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search.filters.applied.f.access: openAccess × End date: 2009 × Start date: 2000 × Has files: Yes × Type: Text × WGL Institute: PIK ×

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Now showing 1 - 10 of 65
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    Integrierte Abschätzung der Änderungen der thermohalinen Zirkulation (INTEGRATION) : Schlussbericht INTEGRATION
    (Hannover : Technische Informationsbibliothek (TIB), 2006) Kuhlbrodt, Till; Rahmstorf, Stefan
    [no abstract available]
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    Timing cellular decision making under noise via cell-cell communication
    (San Francisco, CA : Public Library of Science (PLoS), 2009) Koseska, A.; Zaikin, A.; Kurths, J.; García-Ojalvo, J.
    Many cellular processes require decision making mechanisms, which must act reliably even in the unavoidable presence of substantial amounts of noise. However, the multistable genetic switches that underlie most decision-making processes are dominated by fluctuations that can induce random jumps between alternative cellular states. Here we show, via theoretical modeling of a population of noise-driven bistable genetic switches, that reliable timing of decision-making processes can be accomplished for large enough population sizes, as long as cells are globally coupled by chemical means. In the light of these results, we conjecture that cell proliferation, in the presence of cell-cell communication, could provide a mechanism for reliable decision making in the presence of noise, by triggering cellular transitions only when the whole cell population reaches a certain size. In other words, the summation performed by the cell population would average out the noise and reduce its detrimental impact.
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    Reliability of regional climate model simulations of extremes and of long-term climate
    (Göttingen : Copernicus GmbH, 2004) Böhm, U.; Kücken, M.; Hauffe, D.; Gerstengarbe, E.-W.; Werner, P.C.; Flechsig, M.; Keuler, K.; Block, A.; Ahrens, W.; Nocke, T.
    We present two case studies that demonstrate how a common evaluation methodology can be used to assess the reliability of regional climate model simulations from different fields of research. In Case I, we focused on the agricultural yield loss risk for maize in Northeastern Brazil during a drought linked to an El-Niño event. In Case II, the present-day regional climatic conditions in Europe for a 10-year period are simulated. To comprehensively evaluate the model results for both kinds of investigations, we developed a general methodology. On its basis, we elaborated and implemented modules to assess the quality of model results using both advanced visualization techniques and statistical algorithms. Besides univariate approaches for individual near-surface parameters, we used multivariate statistics to investigate multiple near-surface parameters of interest together. For the latter case, we defined generalized quality measures to quantify the model's accuracy. Furthermore, we elaborated a diagnosis tool applicable for atmospheric variables to assess the model's accuracy in representing the physical processes above the surface under various aspects. By means of this evaluation approach, it could be demonstrated in Case Study I that the accuracy of the applied regional climate model resides at the same level as that we found for another regional model and a global model. Excessive precipitation during the rainy season in coastal regions could be identified as a major contribution leading to this result. In Case Study II, we also identified the accuracy of the investigated mean characteristics for near-surface temperature and precipitation to be comparable to another regional model. In this case, an artificial modulation of the used initial and boundary data during preprocessing could be identified as the major source of error in the simulation. Altogether, the achieved results for the presented investigations indicate the potential of our methodology to be applied as a common test bed to different fields of research in regional climate modeling.
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    Tempting long-memory - on the interpretation of DFA results
    (Göttingen : Copernicus GmbH, 2004) Maraun, D.; Rust, H.W.; Timmer, J.
    We study the inference of long-range correlations by means of Detrended Fluctuation Analysis (DFA) and argue that power-law scaling of the fluctuation function and thus long-memory may not be assumed a priori but have to be established. This requires the investigation of the local slopes. We account for the variability characteristic for stochastic processes by calculating empirical confidence regions. Comparing a long-memory with a short-memory model shows that the inference of long-range correlations from a finite amount of data by means of DFA is not specific. We remark that scaling cannot be concluded from a straight line fit to the fluctuation function in a log-log representation. Furthermore, we show that a local slope larger than α=0.5 for large scales does not necessarily imply long-memory. We also demonstrate, that it is not valid to conclude from a finite scaling region of the fluctuation function to an equivalent scaling region of the autocorrelation function. Finally, we review DFA results for the Prague temperature data set and show that long-range correlations cannot not be concluded unambiguously.
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    Reduction 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.
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    EUROPA - Modelling, Reflecting and Communicating Possible Futures of Europe in the Context of Global Change : Schlussbericht
    (Hannover : Technische Informationsbibliothek, 2005) Jaeger, Carlo; Edenhofer, O.; Haas, A.; Leimbach, M.; Lotze-Campen, H.; Reusswig, F.; Sprinz, D.; Welp, M.
    [no abstract available]
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    Abschätzung der regionalen Kohlenstoffbilanz von mitteleuropäischen Wäldern unter dem Aspekt des Globalen Wandels : Abschlußbericht
    (Hannover : Technische Informationsbibliothek (TIB), 2002) Suckow, Felicitas; Lasch, Petra; Klöcking, Beate; Hauf, Ylva; Badeck, Franz
    [no abstract available]
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    Statistical characteristics of surrogate data based on geophysical measurements
    (Göttingen : Copernicus, 2006) Venema, V.; Bachner, S.; Rust, H.W.; Simmer, C.
    In this study, the statistical properties of a range of measurements are compared with those of their surrogate time series. Seven different records are studied, amongst others, historical time series of mean daily temperature, daily rain sums and runoff from two rivers, and cloud measurements. Seven different algorithms are used to generate the surrogate time series. The best-known method is the iterative amplitude adjusted Fourier transform (IAAFT) algorithm, which is able to reproduce the measured distribution as well as the power spectrum. Using this setup, the measurements and their surrogates are compared with respect to their power spectrum, increment distribution, structure functions, annual percentiles and return values. It is found that the surrogates that reproduce the power spectrum and the distribution of the measurements are able to closely match the increment distributions and the structure functions of the measurements, but this often does not hold for surrogates that only mimic the power spectrum of the measurement. However, even the best performing surrogates do not have asymmetric increment distributions, i.e., they cannot reproduce nonlinear dynamical processes that are asymmetric in time. Furthermore, we have found deviations of the structure functions on small scales.