Mistral and Tramontane wind speed and wind direction patterns in regional climate simulations Anika Obermann, Sophie Bastin, Sophie Belamari, Dario Conte, Miguel Angel Gaertner, Laurent Li, Bodo Ahrens To cite this version: Anika Obermann, Sophie Bastin, Sophie Belamari, Dario Conte, Miguel Angel Gaertner, et al.. Mistral and Tramontane wind speed and wind direction patterns in regional climate simulations. Climate Dynamics, Springer Verlag, 2018, 51 (3), pp.1059-1076. 10.1007/s00382-016-3053-3. hal-01289330 HAL Id: hal-01289330 https://hal.sorbonne-universite.fr/hal-01289330 Submitted on 16 Mar 2016 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Distributed under a Creative Commons Attribution| 4.0 International License Clim Dyn DOI 10.1007/s00382-016-3053-3 Mistral and Tramontane wind speed and wind direction patterns in regional climate simulations Anika Obermann1 · Sophie Bastin2 · Sophie Belamari3 · Dario Conte4 · Miguel Angel Gaertner5 · Laurent Li6 · Bodo Ahrens1 Received: 1 September 2015 / Accepted: 18 February 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract The Mistral and Tramontane are important disentangle the results from large-scale error sources in wind phenomena that occur over southern France and the Mistral and Tramontane simulations, only days with well northwestern Mediterranean Sea. Both winds travel through simulated large-scale sea level pressure field patterns are constricting valleys before flowing out towards the Medi- evaluated. Comparisons with the observations show that terranean Sea. The Mistral and Tramontane are thus inter- the large-scale pressure patterns are well simulated by the esting phenomena, and represent an opportunity to study considered models, but the orographic modifications to channeling effects, as well as the interactions between the the wind systems are not well simulated by the coarse-grid atmosphere and land/ocean surfaces. This study inves- simulations (with a grid spacing of about 50 km), and are tigates Mistral and Tramontane simulations using five reproduced slightly better by the higher resolution simula- regional climate models with grid spacing of about 50 km tions. On days with Mistral and/or Tramontane events, most and smaller. All simulations are driven by ERA-Interim simulations underestimate (by 13 % on average) the wind reanalysis data. Spatial patterns of surface wind, as well speed over the Mediterranean Sea. This effect is strongest as wind development and error propagation along the at the lateral borders of the main flow—the flow width is wind tracks from inland France to offshore during Mistral underestimated. All simulations of this study show a clock- and Tramontane events, are presented and discussed. To wise wind direction bias over the sea during Mistral and Tramontane events. Simulations with smaller grid spacing show smaller biases than their coarse-grid counterparts. This paper is a contribution to the special issue on Med- CORDEX, an international coordinated initiative dedicated to the multi-component regional climate modelling (atmosphere, ocean, Keywords Regional climate models · Evaluation · Model land surface, river) of the Mediterranean under the umbrella intercomparison · Mistral · Tramontane · Bayesian network of HyMeX, CORDEX, and Med-CLIVAR and coordinated by Samuel Somot, Paolo Ruti, Erika Coppola, Gianmaria Sannino, Bodo Ahrens, and Gabriel Jordà. 1 Introduction * Anika Obermann [email protected]‑frankfurt.de The Mistral and Tramontane are mesoscale winds in the Mediterranean region that travel through valleys in 1 Institut für Atmosphäre und Umwelt, Goethe Universität southern France. The cold and dry Mistral blows from Frankfurt, Altenhöferallee 1, 60438 Frankfurt am Main, Germany the north to northwest, and travels down the Rhône val- ley, between the Alps and Massif Central, which opens 2 LATMOS/IPSL, 11 bd d’Alembert, 78280 Guyancourt, France to the Gulf of Lion. The Tramontane travels the Aude valley between the Massif Central and Pyrenees. Both 3 CNRM/GAME, Météo-France/CNRS, Toulouse, France valleys (areas outlined in blue in Fig. 1) form a constric- 4 CMCC, Via Augusto Imperatore 16, 73100 Lecce, Italy tion before opening towards the Mediterranean Sea, and 5 UCLM, AVDA. CARLOS III, S/N E45071, Toledo, Spain are therefore interesting areas for studying channeling 6 LMD/UPMC/CNRS, 4 Place Jussieu, 75005 Paris, France effects. Over the sea, these winds cause deep-water 1 3 A. Obermann et al. generation, and thus impact the hydrological cycle of the 2 Observational data Mediterranean Sea (Schott et al. 1996; Béranger et al. 2010). Accurate forecasting of wind speeds is important Mistral and Tramontane time series and two gridded obser- for assessing the risk of damage from strong winds, to vational surface wind data sets are used in this study, one evaluate possible sites for wind energy production, and for evaluation over France, and one for evaluation over the many other purposes. The Mistral and Tramontane occur Mediterranean Sea. in similar synoptic situations, and consequently often occur at the same time (Georgelin et al. 1994; Guenard 2.1 Mistral and Tramontane areas et al. 2005). They are most likely to occur in winter (Jacq et al. 2005). Figure 1 shows the western Mediterranean Sea area. Alti- In this study, 9 years (2000–2008) of surface wind simu- tudes and distances to the coast are used to identify Mistral lations using five regional climate models were evaluated. and Tramontane-affected regions in France and over the Simulations driven by ERA-Interim at several resolutions Mediterranean Sea, as explained below. were conducted within the Med-CORDEX project (Ruti This study deals with areas below 600 m altitude in the et al. 2015) and HyMeX programme (Drobinski et al. Rhône and Aude valleys, which are less than 270 km away 2014). The grid spacings of the simulations (0.44◦ and from the coast of the Mediterranean Sea (outlined in blue in smaller) are appropriate for modeling mesoscale winds Fig. 1). The altitude information came from ETOPO1, a 1 such as the Mistral and Tramontane, which can extend arc-minute global relief model of Earth’s surface (Amante several 100 km over the Mediterranean Sea. However, the and Eakins 2009), interpolated to a 0.1◦ grid. The distance constrictions and channeling effects in the Rhône and Aude to the coast was calculated for each land grid cell within valleys have too complex topography to be well repre- this area. The narrowest parts of both valleys are about sented in 0.44◦ simulations. 40 km wide, which is close to the grid spacing of the 0.44◦ To the authors’ knowledge, this is the first multi-model simulations. The area outlined in gray in Fig. 1 indicates evaluation of regional climate models in terms of Mistral the part of the Mediterranean Sea that is of interest in this and Tramontane events covering several years. Several study. It includes the main parts of the western Mediter- case studies have been performed on Mistral events (Gue- ranean Basin that are influenced by Mistral and Tramon- nard et al. 2005; Drobinski et al. 2005) and their interac- tane winds. The areas south of the Balearic Islands and tion with sea breezes (Bastin et al. 2006) and heavy pre- cipitation events (Berthou et al. 2014, 2015). Tramontane events have also been studied (Drobinski et al. 2001). An introduction to other phenomena connected to the Mistral and Tramontane is given in Drobinski et al. (2005) and ref- erences therein. This study surveys the Mistral and Tramontane spatial patterns as well as the error propagation along the valleys and over the Mediterranean Sea. Errors that occur far up in the valleys might increase or counteract errors that occur further downstream. Three possible sources of errors are surveyed: large-scale pressure patterns, processes in the valleys, and processes above the Mediterranean Sea. Sur- face wind speed and direction (i.e., of winds 10 m above ground), as well as sea level pressure over southern France and the western Mediterranean Sea, are compared to grid- ded observation data sets and reanalysis data. To obtain an objective comparison, and to exclude days on which the large-scale sea level pressure fields are not well simulated, the days that are used for comparison are determined by a classification algorithm. Fig. 1 Orography (shaded in red) and bathymetry (shaded in blue) This paper is structured as follows. The measurement from ETOPO1 (Amante and Eakins 2009) in Mistral and Tramon- and simulation data are discussed in Sects. 2 and 3. Then, tane regions (in m). Analysis areas in Mistral and Tramontane valleys the methods used are explained in Sect. 4, followed by the (outlined in blue) and Mediterranean Sea (outlined in gray), location of stations for gust time series in Mistral area (orange symbols) and results in Sect. 5 and a discussion in Sect. 6. The last sec- Tramontane area (turquoise symbols) in the valleys (circles), in the tion contains a summary and conclusion. plains (triangles), and close to the coast (squares) 1 3 Mistral and Tramontane wind speed and wind direction patterns in regional climate simulations southeast of Corsica and Sardinia were excluded because • Coast at Marignane, Toulon, or Cap Cepet (orange the islands modify the wind speed by changes in surface squares). roughness and orographic effects. For an observed Tramontane day, gusts must have been 2.2 Mistral and Tramontane time series present at least at one station in each of two areas: The daily gust time series in the Mistral and Tramontane • Carcassonne or Perpignan (turquoise circles in Fig.