Sur la chronologie des fronts froids

Résumé L'étude chronologique des fronts froids en Suisse romande fait apparaître: a) une faible variation annuelle de fréquence à deux maxima très peu prononcés en hiver et en été, et b) une variation diurne beaucoup plus accusée avec deux maxima vers 6 h et vers 16 h.

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Summary Chronological studies of the cold fronts in the French part of Switzerland reveal a) a feeble annual variation of frequency with two maxima only weakly pronounced in winter and summer, and b) a much larger diurnal variation with two maxima at 6 h and 16 h.

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Zusammenfassung Ein chronologisches Studium der Kaltfronten in der französischen Schweiz läßt a) eine schwache Jahresschwankung der Frequenz mit zwei nur schwach ausgedrückten Maxima im Winter und im Sommer, und b) eine bedeutend stärkere Tagesschwankung mit Maxima gegen 6 und 16 Uhr erkennen.

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Avec 2 Figures     

Fractionation change of hydrogen isotopes in trees due to atmospheric pollutants

Atmospheric pollution effects on hydrogen fractionation in trees are demonstrated for the first time in this study. The δ²H ring-cellulose series obtained for black spruce trees at a site near a SO₂-emitting smelter display short-term fluctuations superimposed on a first order −11‰ shift coincident with the onset of smelter operations. The isotopic depletion in trees exposed to various levels of SO₂ diminish with increasing distance relative to the location of the smelter, and it is not detected at the most distant selected stand, 116 km downwind from the point source. Both the spatial and temporal isotopic trends likely result from the combined effects of leaf transpiration, biochemical processes and water uptake by the root system. The spatial and temporal trends of δ²H values are the reverse of the δ¹³C trends previously obtained for the same tree ring series. These coupled isotopic fractionations underline an important response of trees to pollution stresses likely generated by ambient atmospheric SO₂ concentrations. The degradation of global air quality during the last 150 yr likely generated large scale modifications in the way terrestrial vegetation grows. In this respect, hydrogen dendrogeochemistry combined with other tracers such as C isotope ratios constitutes a new tool to evaluate the past behavior of forest ecosystems in terms of C uptake and acclimation to various types of atmospheric pollution.     

Large-eddy Simulation of the Dispersion of Solid Particles in a Turbulent Boundary Layer

A large-eddy simulation (LES) with the dynamic Smagorinsky-Germano subgrid-scale (SGS) model is used to study the dispersion of solid particles in a turbulent boundary layer. Solid particles are tracked in a Lagrangian way. The instantaneous velocity of the surrounding fluid is considered to have a large-scale part (directly computed by the LES) and a small-scale part. The SGS velocity of the surrounding fluid is given by a three-dimensional Langevin model written in terms of SGS statistics at a mesh level. An appropriate Lagrangian correlation time scale is considered in order to include the influences of gravity and inertia of the solid particle. Inter-particle collisions and the influence of particles on the mean flow are also taken into account. The results of the LES are compared with the wind-tunnel experiments of Nalpanis et al. (1993 J Fluid Mech 251: 661–685) and of Tanière et al. (1997 Exp in Fluids 23:463–471) on sand particles in saltation and in modified saltation, respectively.     

Ionospheric storm effects in the nighttime E region caused by neutralized ring current particles

During magnetic storms an anomalous increase in the ionization density of the nighttime E region is observed at low and middle latitudes. It has been suggested that this effect is caused by the precipitation of neutralized ring current particles. Here a coupled ring current decay-ionosphere model is used to confirm the validity of this explanation.     

La végétation macrophytique d'un lac hautement eutrophe du Plateau suisse: Le Baldegger See

The study of the qualitative and quantitative distribution of the Baldeggersee macrophytic vegetation is undertaken in 1980 with the help of colour aerial photographies and verification on the field. The comparison with old data allows then a short survey of the macrophytic evolution in relation to the eutrophication.      

Cristallochimie de l'owyheeite: nouvelles données

Résumé L'étude chimique comparée, à la microsonde électronique, d'owyheeite de treize gisements différents a révélé des fluctuations sensibles des teneurs en éléments majeurs, en liaison avec la nature des sulfosels associés, ainsi que la présence occasionnelle de différents éléments mineurs: Cu et As (très fréquents), Sn et Bi (moins fréquents), Tl et Se (exceptionnels). Compte tenu du mode de substitution de ces éléments mineurs, la projection des analyses dans le système Pb₂S₂–Ag₂S–Sb₂S₃ délimite un champ restreint de solution solide dont l'allongement s'accorde avec la substitution 2 Pb²⁺Ag⁺+Sb³⁺. Ce champ de solution solide englobe la composition de la variété téremkovite, mais exclue la formule donnée initialement parShannon. La formule Pb_10–2x Sb_11+x Ag_3+x S₂₈ (–0,13x+0,20) rend compte des termes à la résolution de la Ag/Sb maximal; l'obtention d'une formule plus générale reste subordonée à la résolution de la structure cristalline. Aucun élément mineur ne peut jouer à lui seul le rôle de stabilisateur de la structure, mais ce même rôle peut être joué complémentairement par différents éléments, tels que Cu et Sn. Les clichés de microdiffraction électronique montrent des phénomènes de sur-structure plus ou moins prononcés, en liaison avec la concentration en (Ag+Cu), ou avec le degré de finesse du maclage complexe spécifique de l'owyheeite.

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Crystal chemistry of owyheeite: New data

Summary Comparative electron microprobe study of owyheeite from thirteen ore deposits shows significant variations in the concentration of major elements. These can be correlated with the chemical composition of the associated sulfosalts. Sometimes, some minor elements may be present: Cu and As (very frequent), Sn and Bi (less frequent), Tl and Se (exceptional). Considering certain substitution rules for minor elements, the analyses plotted in the Pb₂S₂–Ag₂S–Sb₂S₃ system show a limited solid-solution field, the elongation of which follows the substitution 2 Pb²⁺Ag⁺+Sb³⁺. This solid-solution field includes the composition of the variety teremkovite, but excludes the initial formula for owyheeite proposed byShannon. The formula Pb_10–2x Sb_11+x Ag_3+x S₂₈ (–0.13x+0.20) describes compositional variation of members with the highest Ag/Sb ratio; a more general formula would require the determination of the crystal structure. No individual minor element appears to be responsible for stabilizing owyheeite. The stabilization may be due to a combined effect of two elements, such as Cu and Sn. Electron diffraction patterns show superstructure reflections which are related to the (Ag+Cu) concentration, as well as to the scale of the complex twinning characteristic of owyheeite.

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Avec 3 Figures     

Variability of the ice export through Fram Strait in 1993–98: the winter 1994/95 anomaly

The origin of the large positive anomaly of the Fram Strait sea ice export which occurred in winter 1994/95 is analysed on the basis of a model simulation of the Arctic sea ice cover over the period 1993-98. The overall intra-annual and interannual variability in the model is in good agreement with observational estimates and the 1994/95 anomaly is well reproduced with an amplitude amounting to half of the mean winter value. Model results suggest that, concomitant to anomalous export velocities, larger than usual ice thickness in the strait contributes to the outstanding amplitude of the anomaly. Analysis on the ice thickness evolution in the strait indicates that the thick ice advected in Fram Strait at the end of the fall of 1994 originates in the anomalous cyclonic wind stress which prevailed during the preceding summer. This anomalous wind stress resulted in persistent convergence of the ice flow against the northern coasts of Canada and Greenland and in the formation of a large thickness anomaly north of Greenland. The anomaly then feeds the Fram Strait ice flow during those following winter months when the local wind forcing in the strait favours ice drift from the north-west. Our results suggest that short-term wind stress variations resulting in local thickness changes to the north of Fram Strait can lead to substantial variability of the Fram Strait ice export.     

Multi-technique monitoring of ocean tide loading in northern France

We have organised afield study of ocean tide loading in the northwestern part of France, where tidal amplitudes are known to be among the highest in the world. GPS and gravimetric techniques have already proved their capability to measure such weak and high-frequency signals. In this study, these classical observations are complemented with less usual techniques, such as tiltmeter and Satellite Laser Ranging (SLR) measurements. We present here the preliminary results for a common period of observations spanning from 12–19 May 2004. Additional measurements from the French Transportable Laser Ranging Station (FTLRS) were available during September and October 2004. Observation residuals are computed as the difference between the observed and the predicted time signals. We obtain small RMS residuals for GPS measurements (2.5/3.1/4.5 mm for the eastward, northward and upward components), for absolute and relative gravimetry (9 nm/s² and 13 nm/s²) and for tiltmeters (0.05 μrad for EW component). We also fit the amplitude of the main M2 tidal constituent to FTLRS observations and we find a value of 3.731 cm, which is comparable to the theoretical value.