1,721,018 research outputs found
Study of optical and radiative properties of cirrus clouds deduced from the synergy between active and passive measurements : application in the context of the A-Train and future spatial missions
No full textLes nuages de glace de type cirrus sont reconnus comme ayant un impact radiatif important mais encore mal déterminé sur le système Terre-atmosphère. Récemment, la constellation de satellites A-Train a efficacement contribué à leur étude, grâce un panel d’instruments en parfaite synergie les uns avec les autres. Dans cette thèse, nous proposons d’utiliser l’instrumentation de l’A-Train afin d’étudier les propriétés optiques et radiatives des cirrus. Dans un premier temps, des données issues de deux campagnes aéroportées sont utilisées, afin d’effectuer une validation des mesures du radiomètre infrarouge spatial IIR. Nous montrons de très bonnes similitudes entre les mesures radiométriques aéroportées et spatiales, ce qui permet de conclure à la validation de ces dernières. Une seconde étude présente un algorithme développé dans le but de restituer l’épaisseur optique des cirrus et la dimension effective des cristaux qui les composent, à partir des mesures de IIR. Nous montrons que ses résultats sont en accord avec des mesures in situ et des produits opérationnels, mais remarquons cependant que leur qualité pourrait être améliorée dans l’hypothèse d’une meilleure connaissance des propriétés de nuages d’eau liquide sous-jacents. Une troisième étude propose donc une amélioration de cet algorithme, permettant de restituer simultanément les propriétés d’une couche de nuage de glace et de deux couches de nuage d’eau liquide. Cet algorithme ’multi-couches’ est appliqué sur un grand nombre de cas, de manière à juger efficacement de la qualité de ses résultats. Des comparaisons avec divers produits opérationnels montrent une bonne cohérence de nos restitutions.Cirrus are cloud types that are recognized to have a strong but still poorly understood impact on the Earth-atmosphere radiation balance. Recently, the A-Train satellite constellation has efficiently contributed to the study of these clouds, due to a multitude of instruments in perfect synergism. In this PhD research, several A-Train instruments have been used to study the optical and radiative properties of cirrus. Firstly, with the help of the data from two airborne campaigns, we have performed a validation of the measurements from the Infrared Imaging Radiometer (IIR) onboard CALIPSO. We have observed strong similarities between the airborne and space borne radiometric measurements, which allows validating the latter. A second study presents an algorithm developed in order to perform retrievals of the optical thickness of cirrus and the effective size of their ice crystals using IIR measurements. We demonstrate that these retrievals are perfectly coherent with in situ measurements and operational products of IIR. It is nevertheless observed that better constraints on the properties of liquid water clouds underneath cirrus layers could significantly improve the retrievals. Therefore, a third study presents a modification of this algorithm, allowing us to simultaneously retrieve the properties of one cirrus layer and two layers of liquid water clouds underneath. This ‘multi-layer’ algorithm is applied to a large amount of cases in order to assess its effectiveness. Comparisons with several operational products indicate a good coherence of our retrievals
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
Satellite retrievals improved by advanced atmospheric modelling: The S3COM algorithm
No full textInternational audienceBetter constrained retrievals of cloud parameters from satellite observations are essential to assess the representation of cloud microphysics and processes in climate models. The cloud droplet number concentration (CDNC) is one of these most important microphysical properties of liquid clouds for understanding and for quantifying the effective radiative forcing by aerosol-cloud interactions (ERFaci). Nevertheless, this key parameter still remains poorly known and is not yet operationally provided from current standard satellite retrievals. Our approach relies on an innovative determination of CDNC from satellite observations in combination with advanced atmospheric modelling. We introduce our new, community-based tool: the Satellite Simulator and Sandbox for Cloud Observation and Modelling (S3COM). We use realistic cloud situations obtained from the ICOsahedral Nonhydrostatic Large Eddy Model (ICON-LEM) to simulate top of atmosphere radiances with the Radiative Transfer for TOVS (RTTOV), from visible to infrared, observed by the Moderate Resolution Imaging Spectroradiometer (MODIS). Preliminary results on synthetic MODIS data simulated by S3COM are presented and discussed.Des restitutions mieux contraintes des paramètres nuageux à partir des observations satellitaires sont essentielles pour évaluer la représentation de la microphysique et des processus des nuages dans les modèles de climat. La concentration en nombre de gouttelettes nuageuses est l'une des propriétés microphysiques les plus importantes des nuages liquides pour comprendre et quantifier l'impact du forçage radiatif effectif lié aux interactions entre les aérosols et les nuages. Néanmoins, ce paramètre clé reste encore mal connu et n’est pas encore un produit opérationnel des restitutions satellitaires standards actuelles. Notre approche s'appuie sur une détermination innovante de la concentration en nombre de gouttelettes nuageuses à partir d’observations satellitaires en combinaison avec une modélisation atmosphérique avancée. Nous présentons notre nouvel outil communautaire : le simulateur satellitaire pour l’observation et la modélisation des nuages (S3COM). Nous utilisons des scènes nuageuses réalistes obtenues à partir du modèle non hydrostatique à haute résolution ICON-LEM pour simuler les luminances au sommet de l'atmosphère, du visible à l'infrarouge, observées par le radiomètre spectral pour imagerie de résolution moyenne MODIS, avec le modèle de transfert radiatif rapide pour les instruments passifs RTTOV. Les résultats préliminaires sur les données synthétiques MODIS simulées par S3COM sont présentés et discutés
On the applicability of physical optics in the mm-wave region of the electromagnetic spectrum
No full textFrom 2022 onwards, space-based satellites will measure the upwelling microwave and sub-millimetre emission from the Earth. The purpose of these new Earth observation satellites is to measure the total integrated ice column amount in the atmosphere to constrain climate model predicts of the amount of ice mass that is contained in the earth’s atmosphere. To interpret these new measurements from across the microwave spectral region and invert them to find the total amount of ice mass, it will be necessary to construct realistic models of ice crystals contained in cirrus clouds and efficiently compute their light scattering properties. However, it is possible that in large-scale storm clouds there can exist ice particles with sizes easily in excess of several cm, such sizes of ice crystals preclude the current application of electromagnetic methods to compute their light scattering properties. In this abstract, we explore the applicability, in size parameter space, of a novel physical optics method to compute the integral optical properties and phase functions of highly irregular ice crystals. The classical approach to geometric optics is to add Fraunhofer diffraction at the particle cross-section to the ray tracing result to obtain the full scattering phase function of the particle. In the approach presented here, developed by Hesse and called the Ray Tracing Diffraction on Facets method (RTDF), not only is Fraunhofer diffraction at the cross-section of the particle included but also diffraction at each of the facets on the ray tracing paths. We will show the applicability of the RTDF method down to size parameters of only 18 in the mm-wave and sub-mm-wave spectral regions, where the size parameter is defined by (pi x the crystal maximum dimension)/wavelength. This result means that the more onerous electromagnetic methods need only be applied at up to size parameters of 18, which excludes significant computational cost in time and memory loading. The figure below shows the phase functions of a randomly oriented hexagonal ice column of size parameter 18 at 874 GHz calculated using T-matrix (red line), RTDF (green line), and classical geometric optics (blue line). It is apparent from the figure that the addition of diffraction at each of the facets results in a much closer solution to T-matrix than classical geometric optics. The classical method is shown to over-estimate the halo and the backscattering amplitude by factors and orders of magnitude, respectively. In the presentation, results from other ice crystal shapes will be presented, which show similar results to those shown in the figure above
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
Characterisation of low-base and mid-base clouds and their thermodynamic phase over the Southern Ocean and Arctic marine regions
Full text linkThe thermodynamic phase of clouds in low and middle levels over the Southern Ocean and the Arctic marine regions is poorly known, leading to uncertainties in the radiation budget in weather and climate models. To improve the knowledge of the cloud phase, we analyse 2 years of the raDAR-liDAR (DARDAR) dataset based on active satellite instruments. We classify clouds according to their base and top height and focus on low-, mid-, and mid- to low-level clouds as they are the most frequent in the mixed-phase temperature regime. Low-level single-layer clouds occur in 8 %–15 % of all profiles, but single-layer clouds spanning the mid-level also amount to approx. 15 %. Liquid clouds show mainly a smaller vertical extent but a horizontally larger extent compared to ice clouds. The results show the highest liquid fractions for low-level and mid-level clouds. Two local minima in the liquid fraction are observed around cloud top temperatures of −15 and −5 °C. Mid-level and mid- to low-level clouds over the Southern Ocean and low-level clouds in both polar regions show higher liquid fractions if they occur over sea ice compared to the open ocean. Low-level clouds and mid- to low-level clouds with high sea salt concentrations, used as a proxy for sea spray, show reduced liquid fractions. In mid-level clouds, dust shows the largest correlations with liquid fraction, with a lower liquid fraction for a higher dust aerosol concentration. Low-level clouds clearly show the largest contribution to the shortwave cloud radiative effect in both polar regions, followed by mid- to low-level clouds
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