1,721,243 research outputs found
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
Scales of variability of atmospheric aerosols
Full text linkAerosols have a significant effect on the global radiation budget through their interactions with radiation and clouds. However, estimates of their effect are the dominant source of uncertainty in current estimates of total anthropogenic effect on climate. A major cause of this uncertainty is the high degree of variability of aerosol properties and processes that affect their lifetime. Prediction of the aerosol effect on climate depends on the ability of three-dimensional numerical models to accurately estimate aerosol properties. However, a limitation of traditional grid-based models is their inability to resolve variability on scales smaller than a grid box. Past research has shown that significant aerosol variability exists on scales smaller than these grid-boxes, which can lead to discrepancies between observations and aerosol models. This thesis uses a synthesis of aerosol observations, global climate model (GCM) data, and a new aerosol modelling technique implemented within a regional-scale model to quantify the important scales of aerosol variability and the extent to which different sub-grid scale processes contribute to discrepancies in aerosol modelling. Analysis of black carbon (BC) plumes from aircraft observations shows that BC plumes represent a large portion of total BC mass and typically exist on scales of 65{ 100 km. Comparison of observed plume scales to those simulated by GCMs at multiple resolutions show that GCMs overestimate the scales of along- ight-track variability by 64% at the highest resolution. Variability is shown to be greater near sources than in remote regions, indicating that models may benefit from higher resolutions in regions of high emissions. Additionally, GCMs at all resolutions show higher variability in the latitudinal direction than the longitudinal direction, suggesting that capturing latitudinal variability may result in greater improvements in aerosol modelling. This work additionally presents a novel technique to allow one to isolate the effect of aerosol variability from other sources of variability within the model. Processes most affected by neglecting aerosol sub-grid variability are gas-phase chemistry and aerosol uptake of water through the aerosol/gas equilibrium reactions. The inherent non-linearities in these processes result in large changes in aerosol parameters when aerosol and gaseous species are artificially mixed over large spatial scales. These changes in aerosol and gas concentrations are exaggerated by convective transport, which transports these altered concentrations to altitudes where their effect is more pronounced. Future aerosol model development should focus on accounting for the effect of sub-grid variability on these processes at global scales in order to improve model predictions of the aerosol effect on climate
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
Investigating aerosol-cloud interactions
Full text linkMicrophysical and dynamical interactions between aerosols and clouds are associated with some of the largest uncertainties in projections of future climate. Many possible aerosol effects on clouds have been suggested, but large uncertainties remain. In order to improve model projections of future climate, it is essential that we improve our quantitative understanding of anthropogenic aerosol effects. Several studies investigating interactions between satellite-observed aerosol and cloud properties have been published in recent years. However, the observed relationships are not necessarily due to aerosol effects on clouds. They may be due to cloud and precipitation effects on aerosol, meteorological covariation, observational data errors or methodological errors. An analysis of methodological errors arising through climatological spatial gradients is performed. For region sizes larger than 4°×4°, commonly used in the literature, spurious spatial variations in retrieved cloud and aerosol properties are found to introduce widespread significant errors to calculations of aerosol-cloud relationships. Small scale analysis prior to error-weighted aggregation to larger region sizes is recommended. Appropriate ways of quantifying relationships between aerosol optical depth (τ) and cloud properties are considered, and results are presented for three satellite datasets. There is much disagreement in observed relationships between τ and liquid cloud droplet number concentration and between τ and liquid cloud droplet effective radius, particularly over land. However, all three satellite datasets are in agreement about strong positive relationships between τ and cloud top height and between τ and cloud fraction (f_c). Using reanalysis τ data, which are less affected by retrieval artifacts, it is suggested that a large part of the observed f_c-τ signal may be due to cloud contamination of τ. General circulation model simulations further demonstrate that positive f_c-τ relationships may primarily arise due to covariation with relative humidity, and that negative f_c-τ relationships may arise due to scavenging of aerosol by precipitation. A new method of investigating the contribution of meteorological covariation to the observed relationships is introduced. Extratropical cyclone storm-centric composites of retrieved aerosol and cloud properties are investigated. A storm-centric description of the synoptics is found to be capable of explaining spurious f_c-τ relationships, although the spurious relationships explained are considerably smaller than observed relationships
Aerosol effects on microphysical processes and deep convective clouds
Full text linkAerosolâcloud interactions are an essential feature of the Earthâs climate system. How-
ever, aerosol effects on deep convection are highly uncertain. Different conceptual models
for the effects of aerosols on deep convective clouds have been proposed, and assessments
based on both models and observations show a wide range of results. This thesis aims at
unravelling the cloud microphysical pathways involved in these interactions using a hier-
archy of different model simulations and novel analysis tools. A detailed pathway ana-
lysis based on microphysical process rates for individually tracked clouds is developed
and applied to idealised supercell simulations with three different microphysics schemes
in a cloud-resolving model (CRM). This reveals both consistent responses between the
schemes, e.g. a suppression of warm rain formation and elevated freezing, and signific-
ant differences between the schemes due to the definition of hydrometeor classes and
microphysical processes. The cloud tracking framework tobac is developed to provide
a consistent way to perform analyses resolving the time evolution of individual clouds
in a wide range of datasets. Its application is demonstrated for both CRM simulation
results and geostationary satellite data. The cloud tracking and the pathway analysis are
combined to investigate deep convective clouds in a large case study simulation and their
aerosol response for two different CRMs. Separating the tracked clouds into different
categories and compositing along a relative time axis allows for a detailed assessment
of the cloud microphysics that resolves the time evolution of the clouds. Despite some
similar aerosol responses like a suppression of warm-rain formation and surface precipit-
ation, the analyses highlight significant differences in the cloud types and cloud evolution
simulated by the two models, especially regarding the mixed- and ice-phase processes.
The detailed investigation of the microphysical evolution of individually tracked clouds
reveals important pathways for aerosol effects on deep convective clouds and substantial
uncertainties that arise from the representation of microphysical processes in numerical
models.</p
Aerosol-cloud-precipitation interactions
Full text linkAerosols are thought to have a large effect on the climate, especially through their interactions with clouds. The magnitude and in some cases the sign of aerosol effects on cloud and precipitation are highly uncertain. Part of the uncertainty comes from the multiple competing effects that aerosols have been proposed to have on cloud properties. In addition, covariation of clouds and aerosol properties with changing meteorological conditions has the ability to generate spurious correlations between cloud and aerosol properties. This work presents a new way to investigate aerosol-cloud-precipitation interactions while accounting for the influence of meteorology on cloud and aerosol. The clouds are separated into cloud regimes, which have similar retrieved cloud properties, to investigate the regime dependence of aerosol-cloud-precipitation interactions. The strong aerosol optical depth (AOD)- cloud fraction (CF) correlation is shown to have the ability to generate spurious correlations. The AOD-CF correlation is accounted for by investigating the frequency of transitions between cloud regimes in different aerosol environments. This time-dependent analysis is also extended to investigate the development of precipitation from each of the regimes as a function of their aerosol environment. A modification of the regime transition frequencies consistent with an increase in stratocumulus persistence over ocean is found with increasing AI (aerosol index). Increases in transitions into the deep convective regime and in the precipitation rate consistent with an aerosol invigoration effect are also found over land. Comparisons to model output suggest that a large fraction of the observed effect on the stratocumulus persistence may be due to aerosol indirect effects. The model is not able to reproduce the observed effects on convective cloud, most likely due to the lack of parametrised effects of aerosol on convection. The magnitude of these effects is considerably smaller than correlations found by previous studies, emphasising the importance of meteorological covariation on observed aerosol-cloud-precipitation interactions
Understanding precipitation responses to externally driven climate change
Full text linkPrecipitation plays a fundamental role in transferring water and energy in the climate system. Human activities significantly impact the hydrological cycle, but the magnitude of global and regional changes remains uncertain due to the complex nature of microphysical-dynamical interactions. This thesis aims to improve the understanding of precipitation responses to climate change through a combination of bottom-up (process-driven) and top-down (energetic) approaches.
The relationship between aerosol-induced changes in atmospheric energetics and precipitation responses across different scales is studied in terms of fast (radiatively or microphysically mediated) and slow (temperature-mediated) responses. This thesis proposes an energetic framework to decompose rainfall changes into contributions from clouds, aerosols, and clear-clean sky (without aerosols and clouds). It provides a way to better interpret and quantify the precipitation changes caused by aerosol perturbations. Further investigations show that the increase rate in global-mean precipitation with temperature (hydrological sensitivity; η) is dependent on the spatial pattern of sea surface temperature (SST) change, which has been overlooked before. Warming in strong tropical ascending regions can produce large η via enhanced global circulation and atmospheric radiative cooling. The warming pattern effect significantly contributes to the spread of η in the current generation of general circulation models (GCMs). After accounting for the pattern effect, the reconstructed global-mean precipitation agrees much better with observations, evidencing the importance of SST patterns on global-mean precipitation changes. Furthermore, we extend the analysis to regional rainfall by showing that anthropogenic aerosols modulate the Sahel rainfall variability through their impacts on regional SST. We show analysis supporting the chain of processes whereby changes in anthropogenic aerosol emissions alter net radiative fluxes and sea surface temperature variability in the North Atlantic Ocean, leading to a shift of the intertropical convergence zone (ITCZ), changes in the West African monsoon, and eventually changes in Sahel rainfall. These findings highlight the importance of accurate representation of regional aerosol radiative effects for the simulation of Sahel rainfall variability
High time-resolution observations of convective cloud lifecycles
Full text linkQuantifying the variability of convective cloud is of fundamental importance in order
to understand and accurately model global weather and climate systems. While
broad features of the diurnal and lifecycles of convective cloud are understood, the
spatial and temporal variability of convective properties are not well characterised
on the global scale, particularly over large areas of Africa. Due to the complexity of
the processes involved, models also struggle to realistically simulate the spatial and
temporal variability of convective cloud.
Observations provide insight into the properties of convective cloud and valuable
information about avenues for model development. Due to the wide range of spatial
and temporal scales involved in convection, the continuous nature of observations
from geostationary satellites makes them ideal for investigating convection. This
study uses high temporal resolution data from the geostationary Spinning Enhanced
Visible and Infrared Imager (SEVIRI) instrument to quantify seasonal and diurnal
cycles of cloud top temperature (CTT) and the lifecycle of individually tracked
convective clouds over a large area and period of time.
This study demonstrates that biases in SEVIRI CTT retrievals vary from less than
5 K over the southeast Atlantic Ocean, up to 30 K over central Africa at night, while
biases can also differ by up to 30 K between night and daytime retrievals. This
highlights the importance of considering spatial and diurnal variations in retrieval
errors. Keeping these biases in mind, it is shown that the diurnal cycle of cloud tops
is measured accurately in regions of stratiform cloud, while quantifying the diurnal
cycle over the tropics and regions of desert is more difficult.
SEVIRI observations are used to track individual convective cloud cores and anvils
across sub-Saharan Africa. The distributions, diurnal cycles, lifecycles and spatial
and seasonal cycles of a number of properties are quantified. Horizontal core and
anvil areas are shown to increase with longer cloud lifetimes, while minimum core
CTTs are shown to decrease with cloud lifetime. The diurnal cycle in convective
activity is shown to have both a morning and an evening peak, while the total anvil
coverage peaks in the late afternoon. Seasonal and spatial variations in core and
anvil area, CTT, time of convective initiation and dissipation are also quantified.
These results will be of interest to those in the observation and modelling communities,
particularly for studies considering the diurnal cycle of convection, or developing
new convective cloud process models and parameterisations.</p
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