25 research outputs found
Predictability of Critical Transitions
Critical transitions in multistable systems have been discussed as models for
a variety of phenomena ranging from the extinctions of species to
socio-economic changes and climate transitions between ice-ages and warm-ages.
From bifurcation theory we can expect certain critical transitions to be
preceded by a decreased recovery from external perturbations. The consequences
of this critical slowing down have been observed as an increase in variance and
autocorrelation prior to the transition. However especially in the presence of
noise it is not clear, whether these changes in observation variables are
statistically relevant such that they could be used as indicators for critical
transitions. In this contribution we investigate the predictability of critical
transitions in conceptual models. We study the quadratic integrate-and-fire
model and the van der Pol model, under the influence of external noise. We
focus especially on the statistical analysis of the success of predictions and
the overall predictability of the system. The performance of different
indicator variables turns out to be dependent on the specific model under study
and the conditions of accessing it. Furthermore, we study the influence of the
magnitude of transitions on the predictive performance
Fluctuation-induced Distributed Resonances in Oscillatory Networks
Self-organized network dynamics prevails for systems across physics, biology
and engineering. How external signals generate distributed responses in
networked systems fundamentally underlies their function, yet is far from fully
understood. Here we analyze the dynamic response patterns of oscillatory
networks to fluctuating input signals. We disentangle the impact of the signal
distribution across the network, the signals' frequency contents and the
network topology. We analytically derive qualitatively different dynamic
response patterns and find three frequency regimes: homogeneous responses at
low frequencies, topology-dependent resonances at intermediate frequencies, and
localized responses at high frequencies. The theory faithfully predicts the
network-wide collective responses to regular and irregular, localized and
distributed simulated signals, as well as to real input signals to power grids
recorded from renewable-energy supplies. These results not only provide general
insights into the formation of dynamic response patterns in networked systems
but also suggest regime- and topology-specific design principles underlying
network function.Comment: 7 pages, 4 figure
High-Intensity Discharge Lamp and Duffing Oscillator - Similarities and Differences
The processes inside the arc tube of high-intensity discharge lamps are
investigated by finite element simulations. The behavior of the gas mixture
inside the arc tube is governed by differential equations describing mass,
energy and charge conservation as well as the Helmholtz equation for the
acoustic pressure and the Navier-Stokes equation for the flow driven by the
buoyancy and the acoustic streaming force. The model is highly nonlinear and
requires a recursion procedure to account for the impact of acoustic streaming
on the temperature and other fields. The investigations reveal the presence of
a hysteresis and the corresponding jump phenomenon, quite similar to a Duffing
oscillator. The similarities and, in particular, the differences of the
nonlinear behavior of the high-intensity discharge lamp to that of a Duffing
oscillator are discussed. For large amplitudes the high-intensity discharge
lamp exhibits a stiffening effect in contrast to the Duffing oscillator.Comment: 14 pages, 8 figure
Logarithmic bred vectors in spatiotemporal chaos: structure and growth
Bred vectors are a type of finite perturbation used in prediction studies of
atmospheric models that exhibit spatially extended chaos. We study the
structure, spatial correlations, and the growth- rates of logarithmic bred
vectors (which are constructed by using a given norm). We find that, after a
suitable transformation, logarithmic bred vectors are roughly piecewise copies
of the leading Lyapunov vector. This fact allows us to deduce a scaling law for
the bred vector growth rate as a function of their amplitude. In addition, we
relate growth rates with the spectrum of Lyapunov exponents corresponding to
the most expanding directions. We illustrate our results with simulations of
the Lorenz '96 model.Comment: 8 pages, 8 figure
Predictability of Extreme Events in Time Series
In this thesis we access the prediction of extreme events observing precursory structures, which were identified using a maximum likelihood approach.
The main goal of this thesis is to investigate the dependence of the quality of a prediction on the magnitude of the events under study.
Until now, this dependence was only sporadically reported for different phenomena without being understood as a general feature of predictions.
We propose the magnitude dependence as a property of a prediction, indicating, whether larger events can be better, harder or equally well predicted than smaller events.
Furthermore we specify a condition which can characterize the magnitude dependence of a distinguished measure for the quality of a prediction, the Receiver Operator characteristic curve (ROC).
This test condition allows to relate the magnitude dependence of ap rediction task to the joint PDF of events and precursory variables.
If we are able to describe the numerical estimate of this joint PDF by an analytic expression, we can not only characterize the magnitude dependence of events observed so far, but infer the magnitude dependence of events, larger then the observed events.
Having the test condition specified, we study the magnitude dependence for the prediction of increments and threshold crossings in sequences of random variables and short- and long-range correlated stochastic processes.
In dependence of the distribution of the process under study we obtain different magnitude dependences for the prediction of increments in Gaussian, exponentially, symmetrized exponentially, power-law and symmetrized power-law distributed processes.
For threshold crossings we obtain the same magnitude dependence for all distributions studied.
Furthermore we study the dependence on the event magnitude for the prediction of increments and threshold crossings in velocity increments, measured in a free jet flow and in wind-speed measurements.
Additionally we introduce a method of post-processing the output of ensemble weather forecast models in order to identify precursory behavior, which could indicate failures of weather forecasts.
We then study not only the success of this method, but also the magnitude dependence.
keywords: extreme events, statistical inference, prediction via precursors, ROC curves, likelihood ratio, magnitude dependenc
Model-free inference of direct network interactions from nonlinear collective dynamics
The topology of interactions in network dynamical systems fundamentally
underlies their function. Accelerating technological progress creates massively
available data about collective nonlinear dynamics in physical, biological, and
technological systems. Detecting direct interaction patterns from those
dynamics still constitutes a major open problem. In particular, current
nonlinear dynamics approaches mostly require to know a priori a model of the
(often high dimensional) system dynamics. Here we develop a model-independent
framework for inferring direct interactions solely from recording the nonlinear
collective dynamics generated. Introducing an explicit dependency matrix in
combination with a block-orthogonal regression algorithm, the approach works
reliably across many dynamical regimes, including transient dynamics toward
steady states, periodic and non-periodic dynamics, and chaos. Together with its
capabilities to reveal network (two point) as well as hypernetwork (e.g., three
point) interactions, this framework may thus open up nonlinear dynamics options
of inferring direct interaction patterns across systems where no model is
known.Comment: 10 pages, 7 figure
Reactions to extreme events: moving threshold model
In spite of precautions to avoid the harmful effects of extreme events, we
experience recurrently phenomena that overcome the preventive barriers. These
barriers usually increase drastically right after the occurrence of such
extreme events, but steadily decay in their absence. In this paper we consider
a simple model that mimics the evolution of the protection barriers to study
the efficiency of the system's reaction to extreme events and how it changes
our perception of the sequence of extreme events itself. We obtain that the
usual method of fighting extreme events introduces a periodicity in their
occurrence and is generally less efficient than the use of a constant barrier.
On the other hand, it shows a good adaptation to the presence of slow
non-stationarities.Comment: 14 pages and 7 figure
Precursors of extreme increments
We investigate precursors and predictability of extreme increments in a time
series. The events we are focusing on consist in large increments within
successive time steps. We are especially interested in understanding how the
quality of the predictions depends on the strategy to choose precursors, on the
size of the event and on the correlation strength. We study the prediction of
extreme increments analytically in an AR(1) process, and numerically in wind
speed recordings and long-range correlated ARMA data. We evaluate the success
of predictions via receiver operator characteristics (ROC-curves). Furthermore,
we observe an increase of the quality of predictions with increasing event size
and with decreasing correlation in all examples. Both effects can be understood
by using the likelihood ratio as a summary index for smooth ROC-curves