Free convection and turbulent fluxes over complex terrain
- The impact of complex terrain on the land-atmosphere exchange is investigated in this thesis. Here, free convection, a very effective vertical transport mechanism as turbulence is predominantly driven by buoyant forces, is explicitly addressed. Recently, it was shown for certain situations over complex terrain that free convective injections of surface layer air masses into the atmospheric boundary layer (ABL) can alter the ABL properties significantly. This study aims at the general identification and description of such situations of near-ground free convection conditions (FCCs) over complex terrain. For this purpose, data obtained during the COPS (Convective and Orographically induced Precipitation Study) field campaign in summer 2007 were used. Within this project, several surface flux measurement stations were installed, mainly in valleys and on mountaintops of the Black Forest, southwestern Germany. Turbulent fluxes were calculated with the eddy-covariance (EC) method and were used to detect FCCs with the help of a stability parameter. The flux measurements were further combined with ABL profiling measurements (Sodar/RASS) and a large-eddy simulation (LES) model in order to investigate the impact of FCCs on ABL properties. The effect of complex terrain on the energy balance closure and on spatial and temporal flux differences was also studied with these flux data.
FCCs were detected on about 25% of the days during the three month COPS experiment. In situations of weak synoptic forcing, thermally driven orographic (e.g. valley winds) or local wind systems developed over the complex terrain due to heating differences. During the adaption of these wind systems to changing heating differences (e.g. during the reversal of the valley wind from down- to up-valley winds in the morning), the horizontal wind vanished. If, at the same time, the buoyancy flux was positive and enhanced, buoyant forces exceeded the usually prevailing shear forces in the surface layer and FCCs were detected. Moreover, it was demonstrated that FCCs are not restricted to the COPS region. Also, a data set of Nam Co station on the Tibetan Plateau showed FCCs during the reversal of a thermally driven land-lake breeze. However, at this high-altitude site, FCCs were more often detected in the afternoon compared to the COPS region due to the frequent change of heating differences during cloud cover periods.
The Sodar/RASS as well as the LES model showed the presence of coherent updraft
structures in the developing early-morning convective boundary layer (CBL) in the Kinzig valley (Black Forest) during FCCs. Spectral analysis of the EC data in these situations indicated the existence of large-eddy turbulent scales – typical for thermal updrafts in the CBL – already close to the ground. An ensemble and time mean analysis of the simulated flow field in the valley further confirmed that the Sodar/RASS was located preferably in an updraft region during FCCs. In a CBL over flat homogeneous terrain, the locations of convective structures would occur randomly. However, over the complex orography of the Kinzig valley, the updraft structures were found to develop in
quasi-stationary patterns at specific locations relative to the surrounding mountain ridges. The model further showed that the flux through the valley boundary layer is mainly determined by the flux within these coherent updrafts. In combination with the Sodar/RASS observations, the model also showed that these updrafts deeply penetrated into the stably stratified valley boundary layer up to approximately the height of the surrounding mountains leading to an effective upward counter-gradient transport of surface layer air mass properties during FCCs.
The analysis of the turbulent fluxes at the different COPS sites showed that the flux values were strongly determined by varying land surface characteristics. Also an increase of the Bowen ratio with increasing altitude could be detected. These findings are in accordance with former studies in this area. As expected, the energy balance was found to be unclosed on average during the entire COPS period, with values of the residual typical for heterogeneous landscapes. However, regarding only the periods with FCCs, no residual occurred on average. This is due to the fact that the landscape
heterogeneity is of minor importance in case of the more vertical oriented exchange regime during FCCs, so that missing advective flux components became strongly reduced in these situations. Moreover, it was found that in comparable periods with no FCCs, flux components were missing with exactly the proportions of the buoyancy flux ratio, thus suggesting a correction of the energy balance according to the buoyancy flux ratio approach. These results support recent publications on the energy balance closure
Interactions between hydrology and biogeochemistry within riparian wetlands
- Interactions between hydrology and biogeochemistry at various spatio-temporal scales are important control mechanisms within terrestrial and aquatic ecosystems and exist among different compartments and transition interfaces. Understanding the fundamental mechanistic couplings between hydrological and biogeochemical processes and how these couplings feed back into ecosystem services and functions is an interdisciplinary challenge that must be addressed especially in the context of humanly mediated climate change. Riparian wetlands, as a transition zone between terrestrial and aquatic ecosystems, occupy large fractions of terrestrial ecosystems and provide important ecohydrological services. Due to their anoxic environments, riparian wetlands are able to store significant amounts of carbon as peat and act as an effective nutrient sink e.g. for sulfur, phosphorous and nitrogen. Riparian wetlands are characterized by highly dynamical interactions between hydrologically controlled transport mechanisms and biogeochemically controlled substrate availability, which governs nutrient cycling as well as the sink and source functions of wetlands. Generally, these interactions and their potential implications on ecosystem functions are only poorly understood. The representation of the tight couplings between hydrology and biogeochemistry in mechanistic models is a very challenging task because they have revealed a complexity which is often beyond the capabilities of current models. The objective of this thesis is to investigate interactions between hydrology and biogeochemistry in riparian wetlands and to understand their potential implications for internal biogeochemical process distributions and solute mobilization. Additionally, one major focus of the thesis is the attempt to represent such fundamental couplings in a process-based, hydrological/biogeochemical modeling approach. To this end, this thesis uses a combination of field and virtual experiments, as well as catchment-scale numerical modeling, performed for the Lehstenbach catchment, which was exemplarily chosen as main study site.
Results from the virtual experiments show very complex small-scale hydrological dynamics within the riparian areas. Here, runoff generation processes are strongly influenced by the spatial structure of the wetland-typical micro-topography (hummocks and hollows). Surface flow is episodically generated by a highly dynamical, threshold-controlled process where extended surface flow networks drain large fractions of the wetland's area. During intensive rainstorm events these surface flow networks, which contribute to stream discharge due to a fill and spill mechanism, dominate runoff generation. These fast flow components are characterized by very low residence times (minutes to hours) and once they are activated, the surface flow networks are able to rapidly mobilize large amounts of solutes, like nitrate or dissolved organic carbon (DOC), out of the wetlands by bypassing deeper anoxic layers. The importance of fast flow components for the catchment-scale mobilization of DOC was further confirmed by field investigations and catchment-scale numerical modeling. High frequency measurements of DOC in runoff of the Lehstenbach catchment revealed that DOC export is subject to substantial short term variations at an hourly to daily timescale. During intense rainstorms, DOC concentrations are up to ten times higher (up to 40 mg/L) compared to low flow conditions (~3-5 mg/L). Short term variations together with the dramatic rise of DOC concentrations in runoff during rainstorms can be explained by the episodically activation of fast flow components in the wetland areas. At the catchment-scale, application of a hydraulic mixing-cell (HMC) methodology in combination with numerical modeling has revealed that fast flow components like saturated overland flow are exclusively generated in the wetland areas during intensive rainstorm events. On an annual basis, exemplarily for the hydrological year 2001, the HMC analysis quantified the relative contribution of saturated overland flow related to the total discharge with 19.5%, which highlights the importance of riparian wetlands for catchment-scale runoff generation.
Virtual experiments, additionally show that distinct shifts between surface and subsurface flow dominance, as a result of small-scale micro-topographic driven runoff generation in the wetlands, are responsible for very complex three-dimensional subsurface flow patterns showing a wide range of subsurface residence times. To investigate how these micro-topography induced subsurface flow patterns, together with the non-uniform hydrological and biogeogeochemical boundary conditions, affect the internal re-distribution and transformation of redox-sensitive species (like nitrate, sulfate or iron) a coupled hydrological/biogeogeochemical model was developed. In the model, wetland-typical biogeochemical processes are represented in a sequential stream tube approach where redox-sensitive processes are implemented as kinetic reactions. Simulations show the formation of local hot spots for redox-sensitive processes within the subsurface as a result of the complex subsurface flow paths and the transport-limited availability of electron acceptors and donors. Formation of hot spots was simulated for all key reduction processes including iron(III)-/sulfate reduction and denitrification as well as for the corresponding re-oxidation processes. These results offer a new perspective on hydrologically controlled biogeochemical transformation processes in riparian wetlands, which provides a dynamic framework to explain process heterogeneity in wetland soils and variability in process rates over space and time.
Findings from this thesis clearly prove how useful interdisciplinary approaches are in understanding processes and mechanisms in ecosystems and how important functions of ecosystems are affected by couplings among those. However, a lot of knowledge gaps still exist in understanding the nature of dependency between water and nutrient cycles across scales and how these interacting cycles feed back into humanly-mediated climate change in ecosystems. Development of new interdisciplinary methodologies and frameworks as well as an integrated way of thinking across the boundaries of the different environmental disciplines is necessary to address the grand challenges associated with climate change.
Tibet Plateau Atmosphere-Ecology-Glaciology Cluster Joint Kobresia Ecosystem Experiment: Documentation of the second Intensive Observation Period, Summer 2012 in KEMA, Tibet
- Experiment documentation of the second joined Kobresia ecosystem experiment conducted by the Atmosphere-Ecology-Glaciology Cluster within DFG SPP 1372 (Tibetan Plateau)in Kema, Tibet, China. The report provides background information about the field side, conducted measurements and participants.
The Re-implementation of Sharia in Northern Nigeria and the Education of Muslim Women 1999-2007
- This study investigates the impact of the re-implementation of Sharia on the education of Muslim women between 1999 and 2007, in four states in northern Nigeria namely, Zamfara, Kano, Bauchi and Kaduna. The study fills the research gap in the description of the status of women with respect to education under the Sharia dispensation re-introduced in 1999. Before this era, scholarly works have documented the state of female education in northern Nigeria; however, none deals directly with post re-implementation and its impact on Muslim women’s education. This period is particularly interesting because it coincides with the return of the country to democratic rule after a string of military rules. Using interviews with Sharia proponents, school administrators, women and reports from Sharia Commissions in these states, the work establishes that while Zamfara and Kano States underwent transformations in the educational sector as a result of the programs introduced by the Sharia governments, the changes in the educational domain in Bauchi and Kaduna were exceedingly triggered by the return of democracy in Nigeria after decades of military rule. The results arrived at in this study reveal interesting facets of the relationship between Sharia and the education of Muslim women. The research shows that proponents of Sharia used the premise (also supported by verses from the Qur’an and Prophet Mohammed’s teachings) that education is an important part of Islam to which both males and females have equal rights, to sell the Islamic-based educational programs introduced by the Sharia regime. Using religion to promote these programs, the predominantly male Sharia bodies, mitigate the often tough cultural practices and religious dogmas that have prevented women from having full access to formal, western education. However, the type of education offered to women under these Sharia programs is a hybrid of formal education and Islamic education which pays strict attention to such cultural and religious practices like the wearing of the hijab, the separation of males and females in schools, the teaching of practical, home craft skills such as knitting and baking to the women. So, instead of the education taking women out of the home domain in which they have been restricted in the past, it rather further maintains them while providing them access to formal education which they will hardly use in any public career.
Crystalline-core micelles based on triblock terpolymers with polyethylene middle blocks
- This thesis is focused on the crystallization-induced structure formation of polyethylene containing triblock terpolymers in organic solvents to surface-compartmentalized worm-like crystalline-core micelles (wCCMs). Obtaining profound knowledge of the parameters controlling the self-assembly process allowed the production of a variety of complex one-dimensional micellar architectures with many potential applications, such as adaptive surfactants.
At first, the basic parameters that control the crystallization-induced self-assembly were explored using symmetric polystyrene-block-polyethylene-block-poly(methyl methacrylate) (PS-b-PE-b-PMMA) triblock terpolymers and a PS-b-PE-b-PS triblock copolymer. In good solvents for the PE block, e.g. THF and toluene, the selective formation of wCCMs was observed over a wide range of concentration, applied crystallization temperature and polymer composition. Whereas wCCMs produced by PS-b-PE-b-PS showed a homogeneous PS corona, a patch-like compartmentalization of the corona was observed if the micelles were formed by PS-b-PE-b-PMMA. As THF shows equal solvent quality for both corona blocks, wCCMs with almost alternating PS and PMMA compartments of about 15 nm were observed in this solvent. However, if structure formation was conducted in bad solvents for PE, such as dioxane or dimethylacetamide, spherical micelles with amorphous PE cores were formed already before crystallization. Hence, the subsequent crystallization of PE resulted in spherical CCMs with a patchy or a homogeneous corona depending on the used triblock. These findings allow the highly selective production of stable spherical or worm-like CCMs from the same polymer.
As the corona structure of the patchy micelles self-assembled from triblock terpolymers was mainly deduced from transmission electron microscopy (TEM) performed on dried samples, a small-angle neutron scattering (SANS) study was performed in order to elucidate the morphology in solution. Therefore a partly deuterated triblock terpolymer was synthesized and measured at different contrasts to allow the selective detection of the different corona compartments. The resulting SANS curves could be interpreted using a form factor model for core-shell cylinders with alternating PS and PMMA hemishells including interparticle interactions, thus validating the TEM observations. Notably, Janus-type and patchy cylinders can be clearly distinguished using the applied form factor model.
Moreover, the controlled formation of wCCMs with tunable corona composition and structure was achieved using the cocrystallization of different triblock copolymers. Via random cocrystallization of PS-b-PE-b-PMMA and PS-b-PE-b-PS the corona morphology could be tuned continuously from a mixed corona at low PMMA content over spherical PMMA patches of increasing number and size to alternating PS and PMMA patches. This approach allows to manufacture wCCMs with predefined corona structure omitting the need to synthesize a new tailor-made triblock terpolymer for every desired morphology.
By establishing the controlled crystallization-driven self-assembly of triblock terpolymers with PE middle blocks, it was further possible to prepare wCCMs with predefined average lengths up to 500 nm and length polydispersities as low as Lw/Ln = 1.1. Here, self-assembled spherical CCMs of PS-b-PE-b-PS were used as seeds for the controlled growth of PS-b-PE-b-PS unimers. Upon further addition of PS-b-PE-b-PMMA unimers these grew epitaxially onto the preexisting wCCMs, resulting in triblock co-micelles that consisted of middle blocks with a homogeneous PS corona and outer blocks with alternating PS/PMMA compartments. These structures represent not only the first block co-micelles including blocks with a patchy corona, but also the first ones produced from purely organic block copolymers.
In view of application, the ability of patchy wCCMs formed by PS-b-PE-b-PMMA to stabilize interfaces was investigated using pendant-drop tensiometry. The observed reduction of the interfacial tension at the toluene/water interface was significantly higher than that of comparable triblock terpolymer single chains and that of wCCMs with a homogeneous PS corona. Interestingly, the obtained equilibrium interfacial tension equaled that of Janus cylinders with similar dimensions. To explain this unexpected finding the corona chains were proposed to adapt to the interface via selective collapse and shielding of the incompatible part of the corona chains. Studying wCCMs formed by several triblock terpolymers with different compositions, the interfacial activity was found to increase with increasing overall length of the corona chains, and to a certain extent with the molar fraction of PS units in the corona.
Silent Sound Art: Performing the Unheard
- This article is a reflection on silent sound art, exemplified here by the works of Peter Ablinger and Akio Suzuki, in the context of a partially historical con-sideration of the participatory subject in Installation Art, with a primary focus on artistic movements and selected works from the late 1950s to the early 1970s by John Cage, George Brecht, and La Monte Young.
Applicability of weight-shift microlight aircraft for measuring the turbulent exchange above complex terrain
- The possibility to reliably observe the exchange of heat and moisture between the land surface and the atmosphere is vital to our understanding of the regional and global cycling of energy and water. While ground-based flux measurements can be made continuously for long periods, they only represent a small landscape unit. On the other hand, aircraft-based measurements have the ability to directly measure the exchange over large areas. Especially over heterogeneous landscapes the spatio-temporal characteristics of both approaches complement each other. However, complex terrestrial ecosystems are sparsely investigated to date, in particular over topographically structured terrain. This can be attributed to; (i) limitations in the description of boundary layer processes over non-homogenous terrain, and (ii) a lack of applicable measurement platforms and techniques to study these processes. In pursue of a resolution strategy, this dissertation investigates the applicability of weight-shift microlight aircraft (WSMA) to gain new insights in the spatial variability of heat and moisture exchange over complex terrain.
WSMA are comparatively cheap in procurement and maintenance, and their unique structure provides exceptional transportability and climb rate. These structural features qualify the WSMA for terrain-following flight over complex and inaccessible terrain, but potentially influence measurements aboard the aircraft. In this dissertation a WSMA with a scientific payload enabling fast measurements of the 3D wind, temperature, water vapor concentration, position, and the radiative flux is used to;
(i) Quantify the WSMA wind measurement uncertainty. A novel time-domain procedure is developed, which improves the accuracy of the WSMA wind measurement by 63% for the horizontal- and 72% for the vertical wind components. The resulting precisions are ±0.09 m s−1 and ±0.04 m s−1, and the agreement with ground-based measurements is in the order of ±0.4 m s−1 and ±0.3 m s−1 (root mean square deviation), respectively.
(ii) Quantify the WSMA eddy-covariance flux measurement uncertainty. From uncertainty propagation the smallest resolvable changes in friction velocity (0.02 m s−1), and sensible- (5 W m−2) and latent (3 W m−2) heat flux are estimated. In comparison to tower measurements, the WSMA observes higher fluxes (17–21%). The differences are not statistically significant, and can be explained by the tower setup and non-propagating eddies.
(iii) Spatially resolve and regionalize the heat and moisture exchange above a complex landscape. Wavelet decomposition of the turbulence data is used to yield a flux observation each 90 m along the flight path. For each flux observation the biophysical surface properties in the flux footprint are determined. An environmental response function between the flux observations and biophysical and meteorological drivers is then inferred using a machine learning technique. This function is used to produce regional maps of the heat and moisture exchange to an accuracy of ≤18% and a precision of ≤5% for individual land covers.
Hence this dissertation provides the necessary basis for using WSMA to investigate the mechanisms of turbulent exchange over heterogeneous and topographically structured terrain. Moreover, the developed algorithms are generally applicable to (i) partitioning flux uncertainty and environmental variability, (ii) extrapolating flux measurements, (iii) assessing the spatial representativeness of long-term tower flux measurements, and (iv) designing, constraining and evaluating flux algorithms for remote sensing and numerical modeling applications.
Documentation of the Atmospheric Boundary Layer Experiment, Nam Tso, Tibet, 08th of July – 08th of August 2012
- no abstract
Effects of copper on calcium metabolism and detoxification mechanisms in freshwater bivalve species of Anodonta
- Copper (Cu) is one of the metals contaminating European fresh water ecosystems. Filter feeding bivalves have high bioaccumulation potential for transition metals as Cu. While copper is an essential micronutrient for living organisms, it causes serious metabolic and physiological impairments when in excess.
The objectives of this thesis are to get knowledge on toxic effects and detoxification mechanisms of copper in Anodonta cygnea and Anodonta anatina, two mussel species widely distributed in continental waters. Because Ca plays a fundamental role in shell formation and in numerous biological processes, Cu2+ effects on cellular plasma membrane calcium transport were studied first. In the second step, the investigations focused on Cu2+ detoxification mechanism involving cysteine (Cys) rich compounds known to play a major role in homeostasis of essential trace metals and in cellular metal detoxification.
Under our experimental conditions, copper inhibition of Ca2+-ATPase activity was observed in the gills and the kidneys, and inhibition of Na+/K+-ATPase in the gills and the digestive gland (DG) upon 4 d of exposure to 0.35 micro mol/L Cu2+. At day 7 of exposure to environmental Cu2+ concentrations total recoveries was observed in the kidneys and the gills for Ca2+-ATPase activity, and in the DG for Na+/K+-ATPase, but not at high doses. Ca and Na transport inhibition may entail disturbance of osmo-regulation and lead to continuous under-supply of Ca. Recoveries of Na+/K+-ATPase and Ca2+-ATPase enzymes function suggest that metal-detoxification is induced.
Phytochelatins (PC) are Cys-rich oligopeptides synthesised by phytochelatin synthase from glutathione in plants and fungi. Phytochelatin synthase genes have recently been identified in invertebrates; this allows us to hypothesize a role of PC in metal detoxification in animals.
In the second part of this work, PC and their precursors as well as metallothionein were analyzed in the gills and in the DG of Anodonta cygnea exposed to Cu2+. Our results showed for the first time the presence of PC2-4 in invertebrates. PC were detected in control mussels not exposed to metal, suggesting a role in essential metal homeostasis. Compared to control, PC2 induction was observed during the first 12 h of Cu2+ exposure. Those results confirm the role of PC as a first line detoxification mechanism in A. cygnea.
Light Harvesting using Metal-Organic and Organic Sensitizers in Hybrid Solar Cells: Synthesis, Characterisation and Application
- This thesis addresses the question how to improve light harvesting with novel tailor-made metal-organic and organic sensitizers for solid-state hybrid solar cell applications. Two approaches are in the focus: 1) the design and synthesis of sensitizers featuring high extinction coefficients over a broad wavelength range and 2) modern device concepts to further enhance or extend the absorption by the combination of two sensitizers. In short: The primary goal was to broaden and boost the optical density of hybrid solar cells. To reach this, novel sensitizer with extended conjugated π-system providing excellent optical properties had to be designed and synthesised in complex multi-step reaction sequences. For ideal sensitizers, further aspects had to be taken into account such as structural demands, electronic properties, and the tendency towards aggregation.
The first part of this thesis deals with the synthesis, characterisation and application of a series of metal-organic ruthenium(II) donor-antenna complexes. In addition to the typically broad absorption of Ru(II)bis(bipyridyl)(NCS)2 complexes in the blue-green region arising from MLCT, these dyes feature much higher extinction coefficients in comparison to a commercially available reference dye lacking any donor-antenna groups. By the application of these Ru(II) complexes in solid-state dye-sensitized solar cells, we found a clear structure-property relationship. The performance - especially the photocurrent density - was significantly improved with increasing extension of the delocalized system of the donor-antenna groups.
To further boost the optical density in hybrid solar cells sensitized with a donor-antenna ruthenium dye (Ru-TPA-NCS), we developed an innovative and technically relevant concept for multichromophore sensitization involving a second sensitizer (TPD-dye). The latter absorbs in a region where Ru-TPA-NCS absorbs weakly. The solar cells were fabricated according to a novel method developed by us. However, the power conversion efficiencies of multichromophore hybrid blend solar cells were initially low due the weak interconnectivity of the TiO2 particles. This issue was addressed by an optimization of the TiO2:spiro-OMeTAD ratio and the addition of PCBM
A further concept dealing with the combination of two sensitizers in solid-state dye-sensitized solar cell was accomplished by co-sensitization of a triphenyldiamine-based dye (TPD-dye) absorbing in the blue region and squaraine dye (SQ-dye) mainly absorbing the red part of the visible spectrum. In this way, the optical response of the device was extended up to 700 nm. Under optimized conditions, a conversion efficiency of 2.41 % could be reached.
To accomplish the desired panchromaticity or even an extension of the absorption up to the NIR region with a single sensitizer, novel BODIPY dyes with excellent optical properties were designed and synthesised. We prepared BODIPYs with donor-groups to extend the delocalized system and integrated a meso-ethynyl bridge between the BODIPY core and the anchoring group to improve the electronic connection between them. For comparison, we synthesised the corresponding BODIPYs without donor-moieties and without ethynyl bridge. The multi-step synthetic routes were optimized, the mechanism of the donor-attachment was clarified and the introduction of the ethynylphenyl group in the meso-position was accomplished for the first time. The optical characterisation of the compounds disclosed an impressively broad and intensive spectral response, especially for a meso-ethynylphenyl BODIPY with donor-groups. This dye absorbs up to 1030 nm with high extinction coefficients. This makes suitable functionalised BODIPYs promising candidates for solar cell applications.
The next part took advantage of the excellent optical properties of BODIPYs and expanded the topic towards the concept of energy transfer. Here, an unattached energy donor dye provides additional adsorption and transfers the energy to a sensitizing acceptor dye. Indeed, for appropriate combinations an additional contribution to the external quantum efficiency was found in the absorption region of the energy donor dye.
Furthermore, a review chapter covering all aspects of dye-sensitized solar cells and the sensitizers is added as appendix.
In summary, this thesis presents the successful design, synthesis and characterisation of both metal-organic and organic sensitizers including ruthenium complexes, triphenyldiamine-based dyes, a squaraine sensitizer and BODIPY dyes. The sensitizers (either individually or in combination with complementary sensitizers) provide excellent optical properties for the application in solar cells. The applicability of these sensitizers was successfully demonstrated in standard solid-state dye-sensitized solar cells, in newly developed multichromophore hybrid blend solar cells, co-sensitized solar cells and in solid-state dye-sensitized solar cells taking advantage of energy transfer.