Spatial mode estimation for functional random fields with application to bioturbation problem   总被引：1，自引：1，他引：0  

Sophie Dabo-Niang  Anne-Francoise Yao  Laura Pischedda  Philippe Cuny  Franck Gilbert 《Stochastic Environmental Research and Risk Assessment (SERRA)》2010,24(4):487-497

This work provides a useful tool to study the effects of bioturbation on the distribution of oxygen within sediments. We propose here heterogeneity measurements based on functional spatial mode. To obtain the mode, one usually needs to estimate the spatial probability density. The approach considered here consists in looking each observation as a curve that represents the history of the oxygen concentration at a fixed pixel.  相似文献   

The 1995–96 decline of R Coronae Borealis: high-resolution optical spectroscopy     

N. Kameswara Rao  David L. Lambert  Mark T. Adams  David R. Doss  Guillermo Gonzalez  Artie P. Hatzes  C. Renée James  C. M. Johns-Krull  R. Earle Luck  Gajendra Pandey  Klaus Reinsch  Jocelyn Tomkin  Vincent M. Woolf 《Monthly notices of the Royal Astronomical Society》1999,310(3):717-744

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Biogeochemistry and dynamics of settling particle fluxes at the Antikythira Strait (Eastern Mediterranean)     

Philippe Kerherv  Serge Heussner  Bruno Charrire  Spyros Stavrakakis  Jean-Luc Ferrand  Andr Monaco  Nicole Delsaut 《Progress in Oceanography》1999,44(4):295

For the first time, a 12-month trap experiment was conducted on both sides of the strait between Crete and Antikythira Island (Eastern Mediterranean Sea) from June 1994 to June 1995 as part of the PELAGOS experiment. Analyses of major chemical constituents, including carbohydrates and stable lead isotopes and Scanning Electron Microscope studies were performed on the trap samples. Total mass fluxes varied between 1 and 1273 mg m⁻² d⁻¹. The lowest fluxes observed were in summer and autumn 1994, when stratification of the water column was at its deepest. In general, mass fluxes exhibited very low values throughout this experiment confirming the strong oligotrophy of this area. The mean contents of the major constituents (carbonates, opal, lithogenic fraction) were quite similar during the survey and between traps, with the exception of organic carbon contents, which were highest (7–10%) in summer 1994, i.e. during the period of lowest mass fluxes. During the first 6-month deployment (summer–autumn 1994) there was an important mass flux peak, which was depleted in organic carbon, at the Ionian near-bottom trap. This event coincided with a violent wind episode, which may have caused the resuspension of particles, which were then transported down the steep continental slope on the Ionian side of the strait. A smaller peak in mass flux occurred at the Aegean near-bottom trap, coincident with rainfall. Both these events indicate that environmental factors can control flux variations in an oligotrophic environment. During the second 6-month deployment (winter–spring 1995) there was another important increase in mass fluxes, which occurred at all three traps, although in the Ionian traps mass flux peaks were delayed by one to two sampling intervals. The distance between the two mooring sites gives a rough estimate of a minimum horizontal advection speed of 2 cm s⁻¹ for this particulate transfer from the Aegean to the Ionian area. This estimate is in good agreement with the measured current velocities.  相似文献   

Mesoscale slope current variability in the Gulf of Lions. Interpretation of in-situ measurements using a three-dimensional model     

Gaël André  Pierre Garreau  Philippe Fraunie 《Continental Shelf Research》2009

The ECOLOPHY experiments aimed at investigating physical exchanges between coastal and open sea. They were carried out in June and December 2005 over the shelf-break in the North-eastern part of the Gulf of Lions (Northwestern Mediterranean Sea). This area is considered to be the generation zone for the eddy and meandering structures of the Northern Current (NC). The objective of the present work is to examine mesoscale variability of this coastal slope current in the light of available data. Numerical modeling is used to support the field data analysis. ADCP current measurements over a one-year period show that mesoscale activity is maximal in late winter, correlating with the seasonal variability of the NC and, also, partly with local winds. Measured currents exhibit mesoscale fluctuations with periods ranging from 3 to 30 days, in agreement with previous analyses. Fluctuations of periods longer than 10 days are found to be mainly oriented in the direction of the mean current, whereas more frequently observed high frequency fluctuations tend to be oriented cross-slope, suggesting a relationship with the NC mesoscale meandering. Moreover, trajectories of surdrift buoys launched in the NC vein exhibit mesoscale phenomena, such as current meanders or eddies and on-shelf intrusions. Numerical modeling provides a synoptic point of view and is used hereafter to support physical interpretation of punctual eulerian or lagrangian measurements. Therefore, modeled hydrodynamic fields are used to analyze surdrift buoy trajectories and computed vertically averaged current and Ertel potential vorticity provide a better understanding of these behaviors.  相似文献   

Efficient approximations for the simulation of density driven flow in porous media     

Philippe Ackerer  Anis Younes 《Advances in water resources》2008

Due to the non-linear coupling between flow and transport equations the simulation of real density driven flow problems requires a lot of computational time and/or heavy equipments. We suggest some approximations and numerical recipes to reduce the CPU costs for these strongly non-linear coupled equations without loss in accuracy.  相似文献   

Late Weichselian relative sea-level changes and ice sheet history in southeast Greenland   总被引：1，自引：0，他引：1  

Antony J. Long  David H. Roberts  Matthew J.R. Simpson  Sue Dawson  Glenn A. Milne  Philippe Huybrechts 《Earth and Planetary Science Letters》2008,272(1-2):8-18

Relative sea-level (RSL) observations from the margins of the Greenland Ice Sheet (GIS) provide information regarding the timing and rate of deglaciation and constraints on geophysical models of ice sheet evolution. In this paper we present the first RSL record for the southeast sector of the GIS based on field observations completed close to Ammassalik. The local marine limit is c. 69 m above sea-level (asl) and is dated to c. 11 k cal. yrs BP (thousand calibrated years before present) and is a minimum date for ice free conditions at the study site. RSL fell to c. 24 m asl by 9.5 k cal. yrs BP and continued to fall at a decreasing rate to reach close to present by 6.5 k cal. yrs BP. Our chronology agrees with radiocarbon dates from offshore cores that indicate ice free conditions on the adjacent mid-shelf by 15 k cal. yrs BP. We compare the new RSL data with predictions generated using two recently published glaciological models of the GIS that differ in the amount and timing of ice loading and unloading over our study area. These two GIS models are coupled to the same Earth viscosity model and background (global) ice model to aid in the data-model comparison. Neither model provides a close fit to the RSL observations. Based on a preliminary sensitivity study using a suite of Earth viscosity models, we conclude that the poor data-model fit is most likely due to an underestimate of the local ice unloading. An improved fit could be achieved by delaying the retreat of a thicker ice sheet across the continental shelf. A thick ice sheet extending well onto the continental shelf is in agreement with other recent observations elsewhere in east and south Greenland.  相似文献   

Frequency analysis of rainfall and streamflow extremes accounting for seasonal and climatic partitions     

Michael Leonard  Andrew Metcalfe  Martin Lambert 《Journal of Hydrology》2008,348(1-2):135-147

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Structure of the Nemrut caldera (Eastern Anatolia, Turkey) and associated hydrothermal fluid circulation     

&#x;nan Ulusoy  Philippe Labazuy  Erkan Aydar  Orkun Ersoy  Evren ubuku 《Journal of Volcanology and Geothermal Research》2008,174(4):269-283

Plio-Quaternary volcanism played an important role in the present physical state of Eastern Anatolia. Mount Nemrut, situated to the west of Lake Van is one of the main volcanic centers in the region, with a spectacular summit caldera 8.5 × 7 km in diameter. The most recent eruptions of the volcano were in 1441, 1597 and 1692. Nemrut Lake covers the western half of the caldera; it is a deep, half-bowl-shaped lake with a maximum depth of 176 m. Numerous eruption centers are exposed within the caldera as a consequence of magma–water interaction. Current activity of Nemrut caldera is revealed as hot springs, fumaroles and a small, hot lake.Self-potential and bathymetric surveys carried out in the caldera were used to characterize the structure of the caldera and the associated hydrothermal fluid circulation. In addition, analyses based on digital elevation models and satellite imagery were used to improve our knowledge about the structure of the caldera. According to SP results, the flanks of the volcano represent “the hydrogeologic zone”, whereas the intra-caldera region is an “active hydrothermal area” where the fluid circulation is controlled by structural discontinuities. There is also a northern fissure zone which exhibits hydrothermal signatures. Nemrut caldera collapsed piecemeal, with three main blocks. Stress controlling the collapse mechanism seems to be highly affected by the regional neotectonic regime. In addition to the historical activity, current hydrothermal and hydrogeologic conditions in the caldera, in which there is a large lake and shallow water table, increase the risk of the quiescent volcano.  相似文献   

Inversion of swell frequency from a 1-year HF radar dataset collected in Brittany (France)   总被引：1，自引：1，他引：0  

Weili Wang  Philippe Forget  Changlong Guan 《Ocean Dynamics》2014,64(10):1447-1456

This article presents long period ocean wave (swell) frequencies inverted from a 13-month dataset of high-frequency (HF) phased array radars and an assessment of these estimates by comparison with WAVEWATCH III model data. The method of swell frequency inversion from high-frequency radar sea echo Doppler spectra is described. Radar data were collected from a two-site HF Wellen Radar (WERA) radar system on the west coast of Brittany (France) operating at 12 MHz. A standard beam-forming processing technique has been used to obtain Doppler spectra of processed radar cells. Swell frequencies are obtained from the frequencies of particular spectral peaks of the second-order continuum in hourly averaged Doppler spectra. The data coverage of effective Doppler spectra considered for swell frequency estimates shows the influence of islands and shallow water effects. Swell estimates from both radar stations are in good agreement. The comparison of radar-derived results to WAVEWATCH III (WW3) estimates shows that radar measurements agree quite well with model results. The bias and standard deviation between two estimates are very small for swells with frequency less than 0.09 Hz (period >11 s), whereas radar estimates are generally lower than model estimates for shorter swells, along with higher standard deviation. Statistical analysis suggests that radar measurement uncertainty explains most of the difference between radar and model estimates. For each swell event, time series of frequency exhibits a quasi-linear frequency increase which is associated with the dispersive property of wave phase velocity. The use of swell frequency estimates from both radars on common radar cells only slightly increases the accuracy of swell frequency measurement.  相似文献   

Optimal softening for force calculations in collisionless N-body simulations     

E. Athanassoula  E. Fady  J. C. Lambert  A. Bosma 《Monthly notices of the Royal Astronomical Society》2000,314(3):475-488

In N -body simulations the force calculated between particles representing a given mass distribution is usually softened, to diminish the effect of graininess. In this paper we study the effect of such a smoothing, with the aim of finding an optimal value of the softening parameter. As already shown by Merritt, for too small a softening the estimates of the forces will be too noisy, while for too large a softening the force estimates are systematically misrepresented. In between there is an optimal softening, for which the forces in the configuration best approach the true forces. The value of this optimal softening depends both on the mass distribution and on the number of particles used to represent it. For a higher number of particles the optimal softening is smaller. More concentrated mass distributions necessitate smaller softening, but the softened forces are never as good an approximation of the true forces as for non-centrally concentrated configurations. We give good estimates of the optimal softening for homogeneous spheres, Plummer spheres and Dehnen spheres. We also give a rough estimate of this quantity for other mass distributions, based on the harmonic mean distance to the k th neighbour ( k =1,…,12), the mean being taken over all particles in the configuration. Comparing homogeneous Ferrers' ellipsoids of different shapes we show that the axial ratios do not influence the value of the optimal softening. Finally we compare two different types of softening, a spline softening and a generalization of the standard Plummer softening to higher values of the exponent. We find that the spline softening fares roughly as well as the higher powers of the power-law softening and both give a better representation of the forces than the standard Plummer softening.  相似文献