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- ItemNeedle beams: a review(Abingdon : Taylor and Francis Ltd., 2020) Grunwald R.; Bock M.Needle beams are highly attractive for applications which take advantage from a spatial and temporal localization of photons. High intensities, high resolution and extended depth of focus lead to fundamental advances in the optical system performance. Ultrashort, fringe-free, self-reconstructing nondiffracting pulses with undistorted temporal transfer are obtained by generating truncated Bessel beams under self-apodization conditions. Nondiffracting Talbot self-imaging of needle beam arrays enables to transfer near field information to the Fraunhofer zone. With addressable arrays of needle beams, reconfigurable time-wavefront sensors are built up. Moreover, spatial light modulators and flexible axicons are used to realize structured, highly localized wavepackets, accelerating beams and nondiffracting images. © 2020, © 2020 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
- ItemInfrared spectroscopy in superfluid helium droplets(Abingdon : Taylor and Francis Ltd., 2019) Verma D.; Tanyag R.M.P.; O’Connell S.M.O.; Vilesov A.F.For more than two decades, encapsulation in superfluid helium nanodroplets has served as a reliable technique for probing the structure and dynamics of molecules and clusters at a low temperature of ≈0.37 K. Due to weak interactions between molecules and the host liquid helium, good spectral resolution can usually be achieved, making helium droplets an ideal matrix for spectroscopy in a wide spectral range from infrared to ultraviolet. Furthermore, rotational structure in the spectra of small molecules provides a unique probe for interactions with the superfluid on an atomic scale. This review presents a summary of results and a discussion of recent experimental developments in helium droplet spectroscopy with the emphasis laid on infrared studies. Initially, studies focused on single molecules and have been expanded to larger species, such as metal-molecular clusters, biomolecules, free radicals, ions, and proteins. © 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group.
- ItemReduction of biosphere life span as a consequence of geodynamics(Abingdon : Taylor and Francis Ltd., 2000) Franck, S.; Block, A.; Von Bloh, W.; Bounama, C.; Schellnhuber, H.J.; Svirezhev, Y.The long-term co-evolution of the geosphere-biosphere complex from the Proterozoic up to 1.5 billion years into the planet's future is investigated using a conceptual earth system model including the basic geodynamic processes. The model focusses on the global carbon cycles as mediated by life and driven by increasing solar luminosity and plate tectonics. The main CO2 sink, the weathering of silicates, is calculated as a function of biologic activity, global run-off and continental growth. The main CO2 source, tectonic processes dominated by sea-floor spreading, is determined using a novel semi-empirical scheme. Thus, a geodynamic extension of previous geostatic approaches can be achieved. As a major result of extensive numerical investigations, the 'terrestrial life corridor', i.e., the biogeophysical domain supporting a photosynthesis-based ecosphere in the planetary past and in the future, can be identified. Our findings imply, in particular, that the remaining life-span of the biosphere is considerably shorter (by a few hundred million years) than the value computed with geostatic models by other groups. The 'habitable-zone concept' is also revisited, revealing the band of orbital distances from the sun warranting earth-like conditions. It turns out that this habitable zone collapses completely in some 1.4 billion years from now as a consequence of geodynamics.
- ItemSelf-stabilization of the biosphere under global change: A tutorial geophysiological approach(Abingdon : Taylor and Francis Ltd., 1997) Von Bloh, W.; Block, A.; Schellnhuber, H.J.A 2-dimensional extension of the simple Lovelock-Watson model for geosphere-biosphere feed-back is introduced and discussed. Our enriched version also takes into account various pertinent physical, biological, and civilisatory processes like lateral heat transport, species competition, mutation, germination, and habitat fragmentation. The model is used as a caricature of the Earth System, which allows potential response mechanisms of the biosphere to environmental stress (as generated, e.g., by global warming or anthropogenic land-cover change) to be investigated qualitatively. Based on a cellular automaton representation of the system, extensive calculations are performed. They reveal a number of remarkable and, partially, counter-intuitive phenomena: our model biosphere is able to control almost perfectly the geophysical conditions for its own existence. If the environmental stress exceeds certain thresholds, however, life breaks down on the artificial planet via a first-order phase transition, i.e., in a non-reversible way. There is a close connection between self-stabilizing capacity, biodiversity and geometry of habitat fragmentation. It turns out, in particular, that unrestricted Darwinian competition, which reduces the number of co-existing species, is the best guarantee for survival of the artificial ecosphere as a whole.
- ItemOn freshwater-dependent bifurcations in box models of the interhemispheric thermohaline circulation(Abingdon : Taylor and Francis Ltd., 2002) Titz, S.; Kuhlbrodt, T.; Rahmstorf, S.; Feudel, U.Conceptual box models of the interhemispheric thermohaline circulation are studied with respect to bifurcations. Freshwater fluxes are the main control parameters of the system: they determine the stable states and transitions between stable states of the large-scale thermohaline circulation. In this study of interhemispheric box models both numerical and analytical methods are used to investigate transition mechanisms of the thermohaline circulation. The box model examined first is an interhemispheric four-box model. It is shown that the two bifurcations where the present THC can become unstable, the saddle-node and the Hopf bifurcation, depend in a different way on hemispheric freshwater fluxes. A reduction of the model variables leads to the conclusion that two fixed freshwater fluxes between three surface boxes are the model feature responsible for the bifurcation behavior found. The significance of the Hopf bifurcation for the stability of the thermohaline circulation is discussed.
- ItemLong-term evolution of the global carbon cycle: Historic minimum of global surface temperature at present(Abingdon : Taylor and Francis Ltd., 2002) Franck, S.; Kossacki, K.J.; Von Bloh, W.; Bounama, C.We present a minimal model for the global carbon cycle of the Earth containing the reservoirs mantle, ocean floor, continental crust, continental biosphere, and the kerogen, as well as the aggregated reservoir ocean and atmosphere. This model is coupled to a parameterised mantle convection model for describing the thermal and degassing history of the Earth. In this study the evolution of the mean global surface temperature, the biomass, and reservoir sizes over the whole history and future of the Earth under a maturing Sun is investigated. We obtain reasonable values for the present distribution of carbon in the surface reservoirs of the Earth and find that the parameterisation of the hydrothermal flux and the evolution of the ocean pH in the past has a strong influence on the atmospheric carbon reservoir and surface temperature. The different parameterisations give a rather hot as well as a freezing climate on the early Earth (Hadean and early Archaean). Nevertheless, we find a pronounced global minimum of mean surface temperature at the present state at 4.6 Gyr. In the long-term future the external forcing by increasing insolation dominates and the biosphere extincts in about 1.2 Ga. Our study has the implication that the Earth system is just before the point of evolution where this external forcing takes over the main influence from geodynamic effects acting in the past.
- ItemPolynyas in a high-resolution dynamic-thermodynamic sea ice model and their parameterization using flux models(Abingdon : Taylor and Francis Ltd., 2001) Bjornsson, H.; Willmott, A.J.; Mysak, L.A.; Morales Maqueda, M.A.This paper presents an analysis of the solutions for a steady state latent heat polynya generated by an applied wind stress acting over a semi-enclosed channel using: (a) a dynamic-thermodynamic sea ice model, and (b) a steady state flux model. We examine what processes in the sea ice model are responsible for the maintenance of the polynya and how sensitive the results are to the choice of rheological parameters. We find that when the ice is driven onshore by an applied wind stress, a consolidated ice pack forms downwind of a zone of strong convergence in the ice velocities. The build-up of internal stresses within the consolidated ice pack becomes a crucial factor in the formation of this zone and results in a distinct polynya edge. Furthermore, within the ice pack the across-channel ice velocity varies with the across-channel distance. It is demonstrated that provided this velocity is well represented, the steady state polynya flux model solutions are in close agreement with those of the sea ice model. Experiments with the sea ice model also show that the polynya shape and area are insensitive to (a) the sea ice rheology; (b) the imposition of either free- slip or no-slip boundary conditions. These findings are used in the development of a simplified model of the consolidated ice pack dynamics, the output of which is then compared with the sea ice model results. Finally, we discuss the relevance of this study for the modelling of the North Water Polynya in northern Baffin Bay.
- ItemOn statistics of the free-troposphere synoptic component: An evaluation of skewnesses and mixed third-order moments contribution to the synoptic-scale dynamics and fluxes of heat and humidity(Abingdon : Taylor and Francis Ltd., 2008) Petoukhov, V.; Eliseev, A.V.; Klein, R.; Oesterle, H.Based on the ERA40 data for 1976-2002 we calculated skewnesses and mixed third-order statistical moments (TOMs) for the synoptic variations [with (2.5-6) d timescales]of horizontal winds, temperature, vertical velocity and the specific humidity in Eulerian coordinates. All these variables show skewnesses which markedly deviate from zero, basically at the entries and the outlets of the mid-latitude storm tracks. In these regions, high values of skewness for vertical velocity, temperature and the specific humidity are revealed throughout the entire free troposphere, while the marked skewnesses for horizontal winds are traced in the lower free troposphere. We found a notable deviation of the synoptic-component statistics from the Gaussian statistics. We also made an estimate of the contribution from TOMs to the prognostic equations for the synoptic-scale kinetic energy and the meridional fluxes of sensible and latent heat, which appeared to be non-negligible, mainly in the storm tracks in winter. Our analysis attests that the most pronounced contribution of TOMs to the aforementioned equations comes from the self-advection by the horizontal synoptic-scale motions, while the TOMs induced by the metric terms in the original equations, and specifically the TOMs associated with the vertical self-advection by the synoptic-scale motions, are much less important.
- ItemLong-term changes in the seasonality of Baltic sea level(Abingdon : Taylor and Francis Ltd., 2016) Barbosa, S.M.; Donner, R.V.
- ItemA new validation scheme for the evaluation of multiparameter fields(Abingdon : Taylor and Francis Ltd., 2005) Gerstengarbe, F.-W.; Kücken, M.; Werner, P.C.On the basis of an extended cluster analysis algorithm, we present a new validation method for the evaluation of simulation experiments characterized by more than one parameter. This method allows the assessment of any parameter combination in space and time. As an example for the effectiveness of the algorithm, the results of two regional climate model runs and observational data have been tested and interpreted.