Determination des echanges verticaux moyens dans la couche limite planetaire a partir des donnees statistiques obtenues par les radiosondages de temperature

We have studied the vertical structure of the planetary boundary layer (PBL) as well as the vertical exchanges between this layer and the free atmosphere, using average macroscopic temperature data obtained from radiosondes. For this study we have used, for seven months in 1972, twice-daily radiosondes (00 and 12 H) from Trappes (Paris area) and PointK (Atlantic Ocean). The vertical structure of the PBL is given in the first part of the present work in terms of monthly average statistical parameters (vertical temperature gradient, frequency and level of inversion layers, frequency and thickness of mixing layers). We have thus demonstrated for the continental station, the influence of the daily cycle on the vertical temperature gradient; we have determined the monthlyH _M level above which the daily variation is not noticed. However, for the oceanic station, the absence of a daily cycle makes the temperature gradients at 00 and 12 H identical. The study of temperature inversion layers clearly indicates a high probability of their existence between 1500 and 2000 m; this probability is more than 80% both in summer at PointK, and in winter at Trappes. Similarly, we have demonstrated the annual evolution of the level of these elevated inversions at the two stations. An identical process has been performed in the case of the mixing layers. In the second part of our study, we have used a relationship between the vertical temperature gradient and the coefficients of matter exchange (K _z ), obtained from natural radioactive tracers (Guedaliaet al., 1974). Statistics have thus been obtained on the values ofK _z in the various layers above the two stations. These statistics prove that for the two stations and above 1500 m, values of the coefficients between 1 and 5 m² s^?1 are the most frequent; on the other hand, below 1500 m, the distribution of the coefficientsK _z offers different characteristics according to the month and to the station considered. Finally, we have used the concept of ‘equivalent coefficient’ -K _e - to characterize the exchanges between two levels considered as a whole. We have made a comparison of the values ofK _e when in the 0–1000 m layer and when in the 0–2000 m layer. The equivalent coefficientK _e allows us to compare the average exchanges above the two areas; thus, in summer, between the 0 and 1000 m level, the exchanges are more important above Trappes than they are above Point K. On the other hand, whatever may be the vertical structure of the PBL below, the value ofK _e in the 0–2000 m-layer is always between 1 and 5 m² s^?1. A generalisation of such a study applied to better chosen continental and oceanic sites would allow a comprehensive view of the structure of the PBL as well as of the turbulent exchanges between the PBL and the free atmosphere.     

Recent research on the Brabant Massif

Summary The Brabant Massif is the southeastern part of the Caledonian fold belt known as the Anglo-Brabant Massif, which extends from East Anglia to central Belgium and is a major constituting part of the Eastern Avalonia microcontinent. Recent research in Lower Palaeozoic stratigraphy and in geophysical interpretation led to a new subcrop map of the Brabant Massif. Palaeogeographical analysis supports the rapid movement of this Massif to lower latitudes and towards Baltica in Ordovician times. Structurally, the Brabant Massif appears as a faulted antiform, flanked longitudinally to the southwest by a magmatic arc of late Ordovician to early Silurian age. An elongated gravity low points to concealed granitic intrusions below a part of the magmatic arc.     

A non-mass-dependent isotopic fractionation effect

Isotopic fractionation is discussed in terms of collision cross sections. Under thermal equilibrium conditions, isotopically non-reactive collisions and collisions involving indistinguishable isotopes have no effect on the concentration of isotopic species: isotopic exchange reactions give rise to the well known mass-dependent isotopic fractionation (MDF). Under non-thermal equilibrium conditions, the non-reactive and indistinguishable cross sections must be taken into account since they participate in the loss of energy (thermalisation) of the hot species. If an isotope exchange between hot and cold species occurs during the thermalisation, the relative isotopic abundances contribute to the reaction rates and the MDF rule is not valid anymore. A general formulation of the isotopic fractionation, accounting for thermal and non-thermal situations, is proposed. The experimental results obtained during the synthesis of ozone [1] are well explained by the present treatment, which could be extended to other isotopic anomalies observed in meteorites.     

Le principe d'extremum et la stabilité de certains états de mouvement de l'air atmosphérique

Résumé Une particule qui ne subit que des transformations adiabatiques, occupe sa position d'équilibre au sein d'une masse d'air en équilibre hydrostatique, lorsque la sommeS de son enthalpie spécifique et de son énergie potentielle par unité de masse est extremum. Suivant que cet extremum est un minimum ou un maximum, l'état d'équilibre hydrostatique est stable ou instable. On peut généraliser cette proposition au cas de l'état de mouvement géostrophique, à condition d'ajouter, à la sommeS, l'énergie cinétique longitudinale, par unité de masse, de la particule. On peut étendre immédiatement ce dernier résultat au cas du tourbillon circulaire permanent.

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Summary A particle of air which undergoes but adiabatic changes of state occupies its position of equilibrium inside a mass of air in hydrostatic equilibrium if the sumS of its specific enthalpy and of its potential energy per unit of mass assumes an extreme value. According as this extreme is a minimum or a maximum the state of hydrostatic equilibrium is stable or unstable. This postulate can be generalized in case of geostrophic state of motion on condition that the kinetic longitudinal energy of the particle per unit of mass is added to the sumS. This last result can be immediately extended on the case of a permanent circular vortex.

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Zusammenfassung Ein Luftpartikelchen, das ausschließlich adiabatischen Zustandsänderungen unterliegt, nimmt seine Gleichgewichtslage im Innern einer im hydrostatischen Gleichgewicht befindlichen Luftmasse ein, wenn die SummeS seiner spezifischen Enthalpie und seiner potentiellen Energie in der Masseneinheit ein Extrem wird; je nachdem dieses Extrem ein Minimum oder ein Maximum ist, ist das hydrostatische Gleichgewicht stabil oder instabil. Diese Forderung läßt sich für den Fall der geostrophischen Bewegung verallgemeinern, indem man zur SummeS die kinetische Longitudinalenergie der Partikel (je Maßeinheit) addiert. Dieses letztere Resultat läßt sich unmittelbar auf den Fall des permanenten Kreiswirbels ausdehnen.

     

Plio-Quaternary reactivation of the Neogene margin off NW Algiers, Algeria: The Khayr al Din bank

The Algiers region, northern Algeria, is known to be seismically active, with recurrent large (M>6) earthquakes. Because of the lack of high-resolution bathymetry, the offshore structures remained for a long time poorly known. Thanks to a new marine data base (MARADJA 2003 cruise), the offshore part of the margin is accurately mapped, and new active and recent structures are described. West of the bay of Algiers, the margin enlarges, forming the Khayr al Din bank, interpreted as a tilted block of the passive margin born during the opening of the Algero-Provençal basin. At the slope break, a 80 km-long fault-tip Quaternary fold, namely the Khayr al Din fault, extends at the foot of the margin off NW Algiers and represents the largest active structure of the coastal area, together with the Sahel anticline. We also map for the first time a set of overlapping, en echelon active folds in the upper part of the Khayr al Din bank, located off previously known active structures on land. Most of these faults represent actually a threat for the Algiers region in terms of seismic hazard but also geological hazards, such as tsunamis, as most of them depicts significant dimensions and slip rates. The highest long-term horizontal shortening rate is found on the Khayr al Din fault and is estimated at 0.5 ± 0.1 mm/yr, with a maximal magnitude of 7.3, which provides one of the highest seismogenic potential in the region.A new tectonic framework for the Algiers region is proposed, in which the main south-dipping offshore structure, of opposite vergence relative to most thrusts on land, appears to be nowadays the main driving fault system, as also found further east in the Boumerdès (M 6.8) 2003 rupture zone. The overall apparent pop-up structure of the recent and active faults may result from a progressive migration of the plate limit from the Late Miocene, north-dipping suture zone on land, to the Quaternary, south-dipping main Khayr al Din fault at sea, suggesting a process of subduction inception.