bluuebunny/crossref_metadata_2025 huggingface dataset:
Recurrent frequency-size distribution of characteristic events
Authors: Abaimov S. G. , Tiampo K. F. , Turcotte D. L. , Rundle J. B.
Published: April 2010

Abstract. Statistical frequency-size (frequency-magnitude) properties of earthquake occurrence play an important role in seismic hazard assessments. The behavior of earthquakes is represented by two different statistics: interoccurrent behavior in a region and recurrent behavior at a given point on a fault (or at a given fault). The interoccurrent frequency-size behavior has been investigated by many authors and generally obeys the power-law Gutenberg-Richter distribution to a good approximation. It is expected that the recurrent frequency-size behavior should obey different statistics. However, this problem has received little attention because historic earthquake sequences do not contain enough events to reconstruct the necessary statistics. To overcome this lack of data, this paper investigates the recurrent frequency-size behavior for several problems. First, the sequences of creep events on a creeping section of the San Andreas fault are investigated. The applicability of the Brownian passage-time, lognormal, and Weibull distributions to the recurrent frequency-size statistics of slip events is tested and the Weibull distribution is found to be the best-fit distribution. To verify this result the behaviors of numerical slider-block and sand-pile models are investigated and the Weibull distribution is confirmed as the applicable distribution for these models as well. Exponents β of the best-fit Weibull distributions for the observed creep event sequences and for the slider-block model are found to have similar values ranging from 1.6 to 2.2 with the corresponding aperiodicities CV of the applied distribution ranging from 0.47 to 0.64. We also note similarities between recurrent time-interval statistics and recurrent frequency-size statistics.
Quantitative analysis of randomness exhibited by river channels using chaos game technique: Mississippi, Amazon, Sava and Danube case studies
Authors: Žibret G. , Verbovšek T.
Published: April 2010

Abstract. This paper presents a numerical evaluation of the randomness which can be observed in the geometry of major river channels. The method used is based upon that of generating a Sierpinski triangle via the chaos game technique, played with the sequence representing the river topography. The property of the Sierpinski triangle is that it can be constructed only by playing a chaos game with random values. Periodic or chaotic sequences always produce an incomplete triangle. The quantitative data about the scale of the random behaviour of the river channel pathway was evaluated by determination of the completeness of the triangle, generated on the basis of sequences representing the river channel, and measured by its fractal dimension. The results show that the most random behaviour is observed for the Danube River when sampled every 715 m. By comparing the maximum dimension of the obtained Sierpinski triangle with the gradient of the river we can see a strong correlation between a higher gradient corresponding to lower random behaviour. Another connection can be seen when comparing the length of the segment where the river shows the most random flow with the total length of the river. The shorter the river, the denser the sampling rate of observations has to be in order to obtain a maximum degree of randomness. From the comparison of natural rivers with the computer-generated pathways the most similar results have been produced by a complex superposition of different sine waves. By adding a small amount of noise to this function, the fractal dimensions of the generated complex curves are the most similar to the natural ones, but the general shape of the natural curve is more similar to the generated complex one without the noise.
Climate spectrum estimation in the presence of timescale errors
Authors: Mudelsee M. , Scholz D. , Röthlisberger R. , Fleitmann D. , Mangini A. , Wolff E. W.
Published: April 2010

Abstract. We introduce an algorithm (called REDFITmc2) for spectrum estimation in the presence of timescale errors. It is based on the Lomb-Scargle periodogram for unevenly spaced time series, in combination with the Welch's Overlapped Segment Averaging procedure, bootstrap bias correction and persistence estimation. The timescale errors are modelled parametrically and included in the simulations for determining (1) the upper levels of the spectrum of the red-noise AR(1) alternative and (2) the uncertainty of the frequency of a spectral peak. Application of REDFITmc2 to ice core and stalagmite records of palaeoclimate allowed a more realistic evaluation of spectral peaks than when ignoring this source of uncertainty. The results support qualitatively the intuition that stronger effects on the spectrum estimate (decreased detectability and increased frequency uncertainty) occur for higher frequencies. The surplus information brought by algorithm REDFITmc2 is that those effects are quantified. Regarding timescale construction, not only the fixpoints, dating errors and the functional form of the age-depth model play a role. Also the joint distribution of all time points (serial correlation, stratigraphic order) determines spectrum estimation.
Joule heating and anomalous resistivity in the solar corona
Authors: Spangler S. R.
Published: April 2010

Abstract. Recent radioastronomical observations of Faraday rotation in the solar corona can be interpreted as evidence for coronal currents, with values as large as 2.5×109 Amperes (Spangler, 2007). These estimates of currents are used to develop a model for Joule heating in the corona. It is assumed that the currents are concentrated in thin current sheets, as suggested by theories of two dimensional magnetohydrodynamic turbulence. The Spitzer result for the resistivity is adopted as a lower limit to the true resistivity. The calculated volumetric heating rate is compared with an independent theoretical estimate by Cranmer et al. (2007). This latter estimate accounts for the dynamic and thermodynamic properties of the corona at a heliocentric distance of several solar radii. Our calculated Joule heating rate is less than the Cranmer et al estimate by at least a factor of 3×105. The currents inferred from the observations of Spangler (2007) are not relevant to coronal heating unless the true resistivity is enormously increased relative to the Spitzer value. However, the same model for turbulent current sheets used to calculate the heating rate also gives an electron drift speed which can be comparable to the electron thermal speed, and larger than the ion acoustic speed. It is therefore possible that the coronal current sheets are unstable to current-driven instabilities which produce high levels of waves, enhance the resistivity and thus the heating rate.
The diffuse ensemble filter
Authors: Yang X. , DelSole T.
Published: April 2010

Abstract. A new class of ensemble filters, called the Diffuse Ensemble Filter (DEnF), is proposed in this paper. The DEnF assumes that the forecast errors orthogonal to the first guess ensemble are uncorrelated with the latter ensemble and have infinite variance. The assumption of infinite variance corresponds to the limit of "complete lack of knowledge" and differs dramatically from the implicit assumption made in most other ensemble filters, which is that the forecast errors orthogonal to the first guess ensemble have vanishing errors. The DEnF is independent of the detailed covariances assumed in the space orthogonal to the ensemble space, and reduces to conventional ensemble square root filters when the number of ensembles exceeds the model dimension. The DEnF is well defined only in data rich regimes and involves the inversion of relatively large matrices, although this barrier might be circumvented by variational methods. Two algorithms for solving the DEnF, namely the Diffuse Ensemble Kalman Filter (DEnKF) and the Diffuse Ensemble Transform Kalman Filter (DETKF), are proposed and found to give comparable results. These filters generally converge to the traditional EnKF and ETKF, respectively, when the ensemble size exceeds the model dimension. Numerical experiments demonstrate that the DEnF eliminates filter collapse, which occurs in ensemble Kalman filters for small ensemble sizes. Also, the use of the DEnF to initialize a conventional square root filter dramatically accelerates the spin-up time for convergence. However, in a perfect model scenario, the DEnF produces larger errors than ensemble square root filters that have covariance localization and inflation. For imperfect forecast models, the DEnF produces smaller errors than the ensemble square root filter with inflation. These experiments suggest that the DEnF has some advantages relative to the ensemble square root filters in the regime of small ensemble size, imperfect model, and copious observations.
Surface mixing and biological activity in the four Eastern Boundary Upwelling Systems
Authors: Rossi V. , López C. , Hernández-García E. , Sudre J. , Garçon V. , Morel Y.
Published: April 2010

Abstract. Eastern Boundary Upwelling Systems (EBUS) are characterized by a high productivity of plankton associated with large commercial fisheries, thus playing key biological and socio-economical roles. Since they are populated by several physical oceanic structures such as filaments and eddies, which interact with the biological processes, it is a major challenge to study this sub- and mesoscale activity in connection with the chlorophyll distribution. The aim of this work is to make a comparative study of these four upwelling systems focussing on their surface stirring, using the Finite Size Lyapunov Exponents (FSLEs), and their biological activity, based on satellite data. First, the spatial distribution of horizontal mixing is analysed from time averages and from probability density functions of FSLEs, which allow us to divide each areas in two different subsystems. Then we studied the temporal variability of surface stirring focussing on the annual and seasonal cycle. We also proposed a ranking of the four EBUS based on the averaged mixing intensity. When investigating the links with chlorophyll concentration, the previous subsystems reveal distinct biological signatures. There is a global negative correlation between surface horizontal mixing and chlorophyll standing stocks over the four areas. To try to better understand this inverse relationship, we consider the vertical dimension by looking at the Ekman-transport and vertical velocities. We suggest the possibility of a changing response of the phytoplankton to sub/mesoscale turbulence, from a negative effect in the very productive coastal areas to a positive one in the open ocean. This study provides new insights for the understanding of the variable biological productivity in the ocean, which results from both dynamics of the marine ecosystem and of the 3-D turbulent medium.
Large-scale instability of a generalized turbulent Kolmogorov flow
Authors: Legras B. , Villone B.
Published: April 2010

Abstract. We present an analytical study of the large scale instability of a generalized turbulent Kolmogorov flow, i.e. a periodic shear flow where the molecular viscosity has been substituted by an eddy viscosity parameterized with the Clark-Smagorinsky model and where the external forcing is adapted to maintain the flow against this dissipation. We employ multiscaling technique assuming a scale separation between the basic scale of such a generalized turbulent Kolmogorov flow and the largest scales of the flow. The main result is that an amplitude equation for the large-scale secondary flow is obtained which exhibits, like for the standard Kolmogorov flow, an instability of the negative viscosity type. We find that the presence of mirror symmetry in the basic flow is a necessary condition and that further propagative and nonlinear contribution are produced otherwise. The result is encouraging for the generic existence of large-scale instabilities of the negative viscosity type in fully turbulent flows.
The stochastic multiplicative cascade structure of deterministic numerical models of the atmosphere
Authors: Stolle J. , Lovejoy S. , Schertzer D.
Published: April 2010

Abstract. By direct statistical analysis we show that over almost all their range of scales and to within typically better than ±1%, atmospheric fields obtained from analyses and numerical integrations of atmospheric models have the multifractal structure predicted by multiplicative cascade models. We quantify this for the horizontal wind, temperature, and humidity fields at 5 different pressure levels for the ERA40 reanalysis, the Canadian Meteorological Centre Global Environmental Multiscale (CMC, GEM) model, as well as the National Oceanographic and Atmospheric Administration Global Forecasting System (NOAA, GFS). We investigate the additional prediction that the cascade belongs to a universal multifractal basin of attraction. By demonstrating a "Levy collapse" of the statistical moments to within ±2 to ±5% over most of the range of scales, we conclude that there is good evidence for this. Finally, we discuss how this stochastic multiplicative cascade structure can be exploited in improving ensemble forecasts.
Acceleration and transport of ions in turbulent current sheets: formation of non-maxwelian energy distribution
Authors: Artemyev A. V. , Zelenyi L. M. , Malova H. V. , Zimbardo G. , Delcourt D.
Published: April 2010

Abstract. The paper is devoted to particle acceleration in turbulent current sheet (CS). Our results show that the mechanism of CS particle interaction with electromagnetic turbulence can explain the formation of power law energy distributions. We study the ratio between adiabatic acceleration of particles in electric field in the presence of stationary turbulence and acceleration due to electric field in the case of dynamic turbulence. The correlation between average energy gained by particles and average particle residence time in the vicinity of the neutral sheet is discussed. It is also demonstrated that particle velocity distributions formed by particle-turbulence interaction are similar in essence to the ones observed near the far reconnection region in the Earth's magnetotail.
Low-frequency variability of the Kuroshio Extension
Authors: Pierini S. , Dijkstra H. A.
Published: April 2010

Abstract. In this paper, we provide a review of recent results targeted at the understanding of the low-frequency variability of the Kuroshio Extension. We provide the background and main arguments of two views which have recently been proposed to explain this variability. In the first view, wind-induced Rossby waves and the effects of mesocale eddies are crucial. The second view is based on low-dimensional equivalent-barotropic large-scale nonlinear dynamics, with neither Rossby wave dynamics nor baroclinic instability being important. Results from models supporting each view are discussed and confronted with results from available observations.
Advection/diffusion of large scale magnetic field in accretion disks
Authors: Lovelace R. V. E. , Bisnovatyi-Kogan G. S. , Rothstein D. M.
Published: April 2010

Abstract. Activity of the nuclei of galaxies and stellar mass systems involving disk accretion to black holes is thought to be due to (1) a small-scale turbulent magnetic field in the disk (due to the magneto-rotational instability or MRI) which gives a large viscosity enhancing accretion, and (2) a large-scale magnetic field which gives rise to matter outflows and/or electromagnetic jets from the disk which also enhances accretion. An important problem with this picture is that the enhanced viscosity is accompanied by an enhanced magnetic diffusivity which acts to prevent the build up of a significant large-scale field. Recent work has pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the MRI is suppressed high in the disk where the magnetic and radiation pressures are larger than the thermal pressure. Here, we calculate the vertical (z) profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity due to the MRI and the fact that the turbulence vanishes at the surface of the disk. We derive a sixth-order differential equation for the radial flow velocity vr(z) which depends mainly on the midplane thermal to magnetic pressure ratio β>1 and the Prandtl number of the turbulence P=viscosity/diffusivity. Boundary conditions at the disk surface take into account a possible magnetic wind or jet and allow for a surface current in the highly conducting surface layer. The stationary solutions we find indicate that a weak (β>1) large-scale field does not diffuse away as suggested by earlier work.
The effect of shear on the generation of gravity waves
Authors: Humi M.
Published: April 2010

Abstract. Previous research regarding the solutions of Long's equation always presumed that the flow far upstream is without shear. In this paper we derive the proper form of this equation when shear is present. We then apply a sequence of transformations to this equation which make it possible to linearize it while preserving its physical contents. We then derive conditions under which the solutions of this linear equation admit the existence (or creation) of gravity waves. We present also a solution of this model equation when the presence of shear in the overall flow is "small".
Kernel estimation and display of a five-dimensional conditional intensity function
Authors: Adelfio G.
Published: April 2010

Abstract. The aim of this paper is to find a convenient and effective method of displaying some second order properties in a neighbourhood of a selected point of the process. The used techniques are based on very general high-dimensional nonparametric smoothing developed to define a more general version of the conditional intensity function introduced in earlier earthquake studies by Vere-Jones (1978).
Vertical mixing and coherent anticyclones in the ocean: the role of stratification
Authors: Koszalka I. , Ceballos L. , Bracco A.
Published: April 2010

Abstract. The role played by wind-forced anticyclones in the vertical transport and mixing at the ocean mesoscale is investigated with a primitive-equation numerical model in an idealized configuration. The focus of this work is to determine how the stratification impacts such transport. The flows, forced only at the surface by an idealized wind forcing, are predominantly horizontal and, on average, quasigeostrophic. Inside vortex cores and intense filaments, however, the dynamics is strongly ageostrophic. Mesoscale anticyclones appear as "islands" of increased penetration of wind energy into the ocean interior and they represent the maxima of available potential energy. The amount of available potential energy is directly correlated with the degree of stratification. The wind energy injected at the surface is transferred at depth through the generation and subsequent straining effect of Vortex Rossby Waves (VRWs), and through near-inertial internal oscillations trapped inside anticyclonic vortices. Both these mechanisms are affected by stratification. Stronger transfer but larger confinement close to the surface is found when the stratification is stronger. For weaker stratification, vertical mixing close to the surface is less intense but below about 150 m attains substantially higher values due to an increased contribution of both VRWs, whose time scale is on the order of few days, and of near-inertial motions, with a time scale of few hours.
A study of the phase instability of quasi-geostrophic Rossby waves on the infinite β-plane to zonal flow perturbations
Authors: Marié L.
Published: April 2010

Abstract. The problem of the linear instability of quasi-geostrophic Rossby waves to zonal flow perturbations is investigated on an infinite β-plane using a phase dynamics formalism. Equations governing the coupled evolutions of a zonal velocity perturbation and phase and amplitude perturbations of a finite-amplitude wave are obtained. The analysis is valid in the limit of infinitesimal, zonally invariant perturbation components, varying slowly in the meridional direction and with respect to time. In the case of a slow sinusoidal meridional variation of the perturbation components, analytical expressions for the perturbation growth rates are obtained, which are checked against numerical codes based on standard Floquet theory.
Variable predictability in deterministic dissipative sandpile
Authors: Shnirman M. G. , Shapoval A. B.
Published: April 2010

Abstract. It is known that some quiescence precedes the strong events in the Bak–Tang–Wiesenfeld sand-pile (Pepke and Carlson, 1994) We introduce dissipation depending on the propagation of the events into this model such that in the constructed model the growth of activity occurs before the strong events. This fact allows the prediction of them in advance with a certain efficiency. This efficiency is variable in time. The best predictability is observed during subcritical time ranges, while the efficiency is definitely worse in the supercritical state.
Cellular automaton for realistic modelling of landslides
Authors: Segre E. , Deangeli C.
Published: April 2010

Abstract. A numerical model is developed for the simulation of debris flow in landslides over a complex three dimensional topography. The model is then validated by comparing a simulation with reported field data. Our model is in fact a realistic elaboration of simpler "sandpile automata", which have in recent years been studied as supposedly paradigmatic of "self-organized criticality". Statistics and scaling properties of the simulation are examined, and show that the model has an intermittent behaviour.
Fractal dimension of lightning discharge
Authors: Sañudo J. , Gómez J. B. , Castaño F. , Pacheco A. F.
Published: April 2010

Abstract. Using a stochastic model, we simulate the process of dielectric breakdown in the atmosphere and calculate the fractal dimension of 3-dimensional lightning patterns. Finite-size effects have been studied. The projections of our patterns on vertical planes fit the experimental fractal dimension obtained from photographic analysis. This work is inspired by a previous work by A.A. Tsonis.
Experimental study of the statistics of slipping events in solid friction
Authors: Ciliberto S. , Laroche C.
Published: April 2010

Abstract. The statistics of the stick-slip motion is studied in two experiments, where elasticity is distributed either on a surface or on a volume. The rough surface is realised by embedding steel spheres in an elastic substrate whereas the volume is constituted by several layers of rubber spheres. Both systems produce a complex dynamics characterized by power law with non-trivial exponents in the distribution of the amplitude of the slipping events and in the power spectra of the friction force time evolution. The dependence of the results on the system size is also studied.
Temporal variations of the fractal properties of seismicity in the western part of the north Anatolian fault zone: possible artifacts due to improvements in station coverage
Authors: Öncel A. O. , Alptekin Ö. , Main I.
Published: April 2010

Abstract. Seismically-active fault zones are complex natural systems exhibiting scale-invariant or fractal correlation between earthquakes in space and time, and a power-law scaling of fault length or earthquake source dimension consistent with the exponent b of the Gutenberg-Richter frequency-magnitude relation. The fractal dimension of seismicity is a measure of the degree of both the heterogeneity of the process (whether fixed or self-generated) and the clustering of seismic activity. Temporal variations of the b-value and the two-point fractal (correlation) dimension Dc have been related to the preparation process for natural earthquakes and rock fracture in the laboratory These statistical scaling properties of seismicity may therefore have the potential at least to be sensitive short- term predictors of major earthquakes. The North Anatolian Fault Zone (NAFZ) is a seismicallyactive dextral strike slip fault zone which forms the northern boundary of the westward moving Anatolian plate. It is splayed into three branches at about 31oE and continues westward toward the northern Aegean sea. In this study, we investigate the temporal variation of Dc and the Gutenberg-Richter b-value for seismicity in the western part of the NAFZ (including the northern Aegean sea) for earthquakes of Ms > 4.5 occurring in the period between 1900 and 1992. b ranges from 0.6-1.6 and Dc from 0.6 to 1.4. The b-value is found to be weakly negatively correlated with Dc (r=-0.56). However the (log of) event rate N is positively correlated with b, with a similar degree of statistical significance (r=0.42), and negatively correlated with Dc (r=-0.48). Since N increases dramatically with improved station coverage since 1970, the observed negative correlation between b and Dc is therefore more likely to be due to this effect than any underlying physical process in this case. We present this as an example of how man-made artefacts of recording can have similar statistical effects to underlying processes.
Organisation of joints and faults from 1-cm to 100-km scales revealed by optimized anisotropic wavelet coefficient method and multifractal analysis
Authors: Ouillon G. , Sornette D. , Castaing C.
Published: April 2010

Abstract. The classical method of statistical physics deduces the macroscopic behaviour of a system from the organization and interactions of its microscopical constituents. This kind of problem can often be solved using procedures deduced from the Renormalization Group Theory, but in some cases, the basic microscopic rail are unknown and one has to deal only with the intrinsic geometry. The wavelet analysis concept appears to be particularly adapted to this kind of situation as it highlights details of a set at a given analyzed scale. As fractures and faults generally define highly anisotropic fields, we defined a new renormalization procedure based on the use of anisotropic wavelets. This approach consists of finding an optimum filter will maximizes wavelet coefficients at each point of the fie] Its intrinsic definition allows us to compute a rose diagram of the main structural directions present in t field at every scale. Scaling properties are determine using a multifractal box-counting analysis improved take account of samples with irregular geometry and finite size. In addition, we present histograms of fault length distribution. Our main observation is that different geometries and scaling laws hold for different rang of scales, separated by boundaries that correlate well with thicknesses of lithological units that constitute the continental crust. At scales involving the deformation of the crystalline crust, we find that faulting displays some singularities similar to those commonly observed in Diffusion- Limited Aggregation processes.
Seismic tomography and mixing in the deep earth
Authors: Peltier W. R. , Pari G. , Dziewonski A. M.
Published: April 2010

Abstract. Recently constructed tomographic models of the lateral heterogeneity of elastic properties in the Earth's mantle are contrasted in terms of their implications concerning the extent to which the endothermic phase transformation at 660 km depth is influencing the radial style of mixing. Previously published whole mantle and split mantle tomographic reconstructions, SH8/WMI3 and SH8/U4L8 respectively, fit the seismic observations equally well but disagree on the extent to which radial mixing may be impeded across this depth horizon. We show that inferences from seismic tomographic images based on the application of diagnostic functions (global and regional variance spectra and the radial correlation function) lead to the conclusion that the mantle circulation is whole mantle in style if model SH8/WM13 is employed. The split mantle tomographic inversion SHS/U4L8 leads to the contradictory conclusion that the mantle circulation is significantly impeded across the 660 km depth horizon. This latter interpretation is reinforced when we employ the new higher resolution split mantle model SH12/U7L5 in our calculations. We demonstrate that the depth-dependent radial heat flow delivered by both of the split models implies the existence of a thermal boundary layer at 660 km depth, and therefore significant layering, whereas that delivered by the whole mantle model does not. By insisting that the depth-dependent viscosity profile of the mantle be compatible with the thermal structure if the flow were layered, we argue that the split mantle tomographic inversions lead to a self-consistent description of geodynamic constraints (geoid and postglacial rebound data).
Limits and characteristics of the multifractal behaviour of a high-resolution rainfall time series
Authors: Olsson J.
Published: April 2010

Abstract. The multifractal properties of a 2-year time series of 8-min rainfall intensity observations are investigated. The empirical probability distribution function suggests a hyperbolic intermittency with divergence of moment of order greater than 2. The power spectrum E(f) of the series obeys a power law form E(f)=f -0.66 in the range of scales 8 min to approximately 3 days. The variation of the average statistical moments with scale shows that the series is characterized by a multifractal behaviour between 8 min and approximately 11 days. The multifractal parameters associated with universality were estimated to be α=0.63 and C1=0,44 by using the Double Trace Moment, DTM, technique. The moment scaling functions obtained from the empirical values and the universal expression are in good agreement in the approximate range 1≤q≤3. Outside of this range, however, differences exist which may be related to either limitations of the data or an inexact estimation of the parameters by DTM. The evident multifractal nature of rainfall time series is encouraging since it may lead to new and improved ways of processing rainfall data used in hydrological calculations.
Successive bifurcations in a shallow-water model applied to the wind-driven ocean circulation
Authors: Speich S. , Dijkstra H. , Ghil M.
Published: April 2010

Abstract. Climate - the "coarse-gridded" state of the coupled ocean - atmosphere system - varies on many time and space scales. The challenge is to relate such variation to specific mechanisms and to produce verifiable quantitative explanations. In this paper, we study the oceanic component of the climate system and, in particular, the different circulation regimes of the mid-latitude win driven ocean on the interannual time scale. These circulations are dominated by two counterrotating, basis scale gyres: subtropical and subpolar. Numerical techniques of bifurcation theory are used to stud the multiplicity and stability of the steady-state solution of a wind-driven, double-gyre, reduced-gravity, shallow water model. Branches of stationary solutions and their linear stability are calculated systematically as parameter are varied. This is one of the first geophysical studies i which such techniques are applied to a dynamical system with tens of thousands of degrees of freedom. Multiple stationary solutions obtain as a result of nonlinear interactions between the two main recirculating cell (cyclonic and anticyclonic) of the large- scale double-gyre flow. These equilibria appear for realistic values of the forcing and dissipation parameters. They undergo Hop bifurcation and transition to aperiodic solutions eventually occurs. The periodic and chaotic behaviour is probably related to an increased number of vorticity cells interaction with each other. A preliminary comparison with observations of the Gulf Stream and Kuroshio Extensions suggests that the intern variability of our simulated mid-latitude ocean is a important factor in the observed interannual variability o these two current systems.
Models of explosive volcanism
Authors: Woods A. W. , Bower S. M. , Bursik M. I.
Published: April 2010

Abstract. We describe a series of models which illustrate the controls upon the evolution of an erupting mixture of ash and gas during an explosive volcanic eruption. For large eruption rates, material typically issues from a crater as a supersonic jet which may entrain and heat sufficient air to become buoyant and form a Plinian eruption column. If a buoyant eruption column is able to form, then this column may ascend to heights of order 10-30 km, depending upon the erupted mass flux. In contrast, for low eruption rates, a shock forms in the crater and the material issues as a slow subsonic flow which generates dense hot ash flows. A new model shows that as such ash flows propagate from the vent, the density of the flow decreases mainly due to sedimentation, until ultimately the residual ash flow becomes buoyant. The distance the flow travels before becoming buoyant increases with the mass flux in the current and the mean size of particles in the current, but decreases with the flow temperature. It also depends upon the mass of air entrained into the collapsing fountain. The mass fraction of solid lifted from such ash flows into the ascending cloud depends mainly upon the mass of air entrained into the collapsing fountain near the volcanic vent. We apply our models to predict run-out distances and deposition patterns produced by erupting volcanoes.
The evolution of a thermocline effected by a turbulent stream
Authors: Druzhinin O. A. , Kazakov V. I. , Matusov P. A. , Ostrovsky L. A.
Published: April 2010

Abstract. The process of thermocline evolution under the action of turbulent stream in the upper layer is investigated in laboratory experiment in thermally stratified tank, the initial temperature profile with pronounced thermocline being similar to that observed in tropic seas. The mean velocity and turbulent energy spatial distributions have been shaped to model the hydrological conditions in strong oceanic currents or wind-induced drag currents. The experiment demonstrates the gradual deepening and transformation of thermocline in a case where no global instability took place (i.e., with Richardson numbers always exceeding 0.3-0.4). The process of thermocline evolution resulted also in recurring "bursts" of microstructure. A numerical experiment based on equations of semi-empirical theory of turbulence shows quantitative agreement with experimental data. Moreover, simple analytical solutions and numerical results show that a layer with marginal stability is formed with Richardson numbers being very close to the stability threshold, so that quite small disturbances in thermocline can result in appearance of internal waves and bursts of turbulence.
Estimations of the nonlinear properties of the internal wave field off the Israel coast
Authors: Pelinovksy E. , Talipova T. , Ivanov V.
Published: April 2010

Abstract. The measurements of the vertical structure of hydrological fields and internal waves on the Levantine Sea's polygon in the Mediterranean, obtained in the 27-th cruise of the RV "Professor Kolesnikov" in 1991, have been used to estimate the kinematic and nonlinear characteristics of the internal wave field. Statistical and spatial distributions of the vertical profiles of the Brunt-Vaisala frequency are described. They have been used to calculate the coefficients of the Korteweg - de Vries equation. This equation forms the main model for nonlinear parameters. It is shown that the variations of the long wave speed propagation and the dispersion parameter are relatively small in comparison with the variation of the nonlinear parameter. Estimations of the nonlinear properties of the internal waves, being measured, based on the calculation of the local Ursell parameter are given. This method can be used for investigation of the internal wave transformation processes in oceanic regions with horizontal variability of the hydrophysical fields (temperature, salinity) and sloped sea floor.
Flux-gradient relationship for turbulent dispersion over complex terrain
Authors: Tampieri F. , Giostra U. , Trombetti F. , Anfossi D. , Ferrero E. , Tinarelli G.
Published: April 2010

Abstract. The transfer of a passive tracer in inhomogeneous turbulent flow is investigated. Starting from Lumley's constitutive equation, we derived an expression for the ratio between the effective eddy diffusivity K and eddy diffusivity K as a function of three length scales characterizing the local turbulence structure, flux variations and turbulence inhomogeneities. The theoretical predictions for the one-dimensional case of inhomogeneous symmetric turbulence were validated through a comparison with the numerical results of a Lagrangian particle model simulating a wind tunnel experiment of dispersion in the lee of an idealized two-dimensional hill. A qualitative agreement is reached between the theoretical evaluation of K and the value obtained from the numerical simulation.
Faults self-organized by repeated earthquakes in a quasi-static antiplane crack model
Authors: Sornette D. , Vanneste C.
Published: April 2010

Abstract. We study a 2D quasi-static discrete crack anti-plane model of a tectonic plate with long range elastic forces and quenched disorder. The plate is driven at its border and the load is transferred to all elements through elastic forces. This model can be considered as belonging to the class of self-organized models which may exhibit spontaneous criticality, with four additional ingredients compared to sandpile models, namely quenched disorder, boundary driving, long range forces and fast time crack rules. In this "crack" model, as in the "dislocation" version previously studied, we find that the occurrence of repeated earthquakes organizes the activity on well-defined fault-like structures. In contrast with the "dislocation" model, after a transient, the time evolution becomes periodic with run-aways ending each cycle. This stems from the "crack" stress transfer rule preventing criticality to organize in favour of cyclic behaviour. For sufficiently large disorder and weak stress drop, these large events are preceded by a complex spacetime history of foreshock activity, characterized by a Gutenberg-Richter power law distribution with universal exponent B = 1±0.05. This is similar to a power law distribution of small nucleating droplets before the nucleation of the macroscopic phase in a first-order phase transition. For large disorder and large stress drop, and for certain specific initial disorder configurations, the stress field becomes frustrated in fast time: out-of-plane deformations (thrust and normal faulting) and/or a genuine dynamics must be introduced to resolve this frustration.
Renormalization group theory of earthquakes
Authors: Saleur H. , Sammis C. G. , Sornette D.
Published: April 2010

Abstract. We study theoretically the physical origin of the proposed discrete scale invariance of earthquake processes, at the origin of the universal log-periodic corrections to scaling, recently discovered in regional seismic activity (Sornette and Sammis (1995)). The discrete scaling symmetries which may be present at smaller scales are shown to be robust on a global scale with respect to disorder. Furthermore, a single complex exponent is sufficient in practice to capture the essential properties of the leading correction to scaling, whose real part may be renormalized by disorder, and thus be specific to the system. We then propose a new mechanism for discrete scale invariance, based on the interplay between dynamics and disorder. The existence of non-linear corrections to the renormalization group flow implies that an earthquake is not an isolated "critical point", but is accompanied by an embedded set of "critical points", its foreshocks and any subsequent shocks for which it may be a foreshock.
Spectral evolution of two-layer weak geostrophic turbulence. Part I: Typical scenarios
Authors: Soomere T.
Published: April 2010

Abstract. Long-time evolution of large-scale geophysical flows is considered in a β-plane approximation. Motions in an infinite 2-layer model ocean are treated as a system of weakly nonlinear Rossby waves (weak geostrophic turbulence). The evolution of the energy spectrum of the barotropic and the baroclinic modes is investigated on the basis of numerical experiments with the kinetic equation for baroclinic Rossby waves. The basic features of free (nonforced inviscid) spectral evolution of baroclinic flows are similar to those of the barotropic motions. A portion of the energy is transferred to a sharp spectral peak while the rest of it is isotropically distributed. The peak corresponds to an intensive nearly zonal barotropic flow. Typically, this well-defined barotropic zonal anisotropy inhibits the reinforcement of its baroclinic analogy. For a certain set of initial conditions (in particular, if the barotropic zonal flow is not present initially), a zonal anisotropy of both modes is generated. The interplay between the multimodal nearly zonal flow components leads to the excitation of large-scale (several times exceeding the scale of the initial state), mostly meridional, baroclinic motions at the expense of the barotropic nearly zonal flow. The underlying mechanism is explained on the level of elementary mixed-triad interaction. The whole wave field retains its essentially baroclinic as well as spectrally broad nature. It evidently tends towards a thermodynamically equilibrated final state, consisting of the superposition of a (usually barotropic, but occasionally multimodal) zonal flow and a wave system with a Raleigh-Jeans spectrum. This evolution takes place as a multi-staged process, with fast convergence of the modal spectra to a local equilibrium followed by a more gradual adjustment of the energy balance between the modes.
Nonlinear interaction between acoustic gravity waves in a rotating atmosphere
Authors: Axelsson P. , Larsson J. , Stenflo L.
Published: April 2010

Abstract. The influence of the Earth's rotation on the resonant interaction of atmospheric waves is investigated. The explicit expressions for the coupling coefficients are presented. They are derived by means of two different techniques; first, by a direct expansion derivation from a set of reduced equations, and second, by a Hamiltonian method.
Multifractal intermittency of Eulerian and Lagrangian turbulence of ocean temperature and plankton fields
Authors: Seuront L. , Schmitt F. , Schertzer D. , Lagadeuc Y. , Lovejoy S.
Published: April 2010

Abstract. In this paper, we present evidence that intermittency of Eulerian and Lagrangian turbulence of ocean temperature and plankton fields is multifractal and furthermore can be analysed with the help of universal multifractals. We analyse time series of temperature and in vivo fluorescence taken from a drifter in the mixed coastal waters of the eastern English Channel. Two analysis techniques are used to compute the fundamental universal multifiractal parameters, which describe all the statistics of the turbulent fluctuations: the analysis of the scale invariant structure function exponent ζ(q) and the Double Trace Moment technique. At small scales, we do not detect any significant difference between the universal multifiractal behavior of temperature and fluorescence in an Eulerian framework. This supports the hypothesis that the latter is passively advected with the flow as the former. On the one hand, we show that large scale measurements are Lagrangian and indeed we obtain for temperature fluctuations a ω2 power spectrum corresponding to the theoretical scaling of a Lagrangian passive scalar. Furthermore, we show that Lagrangian temperature fluctuations are multiscaling and intermittent. On the other hand, the flatter slope at large scales of the fluorescence power spectrum points out that the plankton is at these scales a "biologically active" scalar.
Solar wind low-frequency magnetohydrodynamic turbulence: extended self-similarity and scaling laws
Authors: Carbone V. , Veltri P. , Bruno R.
Published: April 2010

Abstract. In this paper we review some of the work done in investigating the scaling properties of Magnetohydrodynamic turbulence, by using velocity fluctuations measurements performed in the interplanetary space plasma by the Helios spacecraft. The set of scaling exponents ξq for the q-th order velocity structure functions, have been determined by using the Extended Self-Similarity hypothesis. We have found that the q-th order velocity structure function, when plotted vs. the 4-th order structure function, displays a range of self-similarity which extends over all the lengths covered by measurements, thus allowing for a very good determination of ξq. Moreover the results seem to show that the scaling exponents are the same regardless the various observation periods considered. The obtained scaling exponents have been compared with the results of some intermittency models for Kraichnan's turbulence, derived in the framework of infinitely divisible fragmentation processes, showing the good agreement between these models and our observations. Finally, on the basis of the actually available data sets, we show that scaling laws in Solar Wind turbulence seem to be different from turbulent scaling laws in the ordinary fluid flows. This is true for high-order velocity structure functions, while low-order velocity structure functions show the same scaling laws. Since our measurements involve length scales which extend over many order of magnitude where dissipation is practically absent, our results show that Solar Wind turbulence can be regarded as a testing bench for the investigation of general scaling behaviour in turbulent flows. In particular our results strongly support the point of view which attributes a key role to the inertial range dynamics in determining the intermittency characteristics in fluid flows, in contrast with the point of view which attributes intermittency to a finite Reynolds number effect.
Non-Gaussian statistics in space plasma turbulence: fractal properties and pitfalls
Authors: Dudok de Wit T. , Krasnosel'skikh V. V.
Published: April 2010

Abstract. Statistical properties of collisionless plasmas in the vicinity of the Earth's bow shock are investigated with the aim to characterize the intermittent behaviour of non- magnetohydrodynamic turbulence. The structure functions of the fluctuating magnetic field reveal an increasing departure from Gaussianity at small scales, which is similar to that observed in solar wind turbulence and is surprisingly little affected by the abrupt shock transition. While these results may be the signature of a multifractal process, a deeper inspection reveals caveats in such an interpretation. Several effects, including the anisotropy of the wavefield, the violation of the Taylor hypothesis and the occasional occurrence of coherent wave packets, strongly affect the higher order statistical properties. Most of the small differences observed between the up- and downstream sides of the shock can be ascribed to the occurrence of discrete whistler wavetrains, while the wavefield itself is much less intermittent. It is also shown how the finite length of the records prohibits a reliable estimation of structure functions beyond the fourth order. These results preclude an unambiguous identification of underlying models for intermittency.