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Die nachfolgende Dissertation wurde an der Goethe-Universität Frankfurt am Institut für Geowissenschaften (FB 11) in der Arbeitsgruppe Kristallographie und Mineralogie (AG Winkler) verfasst. Die Betreuung der hier durchgeführten Arbeiten erfolgte hauptsächlich durch Prof. B. Winkler in Zusammenarbeit mit Dr. L. Bayarjargal, PD Dr. E. Haussühl und PD Dr. V. Vinograd. Bei dem vorliegenden Manuskript handelt es sich um eine kumulative bzw. publikationsbasierte Dissertation, welche die Forschungsergebnisse verschiedener bereits veröffentlichter wissenschaftlicher Fachartikel zusammenfasst.
Die Arbeit beschreibt verschiedene Synthesen und Untersuchungen an Carbonaten und teilt sich im Wesentlichen in zwei Abschnitte. Zum einen wurden Experimente mit Carbonaten bei Extrembedingungen bzw. unter hohen Drücken und hohen Temperaturen durchgeführt, wie sie auch im Inneren der Erde zu finden sind. Im zweiten Teil wurden Carbonate bei Raumbedingungen synthetisiert und der Einbau von Seltenerdelementen untersucht. Grundsätzlich werden jedoch in beiden Teilen dieser Arbeit die Strukturen und Eigenschaften verschiedener Carbonate und eine mögliche Kationensubstitution bzw. die Synthese isostruktureller Verbindungen erforscht.
Climatology of morphology and cloud-radiative properties of marine low-level mixed-phase clouds
(2023)
Marine stratocumuli cover about 40 - 60% of the ocean surface. They self-organize into different morphological regimes. The two organized cellular regimes are called open and closed mesoscale-cellular convective (MCC) clouds. In mid-to-high latitudes, open and closed cells are the two most frequent types of MCC clouds. In particular, many MCC clouds consist of a mixture of vapor, liquid droplets, and ice particles, referred to as mixed-phase clouds (MPCs). Even for the same cloud fraction, the albedo of open cells is, on average, lower than that of closed MCC clouds. Cloud phase and morphology individually influence the cloud radiative effect. Thus, this thesis investigates the relationships between the cloud phase, MCC organization, cell size, and differences regarding the cloud-radiative effect.
This thesis focuses on space-borne retrievals to achieve extensive temporal and spatial coverage. The liDAR-raDAR (DARDAR) version 2 product collocates two active and one passive satellite: CloudSat, Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), and Moderate Resolution Imaging Spectroradiometer (MODIS). The cloud phase of DARDAR is vertically integrated to establish a single cloud phase at each data point. The MCC classification data set based on the liquid water path (LWP) of MODIS scenes is collocated with the DARDAR product to determine the MCC organization. Cell-size statistics of both MCC clouds are obtained using a marker-based image segmentation method on MODIS reflectance scenes. In addition, based on MODIS reflectance scenes, a convolutional neural network (CNN) is developed to classify open and closed MCC scenes to avoid missing mature MPCs with a low LWP.
The first part of this thesis explores the relationships between cloud phase, morphology, and cloud albedo in the Southern Ocean (SO). At a given cloud-top temperature (CTT), seasonal changes in the mixed-phase fraction, defined as the number of MPCs divided by the sum of MPC and supercooled liquid cloud (SLC) pixels, are stronger than the morphological changes. Therefore, external factors seem to influence these changes instead of morphology. The dependence of cloud phase on cloud-top height (CTH) is more substantial than on CTT in clouds with CTHs below 2.5 km. The previously observed acceleration of closed-to-open transition in MPCs, known as preconditioning, is not the primary driver of climatological cloud morphology statistics in the SO. The morphological differences in cloud albedo are more pronounced in SLCs than in MPCs. This change in albedo alters the cloud radiative effect in the SO by 21Wm−2 to 39Wm−2 depending onseason and cloud phase.
Open and closed MCC clouds exhibit larger equivalent cell diameters in the MPCs than in SLCs in austral summer, whereas, in austral winter, the SLCs are larger. The cell’s aspect ratio accounts for varying CTHs. Closed cells have smaller aspect ratios than open cells, so their cell diameter is smaller, independent of CTH. While the seasonal differences in closed cells are due to changes in CTH, the seasonal aspect ratio differences in open cells are mainly caused by MPCs. With increasing aspect ratios, the cloud albedo decreases in both open and closed MCC clouds, with the most substantial decrease in open MPCs clouds. This leads to cloud-radiative changes of 60 - 75Wm−2 in the SO, depending on cloud phase and aspect ratio.
The established CNN exhibits a good accuracy of 80.6%, with even higher accuracies in the Open (85.5%) and Closed (87.3%) categories. The global MCC climatology based on the CNN generally agrees well with previous MCC distributions. The most notable difference occurs in the Northern Hemisphere (NH) in boreal winter, with a higher occurrence frequency of closed and open MCC clouds. This might indicate missing MPCs in previous studies based on the LWP and some restricted to warm cloud scenes. Thus, the developed CNN seems to better represent the different morphologies in MPCs than in previous classifications.
In conclusion, this thesis shows that understanding the dependencies of cloud phase, cloud morphology, and cell size is important to enhance predictions of the cloud-radiative effect and thus, it is important to evaluate how cloud phase, cloud morphology, and cellsize change in a warming climate.
Semi-arid African ecosystems influence trends and variability in global terrestrial carbon dynamics. However, there are uncertainties in potential effects of future climates for semi-arid ecosystems, especially for niche ecosystems. At the same time, African ecosystems provide the livelihoods and ecosystem services for around 1.4 billion people. Future population growth and associated changes in land use pose a challenge for the protection of African biodiversity. Therefore, this work focussed on future impacts of climate change on African ecosystems and carbon dynamics and also for African protected areas (PAs), where they may cooccur with other global change factors. Another focus was on uncertainties associated with future projections and with modelling the Nama Karoo, as an example of a semi-arid niche ecosystem. Dynamic vegetation models (DVMs) were the main research tool.
In Chapter 2, we analysed climate change impacts on African ecosystems and carbon pools until the end of the 21st century and associated uncertainties based on an ensemble of vegetation simulations with the DVM adaptive dynamic vegetation model (aDGVM). We investigated the impact of increased atmospheric CO2 concentrations and two climate change scenarios (medium (RCP4.5) and high emissions (RCP8.5); RCP - representative concentration pathway) on vegetation changes. Differences in the simulated vegetation were primarily driven by assumptions about the influence of CO2 on plants. Elevated CO2 concentrations led to increased total aboveground vegetation biomass and shrub encroachment into grasslands and savannas for both climate scenarios. In simulations without the direct influence of CO2 on plants, there was hardly any shrub encroachment and vegetation biomass decreased or varied between a slight decrease in some cases and a slight increase in others. Based on these results, biome changes due to climate change are likely in Africa in the future. Due to the large uncertainties in future projections, strategies to adapt to climate change must be flexible.
The simulated vegetation in Chapter 2 represented potential, natural vegetation and is particularly suitable to investigate PAs. However, PAs do not exist isolated from their environment and social developments. In Chapter 3, the vegetation projections with CO2 effect from Chapter 2 were combined with projections for population density and land use. Except for many PAs in North Africa, most PAs were adversely affected by at least one of the three drivers by the end of the 21st century in both investigated scenarios ("middle-of-the-road" and "fossil-fuelled development"). Cooccurrence of the drivers varied by region and scenario for PAs. Both scenarios implied increasing challenges for the conservation of African biodiversity in PAs. The impact of climate change on vegetation is likely to be exacerbated by socio-economic change for most African PAs. Strong mitigation of future climate change together with equitable societal development may facilitate successful ecosystem conservation.
The simulations in Chapters 2 and 3 showed large-scale patterns of vegetation change, but their low resolution makes them unsuitable for local analyses. In Chapter 4, the challenges of simulating smaller scale, semi-arid ecosystems and their carbon cycle were analysed for the Nama Karoo with the aDGVM2 and its shrub module. The aDGVM2 is based on the aDGVM, but represents plants more flexibly. In all tested aDGVM2 configurations, the carbon fluxes improved compared to initial simulations but still overestimated them. The measured morphology of the dwarf shrubs and soil water dynamics were not reproduced in aDGVM2. Semi-arid soil water dynamics and coping strategies of semi-arid dwarf shrubs under drought stress are not adequately implemented in the aDGVM2. Further field research on semi-arid water and carbon dynamics of vegetation is necessary to parameterise the aDGVM2 for dwarf shrubs. If these challenges are overcome, DVMs can be a powerful tool for much-needed research on the impacts of climate change on the Nama Karoo.
The analyses have shown that climate change under medium to high emission scenarios is likely to lead to large-scale changes in ecosystems and the carbon balance in Africa. Because lower emissions scenarios come with less uncertainty, climate change adaptation strategies likely need to be less complex or extensive if climate change is minimised. For African PAs, the challenges of climate change may be exacerbated by socio-economic factors to a regionally varying extent. This research suggests that successful ecosystem conservation depends on climate change mitigation measures and ensuring equitable, sustainable development. The shown uncertainties, e.g., in the implementation of the CO2 effect on plants or vegetation dynamics in more niche ecosystems, help to focus future research efforts and increase our understanding of the range of plausible futures we may need to adapt to.
This work describes the development and characterization of two instruments and their data evaluation, which contributes to a better understanding of new particle formation and growth, as well as their interactions with clouds. Both instruments were characterized at the Cosmics Leaving Outdoor Droplets (CLOUD) experiment at the European Center for Nuclear Research (CERN).
Paläoklimarekonstruktionen, die es sich zum Ziel gesetzt haben, Klima-Mensch Interaktionen auf lange Zeitreihen betrachtet zu erforschen, nehmen begünstigt durch die aktuell intensiv geführte Klimadebatte, einen immer größer werdenden Stellenwert in der öffentlichen und wissenschaftlichen Wahrnehmung ein. Denn trotz aller wissenschaftlicher Fortschritte, die in den vergangenen Jahrzehnten im Bereich der modernen Klimaforschung gemacht wurden, bleibt die zuverlässige Vorhersage und Modellierung von zukünftigen Klimaveränderungen noch immer eine der größten Herausforderungen unser heutigen Zeit. Betrachtet man die Karibik exemplarisch in diesem Rahmen, dann prognostizieren viele Modellrechnungen, infolge steigender Ozeantemperaturen, ein deutlich häufigeres Auftreten von tropischen Stürmen und Hurrikanen sowie eine Verschiebung hin zu höheren Sturmstärken. Dieser Trend stellt für die Karibik und viele daran angrenzende Staaten eine der größten Gefahren des modernen Klimawandels dar, den es wissenschaftlich über einen langen Zeitrahmen zu erforschen gilt.
Klimaprognosen stützen sich meist vollständig auf hoch-aufgelöste instrumentelle Datensätze. Diese sind aber alle durch einen wesentlichen Aspekt limitiert. Aufgrund ihrer eingeschränkten Verfügbarkeit (~150 Jahre) fehlt ihnen die erforderliche Tiefe, um die auf langen Zeitskalen operierenden Prozesse der globalen Klimadynamik adäquat abbilden zu können. Betrachtet man das Holozän in seiner Gesamtheit, so wurde die globale Klimadynamik über die vergangenen ~11,700 Jahre von periodisch auftretenden Prozessen und Abläufen gesteuert. Diese wirken grundsätzlich über Zeiträume von mehreren Jahrzehnten, teilweise Jahrhunderten und in einigen Fällen sogar Jahrtausenden. Viele dieser natürlichen Prozesse, können in der kurzen Instrumentellen Ära nicht gänzlich identifiziert und angemessen in Klimamodellen berücksichtig werden. Die alleinige Berücksichtigung der Instrumentellen Ära bietet daher nur eine eingeschränkte Perspektive, um die Ursachen und Abläufe von vergangenen sowie mögliche Folgen von zukünftigen Klimaveränderungen zu verstehen. Um diese Einschränkung zu überwinden, ist es somit erforderlich, dass die geowissenschaftliche Forschung mit Proxymethoden ein zusammenfassendes und mechanistisches Verständnis über alle Holozänen Klimaveränderungen erlangt.
Wenn man sich diese Limitierung, die ansteigenden Ozeantemperaturen und das in der Karibik in den vergangen 20 Jahren vermehrte Auftreten von starken tropischen Zyklonen ins Gedächtnis ruft, ist es nachvollziehbar, dass im Rahmen dieser Doktorarbeit ein zwei Jahrtausende langer und jährlich aufgelöster Klimadatensatz erarbeitet werden soll, der spät Holozäne Variationen von Ozeanoberflächenwasser-temperaturen (SST) und daraus resultierende lang-zeitliche Veränderungen in der Häufigkeit tropischer Zyklone widerspiegelt. In Zentralamerika wird das Ende der Maya Hochkultur (900-1100 n.Chr.) mit drastischen Umweltveränderungen (z.B. Dürren) assoziiert, die während der Mittelalterlichen Warmzeit (MWP; 900-1400 n.Chr.) durch eine globale Klimaveränderung hervorgerufen wurde. Die aus einem „Blue Hole“ abgeleiteten Informationen über Klimavariationen der Vergangenheit können als Referenz für die gegenwärtige Klimakriese verwendet werden.
Als „Blue Hole“ wird eine Karsthöhle bezeichnet, die sich subaerisch während vergangener Meeresspiegeltiefstände im karbonatischen Gerüst eines Riffsystems gebildet hat und in Folge eines Meeresspiegelanstiegs vollständig überflutet wurde. In einigen wenigen marinen „Blue Holes“ treten anoxische Bodenwasserbedingungen auf. Die in diesen anoxischen Karsthöhlen abgelagerten Abfolgen mariner Sedimente können als einzigartiges Klimaarchiv verwendet werden, da sie aufgrund des Fehlens von Bioturbation eine jährliche Schichtung (Warvierung) aufweisen.
In dieser kumulativen Dissertation über das „Great Blue Hole“ werden die Ergebnisse eines 3-jährigen Forschungsprojekts vorgestellt, dass das Ziel verfolgte einen wissenschaftlich herausragenden spät Holozänen Klimadatensatz für die süd-westliche Karibik zu erzeugen. Beim „Great Blue Hole“ handelt es sich um ein weltweit einzigartiges marines Sedimentarchiv für diverse spät Holozäne Klima-veränderungen, das im Zuge dieser Dissertation sowohl nach paläoklimatischen als auch nach sedimentologischen Fragestellungen untersucht wurde. Die vorliegende Doktorarbeit befasst sich im Einzelnen mit (1) der Ausarbeitung eines jährlich aufgelösten Archives für tropische Zyklone, (2) der Entwicklung eines jährlich aufgelösten SST Datensatzes und (3) einer kompositionellen Quantifizierung der sedimentären Abfolgen sowie einer faziell-stratigraphischen Charakterisierung von Schönwetter-Sedimenten und Sturmlagen. Zu jedem dieser drei Aspekte, wurde jeweils ein Fachartikel bei einer anerkannten wissenschaftlichen Fachzeitschrift mit „peer-review“ Verfahren veröffentlicht.
Der insgesamt 8.55 m lange Sedimentbohrkern („BH6“), der für diese Dissertation untersucht wurde, stammt vom Boden des 125 m tiefen und 320 m breiten „Great Blue Holes“, das sich in der flachen östlichen Lagune des 80 km vor der Küste von Belize (Zentralamerika) gelegenen „Lighthouse Reef“ Atolls befindet. Durch seine besondere Geomorphologie wirkt das, innerhalb des atlantischen „Hurrikan Gürtels“ positionierte, „Great Blue Hole“ wie eine gigantische Sedimentfalle. Die unter Schönwetter-Bedingungen kontinuierlich abgelagerten Abfolgen feinkörniger karbonatischer Sedimente, werden von groben Sturmlagen unterbrochen, die auf „over-wash“ Prozesse von tropischen Zyklonen zurückzuführen sind.
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In this thesis the Quadrupole Electrical Resistivity Tomography (QERT) method is presented as a new measurement concept for profile-based geoelectric field measurements. The concept is based on a tensorial formulation of the apparent resistivity in order to make three-dimensional statements about the underground conductivity structure. For a simple application of the method a number of similarities to the classical dipole-dipole method were made, such as the presentation of the measurement data in a pseudo-section. The added value of the method compared to the classical profile-based methods is especially the differentiation of lateral structures. Anomalies, which are located laterally to the profile, can be detected with respect to their position (left-right) as well as their conductivity contrast. For the practical implementation of the concept a measuring device was developed and constructed, the CR Device. The device uses 64 channels for simultaneous signal recording of voltage and current time series with up to 1 kHz sampling rate. The current injection is freely programmable and allows any survey design. The measurement of the voltages is performed against a common reference (CR) electrode and thus allows the reconstruction of any dipole voltage by difference formation. A complementary, Matlab-based software package completes the measuring system. An evaluation module allows the raw data of the CR device to be read in, processed and displayed in a suitable form. An inversion module allows the inversion of measurement data into a three-dimensional subsurface model. With a modeling module, measurements over any subsurface situation can be simulated and subsequently analysed. A field measurement on a volcanic maar in the Eifel region, Germany, demonstrates the benefits of the method. A QERT profile was set-up tangentially to a conductive anomaly in the centre of the maar. The measurement data were successfully inverted into a geologically coherent 3D resistivity model.
Groundwater is the largest source of accessible freshwater with its dynamics having significantly changed due to human withdrawals, and being projected to continue to as a result of climate change. The pumping of groundwater has led to lowered water tables, decreased base flow, and depletion.
Global hydrological models (GHMs) are used to simulate the global freshwater cycle, assessing impacts of changes in climate and human freshwater use. Currently, groundwater is commonly represented by a bucket-like linear storage component in these models. Bucket models, however, cannot provide information on the location of the groundwater table. Due to this limitation, they can only simulate groundwater discharge to surface water bodies but not recharge from surface water to groundwater and calculate no lateral and vertical groundwater flow whatsoever among grid cells. For instance this may lead to an underestimation of groundwater resources in semiarid areas, where groundwater is often replenished by surface water. In order to overcome these limitations it is necessary to replace the linear groundwater model in GHMs with a hydraulic head gradient-based groundwater flow model
This thesis presents the newly developed global groundwater model G3M and its coupling to the GHM WaterGAP spanning over 70,000 lines of newly developed code. Development and validation of the modeling software are discussed along with numerical challenges. Based on the newly developed software, a global natural equilibrium groundwater model is presented showing better agreements with observations than previous models. Groundwater discharge to rivers is found to be the most dominant flow component globally, compared to flows to other surface water bodies and lateral flows. Furthermore, first global maps of the distribution of gaining and losing surface water bodies are displayed.
For the purpose of determining the uncertainty in model outcomes a sensitivity study is conducted with an innovative approach through applying a global sensitivity analysis for a computationally complex model. First global maps of spatially distributed parameter sensitivities are presented. The results at hand indicate that globally simulated hydraulic heads are equally sensitive to hydraulic conductivity, groundwater recharge and surface water body elevation, even though parameter sensitivities do vary regionally.
A high resolution model of New Zealand is developed to further understand the involved uncertainties connected to the spatial resolution of the global model. This thesis finds that a new understanding is necessary how these models can be evaluated and that a simple increase in spatial resolution is not improving the model performance when compared to observations.
Alongside the assessment of the natural equilibrium, the concept of a fully coupled transient model as integrated storage component replacing the former model in the hydrological model WaterGAP is discussed. First results reveal that the model shows reasonable response to seasonal variability although it contains persistent head trends leading to global overestimates of water table depth due to an incomplete coupling. Nonetheless, WaterGAP-G3M is already able to show plausible long term storage trends for areas that are known to be affected by groundwater depletion. In comparison with two established regional models in the Central Valley the coupled model shows a highly promising simulation of storage declines.
Recently, carbonates have attracted a lot of attention, due to the recognition of their importance in the global carbon cycle. This was enabled by improvement of the experimental techniques that allow for investigating the stability, structure, and physical properties of materials and high-pressures and high-temperatures, that is, they allow for investigating minerals and geochemical processes at the conditions occurring deep inside Earth. Although a lot of research has been focused on carbonates, there are still some open questions regarding their structure and physical properties at such extreme conditions. The aim of this thesis is to establish a deeper understanding of the nature of the phase transitions in carbonates by studying how do the atoms building up the crystal structure vibrate, that is lattice dynamics. The methodology adapted in this study is a combination of experimental and computational methods which allows for a very thorough examination of the problem. The computational approach allows to determine parameters that are elusive or tedious to measure, and the experimental results provide a solid benchmark for the calculations. This tandem of methods has been widely used for investigating lattice dynamics of various materials. In this study it was used to elucidate the structure and properties of carbonates in the deep Earth conditions
The main objective of this PhD work is to assess the impact of fine-scale air-sea interaction on the performance of a regional climate prediction model in marginal sea regions. Focus is on the North and Baltic Seas, the largest marginal sea area in the mid-latitudes. Motivation for this work is to better understand the interaction between the different components of the climate system, namely atmosphere, ocean and sea-ice. In addition to that, the sea regions of interest, the North and Baltic Seas, are orographically complex and cannot be resolved by a global ocean model. The ice coverage on the Baltic Sea is underestimated in the stand-alone atmospheric model COSMO-CLM due to the low water freezing temperature value assumed, which is not applicable for such brackish water body. To fulfil the thesis goal, a new regional coupled atmosphere-ocean-ice system was developed for these two seas, named COSMO-CLM/NEMO. The two-way coupling system involves active feedback from both component models: the limited-area climate model COSMO-CLM and the regional ocean model NEMO-NORDIC.
The coupled system COSMO-CLM/NEMO for the North and Baltic Seas was used to study the impact of sea surface temperature and sea ice on the atmosphere on diffrent topics. The long term impact of the North and Baltic Seas was studied through 15- year long simulations driven by European Center for Medium-Range Weather Forecasts (ECMWF) Interim reanalysis (ERA-Interim) data. Furthermore, to see whether the marginal sea modelling can advance the simulation of extreme climate events, the coupled model was used to reproduce six extreme snowband phenomena over the Baltic Sea in simulations driven by ERA-interim data. Last but not least, the role of the North and Baltic Sea model in improving long-term regional climate prediction was examined. Two sets of experiments with coupled and uncoupled models, each set has five independent decadal hindcasts forced by global climate model, were carried out.
All results were compared with observations and the stand-alone atmospheric model COSMO-CLM results. In all experiments, COSMO-CLM/NEMO showed good agreement with observations. Improvements compared with the uncoupled COSMO-CLM were also found. Coupling was found to affect the air temperature not only around the coupled sea region but also inland. The convective snowbands over the Baltic Sea were successfully reproduced by the coupled model. The high contrast of temperature in the air column, as well as considerably high amounts of surface heat fluxes exchanged between air and sea could not be simulated by COSMO-CLM without the help of reanalysis data. The coupled model also provided better forecasts in decadal scales compared with the uncoupled model and the global model. The added predictability came from the initialized regional seas and better simulated sea surface temperatures by the ocean model.
The impact of the North and Baltic Seas on the climate of the surrounding regions is in certain phases dominated by the North Atlantic Oscillation (NAO) activity. In this thesis, the relation between the NAO and the marginal sea influences was studied. It is confirmed by this study that, in strong phases, the NAO can overpower the impact of the local seas. During dominant phases of NAO, the European climate is mainly governed by large-scale circulation. On the other hand, the local seas play an important role in determining the European climate when NAO is in weak phases.
The added value of the coupled model raises promising perspectives for research in this field. It points to a potential benefit of using the coupled atmosphere-ocean-ice system for climate prediction in the region surrounding the North and Baltic Seas. Along with that, it is still a challenge to complete the model representation of the climate system by adding more climate components (such as a hydrological model). Further improvement of the coupled system can be achieved by coupling for a larger sea region, or by trying to reduce remaining low performance of the coupled model in some areas with a better configuration of the current system.