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Aspekte der zonal asymmetrischen Zirkulation der Südhemisphäre und ihre Repräsentation im MPI Erdsystemmodell
Get PDFDue to the characteristic land-sea distribution of the southern hemisphere (SH)
its atmospheric circulation is generally more zonally symmetric than its northern
hemispheric equivalent. Distinct deviances from its symmetric nature, however,
play an important role with regard to weather and climate on the regional scale, e.g.
on the Antarctic sea ice distribution over the Amundsen Sea (Lefebvre et al., 2004).
Thus the SH represents an ideal research area for the investigation of different
aspects of the zonally asymmetric components of the circulation.
As the Antarctic Oscillation (AAO) is the leading mode of tropospheric variability
an adequate representation in earth system models is essential. Therefore the
first goal of this thesis is evaluating to what extent the AAO and its related precipitation
patterns are represented in the MPI Earth System Model (MPI-ESM). This
is done by comparing the spatial patterns of the AAO (given as EOFs) with their
associated principal components (PCs) and precipitation patterns of the MPI-ESM
to three different reanalyses. A comparison between the leading EOFs of the MPIESM
and ERA-Interim indicates shifted and less pronounced centers of action in
the MPI-ESM. Similarly the spectral density estimates of the associated PCs in the
ESM show reduced variability for periods between 4 to 5 months. The relation between
AAO and SH precipitation is assessed via composite and correlation analysis.
Altogether the MPI-ESM underestimates the relation of AAO and SH precipitation
but exhibits the same sign and spatial distribution of correlation values. These
findings suggest a lack of El Nino-Southern Oscillation (ENSO) variability in the
MPI-ESM and related teleconnections towards Antarctica, which accounts for the
reduced variability over the Amundsen Sea.
One of the most prominent asymmetric features of the SH circulation is the
split jet over Australia and New Zealand in Austral winter. Previous studies have
developed indices to describe to what degree the upper-level mid-latitude westerlies
are split. Furthermore, these studies have investigated the relationship of the
split jet to the AAO and ENSO. The results of these studies, however, are fairly
inconsistent in their message so that the relationship between the SH wintertime
split jet and climate variability indices remains unclear. The scope of this thesis is
a more thorough investigation of this link.
So far, all established split jet indices are based on a definition that is focused
on the specific region in which the jet split is recognizable. In this thesis, the split
jet is considered to be of a hemispheric nature rather than of a regional. Therefore
a new, hemispherically defined index based on the principal components (PCs) of
the zonal wind field for the Austral winter is proposed. A linear combination of
PC2 and PC3 (PSI = PC2 - PC3) of the anomalous monthly (JAS) zonal wind is
used to identify the split jet condition. The newly defined index indicates a strong
coherence with the AAO. A regression analysis with the Multivariate ENSO Index
indicates a non-linear relationship between PSI and ENSO, i.e. split jets occur during strong positive and negative phases of ENSO but hardly under "normal"
conditions.
The 2nd and 3rd PCA-mode of the geopotential height variability in 500 hPa
are defined as the Pacific South American patterns. They exhibit only a weak
correlation with the PSI, but they show a significant correlation with the individual
components (PCs) of the PSI, thereby uncovering an indirect influence on the SH split jet variability. This leads to the conclusion that a positive AAO phase, as
well as both flavors of ENSO and the PSA-1 pattern, produce favorable conditions
for a SH split event.
The MPI-ESM's ability to reproduce the zonally asymmetric components of the
SH circulation was further evaluated by an investigation of the split variability in
the model. Modes larger than the first order in the zonal wind anomalies over
the SH in Austral winter are found to be not distinguishable according to North's
"rule of thumb". Nevertheless, PC1 and PC3 of the 200 hPa zonal wind field are
found to contain the "model-intern" split variability. A composite analysis reveals,
that the split variability is simulated insuffciently in the MPI-ESM. It is deduced
that the split variability of the higher order PCs is mixed by the model. A likely
cause for this difference compared to the reanalysis could be a low-pressure system
over the Amundsen Sea, i.e. the Amundsen Sea Low (ASL). The model lacks ASL
variability due to insuffciently simulated ENSO and its teleconnections which are
known to reach and impact the ASL region. This can be confined by considering
the climatologies and standard deviations of two associated fields (U200 and Z500).
Furthermore, it was analysed to what extent the model is able to reproduce the
zonally asymmetric component by comparing the deviations from the zonal mean
in both fields (U200 and Z500). The zonal asymmetric part of the total variability is
underestimated in the model especially over the ASL region and south of Australia,
i.e. where the split jet is located. A composite analysis of the zonal asymmetric
AAO component (i.e. the transient eddy portion) reveals that the model lacks
substantial parts of the asymmetric AAO component. The composite's spatial distribution
of significant values resembles approximately the PSA patterns in the
reanalysis which are traditionally seen as the primary mechanism for the poleward
transport of tropical signals. As this spatial structure is largely absent in
the model, it is deduced that the deficiencies of the model in simulating the zonal
asymmetric part of the SH circulation originate from the inadequate representation
of ENSO variability including the high-latitude teleconnections to the ASL region.
Consequently, improving the ability of the model to simulate the discussed variability
modes would improve the representation of the SH split variability (and likely
other asymmetric aspects of the SH atmospheric circulation) in the MPI-ESM
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Investigating the Interdependent Influence of Multiple Environmental Features of Urban Green Spaces on Human Thermal Comfort at Microscale Using a Multi-parameter Approach
Get PDFThe three studies in this dissertation investigate the interdependent influence of various environmental features in Urban Green Space (UGS) on human thermal comfort at the microscale. This research pertains to microscale urban climatology. Urban climatology is an interdisciplinary field focused on advising urban construction to achieve the preferred urban climate.
Microscale urban climatological studies have more application potential than those at meso- and local scales. Yet, current limitations in microscale urban climatological research hinder realizing this potential. Firstly, the costs of microscale studies are generally high. Secondly, it is difficult to apply the research results elsewhere. As a results, the high-cost microscale studies must be continuously repeated in various locations. The challenge of generalizing research results is a common limitation in urban climatology. Approaches have been proposed to alleviate this limitation in urban climatological studies at meso- and local scales. However, there is a lack of such attempts at the microscale.
This research aims to propose an approach contributing to the generalization of research results in microscale urban climatological studies. This approach upgrades from the previous single-parameter approach to a multi-parameter approach. The multi-parameter approach views the UGS holistically by exploring the interdependent influence of various environmental features on human thermal comfort. The interdependent influence can enhance the comprehensiveness of results, improve the results generalization, and facilitate the knowledge transfer into specific guidelines for climate-adaptative UGS design and planning.
To explore the interdependent influence, the multi-parameter approach utilizes the factorial experimental design. The microclimate model ENVI-met is used to simulate scenarios in the factorial experiments. The simulations are conducted on idealized and simplified scenarios to minimize the research costs. To control the research cost, the Latin Hypercube Sampling (LHS) design is adopted to sample a subset of scenarios from the factorial experiment for field measurement and simulation. For result applicability, this research evaluates UGS cooling effects using both climatic parameters and biometeorological index -- Physiological Equivalent Temperature (PET). PET values in this research are calculated using the RayMan model.
This thesis comprises three independent studies on the interdependent influence of various environmental features of UGS on human thermal comfort. The following common conclusions can be drawn from these three studies. Firstly, different environmental features influence the cooling effects of UGS in an interdependent way. Specifically, the correlation between an environmental feature and the PCI effect depends on various environmental features. Secondly, the multi-parameter approach is significantly superior to the single-indicator approach in comprehensively and accurately capturing the interdependent influence. Additionally, these three studies provide specific and generalizable suggestions for the climate-adaptative UGS design and planning, demonstrating the capability of the multi-parameter approach to enhance the applicability and generalizability of findings in urban climatological studies.
The characteristics of the multi-parameter approach include the following three aspects. At the practical level, it can provide specific guidelines for practical UGS design and planning; at the research level, it explores the interdependent influence of multiple environmental features on human thermal comfort; and at the philosophical level, it adopts the holistic view. This research is not only an attempt to enhance methodology and promote the application of research results, but also a manifestation of the philosophical paradigm shift in urban climatology
Verbesserung von Dekadischen Vorhersagen durch Ensemble Dispersionsfilterung im Ozean und einer effizienten und systematischen Auswertung
Get PDFDecadal climate predictions have the objective to predict the development of the climate for the following years to decades. Numerical Earth system models are initialized with observational values, similar to the methodology applied in weather forecasting. Additionally, they are forced by boundary conditions, like greenhouse gas scenarios, to project the long term development. This thesis investigates decadal climate predictions with Earth system models and their further improvement.
A decadal prediction system is evaluated and investigated for sources of potential skill. Hence a systematic evaluation strategy is developed. It contains the assessment of accuracy of the ensemble mean and the ensemble spread, and compares decadal experiments with climatology, observations, and climate projections. This initialized reference system leads to good predictive skill in temperature and precipitation forecasts. The evaluation shows that the decadal prediction is scientifically sound, but it also has potential for improvement. The initialization with observed ocean data and the prediction with the ensemble mean of a larger ensemble size turn out to be sources of skill for decadal predictions. The entire assessment is performed within a novel evaluation system called Freva. This system is designed to complement climate modeling by a systematic and efficient assessment. Freva serves as a resource-efficient process framework between the data generation and its evaluation, to detect decadal climate prediction potential.
A new prediction technique called ’Ensemble Dispersion Filter’ is developed. It exploits two important climate prediction paradigms: the ocean’s heat capacity and the advantage of the ensemble mean. The Ensemble Dispersion Filter averages the ocean temperatures of the ensemble members every three months, uses this ensemble mean as a restart condition for each member, and further executes the prediction. The evaluation by the new verification framework shows that the Ensemble Dispersion Filter results in a significant improvement in the predictive skill compared to the unfiltered reference system. Even in comparison with prediction systems of a larger ensemble size and higher resolution, the Ensemble Dispersion Filter system performs better. In particular, the prediction of the global average temperature of the forecast years 2 to 5 shows a significant skill improvement. Compared to the observational climatology forecast, the Ensemble Dispersion Filter experiment has a Mean Squared Error Skill Score of 0.83, while the unfiltered reference system exceeds only 0.68. With major improvements over the Pacific and North Atlantic, the regional distribution of the Ensemble Dispersion Filter experiment is more accurate than the reference. In precipitation forecasts, improvements are seen over the continents. The prediction of the cyclone frequencies improves over the key region of the North Atlantic. Consequently, the thesis demonstrates a substantial advance in research on decadal climate predictions
Variations on the Author
Get PDF“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
Die hydrologischen Auswirkungen explosiver Vulkanausbrüche in der asiatischen Monsunregion
Get PDFThis thesis investigates the spatio-temporal characteristics and the mechanisms of explosive volcanic eruptions’ hydrological effects in subregions of the Asian monsoon region (AMR). Based on two volcanic forcing indices, volcanic eruptions are classi ed according to their northern hemisphere volcanic aerosol injection (NHVAI). Comparisons on the hydrological effect of NHVAI in subregions of AMR are made between tree ring-based hydrological reconstruction data MADA, and the model data from CMIP5 and Last Millennium Ensemble project. MADA and the multi-model ensemble means agree on the drying effect of volcanic eruptions in the monsoon-dominated subregion, while discrepancies can be found in the westerlies-dominated subregion. A better agreement between proxy reconstruction and models is visible one year after the eruption rather than in the eruption year. Less proxy- model discrepancies are apparent in subregions with more available tree-ring chronologies. Results also indicate that the MMEM of CMIP5 is able to reproduce volcanic eruptions’ hydrological effects in southern Asian monsoon region.
Based on the multi-model ensemble mean of CMIP5, the mechanism of the hydrological effects of NHVAI is investigated. The analyses focus on the drying effect in the relatively wettest area (RWA) and the wetting effect in the relatively driest area (RDA). The radiative effect of NHVAI causes a reduction of land-ocean thermal contrast. This changes the transport of water vapor and the formation of atmospheric clouds in the region. The regional temperature and precipitation responses result from the radiative effect of atmospheric clouds. The reduced land-ocean thermal contrast causes a weakened Asian summer monsoon circulation. The transport of water vapor from the ocean to the land is reduced. This results in the drying effect in the RWA due to the reduced formation of atmospheric clouds and precipitation. On the other hand, more water vapor is transported to the RDA due to the change of atmospheric circulation. The vertical motion is also enhanced. This increases the formation of atmospheric clouds and precipitation, and results in the wetting effect in the RDA. Another volcanic classi cation with only southern hemisphere volcanic aerosol injection (SHVAI) is constructed in order to study the climate effect of SHVAI. Results show inversed climate responses to SHVAI compared to NHVAI. Especially in the RWA, the results suggest signi cant drying effect after NHVAI, but slight wetting effect after SHVAI.
In order to investigate the climate effects of volcanic eruptions at different latitudes, volcanic forcing generated from the Easy Volcanic Aerosol (EVA) is implemented into the full-coupled land-ocean general circulation model MPI-ESM-LR. The aim is to simulate a Pinatubo-like eruption at 30°N (NH), equator (EQ) and 30°S (SH). The results con rm the opposite effects on monsoon precipitation after the NH and SH volcanic eruptions. Especially in India, one nds warming and drying effects after the NH eruption, but cooling and wetting effects after the SH eruption. The climate responses to the EQ eruption are similar to that after the SH eruption.
A tendency towards an El Niño event is visible one to three years after the NH eruption. After the SH eruption, a tendency towards the negative phase of the ENSO can be found. One year after the EQ eruption, a tendency towards an El Niño event can be found. The ENSO moves to the negative phase three years after the EQ eruption.
In summary, the mechanism of the climate response in India can be described as follows: The radiative effect of stratospheric aerosol from the NH (SH) volcanic eruption causes decreased (increased) interhemispheric thermal contrast and land-ocean thermal contrast. It strengthens (weakens) the South Asian summer monsoon and forces a southward (north- ward) move of the Intertropical Convergence Zone (ITCZ). This decreases (increases) the water vapor transport from the ocean to the land, and leads to the depletion (formation) of atmospheric cloud and less (more) precipitation in India. The radiative effect of the atmospheric cloud and the physical feedback of the system result further in the warming (cooling) and drying (wetting) effects in India. Results in this study indicate an important role the dynamical response and the resultant change of atmospheric cloud plays on the regional climate responses to volcanic eruptions
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
Get PDFWe 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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