1,721,052 research outputs found
Continuous Middle-Atmospheric Wind Profile Observations by Doppler Microwave Radiometry
No full textObservations of wind profiles in the upper stratosphere /lower mesosphere are challenging as the established measurement techniques based on in situ methods, radars or airglow spectrometers cannot cover this altitude range. Nevertheless, wind information from these altitudes is important for the assessment of middle-atmospheric dynamics in general and as basis for planetary wave or infrasound propagation estimates. Benefitting from recent developments in spectrometers and low-noise amplifiers, microwave radiometry now offers the opportunity to directly and continuously measure horizontal wind profiles at altitudes between 35 and 70 km. This is achieved by retrieving the wind-induced Doppler shifts from pressure broadened atmospheric emission spectra. The typical measurement uncertainties and vertical resolutions of daily average wind profiles lie between 10–20 m/s and 10–16 km, respectively. In this chapter, comparisons of the measured wind profiles to different ECMWF model versions and MERRA re-analysis data are shown. Moreover, the oscillatory behaviour of ECMWF winds is investigated. It appears that the longer period wave activities agree well with the observations, but that the model shows less variability on timescales shorter than 10 days
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
Characterization of atmospheric ambiant noise originating from the ocean : global modelling of microbaroms and comparison with infrasound observations of the International Monitoring System
No full textDes signaux infrasons sont enregistrés en continu par les stations du Système de Surveillance International (SSI). Entre 0.1 et 0.6 Hz, des signaux cohérents d’origine océanique, appelés microbaroms, dominent ces enregistrements. L’objectif de cette thèse est de caractériser ces sources de bruit cohérent pour aider à l’analyse des signaux d’intérêt dans le cadre du Traité d’Interdiction Complète des Essais nucléaires. Basés sur les interactions non linéaires de vagues, les modèles de sources existants considèrent soit un océan de profondeur infinie et un rayonnement dépendant de l’angle d’élévation, soit un rayonnement monopolaire impliquant un effet important de la bathymétrie. Ces modèles sont étendus pour combiner les effets de bathymétrie et de directivité de la source. Le nouveau modèle de source prédit un impact négligeable de la bathymétrie sur les ondes acoustiques rasantes qui représentent l’essentiel de l’énergie acoustique enregistrée. Un modèle global est implémenté en intégrant un terme d’interaction de vagues, un modèle de source et des effets d’atténuation atmosphérique. Une validation quantitative est réalisée en comparant les spectres directionnels modélisés avec l’ensemble des observations du réseau du SSI sur une période de sept ans. Cette thèse montre que le nouveau modèle de source est plus performant que les précédents et que l’intégration de réflexion à la côte dans le modèle de vagues et de l’atténuation atmosphérique dépendante du vent améliorent les prédictions. Au-delà des aspects de surveillance opérationnelle, ce travail ouvre des perspectives pour caractériser en continu, et à l’échelle globale, les effets de propagation dans la moyenne atmosphère.Infrasound signals are continuously detected by the International Monitoring System (IMS) network.Between 0.1 and 0.6 Hz, coherent signals originating from the ocean, known as microbaroms, dominate the recorded signals. This thesis aims at better characterizing these sources of coherent noise for discrimination purpose in the framework of the Comprehensive nuclear Test Ban Treaty. Different theoretical microbarom models had been previously developed based on second-order non-linear interactions of ocean waves. While early theories considered an infinite ocean depth and a source radiation depending on the acoustic wave elevation angle, other works have found a significant effect of the water depth by considering a monopolar radiation. This thesis extends these models by combining the effects of both finite depth and source directivity. The new source model predicts that the water depth has a negligible effect for the near-horizontally propagating acoustic waves that dominate the recorded microbarom signals. A global model of microbaroms arrivals at ground stations is set up including ocean wave models, source models and atmospheric attenuation effects. A quantitative validation is performed by comparing modelling results with microbaroms detected by the IMS infrasound network over seven years.This thesis demonstrates that the new source model performs better than previous ones, and that the predictions are further enhanced by using wind-dependent attenuation and an ocean wave model including coastal reflection.Beyond operational monitoring objectives, this work offers new perspectives to globally and continuously characterize infrasound propagation effect in the middle atmosphere
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Calculating atmospheric gravity wave parameters from infrasound measurements
No full textInfrasound arrays are sensitive enough to be able to detect the subtle pressure changes that occur as an overhead atmospheric gravity wave passes. The array can then provide information regarding the back azimuth, amplitude, frequency and pressure perturbation of the gravity wave. It is shown that by combining this data with meteorological data recorded at the array, further gravity wave parameters can be calculated. Some examples of time series analysis are shown for an infrasound station in the Ivory Coast illustrating how seasonal and daily variations in the weather can change the properties of gravity waves being detected. Ultimately, the parameters calculated using this method can be used by the meteorological community to improve the parametrisation of gravity waves in their models and increase understanding of the diurnal and seasonal variability in gravity wave parameters
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The potential impact of upper stratospheric measurements on sub-seasonal forecasts in the extra-tropics
No full textThis chapter examines the potential improvements in tropospheric weather forecasts that might arise from an enhanced representation of the upper stratospheric state. First, the chapter reviews current operational practice regarding observation of the atmosphere and the relative paucity of observations in the altitude range 40–70 km. Then, we describe some idealised model calculations to quantify the potential gain in skill available from improved monitoring in this region. The idealised model experiments use a relaxation technique with the Hadley Centre General Environment Model, to assess the potential gain in skill from observations both of the whole stratosphere and the upper stratosphere. At weather forecasting timescales (up to forecast day 30), better knowledge of the stratosphere, close to the onset of a sudden stratospheric warming, improves forecasts of the tropospheric northern annular mode. Whole-stratosphere information significantly improved average surface temperature anomalies over northern North America, whilst upper stratosphere information improved anomalies over Central Siberia. These results suggest any new observational technique which can contribute to monitoring of the 40–70 km region would likely benefit tropospheric forecast skill during wintertime
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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