This study assesses the potential for a detection algorithm to identify discriminating analysis-based statistical predictors of a few relevant parameters that can be used to capture heavy precipitation events (HPEs), or, at least, their associated largescale circulation (LSC) patterns in a climate scenario. HPEs are defined from a sample combining 'large-scale' fields from the ECMWF ERA-40 reanalysis with local observations from the Météo-France rain-gauge network. In a first step, LSC patterns considered as significantly favouring HPE over southern France are identified and described with the greatest robustness possible. For that purpose, an objective automatic clustering of the unfiltered 500 hPa geopotential height field is performed. Four clusters are obtained. Among them, the most discriminating for heavy precipitation is characterised by a synoptic-scale deep upper-level low northwest of the area of interest, inducing a southerly flow over the western Mediterranean Sea and southern France. In a second step, other lower-scale parameters are used to refine the characteristics of the clusters. It has been found that the low-level moisture transport is a relevant low-level ingredient to regionally characterise heavy precipitation. Indeed, 'Cévennes' cases are related to more south to southeasterly flows over the Gulf of Lion, whereas 'Languedoc-Roussillon' events occurred preferentially within a more pronounced easterly wind component with two streams of low-level moisture transport. Moreover, in-depth examination of the low-level features reveals that HPEs tend to occur when the wind blows in a specific direction and for the greatest low-level moisture flux over the Gulf of Lion. Finally, the predictive skill of a detection tool for HPEs over southern France, with only synoptic-scale favourable parameters as predictors, is discussed. It is shown that this tool allows selection of HPE situations in more than 70% of cases.
Abstract. This study examines the impact of lateral boundary conditions (LBCs) in convection-permitting (C-P) ensemble simulations with the AROME model driven by the ARPEGE EPS (PEARP). Particular attention is paid to two torrential rainfall episodes, observed on 15-16 June 2010 (the Var case) and 7-8 September 2010 (the Gard-Ardèche case) over the southeastern part of France. Regarding the substantial computing time for convection-permitting models, a methodology of selection of a few LBCs, dedicated for C-P ensemble simulations of heavy precipitation events is evaluated. Several sensitivity experiments are carried out to evaluate the skill of the AROME ensembles, using different approaches for selection of the driving PEARP members. The convective-scale predictability of the Var case is very low and it is driven primarily by a surface low over the Gulf of Lyon inducing a strong convergent low-level flow, and accordingly advecting strong moisture supply from the Mediterranean Sea toward the flooded area. The Gard-Ardèche case is better handled in ensemble simulations as a surface cold front moved slowly eastwards while increasing the low-level water vapour ahead is well reproduced. The selection based on a cluster analysis of the PEARP members generally better performs against a random selection. The consideration of relevant meteorological parameters for the convective events of interest (i.e. geopotential height at 500 hPa and horizontal moisture flux at 925 hPa) refined the cluster analysis. It also helps in better capturing the forecast uncertainty variability which is spatially more localized at the "high-impact region" due to the selection of more mesoscale parameters.
Le premier volet du projet Cyprim vise à mieux identifier les précurseurs des cyclogenèses et des précipitations intenses en région méditerranéenne. Cet article en présente les principaux résultats. Il est question des ingrédients météorologiques à l'origine de ces systèmes, de la grande à la mésoéchelle, des causes des fortes intensités, localisations et parfois stationnarité des précipitations associées. Le coeur de l'article présente des méthodes d'identification des anomalies de tourbillon potentiel liées à l'occurrence d'événements précipitants intenses et dresse une climatologie de ces anomalies. Les perspectives d'exploitation de cette information pour améliorer la qualité des prévisions sont évoquées. Abstract Cyclogenesis and heavy precipitations in the Mediterranean area: origins and characteristics
[1] The Scandinavian blocking (SB) is a well-known quasistationary state of the atmospheric flow and one of the four main weather regimes over the Euro-Atlantic domain in winter. The aim of our study is to analyze the link between surface cyclones, upper-tropospheric Rossby wave breakings and the life cycle of SB using ERA-40 reanalysis. The onset and decay of SB are studied by considering the preferential transitions from the zonal regime to SB and from SB to the Greenland anticyclone regime. During the SB onset, Atlantic cyclones have straight trajectories that extend toward the north of Scandinavia. These surface cyclones are associated with anticyclonic wave breakings in the upper troposphere over Europe. During the SB decay, cyclones are much less present in the eastern part of the Atlantic and most of them present curved trajectories in mid-Atlantic. They are shown to be linked to cyclonic wave breakings to the south of Greenland. Citation: Michel, C., G. Rivière, L. Terray, and B.Joly (2012), The dynamical link between surface cyclones, upper-tropospheric Rossby wave breaking and the life cycle of the Scandinavian blocking, Geophys.
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