Cevennes Area Research Articles

Explicit simulation of electrified clouds: From idealized to real case studies

Original results obtained with the revised version of the explicit cloud electrification module developed in the French mesoscale model MesoNH are presented. The module includes three ingredients: (i) a prognostic budget equation of the electric charges carried by each type of hydrometeor and by the positive and negative ions, (ii) a computation of the electric field by inverting the Gauss equation and (iii) a statistical algorithm to produce intra-cloud and cloud-to-ground discharges. Electric charges are generated by non-inductive (NI) processes involving ice particles that collide in the presence of supercooled water. The geometry of the lightning flashes obeys a fractal law that characterizes the densely branched flash structures with a marked horizontal extent in electrically charged regions of the clouds.For the first time, the electrical module of MesoNH is used to simulate the electrical aspects of a heavy precipitation event over the Cevennes area in the South of France. MesoNH is initialized with “AROME” analyses of Meteo-France for the 6–8 September 2010 event that produced locally 250 to 300mm of precipitation over the Cevennes ridge. A 54hour simulation starting 6 September 2010 at 00 UTC and without electricity, is performed at 1km resolution over a 384×384km2 area in order to check the ability of the model to reproduce the very fine scale structure of the rainfall. MesoNH is then restarted on 7 September 2010 at noon for a 6hour simulation involving the full electrical module. Several aspects of the electrical state of the clouds could be simulated at this scale: the cloud charge structure, the electric field and the lightning flash characteristics. Finally a map of flash density is compared to observations taken from two conventional detection networks (LInet and ATDnet). Both networks suggest that the model produces ten times too many flashes but arguments are put forward to explain this discrepancy, e.g., filtering the less energetic flashes improves the comparison. In conclusion, this study calls for a reference evaluation of the electrical module of MesoNH against statistics of flash data at high resolution in a next planned field experiment in 2012.

Monitoring soil water content and deficit using Electrical Resistivity Tomography (ERT) – A case study in the Cevennes area, France

Summary At the hillslope scale, surface runoff depends to a great extent on the water deficit of the soils, i.e. the difference between the effective porosity and the water content, all through the soil profile. The aim of this study was to assess water deficits using Electrical Resistivity Tomography (ERT). ERT offers several advantages: (i) it is not intrusive, (ii) it exhibits spatial patterns at a decametric or hectometric scale, and (iii) it provides information on both water content and soil depth. The study was conducted on a Mediterranean shaley sandy soil, in the Cevennes (South of France). First, the factors (nature of the pores and organization of the material, water content, soil solution and temperature) on which electrical resistivities depend were analyzed in the laboratory. Soil samples were used to derive the ‘m’ and ‘n’ parameters of Archie’s relation, changes in the soil solution over time, and the effect of temperature on resistivities. These results were then used to interpret ERT in situ at different times of the year. The water content and water deficit obtained from ERT were then compared with local measurements made using Time Domain Reflectometry (TDR) and the comparison was satisfactory. The results of this study show the potential of measuring soil water content and water deficit using electrical resistivities, and describe how uncertainties in temperature, porosity, and fluid conductivity impact ERT-obtained estimates of soil water content and deficit.

Tectonic evolution of the Cevennes para-autochthonous domain of the Hercynian French Massif Central and its bearing on ore deposits formation

Abstract The Cevennes area belongs to the para-autochthonous domain of the Hercynian Belt of the French Massif Central. Three lithological series, namely: sandstone-pelite, black micaschist and gneiss-micaschist, are identified. They form an imbrication of five tectonic units which overthrust the unmetamorphosed Viganais Paleozoic units to the south and the gneissic Mamejean Unit to the north. The structural, metamorphic and magmatic evolution of the Cevennes area is characterized by three events, namely: (1) southward shearing coeval to a MP/MT metamorphism dated around 340 Ma; (2) post nappe anatexis (T<750 degrees C, P>5 kb); (3) Namurian (ca 315 Ma) E-W extensional tectonics and plutonism. The structure of the Mt-Lozere-Borne granitic complex is constrained by new AMS and gravimetric data. The plutons are the driving power of the hydrothermal convective circulations responsible for an early deposition of diffuse arsenopyrite in the thermal aureole. Gold bearing sulfides are afterwards concentrated in quartz veins along brittle normal and wrench faults around the granite. Lastly, ore bearing quartz pebbles are sedimented in the Stephanian Ales coal basin.

Un suivi de <em>Leishmania infantum</em> dans une cohorte de chiens dans une région endémique du sud de la France

Afin d'évaluer la faisabilité d'un essai de vaccin contre la leishmaniose sur le terrain, une cohorte de 50 chiens mâles de race Beagle, non infectés, a été introduite dans un foyer actif de leishmaniose canine, afin de déterminer l'incidence sérologique et clinique. La prospection a été faite dans la région des Cévennes à 460 m d'altitude, dans un site sélectionné pour des considérations écologiques (densité des phlébotomes attendue, hygrométrie, chiens affectés dans les environs du chenil). Les chiens ont été maintenus dans 2 enclos de 25 animaux chacun. Ils ont été examinés mensuellement par inspection clinique et par sérologie, entre le 17 mai 1989 et mi-1991. Un diagnostic parasitologique a été entrepris à chaque cas douteux ou suspect de leishmaniose. Après deux ans, 70 % des chiens étaient séropositifs pour Leishmania infantum et 20 chiens montraient le tableau typique de leishmaniose canine, dont 16 sont morts. Les résultats sont encourageants. Ils permettront de mieux comprendre l'histoire naturelle de la leishmaniose chez les chiens et nous donnent un site parfait pour des essais sur le terrain de vaccins potentiels.