Hydrometeorological multi-model ensemble simulations of the 4 November 2011 flash flood event in Genoa, Italy, in the framework of the DRIHM Project

The e-Science environment developed in the framework of the EU-funded DRIHM project was used to demonstrate its ability to provide relevant, meaningful hydrometeorological forecasts. This was illustrated for the tragic case of 4 November 2011, when Genoa, Italy, was flooded as the result of heavy, convective precipitation that inundated the Bisagno catchment. The Meteorological Model Bridge (MMB), an innovative software component developed within the DRIHM project for the interoperability of meteorological and hydrological models, is a key component of the DRIHM e-Science environment. The MMB allowed three different rainfall-discharge models (DRiFt, RIBS and HBV) to be driven by four mesoscale limited-area atmospheric models (WRF-NMM, WRF-ARW, Meso-NH and AROME) and a downscaling algorithm (RainFARM) in a seamless fashion. In addition to this multi-model configuration, some of the models were run in probabilistic mode, thus giving a comprehensive account of modelling errors and a very large amount of likely hydrometeorological scenarios (> 1500). The multi-model approach proved to be necessary because, whilst various aspects of the event were successfully simulated by different models, none of the models reproduced all of these aspects correctly. It was shown that the resulting set of simulations helped identify key atmospheric processes responsible for the large rainfall accumulations over the Bisagno basin. The DRIHM e-Science environment facilitated an evaluation of the sensitivity to atmospheric and hydrological modelling errors. This showed that both had a significant impact on predicted discharges, the former being larger than the latter. Finally, the usefulness of the set of hydrometeorological simulations was assessed from a flash flood early-warning perspective

A combining analysis of three Catastrophic Precipitating Events over Western Mediterranean region

Within the framework of FLOODsite, specific attention is dedicated to improve our understanding of flash flood. Flash floods differ markedly from floods occurring on larger basins with larger time characteristics. Flash floods are produced by rain accumulations of typically more than 200 mm during less than 6 hours over natural watersheds ranging in area from 25 to 2500 km2. The rising rate of waters of several m.h-1 and the flow velocities of several m.s-1 make these floods extremely dangerous for human lives. Research on flash floods requires a mobilisation of activities and resources at the European level for at least four reasons: i. Flash floods are rare event. In Europe one or two flash floods per year have dramatic consequences. Probably ten times more cases of storms of comparable severity occur during the same period. ii. Flash floods are ill documented events. Each country has its own investigation and archiving rules frequently separating the meteorological and hydrological aspects. iii. The forcing meteorological situations and their climatic trend develop at the continental scale. iv. Socio-economic short and long term strategies mitigating flash floods need to be harmonized across Europe. Within this general context, this report aims at presenting the main results obtained within Action 1.1 of Task 1. Here, the main objective is the understanding of the meteorological processes that govern flash-flood driven storms. Three cases typical of flash-flood (described section 2) occurring in Western Mediterranean are simulated using the local research model MesoNH (section 3). After identifying synoptic scale factors that favour development of quasi-stationary mesoscale convective systems, sensitive experiments together with Lagrangian trajectory analyses have been performed to identify the role of the orography, of the low-level jets and of the convection itself in maintaining stationarity on the simulated storms and its resulting cumulated surface precipitation.Floodsit

Verification of ensemble forecasts of Mediterranean high-impact weather events against satellite observations

International audienceEnsemble forecasts at kilometre scale of two severe storms over the Mediterranean region are verified against satellite observations. In complement to assessing the forecasts against ground-based measurements, brightness temperature (BT) images are computed from forecast fields and directly compared to BTs observed from satellite. The so-called model-to-satellite approach is very effective in identifying systematic errors in the prediction of cloud cover for BTs in the infrared window and in verifying the forecasted convective activity with BTs in the microwave range. This approach is combined with the calculation of meteorological scores for an objective evaluation of ensemble forecasts. The application of the approach is shown in the context of two Mediterranean case studies, a tropical-like storm and a heavy precipitating event. Assessment of cloud cover and convective activity using satellite observations in the infrared (10.8 µm) and microwave regions (183-191 GHz) provides results consistent with other traditional methods using rainfall measurements. In addition, for the tropical-like storm, differences among forecasts occur much earlier in terms of cloud cover and deep convective activity than they do in terms of deepening and track. Further, the underdispersion of the ensemble forecasts of the two high-impact weather events is easily identified with satellite diagnostics. This suggests that such an approach could be a useful method for verifying ensemble forecasts, particularly in data-sparse regions

Hydro-meteorological evaluation of a convection-permitting ensemble prediction system for Mediterranean heavy precipitating events

International audienceAn assessment of the performance of different convection-permitting ensemble prediction systems (EPSs) is performed, with a focus on Heavy Precipitating Events (HPEs). The convective-scale EPS configuration includes perturbations of lateral boundary conditions (LBCs) by using a global ensemble to provide LBCs, initial conditions (ICs) through an ensemble data assimilation technique and perturbations of microphysical parameterisations to account for part of model errors. A probabilistic evaluation is conducted over an 18-day period. A clear improvement is found when uncertainties on LBCs and ICs are considered together, but the chosen microphysical perturbations have no significant impact on probabilistic scores. Innovative evaluation processes for three HPE case studies are implemented. First, maxima diagrams provide a multiscale analysis of intense rainfall. Second, an hydrological evaluation is performed through the computation of discharge forecasts using hourly ensemble precipitation forecasts as an input. All ensembles behave similarly, but differences are found highlighting the impact of microphysical perturbations on HPEs forecasts, especially for cases involving complex small-scale processes

Le programme HyMeX - connaissances et prévision des pluies intenses et crues rapides en région méditerranéenne

Conférence SHF « de la prévision des crues à la gestion de crise », Avignon, FRA, 14-/11/2018 - 16/11/2018National audienceLe programme international de recherche HyMeX (Hydrological cycle in the Mediterranean eXperiment, www.hymex.org), composante du chantier multi-organisme MISTRALS lancé pour 10 ans en 2010, a pour objectif de progresser dans la compréhension du cycle de l'eau en Méditerranée, et notamment d'améliorer les connaissances et la prévision des risques hydrométéorologiques. L'étude et la prévision des épisodes de pluies intenses et crues rapides qui affectent régulièrement le pourtour Méditerranéen sont au coeur du programme HyMeX. Après une présentation de la stratégie générale d'observations qui a combiné campagnes de mesures, renforcement d'observations pendant plusieurs automnes et collectes de données socio-hydrologiques après les évènements majeurs qu'a connu la région sur la période, une revue d'ensemble des avancées scientifiques réalisées depuis le lancement d'HyMeX est proposée tant en termes de connaissances et modélisation des épisodes méditerranéens de pluie intense et de leurs impacts, qu'en termes d'évaluation et amélioration des méthodes et systèmes de prévision, et plus particulièrement les systèmes de prévision d'ensemble hydrométéorologiques à courte échéance basés sur le modèle de prévision météorologique AROME de Météo-France

AROME-WMED, a real-time mesoscale model designed for the HyMeX special observation periods

During autumn 2012 and winter 2013, two special observation periods (SOPs) of the HYdrological cycle in the Mediterranean EXperiment (HyMeX) took place. For the preparatory studies and to support the instrument deployment during the field campaign, a dedicated version of the operational convective-scale Application of Research to Operations at Mesoscale (AROME)-France model was developed: the AROME-WMED (West Mediterranean Sea) model. It covers the western Mediterranean basin with a 48 h forecast range. It provided real-time analyses and forecasts which were sent daily to the HyMeX operational centre to forecast high-precipitation events and to help decision makers on the deployment of meteorological instruments. This paper presents the main features of this numerical weather prediction system in terms of data assimilation and forecast. Some specific data of the HyMeX SOP were assimilated in real time. <br><br> The forecast skill of AROME-WMED is then assessed with objective scores and compared to the operational AROME-France model, for both autumn 2012 (05 September to 06 November 2012) and winter 2013 (01 February to 15 March 2013) SOPs. The overall performance of AROME-WMED is good for the first HyMeX special observation period (SOP1) (i.e. mean 2 m temperature root mean square error (RMSE) of 1.7 °C and mean 2 m relative humidity RMSE of 10 % for the 0–30 h forecast ranges) and similar to those of AROME-France for the 0–30 h common forecast range (maximal absolute difference of 2 m temperature RMSE of 0.2 °C and 0.21 % for the 2 m relative humidity); conversely, for the 24–48 h forecast range it is less accurate (relative loss between 10 and 12 % in 2 m temperature and relative humidity RMSE, and equitable threat score (ETS) for 24 h accumulated rainfall), but it remains useful for scheduling observation deployment. The characteristics of parameters, such as precipitation, temperature or humidity, are illustrated by one heavy precipitation case study that occurred over the south of Spain

Projet Cyprim, partie II : Impact du changement climatique sur les événements de pluie intense du bassin méditerranéen

Un deuxième thème abordé dans le cadre du projet Cyprim vise à caractériser, dans le contexte du changement climatique, l'évolution des phénomènes de pluie intense en région méditerranéenne. À cette fin, une simulation climatique de 1960 à 2099 a été réalisée à l'aide d'un modèle régional couplé océan-atmosphère sous le scénario d'émissions A2 du Giec. Différentes méthodes de descente d'échelle (jusqu'à une échelle très fine de 2km) et de détection d'environnements synoptiques favorables aux précipitation sintenses sont ensuite proposées pour estimer l'impact du changement climatique sur les précipitations et l'hydrologie du sud-est de la France, tant du point de vue saisonnier que lors des épisodes de pluies intenses.A second topic covered by the CYPRIM project(1) aims to characterize the evolution of heavy precipitation events in Mediterranean in the context of climate change. To this end, a continuous climate simulation from 1960 to 2099 has been run using a regional ocean-atmosphere coupled model under IPCC A2 emission scenario. Various techniques of downscaling, down to the very fine 2 km scale, and methods to highlight synoptic environments favourable to heavy rain, have been used to estimate the impact of climate change on precipitation and hydrology over South-East France, both for the whole autumn season and the heavy rain events