Spatial and temporal variability of the frequency of extreme daily rainfall regime in the La Plata Basin during the 20th century   总被引：1，自引：1，他引：0  

Olga C. Penalba  Federico A. Robledo 《Climatic change》2010,98(3-4):531-550

We analyzed trends, interdecadal variability, and the quantification of the changes in the frequency of daily rainfall for two thresholds: 0.1 mm and percentile 75th, using high quality daily series from 52 stations in the La Plata Basin (LPB). We observed increases in the annual frequencies in spatially coherent areas. This coherence was more marked in austral summer, autumn, and spring, during which the greatest increases occurred in southern Brazil, especially during extreme events. In winter, the low and middle basins of the Río Uruguay and Río Paraná showed negative trends, some of which were significant. Interdecadal variability is well defined in the region with more pronounced positive jumps west of the basin between 1950 and 2000. This variability was particularly more marked during periods of extreme rainfall in summer, autumn, and spring, unlike in winter when extreme daily rainfall in the lower Rio Paraná basin decreased by up to 60%. The changes in the past century during extreme rainfall produced modifications in the annual rainfall cycle. The annual cycle of both indices was broader during the last period which is mainly explained by the strong decreases in winter.  相似文献   

Star-formation modes in the solar neighborhood     

Emilio J. Alfaro  Federico Elias  Jesús Cabrera-Caño 《Astrophysics and Space Science》2009,324(2-4):141-146

We perform a study into the spatial and kinematical distribution of young open clusters in the solar neighborhood, distinguishing between Gould Belt and local Galactic disk members. We use a previous estimate of the structural parameters of both systems obtained from a sample of O to B6 stars from Hipparcos. The two star-forming regions that dominate and give the Gould Belt its characteristic inclined shape show a striking difference in their content of star clusters: while Ori OB1 is richly populated by open clusters, not a single one can be found within the boundaries of Sco OB2. This is mirrored in velocity space, translating again into an abundance of clusters in the region of the kinematic space populated by the members of Ori OB1, and a marginal number of them associated with Sco OB2. We interpret all these differences by characterizing the Orion region as a cluster complex typically surrounded by a stellar halo, and the Sco-Cen region as an OB association in the outskirts of the complex. The different contents of star clusters, the different heights above the Galactic plane and the different residual velocities of Ori OB1 and Sco OB2 can be explained in terms of their relative position with respect to the density maximum of the Local Arm in the solar neighborhood. The origin of this feature could have been the interaction of a density wave with the local interstellar medium close to the Galactic co-rotation radius.  相似文献   

Multiple solutions in preliminary orbit determination from three observations     

Giovanni Federico Gronchi 《Celestial Mechanics and Dynamical Astronomy》2009,103(4):301-326

Charlier’s theory (1910) provides a geometric interpretation of the occurrence of multiple solutions in Laplace’s method of preliminary orbit determination, assuming geocentric observations. We introduce a generalization of this theory allowing to take into account topocentric observations, that is observations made from the surface of the rotating Earth. The generalized theory works for both Laplace’s and Gauss’ methods. We also provide a geometric definition of a curve that generalizes Charlier’s limiting curve, separating regions with a different number of solutions. The results are generically different from Charlier’s: they may change according to the value of a parameter that depends on the observations.  相似文献   

Matrix micrite δC and δO reveals synsedimentary marine lithification in Upper Jurassic Ammonitico Rosso limestones (Betic Cordillera, SE Spain)     

Rute Coimbra  Adrian Immenhauser  Federico Olriz 《Sedimentary Geology》2009,219(1-4):332-348

Matrix micrites are a commonly used carbonate archive for the reconstruction of past environmental parameters, but one that is submitted to known limitations. Main reasons for the often ambiguous value of many micrite-based isotope data sets are the unknown origin of the micrite components and their poorly resolved diagenetic history. Here we present carbon and oxygen-isotope data retrieved from Oxfordian to Tithonian Ammonitico Rosso nodular micrites sampled from three sections in the Betic Cordillera (Southern Spain). All three sections were correlated and sampled using a rigorous biostratigraphic framework. A noteworthy feature is that analyzed matrix micrites are more conservative in terms of their isotopic composition than other carbonate materials commonly considered to resist diagenetic alteration under favourable circumstances. Remarkably, this refers not only to δ¹³C ratios, which reflect the typical Late Jurassic global trend, but also to δ¹⁸O ratios that range around 0.3‰. The ¹⁸O-enriched oxygen-isotope ratios are considered to represent diagenetic stabilization of carbonate ooze under the influence of marine porewaters within the sediment–water interphase (i.e., the immature sedimentary section, usually submitted to biogenic activity). This interpretation agrees with the very early lithification of micrite nodules with cements precipitated from marine porewaters, enriched by the dissolution of aragonite skeletals (i.e., ammonite shells). According to the model proposed, low sedimentation rates as well as rapid early marine differential cementation, under the influence of currents and seawater pumping, affected the sediment–water interphase of epioceanic swells where deposition resulted in early lithified Ammonitico Rosso facies. The data obtained show that special care must be taken to prevent oversimplified interpretations of carbonate archives, particularly in the context of epioceanic settings.  相似文献   

N-body integrators with individual time steps from Hierarchical splitting     

Federico I. Pelupessy  Jürgen Jänes  Simon Portegies Zwart 《New Astronomy》2012,17(8):711-719

We review the implementation of individual particle time-stepping for N-body dynamics. We present a class of integrators derived from second order Hamiltonian splitting. In contrast to the usual implementation of individual time-stepping, these integrators are momentum conserving and show excellent energy conservation in conjunction with a symmetrized time step criterion. We use an explicit but approximate formula for the time symmetrization that is compatible with the use of individual time steps. No iterative scheme is necessary. We implement these ideas in the HUAYNO¹ code and present tests of the integrators and show that the presented integration schemes shows good energy conservation, with little or no systematic drift, while conserving momentum and angular momentum to machine precision for long term integrations.  相似文献   

Radar Backscatter from Underdense Meteors and Diffusion Rates     

Werner Singer  Ralph Latteck  Luis Federico Millan  Nick J. Mitchell  Jens Fiedler 《Earth, Moon, and Planets》2008,102(1-4):403-409

Many meteoroids burn up between about 120 km and 70 km, deposit metals and dust and form ionized trails which are detected by radars. Model studies about the influence of neutral or positively charged background dust on the ambipolar diffusion indicate that significant smaller decay times should be observed for weak meteor echoes compared to strong meteor echoes which can affect the estimation of temperatures. The variation of meteor decay times in dependence on echo strength, height, and season was studied using radar observations at 69° N, 22° S, and 67° S. Significantly reduced decay times were found for weak echoes below about 88 km at low latitudes throughout the year, and at high latitudes with the exception of summer. In summer at high latitudes, decreasing decay times of weak and strong meteors are observed at altitudes below about 85 km during the appearance of noctilucent clouds. The impact of reduced decay times on the estimation of neutral temperatures from decay times is discussed.  相似文献   

Interrogating urban experiments     

Federico Caprotti  Robert Cowley 《Urban geography》2017,38(9):1441-1450

The notion of the “urban experiment” has become increasingly prevalent and popular as a guiding concept and trope used by both scholars and policymakers, as well as by corporate actors with a stake in the future of the city. In this paper, we critically engage with this emerging focus on “urban experiments”, and with its articulation through the associated concepts of “living labs”, “future labs”, “urban labs” and the like. A critical engagement with the notion of urban experimentation is now not only useful, but a necessity: we introduce seven specific areas that need critical attention when considering urban experiments: these are focused on normativity, crisis discourses, the definition of “experimental subjects”, boundaries and boundedness, historical precedents, “dark” experiments and non-human experimental agency.  相似文献   

FEM validation of a double porosity elastic model for consolidation of structurally complex clayey soils     

C. Callari  F. Federico 《国际地质力学数值与分析法杂志》2000,24(4):367-402

Laboratory consolidation of structured clayey soils is analysed in this paper. The research is carried out by two different methods. The first one treats the soil as an isotropic homogeneous equivalent Double Porosity (DP) medium. The second method rests on the extensive application of the Finite Element Method (FEM) to combinations of different soils, composing 2D or fully 3D ordered structured media that schematically discretize the complex material. Two reference problems, representing typical situations of 1D laboratory consolidation of structured soils, are considered. For each problem, solution is obtained through integration of the equations governing the consolidation of the DP medium as well as via FEM applied to the ordered schemes composed of different materials. The presence of conventional experimental devices to ensure the drainage of the sample is taken into account through appropriate boundary conditions. Comparison of FEM results with theoretical results clearly points out the ability of the DP model to represent consolidation processes of structurally complex soils. Limits of applicability of the DP model may arise when the rate of fluid exchange between the two porous systems is represented through oversimplified relations. Results of computations, obtained having assigned reasonable values to the meso‐structural and to the experimental apparatus parameters, point out that a partially efficient drainage apparatus strongly influences the distribution along the sample and the time evolution of the interstitial water pressure acting in both systems of pores. Data of consolidation tests in a Rowe's cell on samples of artificially fissured clays reported in the literature are compared with the analytical and numerical results showing a significant agreement. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

Application of surface-wave tomography to mineral exploration: a case study from Siilinjärvi,Finland     

Federico Da Col  Myrto Papadopoulou  Emilia Koivisto  Łukasz Sito  Mikko Savolainen  Laura Valentina Socco 《Geophysical Prospecting》2020,68(1):254-269

In order to assess the feasibility and validity of surface-wave tomography as a tool for mineral exploration, we present an active seismic three-dimensional case study from the Siilinjärvi mine in Eastern Finland. The aim of the survey is to identify the formation carrying the mineralization in an area south of the main pit, which will be mined in the future. Before acquiring the data, we performed an accurate survey design to maximize data coverage and minimize the time for deployment and recollection of the equipment. We extract path-averaged Rayleigh-wave phase-velocity dispersion curves by means of a two-station method. We invert them using a computationally efficient tomographic code which does not require the computation of phase-velocity maps and inverts directly for one-dimensional S-wave velocity models. The retrieved velocities are in good agreement with the data from a borehole in the vicinity, and the pseudo three–dimensional S-wave velocity volume allows us to identify the geological contact between the formation hosting most of the mineralization and the surrounding rock. We conclude that the proposed method is a valid tool, given the small amount of equipment used and the acceptable amount of time required to process the data.  相似文献   

Modeling core-scale permeability anisotropy in highly bioturbated “tight oil” reservoir rocks     

Nisael A. Solano  Mohammad Soroush  Chris R. Clarkson  Federico F. Krause  Jerry L. Jensen 《Computational Geosciences》2017,21(3):567-593

A high-resolution simulation model of a heterogeneous low-permeability rock sample is used to investigate the effects of physical and biogenic sedimentary structures on scaling and anisotropy of absolute permeability at the core scale. Several simulation sub-samples with random locations and volumes were also selected for evaluation of the effects of scale and lithological composition on the calculated permeability. Vertical and horizontal permeability values (from whole core simulation) are in good agreement with routine core analysis (RCA) measurements from offsetting cores. Despite relatively good reservoir quality associated with geobodies of biogenic and relic bedding structures, results from the full diameter core simulation demonstrate that their limited volumetric abundance and restricted connectivity prevent these features from controlling fluid flow in these rocks. In fact, permeability seems to be dominated by the tighter encasing matrix, which exhibits average permeability values very close to those reported from RCA. Geometric averaging offers a better representation for the upscaling of horizontal permeability datasets; whereas, both geometric and harmonic averaging work similarly well for the vertical measurements. The methodology used in this work is particularly applicable to the detailed characterization of reservoir rocks with a high degree of heterogeneity caused by biological reworking and diagenesis.  相似文献