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The Cauchy problem for the linearised Einstein equation and the Goursat problem for wave equations
(2017)
In this thesis, we study two initial value problems arising in general relativity. The first is the Cauchy problem for the linearised Einstein equation on general globally hyperbolic spacetimes, with smooth and distributional initial data. We extend well-known results by showing that given a solution to the linearised constraint equations of arbitrary real Sobolev regularity, there is a globally defined solution, which is unique up to addition of gauge solutions. Two solutions are considered equivalent if they differ by a gauge solution. Our main result is that the equivalence class of solutions depends continuously on the corre- sponding equivalence class of initial data. We also solve the linearised constraint equations in certain cases and show that there exist arbitrarily irregular (non-gauge) solutions to the linearised Einstein equation on Minkowski spacetime and Kasner spacetime.
In the second part, we study the Goursat problem (the characteristic Cauchy problem) for wave equations. We specify initial data on a smooth compact Cauchy horizon, which is a lightlike hypersurface. This problem has not been studied much, since it is an initial value problem on a non-globally hyperbolic spacetime. Our main result is that given a smooth function on a non-empty, smooth, compact, totally geodesic and non-degenerate Cauchy horizon and a so called admissible linear wave equation, there exists a unique solution that is defined on the globally hyperbolic region and restricts to the given function on the Cauchy horizon. Moreover, the solution depends continuously on the initial data. A linear wave equation is called admissible if the first order part satisfies a certain condition on the Cauchy horizon, for example if it vanishes. Interestingly, both existence of solution and uniqueness are false for general wave equations, as examples show. If we drop the non-degeneracy assumption, examples show that existence of solution fails even for the simplest wave equation. The proof requires precise energy estimates for the wave equation close to the Cauchy horizon. In case the Ricci curvature vanishes on the Cauchy horizon, we show that the energy estimates are strong enough to prove local existence and uniqueness for a class of non-linear wave equations. Our results apply in particular to the Taub-NUT spacetime and the Misner spacetime. It has recently been shown that compact Cauchy horizons in spacetimes satisfying the null energy condition are necessarily smooth and totally geodesic. Our results therefore apply if the spacetime satisfies the null energy condition and the Cauchy horizon is compact and non-degenerate.
In littoral zones of lakes, multiple processes determine lake ecology and water quality. Lacustrine groundwater discharge (LGD), most frequently taking place in littoral zones, can transport or mobilize nutrients from the sediments and thus contribute significantly to lake eutrophication. Furthermore, lake littoral zones are the habitat of benthic primary producers, namely submerged macrophytes and periphyton, which play a key role in lake food webs and influence lake water quality. Groundwater-mediated nutrient-influx can potentially affect the asymmetric competition between submerged macrophytes and periphyton for light and nutrients. While rooted macrophytes have superior access to sediment nutrients, periphyton can negatively affect macrophytes by shading. LGD may thus facilitate periphyton production at the expense of macrophyte production, although studies on this hypothesized effect are missing.
The research presented in this thesis is aimed at determining how LGD influences periphyton, macrophytes, and the interactions between these benthic producers. Laboratory experiments were combined with field experiments and measurements in an oligo-mesotrophic hard water lake.
In the first study, a general concept was developed based on a literature review of the existing knowledge regarding the potential effects of LGD on nutrients and inorganic and organic carbon loads to lakes, and the effect of these loads on periphyton and macrophytes. The second study includes a field survey and experiment examining the effects of LGD on periphyton in an oligotrophic, stratified hard water lake (Lake Stechlin). This study shows that LGD, by mobilizing phosphorus from the sediments, significantly promotes epiphyton growth, especially at the end of the summer season when epilimnetic phosphorus concentrations are low. The third study focuses on the potential effects of LGD on submerged macrophytes in Lake Stechlin. This study revealed that LGD may have contributed to an observed change in macrophyte community composition and abundance in the shallow littoral areas of the lake. Finally, a laboratory experiment was conducted which mimicked the conditions of a seepage lake. Groundwater circulation was shown to mobilize nutrients from the sediments, which significantly promoted periphyton growth. Macrophyte growth was negatively affected at high periphyton biomasses, confirming the initial hypothesis.
More generally, this thesis shows that groundwater flowing into nutrient-limited lakes may import or mobilize nutrients. These nutrients first promote periphyton, and subsequently provoke radical changes in macrophyte populations before finally having a possible influence on the lake’s trophic state. Hence, the eutrophying effect of groundwater is delayed and, at moderate nutrient loading rates, partly dampened by benthic primary producers. The present research emphasizes the importance and complexity of littoral processes, and the need to further investigate and monitor the benthic environment. As present and future global changes can significantly affect LGD, the understanding of these complex interactions is required for the sustainable management of lake water quality.
The classical Navier-Stokes equations of hydrodynamics are usually written in terms of vector analysis. More promising is the formulation of these equations in the language of differential forms of degree one. In this way the study of Navier-Stokes equations includes the analysis of the de Rham complex. In particular, the Hodge theory for the de Rham complex enables one to eliminate the pressure from the equations. The Navier-Stokes equations constitute a parabolic system with a nonlinear term which makes sense only for one-forms. A simpler model of dynamics of incompressible viscous fluid is given by Burgers' equation. This work is aimed at the study of invariant structure of the Navier-Stokes equations which is closely related to the algebraic structure of the de Rham complex at step 1. To this end we introduce Navier-Stokes equations related to any elliptic quasicomplex of first order differential operators. These equations are quite similar to the classical Navier-Stokes equations including generalised velocity and pressure vectors. Elimination of the pressure from the generalised Navier-Stokes equations gives a good motivation for the study of the Neumann problem after Spencer for elliptic quasicomplexes. Such a study is also included in the work.We start this work by discussion of Lamé equations within the context of elliptic quasicomplexes on compact manifolds with boundary. The non-stationary Lamé equations form a hyperbolic system. However, the study of the first mixed problem for them gives a good experience to attack the linearised Navier-Stokes equations. On this base we describe a class of non-linear perturbations of the Navier-Stokes equations, for which the solvability results still hold.
Einleitung: Die Erdnussallergie zählt zu den häufigsten Nahrungsmittelallergien im Kindesalter. Bereits kleine Mengen Erdnuss (EN) können zu schweren allergischen Reaktionen führen. EN ist der häufigste Auslöser einer lebensbedrohlichen Anaphylaxie bei Kindern und Jugendlichen. Im Gegensatz zu anderen frühkindlichen Nahrungsmittelallergien entwickeln Patienten mit einer EN-Allergie nur selten eine natürliche Toleranz. Seit mehreren Jahren wird daher an kausalen Therapiemöglichkeiten für EN-Allergiker, insbesondere an der oralen Immuntherapie (OIT), geforscht. Erste kleinere Studien zur OIT bei EN-Allergie zeigten erfolgsversprechende Ergebnisse. Im Rahmen einer randomisierten, doppelblind, Placebo-kontrollierten Studie mit größerer Fallzahl werden in der vorliegenden Arbeit die klinische Wirksamkeit und Sicherheit dieser Therapieoption bei Kindern mit EN-Allergie genauer evaluiert. Des Weiteren werden immunologische Veränderungen sowie die Lebensqualität und Therapiebelastung unter OIT untersucht.
Methoden: Kinder zwischen 3-18 Jahren mit einer IgE-vermittelten EN-Allergie wurden in die Studie eingeschlossen. Vor Beginn der OIT wurde eine orale Provokation mit EN durchgeführt. Die Patienten wurden 1:1 randomisiert und entsprechend der Verum- oder Placebogruppe zugeordnet. Begonnen wurde mit 2-120 mg EN bzw. Placebo pro Tag, abhängig von der Reaktionsdosis bei der oralen Provokation. Zunächst wurde die tägliche OIT-Dosis alle zwei Wochen über etwa 14 Monate langsam bis zu einer Erhaltungsdosis von mindestens 500 mg EN (= 125 mg EN-Protein, ~ 1 kleine EN) bzw. Placebo gesteigert. Die maximal erreichte Dosis wurde dann über zwei Monate täglich zu Hause verabreicht. Im Anschluss erfolgte erneut eine orale Provokation mit EN. Der primäre Endpunkt der Studie war die Anzahl an Patienten der Verum- und Placebogruppe, die unter oraler Provokation nach OIT ≥1200 mg EN vertrugen (=„partielle Desensibilisierung“). Sowohl vor als auch nach OIT wurde ein Hautpricktest mit EN durchgeführt und EN-spezifisches IgE und IgG4 im Serum bestimmt. Außerdem wurden die Basophilenaktivierung sowie die Ausschüttung von T-Zell-spezifischen Zytokinen nach Stimulation mit EN in vitro gemessen. Anhand von Fragebögen wurde die Lebensqualität vor und nach OIT sowie die Therapiebelastung während OIT erfasst.
Ergebnisse: 62 Patienten wurden in die Studie eingeschlossen und randomisiert. Nach etwa 16 Monaten unter OIT zeigten 74,2% (23/31) der Patienten der Verumgruppe und nur 16,1% (5/31) der Placebogruppe eine „partielle Desensibilisierung“ gegenüber EN (p<0,001). Im Median vertrugen Patienten der Verumgruppe 4000 mg EN (~8 kleine EN) unter der Provokation nach OIT wohingegen Patienten der Placebogruppe nur 80 mg EN (~1/6 kleine EN) vertrugen (p<0,001). Fast die Hälfte der Patienten der Verumgruppe (41,9%) tolerierten die Höchstdosis von 18 g EN unter Provokation („komplette Desensibilisierung“). Es zeigte sich ein vergleichbares Sicherheitsprofil unter Verum- und Placebo-OIT in Bezug auf objektive Nebenwirkungen. Unter Verum-OIT kam es jedoch signifikant häufiger zu subjektiven Nebenwirkungen wie oralem Juckreiz oder Bauchschmerzen im Vergleich zu Placebo (3,7% der Verum-OIT-Gaben vs. 0,5% der Placebo-OIT-Gaben, p<0,001). Drei Kinder der Verumgruppe (9,7%) und sieben Kinder der Placebogruppe (22,6%) beendeten die Studie vorzeitig, je zwei Patienten beider Gruppen aufgrund von Nebenwirkungen. Im Gegensatz zu Placebo, zeigten sich unter Verum-OIT signifikante immunologische Veränderungen. So kam es zu einer Abnahme des EN-spezifischen Quaddeldurchmessers im Hautpricktest, einem Anstieg der EN-spezifischen IgG4-Werte im Serum sowie zu einer verminderten EN-spezifischen Zytokinsekretion, insbesondere der Th2-spezifischen Zytokine IL-4 und IL-5. Hinsichtlich der EN-spezifischen IgE-Werte sowie der EN-spezifischen Basophilenaktivierung zeigten sich hingegen keine Veränderungen unter OIT. Die Lebensqualität von Kindern der Verumgruppe war nach OIT signifikant verbessert, jedoch nicht bei Kindern der Placebogruppe. Während der OIT wurde die Therapie von fast allen Kindern (82%) und Müttern (82%) als positiv bewertet (= niedrige Therapiebelastung).
Diskussion: Die EN-OIT führte bei einem Großteil der EN-allergischen Kinder zu einer Desensibilisierung und einer deutlich erhöhten Reaktionsschwelle auf EN. Somit sind die Kinder im Alltag vor akzidentellen Reaktionen auf EN geschützt, was die Lebensqualität der Kinder deutlich verbessert. Unter den kontrollierten Studienbedingungen zeigte sich ein akzeptables Sicherheitsprofil, mit vorrangig milder Symptomatik. Die klinische Desensibilisierung ging mit Veränderungen auf immunologischer Ebene einher. Langzeitstudien zur EN-OIT müssen jedoch abgewartet werden, um die klinische und immunologische Wirksamkeit hinsichtlich einer möglichen langfristigen oralen Toleranzinduktion sowie die Sicherheit unter langfristiger OIT zu untersuchen, bevor das Therapiekonzept in die Praxis übertragen werden kann.
Thermal cis-trans isomerization of azobenzene studied by path sampling and QM/MM stochastic dynamics
(2017)
Azobenzene-based molecular photoswitches have extensively been applied to biological systems, involving photo-control of peptides, lipids and nucleic acids. The isomerization between the stable trans and the metastable cis state of the azo moieties leads to pronounced changes in shape and other physico-chemical properties of the molecules into which they are incorporated. Fast switching can be induced via transitions to excited electronic states and fine-tuned by a large number of different substituents at the phenyl rings. But a rational design of tailor-made azo groups also requires control of their stability in the dark, the half-lifetime of the cis isomer. In computational chemistry, thermally activated barrier crossing on the ground state Born-Oppenheimer surface can efficiently be estimated with Eyring’s transition state theory (TST) approach; the growing complexity of the azo moiety and a rather heterogeneous environment, however, may render some of the underlying simplifying assumptions problematic.
In this dissertation, a computational approach is established to remove two restrictions at once: the environment is modeled explicitly by employing a quantum mechanical/molecular mechanics (QM/MM) description; and the isomerization process is tracked by analyzing complete dynamical pathways between stable states. The suitability of this description is validated by using two test systems, pure azo benzene and a derivative with electron donating and electron withdrawing substituents (“push-pull” azobenzene). Each system is studied in the gas phase, in toluene and in polar DMSO solvent. The azo molecules are treated at the QM level using a very recent, semi-empirical approximation to density functional theory (density functional tight binding approximation). Reactive pathways are sampled by implementing a version of the so-called transition path sampling method (TPS), without introducing any bias into the system dynamics. By analyzing ensembles of reactive trajectories, the change in isomerization pathway from linear inversion to rotation in going from apolar to polar solvent, predicted by the TST approach, could be verified for the push-pull derivative. At the same time, the mere presence of explicit solvation is seen to broaden the distribution of isomerization pathways, an effect TST cannot account for.
Using likelihood maximization based on the TPS shooting history, an improved reaction coordinate was identified as a sine-cosine combination of the central bend angles and the rotation dihedral, r (ω,α,α′). The computational van’t Hoff analysis for the activation entropies was performed to gain further insight into the differential role of solvent for the case of the unsubstituted and the push-pull azobenzene. In agreement with the experiment, it yielded positive activation entropies for azobenzene in the DMSO solvent while negative for the push-pull derivative, reflecting the induced ordering of solvent around the more dipolar transition state associated to the latter compound. Also, the dynamically corrected rate constants were evaluated using the reactive flux approach where an increase comparable to the experimental one was observed for a high polarity medium for both azobenzene derivatives.
Translating innovation
(2017)
This doctoral thesis studies the process of innovation adoption in public administrations, addressing the research question of how an innovation is translated to a local context. The study empirically explores Design Thinking as a new problem-solving approach introduced by a federal government organisation in Singapore. With a focus on user-centeredness, collaboration and iteration Design Thinking seems to offer a new way to engage recipients and other stakeholders of public services as well as to re-think the policy design process from a user’s point of view. Pioneered in the private sector, early adopters of the methodology include civil services in Australia, Denmark, the United Kingdom, the United States as well as Singapore. Hitherto, there is not much evidence on how and for which purposes Design Thinking is used in the public sector.
For the purpose of this study, innovation adoption is framed in an institutionalist perspective addressing how concepts are translated to local contexts. The study rejects simplistic views of the innovation adoption process, in which an idea diffuses to another setting without adaptation. The translation perspective is fruitful because it captures the multidimensionality and ‘messiness’ of innovation adoption. More specifically, the overall research question addressed in this study is: How has Design Thinking been translated to the local context of the public sector organisation under investigation? And from a theoretical point of view: What can we learn from translation theory about innovation adoption processes?
Moreover, there are only few empirical studies of organisations adopting Design Thinking and most of them focus on private organisations. We know very little about how Design Thinking is embedded in public sector organisations. This study therefore provides further empirical evidence of how Design Thinking is used in a public sector organisation, especially with regards to its application to policy work which has so far been under-researched.
An exploratory single case study approach was chosen to provide an in-depth analysis of the innovation adoption process. Based on a purposive, theory-driven sampling approach, a Singaporean Ministry was selected because it represented an organisational setting in which Design Thinking had been embedded for several years, making it a relevant case with regard to the research question. Following a qualitative research design, 28 semi-structured interviews (45-100 minutes) with employees and managers were conducted. The interview data was triangulated with observations and documents, collected during a field research research stay in Singapore.
The empirical study of innovation adoption in a single organisation focused on the intra-organisational perspective, with the aim to capture the variations of translation that occur during the adoption process. In so doing, this study opened the black box often assumed in implementation studies. Second, this research advances translation studies not only by showing variance, but also by deriving explanatory factors. The main differences in the translation of Design Thinking occurred between service delivery and policy divisions, as well as between the first adopter and the rest of the organisation. For the intra-organisational translation of Design Thinking in the Singaporean Ministry the following five factors played a role: task type, mode of adoption, type of expertise, sequence of adoption, and the adoption of similar practices.
Trends in precipitation over Germany and the Rhine basin related to changes in weather patterns
(2017)
Precipitation as the central meteorological feature for agriculture, water security, and human well-being amongst others, has gained special attention ever since. Lack of precipitation may have devastating effects such as crop failure and water scarcity. Abundance of precipitation, on the other hand, may as well result in hazardous events such as flooding and again crop failure. Thus, great effort has been spent on tracking changes in precipitation and relating them to underlying processes. Particularly in the face of global warming and given the link between temperature and atmospheric water holding capacity, research is needed to understand the effect of climate change on precipitation.
The present work aims at understanding past changes in precipitation and other meteorological variables. Trends were detected for various time periods and related to associated changes in large-scale atmospheric circulation. The results derived in this thesis may be used as the foundation for attributing changes in floods to climate change. Assumptions needed for the downscaling of large-scale circulation model output to local climate stations are tested and verified here.
In a first step, changes in precipitation over Germany were detected, focussing not only on precipitation totals, but also on properties of the statistical distribution, transition probabilities as a measure for wet/dry spells, and extreme precipitation events.
Shifting the spatial focus to the Rhine catchment as one of the major water lifelines of Europe and the largest river basin in Germany, detected trends in precipitation and other meteorological variables were analysed in relation to states of an ``optimal'' weather pattern classification. The weather pattern classification was developed seeking the best skill in explaining the variance of local climate variables.
The last question addressed whether observed changes in local climate variables are attributable to changes in the frequency of weather patterns or rather to changes within the patterns itself. A common assumption for a downscaling approach using weather patterns and a stochastic weather generator is that climate change is expressed only as a changed occurrence of patterns with the pattern properties remaining constant. This assumption was validated and the ability of the latest generation of general circulation models to reproduce the weather patterns was evaluated.
% Paper 1
Precipitation changes in Germany in the period 1951-2006 can be summarised briefly as negative in summer and positive in all other seasons. Different precipitation characteristics confirm the trends in total precipitation: while winter mean and extreme precipitation have increased, wet spells tend to be longer as well (expressed as increased probability for a wet day followed by another wet day). For summer the opposite was observed: reduced total precipitation, supported by decreasing mean and extreme precipitation and reflected in an increasing length of dry spells.
Apart from this general summary for the whole of Germany, the spatial distribution within the country is much more differentiated. Increases in winter precipitation are most pronounced in the north-west and south-east of Germany, while precipitation increases are highest in the west for spring and in the south for autumn. Decreasing summer precipitation was observed in most regions of Germany, with particular focus on the south and west.
The seasonal picture, however, was again differently represented in the contributing months, e.g.\ increasing autumn precipitation in the south of Germany is formed by strong trends in the south-west in October and in the south-east in November. These results emphasise the high spatial and temporal organisation of precipitation changes.
% Paper 2
The next step towards attributing precipitation trends to changes in large-scale atmospheric patterns was the derivation of a weather pattern classification that sufficiently stratifies the local climate variables under investigation. Focussing on temperature, radiation, and humidity in addition to precipitation, a classification based on mean sea level pressure, near-surface temperature, and specific humidity was found to have the best skill in explaining the variance of the local variables. A rather high number of 40 patterns was selected, allowing typical pressure patterns being assigned to specific seasons by the associated temperature patterns. While the skill in explaining precipitation variance is rather low, better skill was achieved for radiation and, of course, temperature.
Most of the recent GCMs from the CMIP5 ensemble were found to reproduce these weather patterns sufficiently well in terms of frequency, seasonality, and persistence.
% Paper 3
Finally, the weather patterns were analysed for trends in pattern frequency, seasonality, persistence, and trends in pattern-specific precipitation and temperature. To overcome uncertainties in trend detection resulting from the selected time period, all possible periods in 1901-2010 with a minimum length of 31 years were considered. Thus, the assumption of a constant link between patterns and local weather was tested rigorously. This assumption was found to hold true only partly. While changes in temperature are mainly attributable to changes in pattern frequency, for precipitation a substantial amount of change was detected within individual patterns.
Magnitude and even sign of trends depend highly on the selected time period. The frequency of certain patterns is related to the long-term variability of large-scale circulation modes.
Changes in precipitation were found to be heterogeneous not only in space, but also in time - statements on trends are only valid for the specific time period under investigation. While some part of the trends can be attributed to changes in the large-scale circulation, distinct changes were found within single weather patterns as well.
The results emphasise the need to analyse multiple periods for thorough trend detection wherever possible and add some note of caution to the application of downscaling approaches based on weather patterns, as they might misinterpret the effect of climate change due to neglecting within-type trends.
Trunk loading and back pain
(2017)
An essential function of the trunk is the compensation of external forces and loads in order to guarantee stability. Stabilising the trunk during sudden, repetitive loading in everyday tasks, as well as during performance is important in order to protect against injury. Hence, reduced trunk stability is accepted as a risk factor for the development of back pain (BP). An altered activity pattern including extended response and activation times as well as increased co-contraction of the trunk muscles as well as a reduced range of motion and increased movement variability of the trunk are evident in back pain patients (BPP). These differences to healthy controls (H) have been evaluated primarily in quasi-static test situations involving isolated loading directly to the trunk. Nevertheless, transferability to everyday, dynamic situations is under debate. Therefore, the aim of this project is to analyse 3-dimensional motion and neuromuscular reflex activity of the trunk as response to dynamic trunk loading in healthy (H) and back pain patients (BPP).
A measurement tool was developed to assess trunk stability, consisting of dynamic test situations. During these tests, loading of the trunk is generated by the upper and lower limbs with and without additional perturbation. Therefore, lifting of objects and stumbling while walking are adequate represents. With the help of a 12-lead EMG, neuromuscular activity of the muscles encompassing the trunk was assessed. In addition, three-dimensional trunk motion was analysed using a newly developed multi-segmental trunk model. The set-up was checked for reproducibility as well as validity. Afterwards, the defined measurement set-up was applied to assess trunk stability in comparisons of healthy and back pain patients.
Clinically acceptable to excellent reliability could be shown for the methods (EMG/kinematics) used in the test situations. No changes in trunk motion pattern could be observed in healthy adults during continuous loading (lifting of objects) of different weights. In contrast, sudden loading of the trunk through perturbations to the lower limbs during walking led to an increased neuromuscular activity and ROM of the trunk. Moreover, BPP showed a delayed muscle response time and extended duration until maximum neuromuscular activity in response to sudden walking perturbations compared to healthy controls. In addition, a reduced lateral flexion of the trunk during perturbation could be shown in BPP.
It is concluded that perturbed gait seems suitable to provoke higher demands on trunk stability in adults. The altered neuromuscular and kinematic compensation pattern in back pain patients (BPP) can be interpreted as increased spine loading and reduced trunk stability in patients. Therefore, this novel assessment of trunk stability is suitable to identify deficits in BPP. Assignment of affected BPP to therapy interventions with focus on stabilisation of the trunk aiming to improve neuromuscular control in dynamic situations is implied. Hence, sensorimotor training (SMT) to enhance trunk stability and compensation of unexpected sudden loading should be preferred.
The functioning of the surface water-groundwater interface as buffer, filter and reactive zone is important for water quality, ecological health and resilience of streams and riparian ecosystems. Solute and heat exchange across this interface is driven by the advection of water. Characterizing the flow conditions in the streambed is challenging as flow patterns are often complex and multidimensional, driven by surface hydraulic gradients and groundwater discharge. This thesis presents the results of an integrated approach of studies, ranging from the acquisition of field data, the development of analytical and numerical approaches to analyse vertical temperature profiles to the detailed, fully-integrated 3D numerical modelling of water and heat flux at the reach scale. All techniques were applied in order to characterize exchange flux between stream and groundwater, hyporheic flow paths and temperature patterns.
The study was conducted at a reach-scale section of the lowland Selke River, characterized by distinctive pool riffle sequences and fluvial islands and gravel bars. Continuous time series of hydraulic heads and temperatures were measured at different depths in the river bank, the hyporheic zone and within the river. The analyses of the measured diurnal temperature variation in riverbed sediments provided detailed information about the exchange flux between river and groundwater. Beyond the one-dimensional vertical water flow in the riverbed sediment, hyporheic and parafluvial flow patterns were identified. Subsurface flow direction and magnitude around fluvial islands and gravel bars at the study site strongly depended on the position around the geomorphological structures and on the river stage. Horizontal water flux in the streambed substantially impacted temperature patterns in the streambed. At locations with substantial horizontal fluxes the penetration depths of daily temperature fluctuations was reduced in comparison to purely vertical exchange conditions.
The calibrated and validated 3D fully-integrated model of reach-scale water and heat fluxes across the river-groundwater interface was able to accurately represent the real system. The magnitude and variations of the simulated temperatures matched the observed ones, with an average mean absolute error of 0.7 °C and an average Nash Sutcliffe Efficiency of 0.87. The simulation results showed that the water and heat exchange at the surface water-groundwater interface is highly variable in space and time with zones of daily temperature oscillations penetrating deep into the sediment and spots of daily constant temperature following the average groundwater temperature. The average hyporheic flow path temperature was found to strongly correlate with the flow path residence time (flow path length) and the temperature gradient between river and groundwater. Despite the complexity of these processes, the simulation results allowed the derivation of a general empirical relationship between the hyporheic residence times and temperature patterns. The presented results improve our understanding of the complex spatial and temporal dynamics of water flux and thermal processes within the shallow streambed. Understanding these links provides a general basis from which to assess hyporheic temperature conditions in river reaches.
Die Arktis erwärmt sich schneller als der Rest der Erde. Die Auswirkungen manifestieren sich unter Anderem in einer verstärkten Erwärmung der arktischen Grenzschicht. Diese Arbeit befasst sich mit Wechselwirkungen zwischen synoptischen Zyklonen und der arktischen Atmosphäre auf lokalen bis überregionalen Skalen. Ausgangspunkt dafür sind Messdaten und Modellsimulationen für den Zeitraum der N-ICE2015 Expedition, die von Anfang Januar bis Ende Juni 2015 im arktischen Nordatlantiksektor stattgefunden hat.
Anhand von Radiosondenmessungen lassen sich Auswirkungen von synoptischen Zyklonen am deutlichsten im Winter erkennen, da sie durch die Advektion warmer und feuchter Luftmassen in die Arktis den Zustand der Atmosphäre von einem strahlungs-klaren in einen strahlungs-opaken ändern. Obwohl dieser scharfe Kontrast nur im Winter existiert, zeigt die Analyse, dass der integrierte Wasserdampf als Indikator für die Advektion von Luftmassen aus niedrigen Breiten in die Arktis auch im Frühjahr geeignet ist. Neben der Advektion von Luftmassen wird der Einfluss der Zyklonen auf die statische Stabilität charakterisiert. Beim Vergleich der N-ICE2015 Beobachtungen mit der SHEBA Kampagne (1997/1998), die über dickerem Eis stattfand, finden sich trotz der unterschiedlichen Meereisregime Ähnlichkeiten in der statischen Stabilität der Atmosphäre. Die beobachteten Differenzen in der Stabilität lassen sich auf Unterschiede in der synoptischen Aktivität zurückführen. Dies lässt vermuten, dass die dünnere Eisdecke auf saisonalen Zeitskalen nur einen geringen Einfluss auf die thermodynamische Struktur der arktischen Troposphäre besitzt, solange eine dicke Schneeschicht sie bedeckt. Ein weiterer Vergleich mit den parallel zur N-ICE2015 Kampagne gestarteten Radiosonden der AWIPEV Station in Ny-Åesund, Spitzbergen, macht deutlich, dass die synoptischen Zyklonen oberhalb der Orographie auf saisonalen Zeitskalen das Wettergeschehen bestimmen.
Des Weiteren werden für Februar 2015 die Auswirkungen von in der Vertikalen variiertem Nudging auf die Entwicklung der Zyklonen am Beispiel des hydrostatischen regionalen Klimamodells HIRHAM5 untersucht. Es zeigt sich, dass die Unterschiede zwischen den acht Modellsimulationen mit abnehmender Anzahl der genudgten Level zunehmen. Die größten Differenzen resultieren vornehmlich aus dem zeitlichen Versatz der Entwicklung synoptischer Zyklonen. Zur Korrektur des Zeitversatzes der Zykloneninitiierung genügt es bereits, Nudging in den unterstem 250 m der Troposphäre anzuwenden. Daneben findet sich zwischen den genudgten HIRHAM5-Simulation und den in situ Messungen der gleiche positive Temperaturbias, den auch ERA-Interim besitzt. Das freie HIRHAM hingegen reproduziert das positive Ende der N-ICE2015 Temperaturverteilung gut, besitzt aber einen starken negativen Bias, der sehr wahrscheinlich aus einer Unterschätzung des Feuchtegehalts resultiert. An Beispiel einer Zyklone wird gezeigt, dass Nudging Einfluss auf die Lage der Höhentiefs besitzt, die ihrerseits die Zyklonenentwicklung am Boden beeinflussen. Im Weiteren wird mittels eines für kleine Ensemblegrößen geeigneten Varianzmaßes eine statistische Einschätzung der Wirkung des Nudgings auf die Vertikale getroffen. Es wird festgestellt, dass die Ähnlichkeit der Modellsimulationen in der unteren Troposphäre generell höher ist als darüber und in 500 hPa ein lokales Minimum besitzt.
Im letzten Teil der Analyse wird die Wechselwirkung der oberen und unteren Stratosphäre anhand zuvor betrachteter Zyklonen mit Daten der ERA-Interim Reanalyse untersucht. Lage und Ausrichtung des Polarwirbels erzeugten ab Anfang Februar 2015 eine ungewöhnlich große Meridionalkomponente des Tropopausenjets, die Zugbahnen in die zentrale Arktis begünstigte. Am Beispiel einer Zyklone wird die Übereinstimmung der synoptischen Entwicklung mit den theoretischen Annahmen über den abwärts gerichteten Einfluss der Stratosphäre auf die Troposphäre hervorgehoben. Dabei spielt die nicht-lineare Wechselwirkung zwischen der Orographie Grönlands, einer Intrusion stratosphärischer Luft in die Troposphäre sowie einer in Richtung Arktis propagierender Rossby-Welle eine tragende Rolle. Als Indikator dieser Wechselwirkung werden horizontale Signaturen aus abwechselnd aufsteigender und absinkender Luft innerhalb der Troposphäre identifiziert.