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Comparing projections of future changes in runoff from hydrological and biome models in ISI-MIP
(2013)
Future changes in runoff can have important implications for water resources and flooding. In this study, runoff projections from ISI-MIP (Inter-sectoral Impact Model Intercomparison Project) simulations forced with HadGEM2-ES bias-corrected climate data under the Representative Concentration Pathway 8.5 have been analysed for differences between impact models. Projections of change from a baseline period (1981–2010) to the future (2070–2099) from 12 impacts models which contributed to the hydrological and biomes sectors of ISI-MIP were studied. The biome models differed from the hydrological models by the inclusion of CO2 impacts and most also included a dynamic vegetation distribution. The biome and hydrological models agreed on the sign of runoff change for most regions of the world. However, in West Africa, the hydrological models projected drying, and the biome models a moistening. The biome models tended to produce larger increases and smaller decreases in regionally averaged runoff than the hydrological models, although there is large inter-model spread. The timing of runoff change was similar, but there were differences in magnitude, particularly at peak runoff. The impact of vegetation distribution change was much smaller than the projected change over time, while elevated CO2 had an effect as large as the magnitude of change over time projected by some models in some regions. The effect of CO2 on runoff was not consistent across the models, with two models showing increases and two decreases. There was also more spread in projections from the runs with elevated CO2 than with constant CO2. The biome models which gave increased runoff from elevated CO2 were also those which differed most from the hydrological models. Spatially, regions with most difference between model types tended to be projected to have most effect from elevated CO2, and seasonal differences were also similar, so elevated CO2 can partly explain the differences between hydrological and biome model runoff change projections. Therefore, this shows that a range of impact models should be considered to give the full range of uncertainty in impacts studies.
The LPJ-GUESS dynamic vegetation model uniquely combines an individual- and patch-based representation of vegetation dynamics with ecosystem biogeochemical cycling from regional to global scales. We present an updated version that includes plant and soil N dynamics, analysing the implications of accounting for C-N interactions on predictions and performance of the model. Stand structural dynamics and allometric scaling of tree growth suggested by global databases of forest stand structure and development were well-reproduced by the model in comparison to an earlier multi-model study. Accounting for N cycle dynamics improved the goodness-of-fit for broadleaved forests. N limitation associated with low N mineralisation rates reduces productivity of cold-climate and dry-climate ecosystems relative to mesic temperate and tropical ecosystems. In a model experiment emulating free-air CO2 enrichment (FACE) treatment for forests globally, N-limitation associated with low N mineralisation rates of colder soils reduces CO2-enhancement of NPP for boreal forests, while some temperate and tropical forests exhibit increased NPP enhancement. Under a business-as-usual future climate and emissions scenario, ecosystem C storage globally was projected to increase by c. 10 %; additional N requirements to match this increasing ecosystem C were within the high N supply limit estimated on stoichiometric grounds in an earlier study. Our results highlight the importance of accounting for C-N interactions not only in studies of global terrestrial C cycling, but to understand underlying mechanisms on local scales and in different regional contexts.
Funded by the German Ministry for Education and Research (BMBF) a major research project called MiKlip (Mittelfristige Klimaprognose, Decadal Climate Prediction) was launched and global as well as regional predictive ensemble hindcasts have been generated. The aim of the project is to demonstrate for past climate change whether predictive models have the capability of predicting climate on time scales of decades. This includes the development of a decadal forecast system, on the one hand to support decision making for economy, politics and society for decadal time spans. On the other hand, the scientific aspect is to explore the feasibility and prospects of global and regional forecasts on decadal time scales. The focus of this paper lies on the description of the regional hindcast ensemble for Europe generated by COSMO-CLM and on the assessment of the decadal variability and predictability against observations. To measure decadal variability we remove the long term bias as well as the long term linear trend from the data. Further, we applied low pass filters to the original data to separate the decadal climate signal from high frequency noise. The decadal variability and predictability assessment is applied to temperature and precipitation data for the summer and winter half-year averages/sums. The best results have been found for the prediction of decadal temperature anomalies, i.e. we have detected a distinct predictive skill and reasonable reliability. Hence it is possible to predict regional temperature variability on decadal timescales, However, the situation is less satisfactory for precipitation. Here we have found regions showing good predictability, but also regions without any predictive skill.
To reconstruct ocean circulation changes during specific periods of Earth history, benthic and planktic foraminifera were used as proxies in the different parts of this thesis. Both studied time periods, the Late Cretaceous and the early Pleistocene, are characterized by long-term climate cooling and major changes in ocean circulation. The first part of this thesis concentrated in the Late Cretaceous. During the Late Cretaceous long-term cooling phase, benthic foraminiferal δ18O values show a positive shift lasting about 1.5 Myr (71.5–70 Ma). This shift can be observed on a global scale and has become known as the Campanian-Maastrichtian Boundary Event (CMBE). It is proposed that this δ18O excursion is influenced either by changing intermediate- to deep-water circulation or by temporal build-up of Antarctic ice sheets. Benthic foraminiferal assemblage counts from a southern high-latitudinal site near Antarctica (ODP Site 690) are analyzed to test if the influence of the CMBE on the benthic species composition. One of the two discussed hypotheses for the causation of the δ18O transition is a change in intermediate- to deep-water circulation from low-latitude to high-latitude water masses. This change would result in cooler temperatures, higher oxygen concentration, and possibly lower organic-matter flux at the seafloor, causing a major benthic foraminiferal assemblage change. Another possible explanation of the δ18O transition of the CMBE is significant ice formation on Antarctica. However no major benthic foraminiferal assemblage change would be expected in this case. The benthic foraminiferal assemblage of Site 690 shows a separation of the studied succession into two parts with significantly different species composition. The older part (73.0–70.5 Ma) is dominated by species, which are typical for lower bottom water oxygen concentration and more common in low-latitude assemblages. Species dominating the younger part (70.0–68.0 Ma) are indicators for well-oxygenated bottom waters and more common in high-latitude assemblages. This change in the benthic foraminiferal assemblages is interpreted to represent a shift of low-latitude toward high-latitude dominated intermediateto deep-water sources. A change in oceanic circulation was therefore at least a major component of the CMBE. The Pacific Ocean contributed significantly to the climatic development during the Late Cretaceous cooling period. The contribution of ocean circulation changes in the Pacific Ocean to the Late Cretaceous climatic development in general and the CMBE and Mid-Maastrichtian Event (MME) in particular, however, is poorly understood. Previously measured high resolution planktic and benthic stable isotope data and a neodymium (Nd) isotope record from the Pacific ODP Site 1210 (Shatsky Rise, tropical Pacific Ocean) for the Campanian to Maastrichtian (69.5 to 72.5 Ma) are used to reconstruct changes in surface- and bottom water temperatures as well as changes in the source region of deep- to intermediate waters [see Appendix 4; Jung et al. 2013]. The results of the benthic foraminiferal δ18O and Nd isotope records in combination with Nd isotope records from other studies indicate changes in the intensity of intermediate- to deep ocean circulation in the tropical Pacific across the Campanian-Maastrichtian interval [see Appendix 4; Jung et al. 2013]. During the early Maastrichtian (72.5 to 69.5 Ma), a three-million-year-long period of cooler conditions and a simultaneous change towards less radiogenic Nd isotope signatures is interpreted to represent a period of increased admixture and northward flow of deep waters from the Southern Ocean (Southern Component Water, SCW). This change was probably caused by an intensified formation of deep waters in the Southern Ocean. This was reduced again during the MME (69.5 to 68.5 Ma). This early Maastrichtian cold interval is similar to the CMBEδ13C fall and succeeding δ13C rise towards the MME and is therefore also interpreted to represent tectonically forced, long-term changes in the global carbon cycle and thus a tectonic forcing of the early Maastrichtian climate cooling. Overall, the Campanian-Maastrichtian Nd and stable isotope records of Shatsky Rise indicate changes in ocean circulation that are paralleled by global warming and cooling periods. The fluctuating strength of SCW contribution in the tropical Pacific points towards an increased respectively weakened ocean circulation, which is probably related to the strength of deep-water formation in the Southern Ocean [see Appendix 4; Jung et al. 2013]. For this study, the analysis of benthic foraminiferal assemblages of Site 1210 is carried out for the same time interval (69.5 to 72.5 Ma) as Nd and stable isotopes to evaluate the influence of intermediate- to deep ocean circulation changes on the benthic foraminiferal community. The possible reaction of benthic foraminiferal assemblages is compared to the results of stable isotope and neodymium isotopes. The observed changes in species abundances only partly reflect the circulation changes reconstructed with Nd and stable oxygen istopes. For example, Stensioina spp., Aragonia spp. and Lenticulina spp., cold-water preferring species, start to be increasingly abundant at the beginning of enhanced influence of SCW. However, their abundance pattern does not follow the varying strength of the cold SCW influence at Shatsky Rise. Other species prefer lesser oxygen concentrations and warmer bottom water, e.g. Paralabamina spp. and Globorotalites spp. Paralabamina spp. has its highest relativ abundance at the beginning of the studied succession, where the influence of SCW is small. However, this taxa occurs throughout the record, even though the influence of SCW increases. Globorotalites spp. is even most abundance after the CMBE, where bottom waters are till cold and influenced by SCW. This leads to the conclusion that the varying strength of SCW in the tropical Pacific at Shatsky Rise through the studied interval is not facilitating a significant faunal turnover as has been observed at the South Atlantic Site 690 (Chapter 3). These results of the benthic foraminiferal assemblage analysis suggest a rather minor influence of the SCW on the major environmental factors that are generally influencing benthic foraminiferal communities (e.g., oxygen concentration, organic matter flux to the sea floor, bottom-water temperature). The second major part of this thesis focused on the late Pliocene-earliest Pleistocene. The late Pliocene is characterized by a long-term global cooling trend resulting in a major increase of Arctic ice sheets from around 3 Ma onwards, culminating in the Plio-Pleistocene intensification of the Northern Hemisphere glaciation. At around 2.7 Ma, large amplitude glacial-interglacial excursions (~1‰ δ18O in benthic foraminiferal calcite) in benthic oxygen isotopes can be observed. Marine isotope stage (MIS) 100 at around 2.55 Ma is the first glacial, when widespread ice rafted debris has been found in sediments in the North Atlantic Ocean. To gain a deeper understanding of the climatic evolution of the latest Pliocene-early Pleistocene, it is necessary to improve the reconstructions of North Atlantic paleohydrography, as the North Atlantic provides a key region for global climate. The consequences of the intensification of Northern Hemisphere on the early Pleistocene North Atlantic thermocline stratification and intermediate waters are still poorly understood. However, surface hydrography, the history of the thermocline and development of North Atlantic intermediate waters are well-studied for the Last Glacial Maximum (LGM). These well-known mechanisms responsible for the LGM in comparison with the present-day interglacial North Atlantic are used as an analogue for te early Pleistocene glacialinterglacials cycles. In this study, suborbitally resolved stable oxygen and carbon isotope and Mg/Ca records are measured from a deep-dwelling planktic foraminifera (Globorotaliacrassaformis) from Integrated Ocean Drilling Program Site U1313 (North Atlantic, 41°N) covering marine oxygen isotope stages MIS 103 to 95 (early Pleistocene, 2.6 to 2.4 Ma). The results are interpreted to represent a change in intermediate-water masses on glacialinterglacial timescales. During glacials geochemical records in G. crassaformis (~500–1000 m) bear the imprint of Glacial North Atlantic Intermediate Water (GNAIW), while during interglacials this species reflects the signature of the influence of Mediterranean Outflow Water (MOW) in combination with the subtropical gyre. The comparison of this data with the published records from G. ruber from the same samples facilitates the reconstruction of glacial-interglacial stratification changes of the upper water column at Site U1313. The results show that larger gradients of temperature, salinity and δ13C prevailed during glacials, suggesting a stronger stratification of the upper water column. This can be seen to indicate glacial-interglacial changes in ntermediate water masses in the North Atlantic similar to those reconstructed for the latest Pleistocene. As an additional proxy, the clumped isotope paleothermometer is applied for the Late Cretaceous study as well as for the early Pleistocene. This proxy is commonly assumed to be independent of other factors than temperature. Clumped isotopes are measured for the Late Cretaceous Site 690 on the planktic foraminiferal species Archaeoglobigerina australis and compared to already existing stable oxygen isotopes of this species. This is assumed to enable the reconstruction of paleotemperature independent of ice volume and therefore contribute to the long-lasting discussion whether there was a temporal ice build-up on Antarctic during the Campanian-Maastrichtian cooling period. For the early Pleistocene, the planktic foraminiferal species G. crassaformis is used from Site U1313 from MIS 99 (interglacial) and MIS 98 (glacial). This provides the opportunity to separate ice volume, salinity and temperature effects on the measured δ18O record of G. crassaformis. The results of the clumped isotope measurements reveal comparatively large standard errors. For the Late Cretaceous the standard error of the clumped isotope measurements proved too large to allow any conclusions on the temperature component on the δ18O record of A. australis. For the early Pleistocene, the temperature difference is also too small to be reconstructed with the standard error of the clumped isotope measurements in this study. Measuring many replicates of one sample would minimize the standard error considerably. However, the amount necessary to measure replicates cannot be gained for either time period, as almost all foraminifera were picked from the respective samples. It is concluded that the respective questions may be solved with a different method of clumped isotope analysis requiring less sample material. This method is, for example, available at the ETH Zurich.
Mantle convection is the process by which heat from the Earth’s core is transferred upwards to the surface and it is accepted to explain the dynamics of the Earth’s interior. On geological time-scales, mantle material flows like a viscous fluid as a consequence of the buoyancy forces arising from thermal expansion. Indeed, mantel convection provides a framework which links together the major disciplines, such as seismology, mineral physics, geochemistry tectonic and geology. The numerical model has been applied to understand the dynamic, structure and evaluation of the Earth, and other terrestrial planets and the investigations continue to explore, different aspects of the mantle convection.
In fact, to model this phenomenon, two complementary approaches are possible. On the one hand, one can solve self-consistently the equations of thermal convection, including parameters and employing physical relationships derived from mineral physics. Our understanding of mantle convection depends ultimately upon the success of such fully self-consistent dynamic models in explaining observable features of the flow. Although, these models presently unable to predict the actual convection pattern of the Earth, they are extremely useful to investigate general characteristics of given physical systems. On the other hand, to permit comparison with specific observables associated with the flow, one can consider a more restricted problem. Instead of focusing on the time evolution of mantle flow, if we know a priori the temperature - and hence presumably the density - anomalies that drive the convection, we can try to build a snapshot of the present-day flow pattern, consistent with those anomalies, that can successfully predict the observables. As matter of fact, the aim of this study is to investigate both approaches in comparison with the main geophysical constraints on mantle structure. These constraints include the geoid anomalies, the dynamic surface and core-mantle boundary topography and tectonic plate motions.
The most appropriate mathematical basis functions for describing a bounded and continuous function on a spherical surface are spherical harmonics. We may therefore expand the geodynamic observables in terms of spherical harmonics. We have investigated two methods of the global spherical harmonic analysis by specific attention to the dynamic geoid computation of the geodynamic models. The first method is the quadrature method in which the loss of the orthogonality of the Legendre functions in transition from continues to discrete case is the major drawback to the method. Particularly, we showed that in the absence of the tesseral harmonics, quadrature formulation leads to obtain inaccurate results. The second method is the least-squares which can be considered as the best linear unbiased estimator that provides the exact results. We showed that even with a low resolution grid data it is possible to reconstruct the data and achieve an accurate result by using this method, which is extremely remarkable in three-dimensional global convection studies. However, special care has to be taken since there is some source of errors that might influence the efficiency of this method.
In general, to better understanding of the properties of the mantle, it is useful to assess observable characteristics of plumes in the mantle, including geoid, topography and heat flow anomalies. However, only few studies exist on geoid and topography for axi-symmetric convection and their models were restricted to isoviscous (or stratified) mantle and low Rayleigh numbers. We studied fully coupled depth and temperature dependent Arrhenius type of viscosity in axi-symmetric spherical shell geometry in order to investigate the shape of geoid anomalies and dynamic topography above a plume. Indeed, the topography and geoid anomalies produced from plumes are sensitive to rheology of the mantle and rheology of the plume; both have effects on shape and amplitude of the geoid anomalies. As results we are able to define different classes of plumes by their geoid signals.
Mainly depth-dependent viscosity models show a geoid with negative sign above the plume which can turn to the positive sign by decrease the viscosity contrast. This can be considered as a transition between the strongly depth dependent and the constant viscosity case. Our results basically support the idea by Morgan [1965] and McKenzie [1977]. They have shown the magnitude and even the sign of the total gravity anomaly depend on the spatial variation in effective viscosity. In addition, Hager [1984] has concluded that the total gravity field is depend on the radial distribution of effective viscosity, and a small change in viscosity contrast leads to varying sign of the response function.
In the case of temperature-dependent viscosity, the formation of an immobile lithosphere is a natural result, and the flow as well as the total geoid becomes strongly time dependent. When we increase the activation energy, all geoids associated with the first arriving plumes look like bell shaped whereas for typical plumes, after reaching a statistical steady state, bell-shaped geoids with decreasing amplitude as well as linear flank shaped geoids are observed. It is surprising that in spite of large differences in lateral and depth varying viscosities, the shapes of the geoid anomalies remained rather similar. We also identified different behaviors in the combined model with temperature-and pressure-dependent viscosity. In fact, in spite of the strongly different rheology, the geoid anomalies in all cases were surprisingly similar. Furthermore, we proposed a scaling law for the geoid which makes our results directly applicable to other planets. Moreover, we can apply the results of our calculation to find relations between different rheology and sub-lid temperature, since we know that the mantle temperature can change significantly with variation in pressure-temperature dependent viscosity. It is also possible to define a range of stagnant lid thickness related to the amplitude of the geoid which can be reasonable for study of the lid thickness in Venus or Mars.
Nevertheless, in these series of models, we simplified a number of complexities within the Earth. One of the most important of such simplification is the Boussinesq approximation. This approximation is valid if the temperature scale height (i.e. the depth over which temperature increases by a factor of “ ” due to adiabatic compression) is much greater than the convection depth. However, a temperature scale height in the Earth’s mantle is at best only slightly greater than the mantle depth. Hence, the Boussinesq approximation could mask some very important stratification and compressibility effects that influence both the spatial and temporal structure of the convection. Therefore, in more advance models we considered compressibility in our mantle convection models, assuming that density vary both radially and laterally, being determined as a function of pressure and temperature through an appropriate equation of the state. Moreover, thermodynamic properties assumed to be a function of depth.
We examined the details of the structure of the spherical axi-symmetric Anelastic Liquid Approximation model (ALA) with special attention to the Arrhenius rheology, and compare it to the cases of compressible convection without depth dependent thermodynamical properties, and to cases of the extended Boussinesq approximation. At the same time, the effects of the interaction between temperature and pressure-dependent viscosity and thermodynamic parameters in the compressible mantle convection on the geoid and topography have been studied. We showed that assuming compressible convection with depth-dependent thermodynamic properties strongly influence the geoid undulations. Using compressible convection with constant thermodynamic properties is physically inconsistent and may lead to spurious results for the geoid and convection pattern. Indeed, by a systematic study of different approaches of compressibility in the spherical shell convection for different Arrhenius viscosity laws we proved that only in the unrealistic case of zero activation energy the different compressibility modes result in comparable convection and geoid patterns. In all other rheological cases, large differences have been obtained, that stressing the important role of consistent compressible thermodynamic properties for mantle convection.
In addition, we examine the impact of compressibility as well as different rheologies on the power law relation that connects the Nusselt number to the Rayleigh number. We have discovered that the power law index of the relationship is controlled by the rheology, independent of which approximation is used. Instead, the bound of this relation is controlled by a combination of different approximation and rheology.
Next, instead of focusing on the time evolution of mantle flow, we have carried out three-dimensional spherical shell models of mantle circulation to investigate the effects of joint radial and lateral viscosity variations on the Earth’s non-hydrostatic geoid, surface and core-mantle boundary topographies. These models include realistic lateral viscosity variations (LVV) in the lithosphere, upper mantle and lower mantle in combination with different stratified viscosity structures. We have demonstrated that the contradictory results concerning the effects of LVV can be clarified by the most straight-forward problem in geoid modeling; namely, rather poorly known stratified viscosity structure. We explored three classes of dynamic geoid models due to lateral viscosity variations. In the first class, the LVV strongly improved the fit to the observed geoid. Indeed, when the viscosity contrast between lower and upper mantles is not large enough to produce a good fit to geoid the LVVs are able to perform this action by adjusting amplitudes, so that it becomes comparable with observation. In the second class, inducing the LVV moderately improved the fit. Actually, when the geoid induced by a stratified viscosity structure already has a good correlation with observation, then the LVV causes its amplitude to further improve. In the last class, if the viscosity contrast between upper and lower mantle would be high enough, inducing LVV deteriorate the fit to the observed geoid.. Indeed, depending on the stratified viscosity, inducing the LVV may take place in one of these categories.
We also quantified the effects of LVV in the mantle and lithosphere individually. We found that the presence of LVV in the mantle (upper and lower) improves the fit to the observed geoid regardless of stratified viscosity. While LVV in the lithosphere is a crucial parameter, and dependent of the stratified viscosity, may increase or decrease the geoid fit. In fact, when the lower mantle considers being viscous enough, it would support the negative buoyancy of subducting slabs. Thus, it transmits some of the stress back to the top boundary and causes a weak coupling between slab and surface. Therefore, by including the low viscous plate boundaries in this model, the slabs and overriding plates decouples and the fit to the observed geoid degrades. In contrast, when the lower mantle viscosity is not sufficiently stiff, the presence of the low viscous plate boundaries assists to weaken the strong mechanical coupling between slab and surface. Hence, a better fit achieved.
In this study we show how size-resolved measurements of aerosol particles and cloud condensation nuclei (CCN) can be used to characterize the supersaturation of water vapor in a cloud. The method was developed and applied for the investigation of a cloud event during the ACRIDICON-Zugspitze campaign (17 September to 4 October 2012) at the high-alpine research station Schneefernerhaus (German Alps, 2650 m a.s.l.). Number size distributions of total and interstitial aerosol particles were measured with a scanning mobility particle sizer (SMPS), and size-resolved CCN efficiency spectra were recorded with a CCN counter system operated at different supersaturation levels.
During the evolution of a cloud, aerosol particles are exposed to different supersaturation levels. We outline and compare different estimates for the lower and upper bounds (Slow, Shigh) and the average value (Savg) of peak supersaturation encountered by the particles in the cloud. For the investigated cloud event, we derived Slow ≈ 0.19–0.25%, Shigh ≈ 0.90–1.64% and Savg ≈ 0.38–0.84%. Estimates of Slow, Shigh and Savg based on aerosol size distribution data require specific knowledge or assumptions of aerosol hygroscopicity, which are not required for the derivation of Slow and Savg from the size-resolved CCN efficiency spectra.
Das Thema dieser Arbeit war die Untersuchung der natürlichen Variationen von den zwei primordialen Uranisotopen (238U und 235U) mit einem Schwerpunkt auf Proben, die (1) die kontinentale Kruste und ihre Verwitterungsprodukte (d.h. Granite, Shales und Flusswasser) repräsentieren, (2) Produkte der hydrothermalen Alteration vom mittelozeanischen Rücken widerspiegeln (d.h. alterierte Basalte, Karbonatgänge und hydrothermales Wasser) und (3) aus abgegrenzten euxinischen Becken (d.h. Proben aus der Wassersäule und den dazugehörigen Sedimenten) stammen. Das allgemeine Ziel war das Verständnis, unter welchen Bedingungen und Mechanismen eine Fraktionierung der zwei häufigsten Uranisotope (238U und 235U) in der Natur erfolgt, zu verbessern.
Die untersuchten Haupt- und Nebenflüsse unterscheiden sich sowohl in Ihrer Urankonzentration (c(U)) als auch in Ihrer Uranisotopenzusammensetzung (δ238U), wobei die Nebenflüsse eine geringere Urankonzentration (0.87 nmol/kg bis 3.08 nmol/kg) und eine schwerere Uranisotopenzusammensetzung aufweisen (-0.29 ‰ bis +0.01 ‰ im δ238U) im Vergleich zu den Hauptflüssen (c(U) = 5.19 nmol/kg bis 11.69 nmol/kg und d238U = -0.31 ‰ bis +0.13 ‰) aufweisen. Die untersuchten Gesteinsproben fallen alle in einen recht schmalen Bereich von δ238U, zwischen -0.45 ‰ und -0.21 ‰, mit einem Durchschnittswert von -0.30 ‰ ± 0.04 ‰ (doppelte Standardabweichung). Deren Uranisotopenvariationen sind unabhängig von der Urankonzentration (11.8 µg/g bis 1.3 µg/g), dem Alter (3.80 Ga bis 328 Ma), der Probenlokalität und Grad der Differenzierung. Basierend auf den Ergebnissen der Hauptflüsse, die die Uranhauptquelle für den Ozean darstellen, schlagen wir für zukünftige Berechnungen in der Massenbilanz des Urans einen neuen Wert als beste Abschätzung für die Quelle des Urans im Ozean vor, δ238U = -0.23 ‰.
Die Produkte der hydrothermalen Alteration, alterierte Basalte und Kalziumkarbonatgänge, zeigten etwas stärkere Isotopenvariationen (δ238U zwischen -0.63 ‰ und +0.27 ‰) als erwartet und die hydrothermalen Fluide wiesen eine etwas leichtere Uranisotopenzusammensetzung als Meerwasser ((-0.43 ± 0.25) ‰ vs. (-0.37 ± 0.03) ‰) auf. Diese Ergebnisse sind in Übereinstimmung mit einem Modell, dass annimmt, dass die beobachtete Isotopenfraktionierung hauptsächlich ein Ergebnis von Redoxprozessen ist, z.B. die partielle Reduktion von löslichem UVI aus dem Meerwasser während der hydrothermalen Alteration, was zu einer Anreicherung der schweren Uranisotope in der reduzierten Uranspezies (UIV) führt und 2) das bevorzugte Entfernen von UIV aus den hydrothermalen Fluid und der Einbau in die alterierte ozeanische Kruste. Durch diesen Prozess wird das hydrothermale Fluid an schweren Uranisotopen verarmt und somit würden auch die alterierten Basalte und Karbonate ein niedriges δ238U aufweisen, wenn sie mit dem isotopisch leichten hydrothermalen Fluid in Kontakt gekommen sind.
Die Untersuchung von Wasser- und Sedimentproben aus der Ostsee und dem anoxischen Kyllaren Fjord (Norwegen) auf deren Uran- und Mo-Isotopenzusammensetzung zeigte, dass die Uranisotopenzusammensetzung der Sedimente abhängt von (1) dem Ausmaß des Uranaustrags aus der Wassersäule (in einer ähnlichen Art und Weise wie bei den Molybdänisotopen) und (2) der Sedimentationsrate, d.h. der Fraktion von authigenem- relativ zum dedritischen Uran in den Sedimenten. Aufgrund der hohen Sedimentationsrate zeigen die Sedimente aus dem Kyllaren Fjord nur eine moderate authigene Urananreicherung und eine leichtere Uranisotopenzusammensetzung als Sedimente aus dem Schwarzen Meer. In den anoxischen Becken der Ostsee erfolgt dagegen eine starke Mo- und schwache U-Isotopenfraktionierung zwischen Wasser und Sediment. Durch die regelmäßigen auftretenden Spülereignisse mit sauerstoffreichem Wasser wurden vermutlich die ursprünglichen anoxischen Mo- und U-Isotopensignaturen der Sedimente verändert. Demzufolge müssen die Sedimente durchgehend anoxischen Bedingungen ausgesetzt sein, um eine Mo- und U-Isotopensignatur von den Redoxbedingungen während der Ablagerungen zu speichern.
Der Vergleich zwischen Molybdän- und Uranisotopen in der Ostsee und dem anoxischen Kyllaren Fjord zeigte, dass sich Uran- und Molybdänisotope in stark euxinischen Wassersäulen (c(H2S) > 11 µmol/L) entgegengesetzt verhalten. Dementsprechend ergänzen sich die beiden Isotopensysteme und können genutzt werden, um die Ablagerungsbedingungen in abgeschlossenen Becken und die Redoxentwicklung des Paläoozeans zu untersuchen.
Gridded maps of meteorological variables are needed for the evaluation of weather and climate models and for climate change monitoring. In order to produce them, values at locations where no observing stations are available need to be estimated from point-wise observations. For the interpolation of meteorological observations deterministic and stochastic methods are often combined. Deterministic methods can account for ancillary information such as elevation, continentality or satellite observations. Stochastic methods such as kriging reproduce observed values at the station locations and also account for spatial variability. In the first two studies of this thesis, a flexible interpolation method for the gridding of locally observed daily extreme temperatures is developed that also provides an optimal estimate of the interpolation ncertainty. In the third study, an observational dataset is created using this interpolation method and then applied to evaluate a climate simulation for Africa.
In the first study, the Regression-Kriging-Kriging (RKK) method is tested for the interpolation of daily minimum and maximum temperatures (Tmin and Tmax) in different regions in Europe. RKK accounts for elevation, continentality index and zonal mean temperature and is applicable in regions of differing station density and climate. The accuracy of RKK is compared to Inverse Distance Weighting, a common deterministic interpolation method, and to Ordinary Kriging, a common stochastic interpolation method. The first step in RKK is to use regression kriging, in which multiple linear regression accounts for topographical effects on the temperature field and kriging minimizes the regression error, to interpolate climatological means. In the second step daily deviations from the monthly climatology are interpolated using simple kriging. Owing to the large climatological differences across the investigation area the interpolation is performed in homogeneous subregions defined according to the Köppen-Geiger climate classification. Cross validation demonstrates the superiority of RKK over the simpler algorithms in terms of accuracy and preservation of spatial variability. The interpolation performance however strongly varies across Europe, being considerably higher over Central Europe (highest station density) than over Greenland (few stations along the coast line). This illustrates the strong impact of the station density on the accuracy of the interpolation result. Satellites provide comprehensive observations of climate variables such as land surface temperature (LST) and cloud cover (CC). However, LST is associated with high uncertainty (standard error ~ 1-2°C), preventing its direct application in meteorology and climatology. The second study investigates the usefulness of LST and CC as predictors for the gridding of daily Tmin and Tmax. The RKK algorithm is compared with similar interpolation methods that apply LST and CC in addition to the predictors used with the RKK algorithm. The investigation is conducted in two regions, Central Europe and the Iberian Peninsula, which differ strongly in average cloud cover (Central Europe is approximately 30% cloud free and the Iberian Peninsula approximately 60 % cloud free). RKKLST (in which monthly mean LST is used as an additional predictor) yields for Central Europe no clear improvement over RKK, yet it reduces the interpolation error over the Iberian Peninsula. This finding can be explained by the higher percentage of cloud free pixels over that region in summer which enables a more robust determination of monthly mean LST. Adding a regression step for daily anomalies (using the predictor CC) yields the RKRK method and improves the preservation of spatial variability over the Iberian Peninsula. Moreover, a successive reduction of the station number (from 140 to 10 stations) reveals an increasing superiority of RKKLST and RKRK over RKK in both regions.
The application of a gridded observational dataset for climate monitoring or climate model validation requires knowledge of the uncertainties associated with the dataset. The estimation of the interpolation uncertainty, here the inter quartile range is the used uncertainty measure, is therefore an important issue within the frame of this thesis. By means of cross validation it is shown that the largest uncertainties occur in regions of low station density (e.g. Greenland), in mountainous regions and along coastlines (in these regions model evaluation results should be interpreted carefully). The magnitude of the interpolation error mainly depends on the station density, while the complexity of terrain has substantially less influence. On average over all regions and investigation days the target precision of the uncertainty estimate is reached. However, on local scales and for single days it can be clearly over- or underestimated. The application of satellite-derived predictors (LST and CC) yields no noteworthy improvement of the uncertainty estimate.
In the last study two regional climate simulations for Africa using the ERA-Interim driven COSMO-CLM (CCLM) model at two different horizontal resolutions (0.22° and 0.44°) are validated. It is assessed whether observed patterns and statistical properties of daily Tmin and Tmax are correctly represented in the model. The ERA-Interim reanalysis and a specially created observational dataset are used as reference. The observational dataset is generated by applying the RKRK algorithm (developed within the second study). The investigations show an occasionally large bias in Tmin and Tmax. The hemispheric summers are generally too warm and the temporal variability in temperature is too high, particularly over extra tropical Africa. The diurnal temperature range is overestimated by about 2°C in the northern subtropics but underestimated by about 2°C over large parts of the African tropics. CCLM reproduces the observed frequency distribution of daily Tmin and Tmax in all African climate regions, and the extreme values in the lower percentiles (5, 10, 20%) for Tmin are well simulated. The higher percentiles (80, 90, 95%) for Tmax are however overestimated by 2-5°C. For both Tmin and Tmax the 0.22° simulation is on average 0.5°C warmer than the 0.44° simulation. Additionally, the higher percentiles are about 1°C warmer for both Tmin and Tmax in the higher resolution run, while the lower percentiles in both runs match very well. Although the temperature pattern is represented in more detail along the coastlines and in topographically complex regions, the higher resolution simulation yields no qualitative improvement.
To summarize, the choice of the appropriate algorithm mainly depends on the interpolation conditions. In cases where the station density is high across the target region and the predictor space is adequately covered by observing stations, the computationally less demanding RK algorithm should be preferred. In regions where the station density is low the more robust RKRK algorithm should be the first choice. Due to the strong physical relation of both CC and LST to Tmin and Tmax the missing information is at least partially compensated for. The estimation of the interpolation uncertainty could be improved by applying a normal score transformation to the data prior to a kriging step. This is because the kriging assumption that the increments of the variable of interest are second order stationary can be approximately met by a normal score transformation.
This study presents a method for adjusting long-term climate data records (CDRs) for the integrated use with near-real-time data using the example of surface incoming solar irradiance (SIS). Recently, a 23-year long (1983–2005) continuous SIS CDR has been generated based on the visible channel (0.45–1 μm) of the MVIRI radiometers onboard the geostationary Meteosat First Generation Platform. The CDR is available from the EUMETSAT Satellite Application Facility on Climate Monitoring (CM SAF). Here, it is assessed whether a homogeneous extension of the SIS CDR to the present is possible with operationally generated surface radiation data provided by CM SAF using the SEVIRI and GERB instruments onboard the Meteosat Second Generation satellites. Three extended CM SAF SIS CDR versions consisting of MVIRI-derived SIS (1983–2005) and three different SIS products derived from the SEVIRI and GERB instruments onboard the MSG satellites (2006 onwards) were tested. A procedure to detect shift inhomogeneities in the extended data record (1983–present) was applied that combines the Standard Normal Homogeneity Test (SNHT) and a penalized maximal T-test with visual inspection. Shift detection was done by comparing the SIS time series with the ground stations mean, in accordance with statistical significance. Several stations of the Baseline Surface Radiation Network (BSRN) and about 50 stations of the Global Energy Balance Archive (GEBA) over Europe were used as the ground-based reference. The analysis indicates several breaks in the data record between 1987 and 1994 probably due to artefacts in the raw data and instrument failures. After 2005 the MVIRI radiometer was replaced by the narrow-band SEVIRI and the broadband GERB radiometers and a new retrieval algorithm was applied. This induces significant challenges for the homogenisation across the satellite generations. Homogenisation is performed by applying a mean-shift correction depending on the shift size of any segment between two break points to the last segment (2006–present). Corrections are applied to the most significant breaks that can be related to satellite changes. This study focuses on the European region, but the methods can be generalized to other regions. To account for seasonal dependence of the mean-shifts the correction was performed independently for each calendar month. In comparison to the ground-based reference the homogenised data record shows an improvement over the original data record in terms of anomaly correlation and bias. In general the method can also be applied for the adjustment of satellite datasets addressing other variables to bridge the gap between CDRs and near-real-time data.
When studying new particle formation, the uncertainty in determining the "true" nucleation rate is considerably reduced when using condensation particle counters (CPCs) capable of measuring concentrations of aerosol particles at sizes close to or even at the critical cluster size (1–2 nm). Recently, CPCs able to reliably detect particles below 2 nm in size and even close to 1 nm became available. Using these instruments, the corrections needed for calculating nucleation rates are substantially reduced compared to scaling the observed formation rate to the nucleation rate at the critical cluster size. However, this improved instrumentation requires a careful characterization of their cut-off size and the shape of the detection efficiency curve because relatively small shifts in the cut-off size can translate into larger relative errors when measuring particles close to the cut-off size.
Here we describe the development of two continuous-flow CPCs using diethylene glycol (DEG) as the working fluid. The design is based on two TSI 3776 counters. Several sets of measurements to characterize their performance at different temperature settings were carried out. Furthermore, two mixing-type particle size magnifiers (PSM) A09 from Airmodus were characterized in parallel. One PSM was operated at the highest mixing ratio (1 L min−1 saturator flow), and the other was operated in a scanning mode, where the mixing ratios are changed periodically, resulting in a range of cut-off sizes. The mixing ratios are determined by varying the saturator flow, where the aerosol flow stays constant at 2.5 L min−1. Different test aerosols were generated using a nano-differential mobility analyser (nano-DMA) or a high-resolution DMA, to obtain detection efficiency curves for all four CPCs. One calibration setup included a high-resolution mass spectrometer (APi-TOF) for the determination of the chemical composition of the generated clusters. The lowest cut-off sizes were achieved with negatively charged ammonium sulfate clusters, resulting in cut-offs of 1.4 nm for the laminar flow CPCs and 1.2 and 1.1 nm for the PSMs. A comparison of one of the laminar-flow CPCs and one of the PSMs measuring ambient and laboratory air showed good agreement between the instruments.