1,721,013 research outputs found
Satellite gravimetry for climate model evaluation
Full text linkGlobal coupled Earth System Models (ESMs) are important for predicting future climate conditions. To assess the quality and reliability of ESMs it is crucial to evaluate them against independent observations. In this thesis, land water storage-related variables from ESMs taking part in the Coupled Model Intercomparison Project Phases 5 and 6 (CMIP5 and CMIP6) are compared to observed terrestrial water storage (TWS) changes from the Gravity Recovery And Climate Experiment (GRACE) and its Follow-On (GRACE-FO) satellite missions. This thesis is the first study to provide a comprehensive assessment of ways in which space gravimetric measurements can be utilized to evaluate coupled ESMs, thereby tackling several challenges of such a comparison:
Apart from external forcing data (e.g., the Sun's energy and greenhouse gas concentrations), CMIP ESMs are not constrained by observations, but evolve freely over time after being initialized. As a consequence, over long time spans they reproduce the climate variability in a statistical sense only, but not the exact timing of particular events. Thus, when analyzing (long-term) climate projections, a direct comparison to observed time series is not feasible, but higher order metrics as the linear trend, seasonal cycle, and interannual variability have to be utilized. Furthermore, discrepancies between observed TWS (full integrated water column, possibly superimposed by non-hydrological mass changes) and modeled TWS (representing only soil moisture and snow) regionally hamper the interpretation of results.
In the comparison of a CMIP5 ESM ensemble with GRACE observations, simulated long-term wetting and drying trends are found to be consistent with observed TWS trends in several regions of the world (e.g., the Mediterranean, Southwestern United States, Central Asia). However, it is shown that interannual variations obscuring long-term trends in TWS can have a large influence over 30 years and more, which regionally prevents reliable conclusions about long-term wetting or drying from the short GRACE time series. In addition to long-term trends, the seasonal cycle and interannual variations of TWS in a CMIP6 multi-model ensemble are assessed with respect to present-day observed conditions. The model data are also analyzed for potential future changes of TWS variability, as these might be an important target for a future gravity mission with enhanced sensitivity. In contrast to long-term climate projections, decadal predictions can directly be compared on time series level, because they are frequently initialized with observed states of atmosphere and oceans. To this end, a GRACE-based TWS reconstruction is used to evaluate decadal climate predictions from CMIP5 and CMIP6 with the result that they provide skillful forecasts of TWS anomalies in markedly humid climates for two years and more into the future.
Overall, this thesis highlights mutual benefits of climate modeling and geodesy: On the one hand, satellite-observed TWS has a great potential to validate the performance or to hint at shortcomings of ESMs for land water storage-related variables. On the other hand, ESM output can provide information on expected climate signals in TWS, which is important for future plans in space gravimetry aiming at the long-term monitoring of climate variations
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
Lecture Notes from the Summer School of DFG SPP1257 Global Water Cycle : September 12-16, 2011, [Mayschoss/Ahr]
No full textIn 2006, the German Research Association DFG had established the coordinated Priority Program SPP1257 Mass distribution and Mass Transport in the Earth System. According to DFG’s philosophy, SPP’s are meant to enable broad-scale research in new, emerging fields. The objective of the SPP1257 was to facilitate integrated analysis of novel-type data collected from dedicated gravity field and radar altimetry satellite missions, to improve our knowledge about mass distribution and mass transport processes within the Earth system such as melting of continental ice sheets and glaciers, changes in ocean circulation pattern and in sea level, variations of surface and ground water levels and river discharge, glacial-isostatic adjustment, mantle convection and tectonics, and to investigate interactions between these processes. During six years, many Ph.D. students and postdocs from more than 30 institutions worked together in collaborative projects.
These lecture notes were compiled on the occasion of the summer school Global Hydrological Cycle, organized by the SPP1257 at September 12-16, 2011 in Mayschoss/Ahr, in which about 70 Ph.D. students, postdoc researchers and master students participated.The challenge imposed on thelecturers wasto familiarize students with widely ifferentbackground (geodesy, hydrology, oceanography, geophysics, mathematics) with
• concepts of observation systems and data processing, such as analysis of data from the Gravity Recovery and Climate Experiment (GRACE) gravity mission, and from radar altimetric satellite missions, associated problems such as noise, spatio-temporal sampling and aliasing, data post-processing techniques such as spherical harmonic synthesis and analysis, gridding, smoothing, covariance analysis, EOF analysis, and
• concepts of modelling and interpretation in hydrology and hydro-meteorology, oceanography and sea level, tides, ice sheet modelling, climate dynamics, and solid-Earth geodynamics.
The focus of the summer school wason concepts, and technical proofs were avoided. Lectures were accompanied by exercises, practicals and group work. Last not least, exciting discussions could be continued during barbecue, walks on the Rotweinwanderweg, and in the cellar of the Winzergenossenschaft
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
A global drought monitoring framework using GRACE/-FO data assimilation
No full textExtreme climate phenomena like droughts can lead to a shortage of available water resources that in turn can cause serious consequences such as famines. Monitoring of extremes is extremely important, and past research mainly focused on fluxes like precipitation or streamflows and surface waters because of easy access to measurements. More complicated is the use of subsurface water storages like groundwater for observing water shortages, mostly monitored via in-situ observations that are spatially irregularly distributed. The stations require maintenance and access can be restricted because, e.g., of political reasons. Hydrological models provide simulations of surface and subsurface water as well but they use strong process simplifications, the forcing data are error-prone, and thus the models imperfectly represent reality.
Another possibility to derive information about water storages is via satellite observations. Since 2002, the gravity missions Gravity Recovery And Climate Experiment (GRACE) and its successor GRACE Follow-On (GRACE-FO) provide measurements of surface and subsurface water globally from space that can be represented as Total Water Storage Anomalies (TWSA). Concretely, TWSA is the change in the aggregated volume of water stored in different compartments, among others, rivers, soil moisture, and groundwater. However, due to the orbit constellation, only a spatial resolution of about 300 km is given, which is too coarse for most drought applications. A possibility to simultaneously improve the spatial resolution, disaggregate TWSA into the single water compartments, and improve the models’ realism can be achieved by data assimilation. With data assimilation, the TWSA observations are integrated into a hydrological model and simulated model output is pushed towards the observations.
So far, no global framework exists on how to optimally use water storage information from GRACE/-FO within assimilation for monitoring drought and its water propagation through the hydrological water cycle, i.e. how precipitation deficits lead to a decrease in water storage that in turn affects vegetation growth. Therefore this thesis has three major goals: (1) developing a framework that allows assimilating GRACE/-FO TWSA into the Water Global Assessment and Prognosis (WaterGAP) hydrological model for the first time globally, (2) analyzing dominant hydrological signatures and identifying the signatures of precipitation - water storage - vegetation seasonal maxima and non-seasonal events, and, (3) developing a drought monitoring framework and an unprecedented prototype warning alert system under the consideration of GRACE/-FO and assimilation outputs.
The global GRACE/-FO assimilation into the WaterGAP model is set up with ensemble-based filters, tuned for global application, e.g. to include spatial observation error correlations via localization techniques and provided as Global Land Water Storage (GLWS) release 3.0 – the update of GLWS release 2.0. The analysis of dominant signatures (e.g., linear trends) shows that TWSA from assimilation outputs inherit properties from both, the GRACE/-FO observations and the simulations, thus, present a smooth transition between them. The timing of seasonal or episodic high or low precipitation leading to an increase or decrease in water storage is for most vegetation regimes, as expected, found to be shortest for soil moisture, longer for surface water, and longest for groundwater. Validation with respect to independent data sets shows an overall improvement of GLWS compared to WaterGAP simulations for TWSA and groundwater but the assimilation does not have a major impact on surface water storage and soil moisture as these storages do not significantly improve.
In drought monitoring it is common to define indices that would classify actual drought conditions as “severe”, “moderate”, etc., to facilitate an easy means of communication to decision makers. Therefore, in this thesis, the performance of existing GRACE drought indices is studied. As it is difficult to evaluate such indices, a synthetic study is set up that reveals how trends, accelerations, and noise in GRACE/-FO time series are biasing drought detection. Hereinafter, droughts are analyzed globally and in selected focus regions for TWSA, and subsequently, I investigate subsequent drought events from soil moisture to surface water and groundwater. An approach is developed for determining drought hazard risk maps from assimilation-derived soil moisture and the combination of surface water and groundwater. Finally, a prototype of a warning alert system is set up and analyzed to identify the spatiotemporal dynamics of droughts for hydrological basins. This system might pave the way for future implementations of TWSA or water storages from GRACE/-FO assimilation into existing or future operational early warning systems
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