1,720,971 research outputs found
Biogeochemical Data Assimilation: On The Singular Evolutive Interpolated Kalman-Filter
Full text linkData Assimilation is nowadays a fundamental part in any forecasting geoscience model.
In the field of marine biogeochemistry, the number and quality of observational systems (e.g. satellites, biogeochemical Argo floats, moorings) is constantly improving, but the available information is still far from picturing per se the true state of the marine ecosystem.
Thus, at forecasting and monitoring purpose, Data Assimilation techniques are necessary in order to face the problem of the sea state estimation.
Handling the large dimension of the state vector of the system (order of 10^6) remains an issue, and many attempts have been done to reduce the complexity of the problem, adding hypothesis and approximations in order to obtain fast Data Assimilation algorithms. The 3D-VAR, a Data Assimilation method based on the variational approach, is one of these results and it is adopted in the EU Copernicus system for monitoring and forecasting the biogeochemical state of the Mediterranean Sea.
Aside from variational, the other main Data Assimilation approach is the Kalman-Filter.
This thesis is focused on biogeochemical marine Data Assimilation, and has a double purpose.
The first one is to compare the 3D-VAR scheme with the Singular Evolutive Interpolated Kalman-Filter (SEIK). From a theoretical point of view, this is realized using a Bayesian framework to derive differences and similarities as well as strengths and weaknesses of the two methods. This analysis shows that the main differences are in the choice of the state estimator (the mode for the variational and the mean for the Kalman-Filter), and in the Kalman-Filter's capability of keeping and transferring the information through the time steps.
A twin experiment has been used to asses the skill performance of the compared schemes. Tests show that the SEIK is one order of magnitude more precise than the 3D-VAR, in terms of root mean squared distance (RMSD).
The second objective of this work is to develop a novel Data Assimilation method from the SEIK filter, focusing on the effects of the model error and its estimation. Various strategies have been implemented at this purpose, namely an high order sampling technique, a method to take into account SEIK's neglected part of the model error as noise-like observation error, a data driven maximum likelihood algorithm for model error estimation and, finally, an ad hoc computationally cheap smoother.
The twin experiment tests prove that the first two modifications change the behaviour of the SEIK filter only in case of large model error, conferring to the modified SEIK an higher resiliency to divergences. The maximum likelihood strategy estimations obtained good agreement with the estimated real value, with better results if used in pair with the modified version of the SEIK. The smoother further improved the RMSD of the method, with even better results in case of large model error
Interaction between like-charged surfaces mediated by uniformly charged counter-nanoparticles
No full textAn electric interfacial layer appears when the mobile ions or nanoparticles of an electrolyte solution interact with an extended, charged surface. The distribution of ions or nanoparticles is driven by electrostatic interactions and entropy. We consider continuously charged spherical nanoparticles of finite size. At thermodynamic equilibrium, the spatial profile of the concentration is obtained by deriving the appropriate Euler-Lagrange equations. We discuss how various model parameters of the nanoparticles influence structural properties of the electric interfacial layer. We calculate the pressure between two like-charged surfaces embedded in a water solution of continuously charged spherical nanoparticles
Emission dynamic meso-simulation model to evaluate traffic strategies in congested urban networks
No full textUniformly charged nanoparticles between like-charged walls
No full textAn electric interfacial layer is created when the mobile ions or charged nanoparticles of an electrolyte interact with a surface of an extended charged object. The competition between electrostatic attraction and translational entropy loss of the mobile nanoparticles results in a diffuse interfacial layer of nanoparticles close to the charged surface. Due to its simplicity and transparency first wide spread theoretical description of the electric interfacial layer was the Poisson-Boltzmann theory. Numerous improvements were applied that account for charge-charge correlations, steric effects and solvent properties. The present article focuses on spherical mobile nanoparticles which have charge distributed over the surface and have finite size. The nanoparticles are sandwiched between two parallel like-charged walls. We perform the minimization of an appropriate free energy functional, which leads to a non-linear integro-differential equation for the electrostatic potential that is solved numerically. Our model predicts condensation of nanoparticles to oppositely charged surface. For highly charged surfaces and nanoparticles of finite size we observed big differences in the volume charge density profiles between soft and hard spheres. The theoretical predictions are in a good agreement with Monte Carlo simulations
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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