energy from the low-level jet to the vortex.

A 6-models strong European ensemble of Copernicus Atmospheric Monitoring Service (CAMS) was run through the season of 2014 computing the olive pollen dispersion in Europe. The simulations have been compared with observations in 6 countries, members of the European Aeroallergen Network. Analysis was performed for individual models, the ensemble mean and median, and for a dynamically optimized combination of the ensemble members obtained via fusion of the model predictions with observations. The models, generally reproducing the olive season of 2014, showed noticeable deviations from both observations and each other. In particular, the season start was reported too early, by 8 days but for some models the error mounted to almost two weeks. For the season end, the disagreement between the models and the observations varied from a nearly perfect match up to two weeks too late. A series of sensitivity studies performed to understand the origin of the disagreements revealed crucial role of ambient temperature, especially systematic biases in its representation by meteorological models. A simple correction to the heat sum threshold eliminated the season shift but its validity in other years remains to be checked. The short-term features of the concentration time series were reproduced better suggesting that the precipitation events and cold/warm spells, as well as the large-scale transport were represented rather well. Ensemble averaging led to more robust results. The best skill scores were obtained with data fusion, which used the previous-days observations to identify the optimal weighting coefficients of the individual model forecasts. Such combinations were tested for the forecasting period up to 4 days and shown to remain nearly optimal throughout the whole period.

This study is part of the efforts undertaken to resolve the “bad trough/good trough” issue for tropical cyclone (TC) intensity changes and to improve the prediction of these challenging events. Sensitivity experiments are run at 8-km resolution with vortex bogusing to extend the previous analysis of a real case of TC–trough interaction (Dora in 2007). The initial position and intensity of the TC are modified, leaving the trough unchanged to describe a realistic environment. Simulations are designed to analyze the sensitivity of TC prediction to both the variety of TC–trough configurations and the current uncertainty in model analysis of TC intensity and position. Results show that TC intensification under upper-level forcing is greater for stronger vortices. The timing and geometry of the interaction between the two cyclonic potential vorticity anomalies associated with the cutoff low and the TC also play a major role in storm intensification. The intensification rate increases when the TC (initially located 12° northwest of the trough) is displaced 1° closer. By allowing a gradual deformation and equatorward tilting of the trough, both scenarios foster an extended “inflow channel” of cyclonic vorticity at midlevels toward the vortex inner core. Conversely, unfavorable interaction is found for vortices displaced 3° or 4° east or northeast. Variations in environmental forcing relative to the reference simulation illustrate that the relationship between intensity change and the 850–200-hPa wind shear is not systematic and that the 200-hPa divergence, 335–350-K mean potential vorticity, or 200-hPa relative eddy momentum fluxes may be better predictors of TC intensification during TC–trough interactions.

disponible à l'adresse : http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2F2008JAS2618.1; International audience; Midlatitude 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.

This paper describes the pre-operational analysis and forecasting system developed during MACC (Monitoring Atmospheric Composition and Climate) and continued in the MACC-II (Monitoring Atmospheric Composition and Climate: Interim Implementation) European projects to provide air quality services for the European continent. This system is based on seven state-of-the art models developed and run in Europe (CHIMERE, EMEP, EURAD-IM, LOTOS-EUROS, MATCH, MOCAGE and SILAM). These models are used to calculate multi-model ensemble products. The paper gives an overall picture of its status at the end of MACC-II (summer 2014) and analyses the performance of the multi-model ensemble. The MACC-II system provides daily 96 h forecasts with hourly outputs of 10 chemical species/aerosols (O3, NO2, SO2, CO, PM10, PM2.5, NO, NH3, total NMVOCs (non-methane volatile organic compounds) and PAN+PAN precursors) over eight vertical levels from the surface to 5 km height. The hourly analysis at the surface is done a posteriori for the past day using a selection of representative air quality data from European monitoring stations. The performance of the system is assessed daily, weekly and every 3 months (seasonally) through statistical indicators calculated using the available representative air quality data from European monitoring stations. Results for a case study show the ability of the ensemble median to forecast regional ozone pollution events. The seasonal performances of the individual models and of the multi-model ensemble have been monitored since September 2009 for ozone, NO2 and PM10. The statistical indicators for ozone in summer 2014 show that the ensemble median gives on average the best performances compared to the seven models. There is very little degradation of the scores with the forecast day but there is a marked diurnal cycle, similarly to the individual models, that can be related partly to the prescribed diurnal variations of anthropogenic emissions in the models. During summer 2014, the diurnal ozone maximum is underestimated by the ensemble median by about 4 μg m−3 on average. Locally, during the studied ozone episodes, the maxima from the ensemble median are often lower than observations by 30–50 μg m−3. Overall, ozone scores are generally good with average values for the normalised indicators of 0.14 for the modified normalised mean bias and of 0.30 for the fractional gross error. Tests have also shown that the ensemble median is robust to reduction of ensemble size by one, that is, if predictions are unavailable from one model. Scores are also discussed for PM10 for winter 2013–1014. There is an underestimation of most models leading the ensemble median to a mean bias of −4.5 μg m−3. The ensemble median fractional gross error is larger for PM10 (~ 0.52) than for ozone and the correlation is lower (~ 0.35 for PM10 and ~ 0.54 for ozone). This is related to a larger spread of the seven model scores for PM10 than for ozone linked to different levels of complexity of aerosol representation in the individual models. In parallel, a scientific analysis of the results of the seven models and of the ensemble is also done over the Mediterranean area because of the specificity of its meteorology and emissions. The system is robust in terms of the production availability. Major efforts have been done in MACC-II towards the operationalisation of all its components. Foreseen developments and research for improving its performances are discussed in the conclusion.

The assimilation of observations from teledetected images in geophysical models requires one to develop algorithms that would account for the existence of coherent structures. In the context of variational data assimilation, a method is proposed to allow the background to be translated so as to fit structure positions deduced from images. Translation occurs as a first step before assimilating all the observations using a classical assimilation procedure with specific covariances for the translated background. A simple validation is proposed using a dynamical system based on the one-dimensional complex Ginzburg–Landau equation in a regime prone to phase and amplitude errors. Assimilation of observations after background translation leads to better scores and a better representation of extremas than the method without translation.

The rapid intensification of Tropical Cyclone (TC) Dora (2007, southwest Indian Ocean) under upper-level trough forcing is investigated. TC–trough interaction is simulated using a limited-area operational numerical weather prediction model. The interaction between the storm and the trough involves a coupled evolution of vertical wind shear and binary vortex interaction in the horizontal and vertical dimensions. The three-dimensional potential vorticity structure associated with the trough undergoes strong deformation as it approaches the storm. Potential vorticity (PV) is advected toward the tropical cyclone core over a thick layer from 200 to 500 hPa while the TC upper-level flow turns cyclonic from the continuous import of angular momentum. It is found that vortex intensification first occurs inside the eyewall and results from PV superposition in the thick aforementioned layer. The main pathway to further storm intensification is associated with secondary eyewall formation triggered by external forcing. Eddy angular momentum convergence and eddy PV fluxes are responsible for spinning up an outer eyewall over the entire troposphere, while spindown is observed within the primary eyewall. The 8-km-resolution model is able to reproduce the main features of the eyewall replacement cycle observed for TC Dora. The outer eyewall intensifies further through mean vertical advection under dynamically forced upward motion. The processes are illustrated and quantified using various diagnostics.

La Réunion is a volcanic island in a tropical zone, which soil undergoes intense erosion. The possible contribution of rainfall to erosion is analyzed and quantified using one year of UHF radar profiler data located at sea level. Measurements of reflectivity, vertical and horizontal wind allow, with suitable assumptions, to determine raindrop vertical and horizontal energy fluxes, which are both essential parameters for erosion. After calibration of radar rain rates, one-year statistics between May 2009 to April 2010 allow to identify differences in rain vertical profiles depending on the season. During the cool dry season, the mean rain rate is less than 2.5 mm h−1 as high as 1.25 km and it decreases at higher altitudes due to the trade winds inversion. During the warm moist season, the mean rain rate is nearly uniform from ground up to 4 km, around 5 mm h−1. The dynamical and microphysical properties of rainfall events are investigated on three cases that are representative of meteorological events in La Réunion: summer deep convection, a cold front and a winter depression embedded in trade winds. For intense rainfall events, the rain rate deduced from the gamma function is in agreement with the rain rate deduced from the mere Marshall Palmer exponential relationship. For less intense events, the gamma function is necessary to represent rain distribution. The deep-convection event is associated to strong reflectivity reaching as high as 10 km, and strong negative vertical velocity. Wind shear is responsible for a deficiency of radar rain detection at the lower levels. During a cold front event, strong reflectivities reach the trade wind inversion (around 4 km high). The trade wind depression generates moderate rain only as high as 2 km. For all the altitudes, the horizontal kinetic energy fluxes are one order of magnitude stronger that than the vertical kinetic energy fluxes. A simple relationship between the reflectivity factor and vertical kinetic energy fluxes is found for each case study.

Microwave radiances from the humidity sounder SAPHIR at 183GHz onboard the MEGHA-TROPIQUES satellite are simulated in cloudy and rainy conditions (all-sky radiances) using short-range forecasts from the operational limited area model ALADIN-Reunion over the Indian Ocean. The simulation of SAPHIR radiances uses cloud and precipitation profiles from the moist physical parameterizations of ALADIN-Reunion describing deep convection and large-scale precipitation, that are coupled to the fast radiative transfer model RTTOV-SCATT accounting for scattering by hydrometeors. Sensitivity studies show that within cloud systems the SAPHIR sounder is particularly sensitive to solid precipitating hydrometeors. An optimal choice of scattering properties for snow particles is undertaken by a systematic comparison of simulated and observed SAPHIR radiances over a 1-month period in February 2012. This choice has required defining a criterion for rain occurrence from SAPHIR radiances that has been calibrated with the TMPA 3B40RT surface precipitation rate product. Finally, a retrieval technique based on Bayesian inversion is proposed to derive atmospheric profiles within clouds to be assimilated in the ALADIN 3D-Var system in a forthcoming study. The retrieved profiles are evaluated through the simulation of radiances from the microwave imager MADRAS that also flew on MEGHA-TROPIQUES. Keywords: data assimilation, microwave radiances, tropical cyclones (Published: 29 January 2016) Citation: Tellus A 2016, 68, 28620, http://dx.doi.org/10.3402/tellusa.v68.28620

Several tropical cyclone forecasting centers issue uncertainty information with regard to their official track forecasts, generally using the climatological distribution of position error. However, such methods are not able to convey information that depends on the situation. The purpose of the present study is to assess the skill of the Ensemble Prediction System (EPS) from the European Centre for Medium-Range Weather Forecasts (ECMWF) at measuring the uncertainty of up to 3-day track forecasts issued by the Regional Specialized Meteorological Centre (RSMC) La Réunion in the southwestern Indian Ocean. The dispersion of cyclone positions in the EPS is extracted and translated at the RSMC forecast position. The verification relies on existing methods for probabilistic forecasts that are presently adapted to a cyclone-position metric. First, the probability distribution of forecast positions is compared to the climatological distribution using Brier scores. The probabilistic forecasts have better scores than the climatology, particularly after applying a simple calibration scheme. Second, uncertainty circles are built by fixing the probability at 75%. Their skill at detecting small and large error values is assessed. The circles have some skill for large errors up to the 3-day forecast (and maybe after); but the detection of small radii is skillful only up to 2-day forecasts. The applied methodology may be used to assess and to compare the skill of different probabilistic forecasting systems of cyclone position.

The inherent predictability of tropical cyclone tracks has received much attention since the 1980s. It is still an issue because of the recent improvement of track forecasts by numerical models. The aim of this study is to assess this predictability limit globally using an approach devised by Lorenz on several up-to-date numerical models. The differences between forecasts valid at the same instant are considered to be error values; the doubling time of these small errors leads to an estimated upper bound on predictability. This method is here applied on cyclone position forecasts obtained from three different global operational models (from ECMWF, Météo-France, and the Met Office) over the main tropical cyclone basins in the world and during three recent cyclone seasons (2006–09). The resulting estimates of predictability largely exceed the values that are commonly accepted in the literature. The doubling time of small errors is found between 30 and 50 h. An important consequence is that cyclone track forecasts have not reached their predictability limit yet. It is argued that the previous methods for computing the predictability of tropical cyclone tracks did not constrain the environment and the structure of the cyclones initially. But the Lorenz method could still underestimate the inherent predictability of tropical cyclone tracks. The sensitivity of the predictability estimates to the model characteristics is discussed. In particular, the use of wind bogus is suggested to avoid serial correlations between successive forecasts and to accelerate error growth.

International audience; This study focuses on Monsoon Depressions (MD) in the Southwest Indian Ocean (SWIO) basin. A MD is a subcategory of tropical depressions. Some aspects of MD dynamical characteristics differ from those of the classical tropical depressions. A MD displays a broader horizontal extension (>1000 km), and its structure is marked by deep convection clusters that are generally poorly organized. These convective clusters often develop as a wide convection belt in the eastern semicircle of the system. The MDnucleus is characterized by an area of weak winds about 200 km wide and by a hazy central cloud system, surrounded by a larger belt of strong winds. In the SWIO basin, between 2000 and 2008, 5 systems, out of the 100 tropical cyclones that were followed by RSMC La Réunion, comply with these criteria and then have been identified as MD. Using potential vorticity from European Centre for Medium‐Range Weather Forecasts analyses as stratospheric tracer, stratospheric signatures have been identified in the troposphere for the five events, involving different dynamical mechanisms: tropopause fold between an anticyclone and MD, tropopause fold between MD and a jet front system, and cut‐off low. The dynamical structure of MD, especially the external belt of strong winds, seems to favor tropopause disturbances and leads to stratosphere-troposphere exchange.

Mechanisms leading a synoptic surface cyclone to cross an upper-level zonal jet and its subsequent deepening are investigated using a two-layer model on a β plane. The baroclinic interaction of a low-level circular cyclonic perturbation with an upper-level one is first studied in vertical and horizontal cyclonic or anticyclonic uniform shears. A first nonlinear effect acting on the shape and energetics of the perturbations is analyzed. If the background shear is anticyclonic, the perturbations are stretched horizontally; they lose energy barotropically but gain it baroclinically by a well-maintained westward tilt with height. Conversely, if the shear is cyclonic, perturbations remain quite isotropic, but they do not keep a favorable vertical tilt with time and the baroclinic interaction is thus only transient. The latitudinal motion of the perturbations also results from a nonlinear effect. It is found to depend strongly on the background potential vorticity (PV) gradient. This effect is a baroclinic equivalent of the so-called nonlinear barotropic “β drift” and combines the nonlinear advection and vertical stretching terms. These results are confirmed when the anomalies are initially located south of a confined westerly jet. The poleward shift of the lower cyclonic anomaly occurs faster when the vertically averaged PV gradient is strongly positive, which happens when the jet has a large barotropic component. The lower anomaly crosses the jet from the warm to the cold side and deepens afterward. After a detailed description of this regeneration process with the help of an energy budget, it is shown that linear dynamics are not able to reproduce such behavior.

Disponible à l'adresse : http://ams.allenpress.com/perlserv/?request=get-abstract&doi=10.1175%2FMWR2955.1; International audience; In 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.

Le premier volet du projet Cyprim vise a mieux identifier les precurseurs des cyclogeneses et des precipitations intenses en region mediterraneenne. Cet article en presente les principaux resultats. Il est question des ingredients meteorologiques a l'origine de ces systemes, de la grande a la mesoechelle, des causes des fortes intensites, localisations et parfois stationnarite des recipitations associees. Le coeur de l'article presente des methodes d'identification des anomalies de tourbillon potentiel liees a l'occurrence d'evenements precipitants intenses et dresse une climatologie de ces anomalies. Les perspectives d'exploitation de cette information pour ameliorer la qualite des previsions sont evoquees. ENGLISH VERSION: The first part of the CYPRIM project (Cyclogenesis and intense precipitations in the Mediterranean region) aims at a better identification of the precursor structures of cyclogenesis and heavy precipitation in the Mediterranean region. This article presents the main results. It mentions the meteorological ingredients from which these systems originate from the large to themesoscale, the reasons for large intensities, the localization and sometimes stationarity of the associated precipitations. The heart of the article shows methods for identifiying potential vorticity anomalies involved in the occurrence of heavy precipitation events and sets up a climatology of these anomalies.The perspectives of the usefulness of this information for improving forecast quality are discussed.