Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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Early warning of climate tipping points from critical slowing down: comparing methods to improve robustness
Lenton, T.M. ; Livina, V. ; Dakos, V. ; Nes, E.H. van; Scheffer, M. - \ 2012
Philosophical Transactions of the Royal Society. Series A, Mathematical, physical and engineering sciences 370 (2012). - ISSN 1364-503X - p. 1185 - 1204.
catastrophic shifts - system - signal - time - bifurcations - antarctica - ecosystems - states - cycle
We address whether robust early warning signals can, in principle, be provided before a climate tipping point is reached, focusing on methods that seek to detect critical slowing down as a precursor of bifurcation. As a test bed, six previously analysed datasets are reconsidered, three palaeoclimate records approaching abrupt transitions at the end of the last ice age and three models of varying complexity forced through a collapse of the Atlantic thermohaline circulation. Approaches based on examining the lag-1 autocorrelation function or on detrended fluctuation analysis are applied together and compared. The effects of aggregating the data, detrending method, sliding window length and filtering bandwidth are examined. Robust indicators of critical slowing down are found prior to the abrupt warming event at the end of the Younger Dryas, but the indicators are less clear prior to the Bølling-Allerød warming, or glacial termination in Antarctica. Early warnings of thermohaline circulation collapse can be masked by inter-annual variability driven by atmospheric dynamics. However, rapidly decaying modes can be successfully filtered out by using a long bandwidth or by aggregating data. The two methods have complementary strengths and weaknesses and we recommend applying them together to improve the robustness of early warnings
Downstream hydraulic geometry of a tidally influenced river delta
Sassi, M.G. ; Hoitink, A.J.F. ; Brye, B. de; Deleersnijder, E. - \ 2012
Journal of Geophysical Research: Earth Surface 117 (2012). - ISSN 2169-9003 - 13 p.
sediment transport - mahakam delta - flow - estuaries - discharge - model - tides - propagation - channels - bifurcations
Channel geometry in tidally influenced river deltas can show a mixed scaling behavior between that of river and tidal channel networks, as the channel forming discharge is both of river and tidal origin. We present a method of analysis to quantify the tidal signature on delta morphology, by extending the hydraulic geometry concept originally developed for river channel networks to distributary channels subject to tides. Based on results from bathymetric surveys, a systematic analysis is made of the distributary channels in the Mahakam Delta (East Kalimantan, Indonesia). Results from a finite element numerical model are used to analyze the spatial variation of river and tidal discharges throughout the delta. The channel geometry of the fluvial distributary network scales with bifurcation order, until about halfway the radial distance from the delta apex to the sea. In the seaward part of the delta, distributary channels resemble funnel shaped estuarine channels. The break in morphology, which splits the delta into river- and tide-dominated parts, coincides with a break in the ratio between tidal to fluvial discharges. Downstream hydraulic geometry exponents of the cross-sectional area show a transition from the landward part to the seaward part of the delta. The numerical simulations show that the tidal impact on river discharge division at bifurcations increases with the bifurcation order, and that the variation of river discharge throughout the network is largely affected by the tides. The tidal influence is reflected by the systematic variation of downstream hydraulic geometry exponents.
Tidal impact on the division of river discharge and distributary channels in the Mahakam Delta
Sassi, M.G. ; Hoitink, A.J.F. ; Brye, B. de; Vermeulen, B. ; Deleersnijder, E. - \ 2011
Ocean Dynamics 61 (2011)12. - ISSN 1616-7341 - p. 2211 - 2228.
doppler current profiler - sediment transport - multiscale model - flow - propagation - tides - bifurcations - friction
Bifurcations in tidally influenced deltas distribute river discharge over downstream channels, asserting a strong control over terrestrial runoff to the coastal ocean. Whereas the mechanics of river bifurcations is well-understood, junctions in tidal channels have received comparatively little attention in the literature. This paper aims to quantify the tidal impact on subtidal discharge distribution at the bifurcations in the Mahakam Delta, East Kalimantan, Indonesia. The Mahakam Delta is a regular fan-shaped delta, composed of a quasi-symmetric network of rectilinear distributaries and sinuous tidal channels. A depth-averaged version of the unstructured-mesh, finite-element model second-generation Louvain-la-Neuve Ice-ocean Model has been used to simulate the hydrodynamics driven by river discharge and tides in the delta channel network. The model was forced with tides at open sea boundaries and with measured and modeled river discharge at upstream locations. Calibration was performed with water level time series and flow measurements, both spanning a simulation period. Validation was performed by comparing the model results with discharge measurements at the two principal bifurcations in the delta. Results indicate that within 10 to 15 km from the delta apex, the tides alter the river discharge division by about 10% in all bifurcations. The tidal impact increases seaward, with a maximum value of the order of 30%. In general, the effect of tides is to hamper the discharge division that would occur in the case without tides
Modelling bird songs: Voice onset, overtones and registers
Zaccarelli, R. ; Elemans, C.P.H. ; Fitch, W.T. ; Herzel, H. - \ 2006
Acta Acustica United with Acustica 92 (2006)5. - ISSN 1610-1928 - p. 741 - 748.
vocal-fold model - 2-mass model - sound generation - ring doves - tract - syrinx - streptopelia - phonation - vocalizations - bifurcations
We analyze two symmetric two-mass models of the avian syrinx. Our first model applies to songbirds and is a rescaled version of the well-known human two-mass model. Our second model (trapezoidal model) introduces a smoother geometry and is used to simulate the ring dove (Streptopelia risoria) syrinx. Simulations show that both models exhibit self-sustained vibrations. We show that the occurrence of collisions and the intensity of harmonics depend strongly on the configuration of the syrinx. The songbird model does not present instabilities. The trapezoidal model, however, displays coexisting limit-cycles that represent vibrations with, and without collisions at the same pressure. Register-like transitions are accompanied by subharmonics and deterministic chaos.
Dynamics of Vibrating Atomic Force Microscopy
Water, W. van de; Molenaar, J. - \ 2000
Nanotechnology 11 (2000)3. - ISSN 0957-4484 - p. 192 - 199.
impact oscillators - bifurcations - behavior - motion
An atomic force microscope which is operated in the oscillating mode is an example of an impact oscillator. The description of such dynamical systems can be reduced to a mathematical mapping, which displays a square-root singularity. A direct consequence of this property is the emergence of an infinite series of period-adding bifurcations. This extremely characteristic phenomenon should be observed in atomic force microscopes. We consider an atomic force microscope in which the tip-substrate forces are modelled by a liquid-bridge interaction. By integrating the dynamical equations we show that the atomic force microscopy (AFM) dynamical behaviour has the same characteristic bifurcation scenario as the square-root map. We point to the remarkable role of the energy that is dissipated upon impact. We finally suggest ways to improve the operation of AFM
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