4,384 research outputs found

    Tipping Points and Early Warning Signals in the Climate-Carbon System

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    This is a thesis about tipping points and early warning signals. The tipping points investigated are related to various components of the climate-carbon system. In contrast, the work on early warning signals has more generic applications, however in this thesis they are analysed in the context of the climate-carbon system. The thesis begins with an introduction to the climate-carbon system as well as a discussion of tipping points in the Earth system. Then a more mathematical summary of tipping points and early warning signals is given. An investigation into the ‘compost bomb’ is undertaken, in which the spatial structure of soils is accounted for. It is found that a hot summer could cause a compost bomb. The effect of biogeochemical heating on the stability of the global carbon cycle is investigated and it is found to play only a small role. The potential for instabilities in the climate-carbon cycle is further investigated when the dynamic behaviour of the ocean carbon cycle is accounted for. It is found that some CMIP6 models may be close to having an unstable carbon cycle. Spatial early warning signals are investigated in the context of more rapidly forced systems. It is found that spatial early warning signals perform better when the system is rapidly forced compared with time series based early warning signals. The typical assumptions about white noise made when using early warning signals are also studied. It is found that time correlated noise may mask the early warning signal. It is shown that a spectral analysis can avoid this problem.European Commissio

    The terminator region of tidally locked M-dwarf exoplanets in 3-d general circulation models

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    The impressive sensitivity of the James Webb Space Telescope has made it possible to study the atmospheres of planets beyond the solar system. It will soon be followed by space missions aiming specifically at this goal, such as the Ariel mission, Twinkle, and the Habitable Worlds Observatory. One category of exoplanet has drawn interest because of its potential to harbour temperate climates with liquid surface water—and therefore potentially life. These are rocky planets orbiting cool M-class stars, or "M-Earths." Stellar population trends and observing biases lead to a high proportion of potentially habitable, terrestrial planets falling into this category. Because of the low temperatures of their host stars, however, habitable worlds of this type are found in close orbits where they are likely to be tidally locked. As the solar system has no tidally locked planets, our knowledge of their atmospheric circulation is currently limited to theoretical modelling. Past modelling work has shown that the asymmetrical irradiation of tidally locked planets results in characteristic circulation regimes which have profound consequences for observations. Atmospheric retrievals, which use statistical methods to fit 1-D atmospheric models to observational data and quantify the confidence of the fit, are not yet able to account for the 3-D nature of this circulation. For planets with large spatial variation in environmental conditions caused by tidal locking, 1-D models are not able to capture the differences and interconnections between planetary regions such as the dayside, nightside, and planetary limb or terminator. In addition, planetary atmospheres exhibit variation over time, potentially resulting in differences in retrieved properties between observing visits or even between different phases of a planet’s orbit. Accounting for 4-D circulation effects in atmospheric retrievals first requires a theoretical understanding of the impact of global-scale phenomena such as atmospheric waves and horizontal transport on conditions at the planetary limb, and then requires incorporation of this knowledge into the retrieval pipeline in the form of, for example, parameterisations. In this thesis, I address the first requirement: the theoretical understanding of the effects of fully modelled 4-D atmospheric circulation on the planetary limb, the region probed by transmission spectroscopy, on tidally locked planets. I focus in particular on effects caused by the global propagation of atmospheric waves and by horizontal transport of clouds and hazes. In Chapter 2, I show that that the atmospheric dynamics on the tidally locked Proxima Centauri b support a longitudinally asymmetric stratospheric wind oscillation (LASO), analogous to Earth’s quasi-biennial oscillation (QBO). The LASO has a vertical extent of 35–55 km, a period of 5–6.5 months, and a peak-to-peak wind speed amplitude of -70 to +130 ms−1 with a maximum at an altitude of 41 km. Unlike the QBO, the LASO displays longitudinal asymmetries related to the asymmetric thermal forcing of the planet and to interactions with the resulting stationary Rossby waves. The equatorial gravity wave sources driving the LASO are localised in the deep convection region at the substellar point and in a jet exit region near the western terminator, unlike the QBO, for which these sources are distributed uniformly around the planet. Longitudinally, the western terminator experiences the highest wind speeds and undergoes reversals earlier than other longitudes. The antistellar point only experiences a weak oscillation with a very brief, low-speed westward phase. The QBO on Earth is associated with fluctuations in the abundances of water vapour and trace gases such as ozone which are also likely to occur on exoplanets if these gases are present. Strong fluctuations in temperature and the abundance of atmospheric species at the terminators will need to be considered when interpreting atmospheric observations of tidally locked exoplanets. In Chapter 3, I investigate the presence of cloud cover at the planetary limb of water-rich Earth-like planets, which is likely to weaken chemical signatures in transmission spectra and impede attempts to characterise these atmospheres. Based on observations of Earth and solar system worlds, exoplanets with atmospheres should have both short-term weather and long-term climate variability, implying that cloud cover may be less during some observing periods. I identify and describe a mechanism driving periodic clear sky events at the terminators in simulations of tidally locked Earth-like planets. A feedback between dayside cloud radiative effects, incoming stellar radiation and heating, and the dynamical state of the atmosphere, especially the zonal wavenumber-1 Rossby wave identified in past work on tidally locked planets, leads to oscillations in Rossby wave phase speeds and in the position of Rossby gyres and results in advection of clouds to or away from the planet’s eastern terminator. I study this oscillation in simulations of Proxima Centauri b, TRAPPIST 1-e, and rapidly rotating versions of these worlds located at the inner edge of their stars’ habitable zones. I simulate time series of the transit depths of the 1.4 µm water feature and 2.7 µm carbon dioxide feature. The impact of atmospheric variability on the transmission spectra is sensitive to the structure of the dayside cloud cover and the location of the Rossby gyres, but none of my simulations have variability significant enough to be detectable with current methods. In Chapter 4, I study the interaction between the atmospheric circulation and photochemical hazes and describe the resulting haze abundances at the terminator. Transmission spectroscopy supports the presence of unknown, light-scattering aerosols in the atmospheres of many exoplanets. The complexity of factors influencing the formation, 3-D transport, radiative impact, and removal of aerosols makes it challenging to match theoretical models to the existing data. My study simplifies these factors to focus on the interaction between planetary general circulation and haze distribution at the planetary limb. I use an intermediate complexity general circulation model, ExoPlaSim, to simulate idealised organic haze particles as radiatively active tracers in the atmospheres of tidally locked terrestrial planets for a range of rotation rates. I find three distinct 3-D spatial haze distributions, corresponding to three circulation regimes, each with a different haze profile at the limb. All regimes display significant terminator asymmetry. In my parameter space, super-Earth-sized planets with rotation periods greater than 13 days have the lowest haze optical depths at the terminator, supporting the choice of slower rotators as observing targets. My thesis supports the existence of characteristic forms of temporal and spatial variability on tidally locked planets which will undoubtedly impact observations and inform our understanding of climate conditions on the surface. Overall, the effects of purely dynamical variability may be too small to be detected for Earth-like planets (but potentially detectable for larger ones). The impact of the atmospheric circulation on the distribution of clouds and hazes, on the other hand, is likely to affect even observations of terrestrial planets due to the highly scattering nature of these aerosols and will need to be accounted for in atmospheric retrievals

    Silicon-Based Optical Sensors for Fungal Pathogen Diagnostics

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    The last years have witnessed a link between the COVID-19 pandemic with increasing numbers of vulnerable patients and globally emerging incidences of severe drug-resistant fungal infections, thus, calling for rapid, reliable, and sensitive diagnostic tools for fungal infections. However, despite strong warnings from health authorities, such as the World Health Organization, concerning the fatal consequences of the global spread of drug-resistant pathogenic fungi, progress in fungal infection diagnosis and therapy is still limited. Today, gold standard methods for revealing resistance and susceptibility in pathogenic fungi, namely antifungal susceptibility testing (AFST), require several days for completion, and thus this lengthy process can adversely affect antifungal therapy and further promote the spread of resistance. In this work, the use of photonic silicon chips consisting of micropatterned diffraction gratings as sensitive sensors for rapid AFST of clinically relevant fungal pathogens is investigated. These photonic chips provide a surface for the colonization of microbial pathogens at a liquid-solid interface and serve as the optical transducer element for label-free monitoring of fungal growth by detecting real-time changes in the white light reflectance. These sensor elements are used to track morphological changes of fungi in the presence of clinically relevant antifungals at varying concentrations to rapidly determine the minimum inhibitory concentration (MIC) values that help to classify pathogens as resistant or susceptible. We show that by careful design of the chip dimensions, this optical method can extend from bacteria, through yeasts, to filamentous fungi for accelerated AFST, which is at least three times faster than current gold standard methods and can provide same-day results. Moreover, a 3D-printed microfluidic gradient generator was designed to complement the assay and provide an integrated system, which can potentially be employed in point-of-care settings. This gradient generator produces the two-fold dilution series of clinically relevant antimicrobials in an automated manner and is interfaced with the photonic silicon chips to include a complete, on-chip, label-free, and phenotypic assay. Using the bacterial species Escherichia coli and ciprofloxacin as a model pathogen-drug combination, MIC values can be expeditiously determined within 90 minutes compared to current clinical practices, which typically require up to 24 h for bacterial species

    Jets and instabilities in forced magnetohydrodynamic flows

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    Magnetic fields are present in the solar system and astrophysical bodies (e.g. the Sun's field, the Earth's field, and the fields of giant planets, stars and galaxies). Our research examines the effect of magnetic fields on these systems, extending the work of Meshalkin and Sinai (1961) & Manfroi and Young (2002). The results will be useful for understanding the effects of the magnetic field in more turbulent regimes, although this study is concerned with the instabilities associated with classical laminar flow. We aim to investigate the role played by the magnetic field in modifying the stability properties of planar-forced fluid flows. In the absence of magnetic fields, the flow found by a body force, and nonlinear interactions with Rossby waves result in the generation of strong zonal flows. However, we find that the presence of a weak magnetic field suppresses the zonal jet generation. Here we study the instabilities of the Kolmogorov flow. We consider u_0=(0,sin x ) as a 2D incompressible flow. In the presence of a mean magnetic field, the dynamics are governed by the Navier–Stokes equations and the induction equation. We perform a classical linear analysis, in which growth rate, stability criteria, and MHD effects are derived. Instabilities are investigated associated with two magnetic field orientations, which can be x-directed (horizontal) or y-directed (vertical}) in our two-dimensional system to give an MHD version of Kolmogorov flow. In a basic equilibrium state magnetic field lines are straight for the case of vertical field and sinusoidal for horizontal field with an additional component of the external force balancing the resulting Lorentz force

    Seismic Noise Interferometry and Distributed Acoustic Sensing (DAS): Inverting for the Firn Layer S ‐Velocity Structure on Rutford Ice Stream, Antarctica

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    Firn densification profiles are an important parameter for ice-sheet mass balance and palaeoclimate studies. One conventional method of investigating firn profiles is using seismic refraction surveys, but these are difficult to upscale to large-area measurements. Distributed acoustic sensing (DAS) presents an opportunity for large-scale seismic measurements of firn with dense spatial sampling and easy deployment, especially when seismic noise is used. We study the feasibility of seismic noise interferometry (SI) on DAS data for characterizing the firn layer at the Rutford Ice Stream, West Antarctica. Dominant seismic energy appears to come from anthropogenic noise and shear-margin crevasses. The DAS cross-correlation interferometry yields noisy Rayleigh wave signals. To overcome this, we present two strategies for cross-correlations: (a) hybrid instruments—correlating a geophone with DAS, and (b) stacking of selected cross-correlation panels picked in the tau-p domain. These approaches are validated with results derived from an active survey. Using the retrieved Rayleigh wave dispersion curve, we inverted for a high-resolution 1D S-wave velocity profile down to a depth of 100 m. The profile shows a “kink” (velocity gradient inflection) at ∼12 m depth, resulting from a change of compaction mechanism. A triangular DAS array is used to investigate directional variation in velocity, which shows no evident variations thus suggesting a lack of azimuthal anisotropy in the firn. Our results demonstrate the potential of using DAS and SI to image the near-surface and present a new approach to derive S-velocity profiles from surface wave inversion in firn studies

    Satellite remote sensing of surface winds, waves, and currents: Where are we now?

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    This review paper reports on the state-of-the-art concerning observations of surface winds, waves, and currents from space and their use for scientific research and subsequent applications. The development of observations of sea state parameters from space dates back to the 1970s, with a significant increase in the number and diversity of space missions since the 1990s. Sensors used to monitor the sea-state parameters from space are mainly based on microwave techniques. They are either specifically designed to monitor surface parameters or are used for their abilities to provide opportunistic measurements complementary to their primary purpose. The principles on which is based on the estimation of the sea surface parameters are first described, including the performance and limitations of each method. Numerous examples and references on the use of these observations for scientific and operational applications are then given. The richness and diversity of these applications are linked to the importance of knowledge of the sea state in many fields. Firstly, surface wind, waves, and currents are significant factors influencing exchanges at the air/sea interface, impacting oceanic and atmospheric boundary layers, contributing to sea level rise at the coasts, and interacting with the sea-ice formation or destruction in the polar zones. Secondly, ocean surface currents combined with wind- and wave- induced drift contribute to the transport of heat, salt, and pollutants. Waves and surface currents also impact sediment transport and erosion in coastal areas. For operational applications, observations of surface parameters are necessary on the one hand to constrain the numerical solutions of predictive models (numerical wave, oceanic, or atmospheric models), and on the other hand to validate their results. In turn, these predictive models are used to guarantee safe, efficient, and successful offshore operations, including the commercial shipping and energy sector, as well as tourism and coastal activities. Long-time series of global sea-state observations are also becoming increasingly important to analyze the impact of climate change on our environment. All these aspects are recalled in the article, relating to both historical and contemporary activities in these fields

    Linear stability and numerical analysis of dipolar vortices and topographic flows

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    The linear stability and numerical analysis of geophysical flow patterns is carried out on the beta-plane in a quasigeostrophic approximation. We consider initial steady state dipoles in a one-and-a-half-layer model that are capable of zonal drift in either direction. Despite previous numerical works suggesting that eastward propagating dipoles are stable, our high resolution simulations identify the spontaneous symmetry breaking of weak dipoles over time. The evolution is associated with a growing critical mode with even symmetry about the zonal axis. On carrying out a linear stability analysis, the critical modes obtained share consistency with the numerical fields. In addition, both methods of analysis show that the linear growth rate is inversely proportional to the dipole intensity. Furthermore, the partner separation becomes more pronounced after the linear growth stage, suggesting that nonlinear effects play a pivotal role in the underlying dynamics. Beyond this, the dynamics of initially tilted dipoles and dipole-rider solutions are considered, while stronger dipoles are further analysed using the method of distillation. Flows over sinusoidal bottom relief are considered in a two-layer model on the quasigeostrophic beta-plane. Fourier mode solutions are assumed for the layer-wise perturbation field in order to carry out a linear stability analysis, from which a coupled eigenproblem is derived between fluid columns for both zonal and meridional bottom irregularities. The presence of zonally oriented multiple ridges stabilises an otherwise unstable homogeneous zonal current with respect to increases in the number of ridges and ridge amplitude. Moreover, a bifurcation occurs in the unstable mode spectra and is dependent on the number of ridges. The critical eigenmodes in this case are found to be eddy chains of alternating sign, and these share remarkable resemblance with those obtained numerically. Meridionally oriented multiple ridges are also considered, but are found not to affect the maximum growth rate directly.Open Acces

    A study of metal-organic framework sorption phenomena in solution.

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    197 p.Metal-organic Frameworks (MOFs) are crystalline porous materials built up from metal ions or clusters connected with organic linkers through strong coordination bonds. Among the most outstanding properties their high porosity, large surface areas and tunable structures arise. In the chapters that make up this thesis, the three most relevant points in Materials Science converge: the synthesis, characterization and applicability of MOFs. On the one hand, the different parameters that affect the shape and shape of some well-known MOFs have been studied. In addition, a new characterization technique named Magnetic Sustentation has been developed for the quantification of adsorption processes taking place in solution, which allows a rapid and direct determination of the mass of captured targeted compounds by a paramagnetic MOF. Moreover, the behaviour of the high flexible MIL-88A(Fe) in the removal of short-chain alcohols from water has been theoretically and experimentally studied as well as the synergetic effect between the polymeric PVDF and MIL-88A(Fe) in prepared membranes. Finally, the first scientific effort to explore the true potentials of diverse kinds of MOFs for capturing CO2 in water has been done in order to set a benchmark in the field

    Investigating the Backscatter of Marine Plastic Litter Using a C- and X-Band Ground Radar, during a Measurement Campaign in Deltares

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    In recent years, marine plastic pollution has seen increased coverage in the public interest and research due to a greater understanding of the scale and impact of plastic pollution within the marine environment. Considering the hazard that plastic waste poses on the environment, marine life, and on humans, remote-sensing techniques could provide timely information on their detection and dynamics. The remote sensing of marine plastic is a relatively new field and research into the capabilities of radar for detecting and monitoring marine plastic pollution is generally limited, with several interactions and mechanisms being largely unknown. Here, we exploit the use of a C- and X-band radar to understand the capabilities of monitoring marine plastics. Our results show that backscattering differences in the C- and X-band between the reference water (called here as “clean”) and the test water filled with plastic can be detected in some conditions (based on statistical analysis). Overall, the results indicate that the X-band frequency performs significantly better than the C-band frequency, with X-band detecting significant differences in backscattering in 48/68 test cases compared with C-band detecting differences in 20/67 test cases. We also find that the difference in backscattering is dependent on the size and shape of the plastic object, as well as the wave conditions which the plastic is moving on. This study provides new insights on the radar capabilities for detecting marine plastic litter and new information which can be used in the planning of future missions and studies on the remote sensing of marine plastic pollution

    Resumen de tesis. Development of ultra-stable characterization techniques for ultrashort laser beams

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    [EN]The dramatic evolution that the laser Science has undergone in the last six decades, and particularly in the field of ultrashort pulses, has been accompanied by breakthrough advances in the development of techniques for their characterization. Despite the large number of characterization techniques developed in the last two decades, it is still necessary to Implement simpler and more robust techniques that can also operate in multiple spectral ranges. This Thesis is devoted to the study and development of temporal and spatiotemporal characterization techniques with compact and stable configurations, which are versatile, e.g., to analyze ultrashort laser beams with different temporal durations and in various spectral regions. The structure of this Thesis is made up of four main blocks. Firstly, a brief introduction to ultrafast optical metrology and the state of the art of spatial, temporal and spatiotemporal characterization techniques is presented in Part I. Then, Part II is focused on the review and extension of the capabilities of the amplitude swing temporal characterization technique, demonstrating three key advances: operation in different spectral regions from visible to near-infrared, with a tunability range spanning of more than one octave; Characterization of ultrashort pulses in the few-cycle regime; or study of the implementation with different amplitude modulation schemes. In addition, a new retrieval algorithm is developed to analyze the amplitude swing traces based on differential evolution strategies. Thirdly, the fundamentals, operation range and experimental demonstration of a spatiotemporal characterization technique based on bulk lateral shearing interferometry are depicted in Part III. Furthermore, the technique is applied to characterize complex spatiotemporal couplings, like constant and time-varying optical vortices or the aberrations when focusing with astigmatic lenses. Finally, the main conclusions of the work and future perspectives are discussed in Part IV[ES]La drástica evolución que ha experimentado la ciencia láser en las últimas seis décadas, y en particular en el campo de los pulsos ultracortos, ha ido acompañada de grandes avances en el desarrollo de técnicas para su caracterización. A pesar de la gran cantidad de métodos de caracterización desarrollados en las últimas dos décadas, aún es necesario implementar sistemas más simples y robustos que también puedan operar en múltiples rangos espectrales. Esta Tesis está dedicada al estudio y desarrollo de técnicas de caracterización temporal y espaciotemporal con configuraciones compactas y estables, que sean versátiles para, por ejemplo, analizar haces láser ultracortos con diferentes duraciones temporales y en diversas regiones espectrales. La estructura de la Tesis está dividida en cuatro bloques principales. En primer lugar, en la Parte I se presenta una breve introducción a la metrología óptica ultrarrápida y los avances de las técnicas de caracterización espacial, temporal y espaciotemporal a lo largo de los últimos años. Después, la Parte II se enfoca en la revisión y extensión de las capacidades de la técnica de caracterización temporal amplitude swing, demostrando tres avances clave: operación en diferentes regiones espectrales desde visible hasta infrarrojo cercano, con un rango de sintonizabilidad que abarca más de una octava; caracterización de pulsos ultracortos en régimen de pocos ciclos; o estudio de diferentes esquemas de modulación de amplitud. Además, se implementa un nuevo algoritmo de reconstrucción para analizar las trazas del sistema amplitude swing basado en las estrategias Differential Evolution. En tercer lugar, en la Parte III se describen los fundamentos, el rango de operación y la demostración experimental de una técnica de caracterización espaciotemporal basada en la interferometría lateral compacta (bulk lateral shearing interferometry). Además, la técnica se emplea para caracterizar acoplamientos espaciotemporales complejos, como son los vórtices ópticos constantes y variables en el tiempo o las aberraciones introducidas al focalizar haces láser con lentes astigmáticas. Finalmente, en la Parte IV se resumen las principales conclusiones y futuras líneas de trabajo.Thesis memory and supplementary material folder: Vídeo 1 ("VideoThesis_GeneralizingAswing_Noise.avi"), Noise analysis amplitude swing configurations video. Vídeo 2 ("VideoThesis_Vortices_ProofOfConcept.avi"), Proof of concept spatiotemporal technique video. Vídeo 3 ("VideoThesis_Vortex_NIR_Lneg1.avi"), Near-infrared vortex video: orbital angular momentum of −1. Vídeo 4 ("VideoThesis_Vortex_NIR_L1.avi"), Near-infrared vortex video: orbital angular momentum of +1. Vídeo 5 ("VideoThesis_Vortex_NIR_L3.avi"), Near-infrared vortex video: orbital angular momentum of +3. Vídeo 6 ("VideoThesis_Vortex_VIS_Lneg2.avi"), Visible vortex video: orbital angular momentum of -2. Vídeo 7 ("VideoThesis_Vortex_VIS_L2.avi"), Visible vortex video: orbital angular momentum of +2. Vídeo 8 ("VideoThesis_Vortex_VIS_L4.avi"), Visible vortex video: orbital angular momentum of +4. Vídeo 9 ("VideoThesis_TimeVaryingVortex_Exp.avi"), Time varying vortex video: Experiment. Vídeo 10 ("VideoThesis_TimeVaryingVortex_Sim.avi"), Time varying vortex video: Simulation. Vídeo 11 ("VideoThesis_Astigmatic_Biconvergent.avi"), Astigmatic beam video: z = da = 49 cm. Vídeo 12 ("VideoThesis_Astigmatic_ConvDiv.avi"), Astigmatic beam video: z = db = 137 cm
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