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51.
A set of global atmospheric simulations has been performed with the ARPEGE-Climat model in order to quantify the contribution of realistic snow conditions to seasonal atmospheric predictability in addition to that of a perfect sea surface temperature (SST) forcing. The focus is on the springtime boreal hemisphere where the combination of a significant snow cover variability and an increasing solar radiation favour the potential snow influence on the surface energy budget. The study covers the whole 1950?C2000 period through the use of an original snow mass reanalysis based on an off-line land surface model and possibly constrained by satellite snow cover observations. Two ensembles of 10-member AMIP-type experiments have been first performed with relaxed versus free snow boundary conditions. The nudging towards the monthly snow mass reanalysis significantly improves both potential and actual predictability of springtime surface air temperature over Central Europe and North America. Yet, the impact is confined to the lower troposphere and there is no clear improvement in the predictability of the large-scale atmospheric circulation. Further constraining the prescribed snow boundary conditions with satellite observations does not change much the results. Finally, using the snow reanalysis only for initializing the model on March 1st also leads to a positive impact on predicted low-level temperatures but with a weaker amplitude and persistence. A conditional skill approach as well as some selected case studies provide some guidelines for interpreting these results and suggest that an underestimated snow cover variability and a misrepresentation of ENSO teleconnections may hamper the benefit of an improved snow initialization in the ARPEGE-Climat model. 相似文献
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Abstract This paper discusses dynamo action in generalisations of the Ponomarenko dynamo at large magnetic Reynolds number. The original Ponomarenko dynamo consists of a spiralling flow in which the stream surfaces are concentric cylinders of circular cross section, and the flow depends only on distance from the axis in cylindrical polar coordinates. In this study, the stream surfaces are allowed to be cylinders of arbitrary cross section, and the flow is only required to be independent of the coordinate along the cylinder axes. For smooth flows alpha and eddy diffusion effects are identified, in terms of the geometry of the stream surfaces, and asymptotic formulae for growth rates in the limit of large magnetic Reynolds number are obtained. Numerical support for these results is presented using direct simulation of dynamo action in selected flows at high conductivity. Finally the case is considered when in spherical polar coordinates the flow is independent of the azimuthal coordinate and the stream surfaces, which are tori, have arbitrary cross sections. 相似文献
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Jan Skaloud Philipp Schaer Yannick Stebler Phillip Tomé 《ISPRS Journal of Photogrammetry and Remote Sensing》2010,65(2):208-217
This paper presents a methodology for the precise registering of airborne laser data directly in flight with an accuracy that is sufficient for the majority of derived products, such as digital terrain models. We first present the strategy that integrates GPS/INS/LiDAR data for generating laser point clouds directly in flight and analyzes their accuracy. The latter requires the implementation of a functional covariance propagation on-line for all the system components (i.e. trajectory, laser, system calibration) to which influences of scanning geometry are added at the end of a flight line. The study of scanning geometry necessitates the classification of vegetation and coarse estimation of the terrain normal. This is achieved by a method that we formerly proposed for off-line quality analysis. The second part of the paper focuses on the positioning component. In high resolution scanning performed close to the terrain, the absolute accuracy of the resulting point cloud depends mainly on the quality of the trajectory which is related to the type of GPS solution (e.g. absolute positioning, DGPS, RTK). To reach sub-decimeter accuracy for the point cloud in the real-time, an RTK-GPS solution is needed. This requires the establishment of a communication link for the transmission of GPS corrections (or measurements). We analyze the usability of RTK-GPS/ALS acquired during several flights using different communication methods in the particular context of helicopter based missions. We focus mainly on the exploitation of nation-wide reference GNSS networks and confirm experimentally that the real-time registration of airborne laser data is feasible with sub-decimeter accuracy. Such quality is sufficient not only for a wide range of applications, but it also opens new opportunities for monitoring missions that require a short reaction time. Finally, we concentrate on situations when the phase and code corrections cannot be transmitted, and the quality of the differential carrier-phase positioning needs to be predicted. We validate the previously introduced indicators of positioning quality by simulated degradation of the input data. 相似文献
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Eric Gloaguen Yannick Branquet Philippe Boulvais Yves Moëlo Jean-Jacques Chauvel Pierre-Jacques Chiappero Eric Marcoux 《Mineralium Deposita》2007,42(4):399-422
In the Saint-Aubin-des-Chateaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects
an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint
produced pervasive and massive sulphidation with stratoid pyritised lenticular bodies within the oolitic ironstone layer.
These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After the massive sulphidation
stage (Fe–As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb–Sb–Au (stage 3) parageneses
in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various
orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification
of structures that developed incrementally via right-lateral simple shear compatible with bulk strain affecting the Central
Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses
reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input
of hot hydrothermal fluids (maximum of 390–350°C) during the main sulphide stage 1. The subsequent stages present a marked
temperature drop (300–275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds
to the underlying Neo-Proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring
deposits suggest that large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to
the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key control for deformation and fluid
flow in the oolitic ironstones, which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic
mineralisation of Saint-Aubin-des-Chateaux appears to be very similar to epigenetic sulphidation described in banded iron
formation-hosted gold deposits. 相似文献
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Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years 总被引:1,自引:0,他引:1
Yannick Garcin David Williamson Laurent Bergonzini Olivier Radakovitch Annie Vincens Guillaume Buchet Joël Guiot Simon Brewer Pierre-Etienne Mathé Amos Majule 《Journal of Paleolimnology》2007,37(4):475-490
The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years.
Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic
carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming
that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response
varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the
clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble
the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation
supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact
is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this
remote basin during colonial and post-colonial times. 相似文献