Transition extra-tropicale d'ouragans en Atlantique Nord et impact sur la prévisibilité d'événements extrêmes en Méditerranée

La transition extra-tropicale est la transformation d'un cyclone tropical en cyclone extra-tropical lors de sa migration vers les latitudes moyennes. Au cours de son interaction avec son nouvel environnement barocline, le cyclone peut se réintensifier par des processus dynamiques et diabatiques complexes. Il peut également avoir un impact en aval par le renforcement d'un train d'ondes de Rossby, qui se propage rapidement et dont le déferlement est souvent à l'origine d'événements extrêmes. La complexité des processus dynamiques et diabatiques de la transition extra-tropicale conduit alors à une réduction de la prévisibilité en aval. Les ouragans Florence et Hélène en Atlantique Nord ont ainsi contribué au déclenchement des événements extrêmes de septembre 2006 en Méditerranée, respectivement un épisode de précipitations intenses et un phénomène plus rare, un méditerragan (ouragan méditerranéen). Hélène s'est distinguée de Florence par des processus diabatiques prépondérants au cours de sa réintensification sur un océan plus chaud, ce qui lui a permis de conserver des caractéristiques tropicales. Ses trois réintensifications en trois jours induites par l'étirement de trois filaments de tourbillon potentiel, ajoutées au développement du méditerragan en aval, font d'Hélène un cas exceptionnel. Les deux événements extrêmes en Méditerranée étaient absents de la prévision déterministe à moyen terme. L'incertitude dans la représentation des processus diabatiques a été examinée car ceux-ci contrôlent l'impact des ouragans sur les ondes de Rossby. Pour la première fois, des simulations ont été réalisées avec le modèle Méso-NH sur un grand domaine avec une résolution explicite de la convection profonde, tirant parti de la parallélisation massive du modèle. Une faible sensibilité à la résolution horizontale du modèle a été trouvée dans les précipitations des ouragans, donc dans leur impact sur les ondes de Rossby et sur les événements extrêmes en Méditerranée. La trajectoire d'Hélène a par contre montré une forte sensibilité à sa synchronisation avec le train d'ondes de Rossby, influencée par la résolution horizontale du modèle. Cette forte sensibilité a été retrouvée dans les prévisions d'ensemble du Centre Européen pour la Prévision Météorologique à Moyen Terme, où le développement du méditerragan a requis la synchronisation d'Hélène avec le train d'ondes de Rossby. La perturbation ciblée des conditions initiales autour d'Hélène et du thalweg en amont a suffi à prévoir le méditerragan à une échéance de 108 h dans une simulation Méso-NH. Ces résultats montrent l'impact possible de transitions extra-tropicales en Atlantique Nord, qui surviennent à la même saison que la plupart des épisodes de précipitations intenses en Méditerranée. La réduction de l'incertitude dans la prévision de la transition extra-tropicale est donc un des enjeux de l'amélioration de la prévision d'événements extrêmes d'automne en Méditerranée.The extratropical transition is the transformation of a tropical cyclone into an extratropical cyclone during its migration towards the midlatitudes. During its interaction with its new baroclinic environment, the cyclone can reintensify through complex dynamical and diabatic processes. It can also have a impact downstream from the strenghtening of a Rossby wave train, which quickly propagates and of which the breaking is often the trigger of extreme weather events. The complexity of dynamical and diabatic processes lead then to a reduction of the downstream predictability. Hurricanes Florence and Helene over the North Atlantic contributed that way to the triggering of extreme weather events in September 2006 in the Mediterranean, respectively, an intense precipitation event and a rarer phenomenon, a Medicane (Mediterranean hurricane). Helene was distinguished from Florence by its predominant diabatic processes during the reintensification over a warmer ocean, from which it maintained tropical characteristics. Helene was an exceptional case from its three reintensifications in three days, induced by the elongation of three filaments of potential vorticity, in addition to the development of the Medicane downstream. Both extreme weather events in the Mediterranean were missed from the midrange deterministic forecast. As diabatic processes control the impact of the hurricanes on the Rossby wave trains, the uncertainty in their description was investigated. For the first time, numerical simulations were performed with the Meso-NH model over a large domain with an explicit resolution of deep convection, taking advantage of the massive parallel computing capabilities of the model. A weak sensitivity to the horizontal resolution of the model was found in the precipitation of the hurricanes, thus in their impact on the Rossby wave trains and in the extreme weather events in the Mediterranean. The track of Helene showed instead a high sensitivity to its phasing with the Rossby wave train, that was influenced by the horizontal resolution of the model. This high sensitivity was found again in the ensemble forecasts from the European Centre for Medium-Range Weather Forecasts, where the developement of the Medicane required the phasing of Helene with the Rossby wave train. Targeted perturbation of initial conditions around Helene and the upstream trough were sufficient to predict the Medicane at 108-h lead time in a Meso-NH simulation. The results show the possible impact of extratropical transitions over the North Atlantic, which take place during the same season as most high precipitation episodes in the Mediterranean. A reduction in the uncertainty of the forecast of an extratropical transition is therefore one of the issues for an improvement of the forecast of extreme weather events in the Autumn season, over the Mediterranean region

Overview and first results of the Wind and Storms Experiment (WASTEX): a field campaign to observe the formation of gusts using a Doppler lidar

Wind gusts are responsible for most damages in winter storms over central Europe, but capturing their small scale and short duration is a challenge for both models and observations. This motivated theWind and Storms Experiment (WASTEX) dedicated to investigate the formation of gusts during the passage of extratropical cyclones. The field campaign took place during the winter 2016–2017 on a former waste deposit located close to Karlsruhe in the Upper Rhine Valley in southwest Germany. Twelve extratropical cyclones were sampled during WASTEX with a Doppler lidar system performing vertical scans in the mean wind direction and complemented with a Doppler C-band radar and a 200m instrumented tower. First results are provided here for the three most intense storms and include a potential sting jet, a unique direct observation of a convective gust and coherent boundary-layer structures of strong winds

Revisiting the synoptic-scale predictability of severe European winter storms using ECMWF ensemble reforecasts

Abstract. New insights into the synoptic-scale predictability of 25 severe European winter storms of the 1995–2015 period are obtained using the homogeneous ensemble reforecast dataset from the European Centre for Medium-Range Weather Forecasts. The predictability of the storms is assessed with different metrics including (a) the track and intensity to investigate the storms\u27 dynamics and (b) the Storm Severity Index to estimate the impact of the associated wind gusts. The storms are well predicted by the whole ensemble up to 2–4 days ahead. At longer lead times, the number of members predicting the observed storms decreases and the ensemble average is not clearly defined for the track and intensity. The Extreme Forecast Index and Shift of Tails are therefore computed from the deviation of the ensemble from the model climate. Based on these indices, the model has some skill in forecasting the area covered by extreme wind gusts up to 10 days, which indicates a clear potential for early warnings. However, large variability is found between the individual storms. The poor predictability of outliers appears related to their physical characteristics such as explosive intensification or small size. Longer datasets with more cases would be needed to further substantiate these points

Interaction d'un cyclone tropical avec une onde de Rossby planétaire

La migration d'un cyclone tropical aux moyennes latitudes est un problème récurrent pour les modèles de prévision du temps. En particulier, l'interaction entre un cyclone tropical et une onde de Rossby planétaire réduit la prévisibilité en aval. Une simulation numérique avec le modèle atmosphérique Méso-NH reproduit cette interaction d'échelle grâce à sa résolution kilométrique étendue à une domaine semi-hémisphérique. Cette résolution élevée permet pour la première fois de décrire explicitement les phénomènes fortement diabatiques associés à la convection du cyclone

Forecasting wind gusts in winter storms using a calibrated convection-permitting ensemble

Windstorms associated with low‐pressure systems from the North Atlantic are the most important natural hazards for central Europe. Although their predictability has generally improved over the last decades, forecasting wind gusts is still challenging, due to the multiple scales involved. One of the first ensemble prediction systems at convection‐permitting resolution, COSMO‐DE‐EPS, offers a novel 2.8‐km dataset over Germany for the 2011–2016 period. The high resolution allows representation of mesoscale features that are barely captured by global models, while the long period allows both investigation of rare storms and application of statistical post‐processing. Ensemble model output statistics based on a truncated logistic distribution substantially improve forecasts of wind gusts in the whole dataset. However, some winter storms exhibit uncharacteristic forecast errors that cannot be reduced by post‐processing. During the passage of the most severe storm, gusts related to a cold jet are predicted relatively well at the time of maximum intensity, whereas those related to a warm jet are poorly predicted at an early phase. Wind gusts are overestimated during two cases of frontal convection, which suggests that even higher resolution is needed to resolve fully the downward mixing of momentum and the stabilization resulting from convective dynamics. In contrast, extreme gusts are underestimated during a rare case involving a possible sting jet, but this arises from the representation of the synoptic rather than the mesoscale. The synoptic scale also controls the ensemble spread, which is inherited mostly from the initial and boundary conditions. This is unsurprising, but leads to high forecast uncertainty in the case of a small, fast‐moving cyclone crossing the model domain. These results illustrate how statistical post‐processing can help identify the limits of predictability across scales in convection‐permitting ensemble forecasts. They may guide the development of regime‐dependent statistical methods to improve forecasts of wind gusts in winter storms further

Dynamics of sting‐jet storm "Egon" over continental Europe: impact of surface properties and model resolution

International audienceIntense Shapiro-Keyser cyclones are often accompanied by a sting jet (SJ), an air stream that descends from the cloud head into the frontal-fracture region and can cause extreme surface gusts. Previous case-studies have concentrated on the North Atlantic and the British Isles. Here we present the first-ever detailed analysis of an SJ over continental Europe and investigate the influence of topography on its dynamical evolution based on observations and high-resolution simulations using the ICOsahedral Nonhydrostatic model (ICON). Windstorm Egon intensified over the English Channel and then tracked from northern France to Poland on 12-13 January 2017, causing gusts of almost 150 km⋅h −1 and important damage. ICON reproduces the storm dynamics, although it delays the explosive deepening, shifts the track southward over Belgium and Germany and underestimates gusts over land. Storm characteristics show weak sensitivity to varying grid spacing between 1.6 and 6.5 km, while switching off the convection parametrization at 3.3 km grid spacing improves correlations with surface observations but deteriorates the mean error. Trajectories reveal typical SJ characteristics such as mid-level descent, strong acceleration and conditional symmetric and other mesoscale instabilities, while evaporative cooling is stronger than in previous cases from the literature, preventing drying during descent. The SJ identification and the occurrence of mesoscale insta-bilities depend considerably on model resolution, convective parametrization, output frequency and employed thresholds for trajectory selection. Sensitivity experiments with modified surface characteristics show that the combined effects of warm-air blocking by the Alps, higher roughness over land and reduced surface fluxes cause Egon to fill more quickly and to move on a faster, more northern track across Germany. While the SJ response is complex, showing some compensating effects, surface gusts strongly increase when roughness is reduced. These results suggest that weather forecasters in continental Europe should be more aware of the potential risks associated with SJs

Remote impact of North Atlantic hurricanes on the Mediterranean during episodes of intense rainfall in autumn 2012

International audienceAutumn is the most favourable season for tropical cyclones to undergo extratropical transition and interact with the midlatitude flow over the North Atlantic. Autumn is also the season when intense rainfall over the Mediterranean is often triggered by Rossby wave breaking. The impact of tropical cyclones on downstream wave breaking is investigated here during three episodes of intense rainfall which were the target of HyMeX (Hydrological cycle in Mediterranean eXperiment) in autumn 2012. Five‐day simulations of hurricanes Leslie , Rafael and Sandy were performed with the Meso‐NH model in a domain encompassing the North Atlantic and the Mediterranean. Control simulations were compared to simulations in which the hurricanes were filtered out from the initial conditions. In each case, the hurricane locally impeded the forward progression of an upstream trough, then reintensified as an extratropical cyclone during the wrap‐up of the trough. The local impact of Leslie and Rafael on the midlatitude flow quickly propagated downstream along a polar jet and amplified Rossby wave breaking but decreased the intensity of the forecast precipitation over the Mediterranean. The local impact of Sandy propagated downstream along a subtropical jet in addition to the polar jet and resulted in a weak impact of the forecast precipitation on the Mediterranean. This study suggests that the interaction of tropical cyclones with the midlatitude flow over the western North Atlantic may be considered a perturbation to, rather than a source of, downstream wave breaking