NPG - recent papershttp://www.nonlin-processes-geophys.net/2017-05-24T16:07:11+02:00Copernicus Publicationshttp://dx.doi.org/10.5194/npg-24-227-2017Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula
<b>Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula</b><br>
Daniel Garaboa-Paz, Nieves Lorenzo, and Vicente Pérez-Muñuzuri<br>
Nonlin. Processes Geophys., 24, 227-235, doi:10.5194/npg-24-227-2017, 2017<br>
This paper evaluates the connection between winter precipitation over the Iberian Peninsula and the large-scale tropospheric mixing over the eastern Atlantic Ocean. Finite-time Lyapunov exponents (FTLEs) have been calculated from 1979 to 2008 to evaluate this mixing. Our study suggests that significant negative correlations exist between summer FTLE anomalies and winter precipitation over Portugal and Spain.
<b>Influence of finite-time Lyapunov exponents on winter precipitation over the Iberian Peninsula</b><br>
Daniel Garaboa-Paz, Nieves Lorenzo, and Vicente Pérez-Muñuzuri<br>
Nonlin. Processes Geophys., 24, 227-235, doi:10.5194/npg-24-227-2017, 2017<br>
Seasonal forecasts have improved during the last decades, mostly
due to an increase in understanding of the coupled ocean–atmosphere
dynamics, and the development of models able to predict the atmosphere
variability. Correlations between different teleconnection patterns and
severe weather in different parts of the world are constantly evolving and
changing. This paper evaluates the connection between winter precipitation
over the Iberian Peninsula and the large-scale tropospheric mixing over the
eastern Atlantic Ocean. Finite-time Lyapunov exponents (FTLEs) have been
calculated from 1979 to 2008 to evaluate this mixing. Our study suggests that
significant negative correlations exist between summer FTLE anomalies and
winter precipitation over Portugal and Spain. To understand the mechanisms
behind this correlation, summer anomalies of the FTLE have also been
correlated with other climatic variables such as the sea surface temperature
(SST), the sea level pressure (SLP) or the geopotential. The East Atlantic
(EA) teleconnection index correlates with the summer FTLE anomalies,
confirming their role as a seasonal predictor for winter precipitation over
the Iberian Peninsula.
Copernicus Electronic Production Support Office2017-05-24T16:07:11+02:002017-05-24T16:07:11+02:00http://dx.doi.org/10.5194/npg-24-215-2017Non-linear effects of pore pressure increase on seismic event generation in a multi-degree-of-freedom rate-and-state model of tectonic fault sliding
<b>Non-linear effects of pore pressure increase on seismic event generation in a multi-degree-of-freedom rate-and-state model of tectonic fault sliding</b><br>
Sergey B. Turuntaev and Vasily Y. Riga<br>
Nonlin. Processes Geophys., 24, 215-225, doi:10.5194/npg-24-215-2017, 2017<br>
The influence of fluid injection on tectonic fault sliding and generation of seismic events was studied in the paper by a multi-degree-of-freedom rate-and-state friction model with a two-parametric friction law. The considered system could exhibit different types of motion. The main seismic activity could appear directly after the start of fluid injection or in the post-injection phase (after some days or months). Such an influence of injection on seismicity is observed in the real cases.
<b>Non-linear effects of pore pressure increase on seismic event generation in a multi-degree-of-freedom rate-and-state model of tectonic fault sliding</b><br>
Sergey B. Turuntaev and Vasily Y. Riga<br>
Nonlin. Processes Geophys., 24, 215-225, doi:10.5194/npg-24-215-2017, 2017<br>
The
influence of fluid injection on tectonic fault sliding and seismic event
generations was studied by a multi-degree-of-freedom rate-and-state friction
model with a two-parametric friction law. A system of blocks (up to 25
blocks) elastically connected to each other and connected by elastic springs
to a constant-velocity moving driver was considered. Variation of the pore
pressure due to fluid injection led to variation of effective stress between
the first block and the substrate. Initially the block system was in a
steady-sliding state; then, its state was changed by the pore pressure
increase. The influence of the model parameters (number of blocks, spring
stiffness, velocity weakening parameter) on the seismicity variations was
considered. Various slip patterns were obtained and analysed.
Copernicus Electronic Production Support Office2017-05-22T16:07:11+02:002017-05-22T16:07:11+02:00http://dx.doi.org/10.5194/npg-24-203-2017Review article: Wave analysis methods for space plasma experiment
<b>Review article: Wave analysis methods for space plasma experiment</b><br>
Yasuhito Narita<br>
Nonlin. Processes Geophys., 24, 203-214, doi:10.5194/npg-24-203-2017, 2017<br>
Various methods in the single-spacecraft data analysis are reviewed to determine physical properties of waves, turbulent fluctuations, and wave-wave and wave-particle interactions in the space plasma environment using the magnetic field, the electric field, and the plasma data.
<b>Review article: Wave analysis methods for space plasma experiment</b><br>
Yasuhito Narita<br>
Nonlin. Processes Geophys., 24, 203-214, doi:10.5194/npg-24-203-2017, 2017<br>
A review of analysis methods is given on
quasi-monochromatic waves, turbulent fluctuations, and wave–wave and
wave–particle interactions for single-spacecraft data in situ in near-Earth
space and interplanetary space, in particular using magnetic field and
electric field data. Energy spectra for different components of the
fluctuating fields, minimum variance analysis, propagation and polarization
properties of electromagnetic waves, wave distribution function, helicity
quantities, higher-order statistics, and detection methods for wave–particle
interactions are explained.
Copernicus Electronic Production Support Office2017-05-12T16:07:11+02:002017-05-12T16:07:11+02:00http://dx.doi.org/10.5194/npg-24-189-2017Trajectory encounter volume as a diagnostic of mixing potential in fluid flows
<b>Trajectory encounter volume as a diagnostic of mixing potential in fluid flows</b><br>
Irina I. Rypina and Lawrence J. Pratt<br>
Nonlin. Processes Geophys., 24, 189-202, doi:10.5194/npg-24-189-2017, 2017<br>
Fluid parcels exchange water properties when coming into contact with each other, leading to mixing. The trajectory encounter volume, defined here as the volume of fluid that passes close to a reference trajectory over a finite time interval, is introduced as a measure of the mixing potential of a flow. Regions with a low encounter volume (the cores of coherent eddies) have a low mixing potential. Regions with a large encounter volume (turbulent or chaotic regions) have a high mixing potential.
<b>Trajectory encounter volume as a diagnostic of mixing potential in fluid flows</b><br>
Irina I. Rypina and Lawrence J. Pratt<br>
Nonlin. Processes Geophys., 24, 189-202, doi:10.5194/npg-24-189-2017, 2017<br>
Fluid parcels can exchange water properties when coming into contact with each
other, leading to mixing. The trajectory encounter mass and a related
simplified quantity, the encounter volume, are introduced as a measure of the
mixing potential of a flow. The encounter volume quantifies the volume of fluid
that passes close to a reference trajectory over a finite time interval.
Regions characterized by a low encounter volume, such as the cores of coherent
eddies, have a low mixing potential, whereas turbulent or chaotic regions
characterized by a large encounter volume have a high mixing potential. The
encounter volume diagnostic is used to characterize the mixing potential in three flows of increasing complexity: the Duffing oscillator, the Bickley jet and
the altimetry-based velocity in the Gulf Stream extension region. An
additional example is presented in which the encounter volume is combined
with the <i>u</i><sup>∗</sup> approach of Pratt et al. (2016) to characterize the
mixing potential for a specific tracer distribution in the Bickley jet flow.
Analytical relationships are derived that connect the encounter volume to the shear
and strain rates for linear shear and linear strain flows, respectively. It
is shown that in both flows the encounter volume is proportional to time.
Copernicus Electronic Production Support Office2017-05-03T16:07:11+02:002017-05-03T16:07:11+02:00http://dx.doi.org/10.5194/npg-24-179-2017Sandpile-based model for capturing magnitude distributions and spatiotemporal clustering and separation in regional earthquakes
<b>Sandpile-based model for capturing magnitude distributions and spatiotemporal clustering and separation in regional earthquakes</b><br>
Rene C. Batac, Antonino A. Paguirigan Jr., Anjali B. Tarun, and Anthony G. Longjas<br>
Nonlin. Processes Geophys., 24, 179-187, doi:10.5194/npg-24-179-2017, 2017<br>
The sandpile-based model is the paradigm model of self-organized criticality (SOC), a mechanism believed to be responsible for the occurrence of scale-free (power-law) distributions in nature. One particular SOC system that is rife with power-law distributions is that of earthquakes, the most widely known of which is the Gutenberg–Richter (GR) law of earthquake energies. Here, we modify the sandpile to be of use in capturing the energy, space, and time statistics of earthquakes simultaneously.
<b>Sandpile-based model for capturing magnitude distributions and spatiotemporal clustering and separation in regional earthquakes</b><br>
Rene C. Batac, Antonino A. Paguirigan Jr., Anjali B. Tarun, and Anthony G. Longjas<br>
Nonlin. Processes Geophys., 24, 179-187, doi:10.5194/npg-24-179-2017, 2017<br>
We propose a cellular automata model for earthquake occurrences patterned
after the sandpile model of self-organized criticality (SOC). By
incorporating a single parameter describing the probability to target the
most susceptible site, the model successfully reproduces the statistical
signatures of seismicity. The energy distributions closely follow power-law
probability density functions (PDFs) with a scaling exponent of around
−1. 6, consistent with the expectations of the Gutenberg–Richter (GR) law,
for a wide range of the targeted triggering probability values. Additionally,
for targeted triggering probabilities within the range 0.004–0.007, we
observe spatiotemporal distributions that show bimodal behavior, which is not
observed previously for the original sandpile. For this critical range of
values for the probability, model statistics show remarkable comparison with
long-period empirical data from earthquakes from different seismogenic
regions. The proposed model has key advantages, the foremost of which is the fact
that it simultaneously captures the energy, space, and time statistics of
earthquakes by just introducing a single parameter, while introducing minimal
parameters in the simple rules of the sandpile. We believe that the
critical targeting probability parameterizes the memory that is inherently
present in earthquake-generating regions.
Copernicus Electronic Production Support Office2017-04-27T16:07:11+02:002017-04-27T16:07:11+02:00http://dx.doi.org/10.5194/npg-2017-17Multistable Slip of a One-degree-of-freedom Spring-slider Model in the Presence of Thermal-pressurized Slip-weakening Friction and Viscosity
<b>Multistable Slip of a One-degree-of-freedom Spring-slider Model in the Presence of Thermal-pressurized Slip-weakening Friction and Viscosity</b><br>
Jeen-Hwa Wang<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-17,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 0 comments)<br>
In this paper, I study the frictional and viscous effects on multi-stable slip of earthquake based on a one-degree-of-freedom model through numerical simulations. Results show three regimes of stable, intermettent, and unstable slip, depending on the values of model parameters.
<b>Multistable Slip of a One-degree-of-freedom Spring-slider Model in the Presence of Thermal-pressurized Slip-weakening Friction and Viscosity</b><br>
Jeen-Hwa Wang<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-17,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 0 comments)<br>
This study is focused on multistable slip of earthquakes based on a one-degree-of-freedom slider-slider model in the presence of thermal-pressurized slip-weakening friction and viscosity by using the normalized equation of motion of the model. The major model parameters are the normalized characteristic displacement, U<sub>c</sub>, of the friction law and the normalized viscosity coefficient, η, between the slider and background plate. Analytic results at small slip suggest that there is a solution regime for η and γ (= 1 / U<sub>c</sub>) to make the slider slip steadily. Numerical simulations exhibit that the time variation in normalized velocity, V / V<sub>max</sub> (V<sub>max</sub> is the maximum velocity), obviously depends on U<sub>c</sub> and η. The effect on the amplitude is stronger due to η than due to U<sub>c</sub>. In the phase portrait of V / V<sub>max</sub> versus the normalized displacement, U / U<sub>max</sub> (U<sub>max</sub> is the maximum displacement), there are two fixed points. The one at large V / V<sub>max</sub> and large U / U<sub>max</sub> is not an attractor; while that at small V / V<sub>max</sub> and small U / U<sub>max</sub> can be an attractor for some values of η and U<sub>c</sub>. When U<sub>c</sub> < 0.55, unstable slip does not exist. When U<sub>c</sub> ≥ 0.55, U<sub>c</sub> and η divide the solution domain into three regimes: stable, intermittent, and unstable (or chaotic) regimes. For a certain U<sub>c</sub>, the three regimes are controlled by a lower bound, η<sub>l</sub>, and an upper bound, η<sub>u</sub>, of η. The values of η<sub>l</sub>, η<sub>u</sub>, and η<sub>u</sub> − η<sub>l</sub> all decrease with increasing U<sub>c</sub>, thus suggesting that it is easier to yield unstable slip for larger U<sub>c</sub> than for smaller U<sub>c</sub> or for larger η than for smaller η. When U<sub>c</sub> < 1, the Fourier spectra calculated from simulation velocity waveforms exhibit several peaks, thus suggesting the existence of nonlinear behavior of the system. When U<sub>c</sub> > 1, the related Fourier spectra show only one peak, thus suggesting linear behavior of the system.
Copernicus Electronic Production Support Office2017-04-25T16:07:11+02:002017-04-25T16:07:11+02:00http://dx.doi.org/10.5194/npg-24-167-2017Dynamics of the Hadley circulation in an axisymmetric model undergoing stratification periodic forcing
<b>Dynamics of the Hadley circulation in an axisymmetric model undergoing stratification periodic forcing</b><br>
Nazario Tartaglione<br>
Nonlin. Processes Geophys., 24, 167-178, doi:10.5194/npg-24-167-2017, 2017<br>
This paper aims to show how the tropical circulation responds to changes of the vertical stratification of the imposed temperature that drives the model. These changes mimic the presence of water vapor cycles. Thus, for simplicity's sake we impose a periodic change of this stratification with variable periods of 10–90 days. The model responds with quasi-periodic oscillations having two or more dominant frequencies. After a long forcing time period, chaotic behavior starts to appear cyclically.
<b>Dynamics of the Hadley circulation in an axisymmetric model undergoing stratification periodic forcing</b><br>
Nazario Tartaglione<br>
Nonlin. Processes Geophys., 24, 167-178, doi:10.5194/npg-24-167-2017, 2017<br>
The time-dependent response of the Hadley circulation to a periodic forcing
is explored via a simplified nonlinear axisymmetric model. Thermal forcing
towards a given equilibrium potential temperature drives the model
atmosphere. The vertical stratification of this temperature is forced to
become periodically neutral with a period <i>t</i><sub>0</sub>. Simulations
performed with values of <i>t</i><sub>0</sub> ranging from 10 to 90 days exhibit
stronger circulation compared to the results of a constant thermal forcing
experiment. As the period increases, a transition occurs first from a
stationary regime, obtained when forcing is constant, to a periodic (and
possibly quasi-periodic) regime, and then to an intermittent regime, albeit
one with a strong periodic component. The stream-function response to
periodic forcing is generally a periodic oscillation, with two main
frequencies dominating: one with a period equal or close to the forcing
period and another with a period that is half of the forcing period. The
former is dominant for values of <i>t</i><sub>0</sub> larger than 30 days, whereas
the latter is prevalent for <i>t</i><sub>0</sub> smaller than 30 days. The periodic
oscillations obtained in this model may be associated with the periodic
oscillations observed in the tropical regions. In this case the periodic
charge and discharge of moisture in the tropical atmosphere, with consequent
change of stratification, may be linked to those oscillations. In the model,
at forcing periods of over 63 days the response of the stream function
periodically enters into a quasi-intermittent regime, exhibiting high-frequency chaotic oscillations that are modulated by the slow timescale of
forcing. Sensitivity experiments for model parameters and configuration were
performed to check whether results obtained are still valid under different
conditions. Although for small changes of parameters the results are still
valid, when parameters depart from the prescribed ones, we can observe change
in the Hadley circulation dynamics.
Copernicus Electronic Production Support Office2017-04-20T16:07:11+02:002017-04-20T16:07:11+02:00http://dx.doi.org/10.5194/npg-2017-16Constraining on the Stationarity of Signal with Time-Frequency Surrogates to
Enhance the Reliability of Singularity Spectrum Attributes of Random Seismic Noise Wavefield
<b>Constraining on the Stationarity of Signal with Time-Frequency Surrogates to
Enhance the Reliability of Singularity Spectrum Attributes of Random Seismic Noise Wavefield</b><br>
Amir Ali Hamed, Hiroe Miyake, Zaher Hossein Shomali, and Ali Moradi<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-16,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 0 comments)<br>
In this paper we examined the inherent stationarity of a signal relative to the different observation scales in the stochastic contexts before feeding the signal into the cycle of Fractal Analysis. This method is based on the comparison between global and local features of the original signal and its synthesized time frequency surrogates; therefor it can effectively improve the accuracy of results.
<b>Constraining on the Stationarity of Signal with Time-Frequency Surrogates to
Enhance the Reliability of Singularity Spectrum Attributes of Random Seismic Noise Wavefield</b><br>
Amir Ali Hamed, Hiroe Miyake, Zaher Hossein Shomali, and Ali Moradi<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-16,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 0 comments)<br>
Existence of a self-afﬁne long range persistence in the seismic noise time series evidences that the current state of system is not in the pure diffused regime and transition from coherent to incoherent motion is still on progress. Rate of this evolving transition can be indirectly linked to the degree of heterogeneity of medium, thus in this paper we tried to gain an insight into the heterogeneity of the medium by analyzing the width, extreme and asymmetrical trend of multifractal spectrum of seismic records. Nonetheless, toward high frequency ranges a seismic signal itself loses its stationarity short while after its recording onset time. Experimentally, the long-range correlation of a stationary time series (with 0 < <i>h</i>(2) < 1) can be discerned from a non-stationary process (with <i>h</i>(2) > 1) by examining the values of scaling exponent <i>h</i>(2), however, changing in the fractal properties in the crossover time scales in time series don’t permit us to ascribe a single amount for <i>h</i>(2) and without executing additional analysis on the stationarity length of signals, direct calculation of such long range correlation and fractal dimensions might be biased. Hence, in this paper we examined the inherent stationarity of a signal relative to the different observation scales in the stochastic contexts before feeding the signal into the cycle of DFA. This method is based on the comparison between global and local features of the original signal and its synthesized time-frequency surrogates; therefor it can effectively improve the accuracy of results. Our approach proves the existence of a high-velocity anomalous feature in the right flank of Sahand inactive volcano where it is surrounded by heterogeneous low-velocity structures and extended to the shallower than ~ 3 km depth beneath this region at the northwestern of Iran.
Copernicus Electronic Production Support Office2017-04-19T16:07:11+02:002017-04-19T16:07:11+02:00http://dx.doi.org/10.5194/npg-24-157-2017Subharmonic resonant excitation of edge waves by breaking surface waves
<b>Subharmonic resonant excitation of edge waves by breaking surface waves</b><br>
Nizar Abcha, Tonglei Zhang, Alexander Ezersky, Efim Pelinovsky, and Ira Didenkulova<br>
Nonlin. Processes Geophys., 24, 157-165, doi:10.5194/npg-24-157-2017, 2017<br>
Parametric excitation of edge waves with a frequency 2 times less than the frequency of surface waves propagating perpendicular to the inclined bottom are investigated in laboratory experiments. The domain of instability on the plane of surface wave parameters (amplitude–frequency) is found. The subcritical instability is observed in the system of parametrically excited edge waves. It is shown that breaking of surface waves initiates turbulent effects and can suppress the parametric generation.
<b>Subharmonic resonant excitation of edge waves by breaking surface waves</b><br>
Nizar Abcha, Tonglei Zhang, Alexander Ezersky, Efim Pelinovsky, and Ira Didenkulova<br>
Nonlin. Processes Geophys., 24, 157-165, doi:10.5194/npg-24-157-2017, 2017<br>
Parametric excitation of edge waves with a frequency
2 times less than the frequency of surface waves propagating perpendicular to
the inclined bottom is investigated in laboratory experiments. The domain of
instability on the plane of surface wave parameters (amplitude–frequency) is
found. The subcritical instability is observed in the system of
parametrically excited edge waves. It is shown that breaking of surface waves
initiates turbulent effects and can suppress the parametric generation of
edge waves.
Copernicus Electronic Production Support Office2017-03-28T16:07:11+02:002017-03-28T16:07:11+02:00http://dx.doi.org/10.5194/npg-2017-15Fractional Brownian motion, the Matérn process, and stochastic
modeling of turbulent dispersion
<b>Fractional Brownian motion, the Matérn process, and stochastic
modeling of turbulent dispersion</b><br>
Jonathan M. Lilly, Adam M. Sykulski, Jeffrey J. Early, and Sofia C. Olhede<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-15,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 2 comments)<br>
This work arose from a desire to understand the nature of particle motions in turbulence. We sought a simple conceptual model that could describe such motions, then realized that this model could be applicable to an array of other problems. The basic idea is to create a string of random numbers, called a stochastic process, that mimics the properties of particle trajectories. This model could be useful in making best use of data from freely-drifting instruments tracking the ocean currents.
<b>Fractional Brownian motion, the Matérn process, and stochastic
modeling of turbulent dispersion</b><br>
Jonathan M. Lilly, Adam M. Sykulski, Jeffrey J. Early, and Sofia C. Olhede<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-15,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 2 comments)<br>
Stochastic process exhibiting power-law slopes in the frequency domain are frequently well modeled by fractional Brownian motion (fBm). In particular, the spectral slope at high frequencies is associated with the degree of small-scale roughness or fractal dimension. However, a broad class of real-world signals have a high-frequency slope, like fBm, but a plateau in the vicinity of zero frequency. This low-frequency plateau, it is shown, implies that the temporal integral of the process exhibits <i>diffusive</i> behavior, dispersing from its initial location at a constant rate. Such processes are not well modeled by fBm, which has a singularity at zero frequency corresponding to an unbounded rate of dispersion. A more appropriate stochastic model is a much lesser-known random process called the Matérn process, which is shown herein to be a <i>damped</i> version of fractional Brownian motion. This article first provides a thorough introduction to fractional Brownian motion, then examines the details of the Matérn process and its relationship to fBm. An algorithm for the simulation of the Matérn process in <i>O</i>(<i>N</i> log <i>N</i>) operations is given. Unlike fBm, the Matérn process is found to provide an excellent match to modeling velocities from particle trajectories in an application to two-dimensional fluid turbulence.
Copernicus Electronic Production Support Office2017-03-28T16:07:11+02:002017-03-28T16:07:11+02:00http://dx.doi.org/10.5194/npg-2017-9Extracting real-crack properties from nonlinear elastic behavior of rocks: abundance of cracks with dominating normal compliance and rocks with negative Poisson's ratio
<b>Extracting real-crack properties from nonlinear elastic behavior of rocks: abundance of cracks with dominating normal compliance and rocks with negative Poisson's ratio</b><br>
Vladimir Y. Zaitsev, Andrey V. Radostin, Elena Pasternak, and Arcady Dyskin<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-9,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 1 comment)<br>
A new method of analysing pressure wave dependences is presented and tested against the published experimental data. Upon the results of examination of more than 90 rock samples it was found that a significant portion of rocks (~ 45 %) exhibiting negative Poisson's ratios at lower pressures. Such a significant number of naturally auxetic rocks suggest that the occurrence of negative Poisson's ratio is not as exotic as assumed previously.
<b>Extracting real-crack properties from nonlinear elastic behavior of rocks: abundance of cracks with dominating normal compliance and rocks with negative Poisson's ratio</b><br>
Vladimir Y. Zaitsev, Andrey V. Radostin, Elena Pasternak, and Arcady Dyskin<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-9,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 1 comment)<br>
Results of examination of experimental data on nonlinear elasticity of rocks using experimental pressure-dependences of P- and S-wave velocities from various literature sources are presented. Overall, over 90 rock samples are considered. Interpretation of the data is performed using an effective-medium description in which cracks are considered as compliant defects (cracks) with independent shear and normal compliances without specifying a particular crack model with an <i>a priori</i> given ratio of the compliances. Comparison with the experimental data indicated abundance of cracks (~ 80 %) with the normal-to-shear compliance ratios significantly exceeding the values typical of conventionally used crack models (such as penny-shape cuts or thin ellipsoidal cracks). Correspondingly, rocks with such cracks demonstrate strongly decreased Poisson's ratio including a significant portion of rocks (~ 45 %) exhibiting negative Poisson's ratios at lower pressures, for which the concentration of not yet closed cracks is maximal. The obtained results indicate the necessity of further development of crack models to account the revealed numerous examples of cracks with strong domination of normal compliance. Discovering such a significant number of naturally auxetic rocks is in contrast with the conventional viewpoint that occurrence of negative Poisson's ratio is an exotic fact that is mostly associated with specially engineered structures.
Copernicus Electronic Production Support Office2017-03-17T16:07:11+01:002017-03-17T16:07:11+01:00http://dx.doi.org/10.5194/npg-24-141-2017Spatial and radiometric characterization of multi-spectrum satellite images through multi-fractal analysis
<b>Spatial and radiometric characterization of multi-spectrum satellite images through multi-fractal analysis</b><br>
Carmelo Alonso, Ana M. Tarquis, Ignacio Zúñiga, and Rosa M. Benito<br>
Nonlin. Processes Geophys., 24, 141-155, doi:10.5194/npg-24-141-2017, 2017<br>
NDVI and EVI vegetation indexes, estimated from satellite images, can been used to estimate root zone soil moisture. However, depending on the spatial and radiometric resolution of the sensors used, estimations could change. In this work, images taken by satellites IKONOS-2 and LANDSAT-7 of the same location are compared on the four bands involved in these vegetation indexes. The results show that spatial resolution has a similar scaling effect in the four bands, but not radiometric resolution.
<b>Spatial and radiometric characterization of multi-spectrum satellite images through multi-fractal analysis</b><br>
Carmelo Alonso, Ana M. Tarquis, Ignacio Zúñiga, and Rosa M. Benito<br>
Nonlin. Processes Geophys., 24, 141-155, doi:10.5194/npg-24-141-2017, 2017<br>
Several studies have shown that vegetation indexes can be used to
estimate root zone soil moisture. Earth surface images, obtained by high-resolution satellites, presently give a lot of information on these
indexes, based on the data of several wavelengths. Because of the potential capacity for
systematic observations at various scales, remote sensing technology extends
the possible data archives from the present time to several decades back.
Because of this advantage, enormous efforts have been made by researchers and
application specialists to delineate vegetation indexes from local scale to
global scale by applying remote sensing imagery.
<br><br>
In this work, four band images have been considered, which are involved in these
vegetation indexes, and were taken by satellites Ikonos-2 and Landsat-7 of the same
geographic location, to study the effect of both spatial (pixel size) and
radiometric (number of bits coding the image) resolution on these wavelength
bands as well as two vegetation indexes: the Normalized Difference
Vegetation Index (NDVI) and the Enhanced Vegetation Index (EVI).
<br><br>
In order to do so, a multi-fractal analysis of these multi-spectral images
was applied in each of these bands and the two indexes derived. The results
showed that spatial resolution has a similar scaling effect in the four
bands, but radiometric resolution has a larger influence in blue and green
bands than in red and near-infrared bands. The NDVI showed a higher
sensitivity to the radiometric resolution than EVI. Both were equally
affected by the spatial resolution.
<br><br>
From both factors, the spatial resolution has a major impact in the
multi-fractal spectrum for all the bands and the vegetation indexes. This
information should be taken in to account when vegetation indexes based on
different satellite sensors are obtained.
Copernicus Electronic Production Support Office2017-03-16T16:07:11+01:002017-03-16T16:07:11+01:00http://dx.doi.org/10.5194/npg-2017-14Retrieval and assimilation of velocities at the ocean surface
<b>Retrieval and assimilation of velocities at the ocean surface</b><br>
Jordi Isern-Fontanet, Joaquim Ballabrera-Poy, Antonio Turiel, and Emilio García-Ladona<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-14,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 3 comments)<br>
Ocean currents play a key role in Earth’s climate, they are of major importance for navigation and human activities at sea, and impact almost all processes that take place in the ocean. Nevertheless, their observation and forecasting are still difficult. Here, we review the main techniques used to derive surface currents from satellite measurements and the existing approaches to assimilate this information into ocean models.
<b>Retrieval and assimilation of velocities at the ocean surface</b><br>
Jordi Isern-Fontanet, Joaquim Ballabrera-Poy, Antonio Turiel, and Emilio García-Ladona<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-14,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 3 comments)<br>
Ocean currents play a key role in Earth’s climate, they are of major importance for navigation and human activities at sea, and impact almost all processes that take place in the ocean. Nevertheless, their observation and forecasting are still difficult. First, direct measurements of ocean currents are difficult to obtain synoptically at global scale. Consequently, it has been necessary to use Sea Surface Height and Sea Surface Temperature measurements and refer to dynamical frameworks to derive the velocity field. Second, the assimilation of the velocity field into numerical models of ocean circulation is difficult mainly due to lack of data. Recent experiments assimilating coastal-based radar data have shown that ocean currents will contribute to increase the forecast skill of surface currents, but require to be applied in multi-data assimilation approaches to allow better identification of the thermohaline structure of the ocean. In this paper we review the current knowledge on these fields and provide global and systematic view on the technologies to retrieve ocean velocities in the upper ocean and the available approaches to assimilate this information into ocean model.
Copernicus Electronic Production Support Office2017-03-09T16:07:11+01:002017-03-09T16:07:11+01:00http://dx.doi.org/10.5194/npg-24-125-2017Insights on the role of accurate state estimation in coupled model parameter estimation by a conceptual climate model study
<b>Insights on the role of accurate state estimation in coupled model parameter estimation by a conceptual climate model study</b><br>
Xiaolin Yu, Shaoqing Zhang, Xiaopei Lin, and Mingkui Li<br>
Nonlin. Processes Geophys., 24, 125-139, doi:10.5194/npg-24-125-2017, 2017<br>
Parameter estimation (PE) with a global coupled data assimilation (CDA) system can improve the runs, but the improvement remains in a limited range. We have to come back to simple models to sort out the sources of noises. Incomplete observations and the chaotic nature of the atmosphere have much stronger influences on the PE through the state estimation (SE) process. Here, we propose the guidelines of how to enhance the signal-to-noise ratio under partial SE status.
<b>Insights on the role of accurate state estimation in coupled model parameter estimation by a conceptual climate model study</b><br>
Xiaolin Yu, Shaoqing Zhang, Xiaopei Lin, and Mingkui Li<br>
Nonlin. Processes Geophys., 24, 125-139, doi:10.5194/npg-24-125-2017, 2017<br>
The uncertainties in values of coupled model parameters
are an important source of model bias that causes model climate drift. The
values can be calibrated by a parameter estimation procedure that projects
observational information onto model parameters. The signal-to-noise ratio
of error covariance between the model state and the parameter being estimated
directly determines whether the parameter estimation succeeds or not. With
a conceptual climate model that couples the stochastic atmosphere and
slow-varying ocean, this study examines the sensitivity of state–parameter
covariance on the accuracy of estimated model states in different model
components of a coupled system. Due to the interaction of multiple timescales,
the fast-varying <q>atmosphere</q> with a chaotic nature is the major
source of the inaccuracy of estimated state–parameter covariance. Thus,
enhancing the estimation accuracy of atmospheric states is very important
for the success of coupled model parameter estimation, especially for the
parameters in the air–sea interaction processes. The impact of
chaotic-to-periodic ratio in state variability on parameter estimation is
also discussed. This simple model study provides a guideline when real
observations are used to optimize model parameters in a coupled general
circulation model for improving climate analysis and predictions.
Copernicus Electronic Production Support Office2017-03-06T16:07:11+01:002017-03-06T16:07:11+01:00http://dx.doi.org/10.5194/npg-2017-11Multiscaled Solitary Waves
<b>Multiscaled Solitary Waves</b><br>
Oleg G. Derzho<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-11,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 4 comments)<br>
It is analytically shown how competing nonlinearities yield new multiscaled (multi humped) structures for internal solitary waves in shallow fluids. These solitary waves only exist for large amplitudes beyond the limit of applicability of the KdV equation or its usual extensions. Multiscaling phenomenon exists or do not exist for almost identical density profiles. Trapped core inside the wave prevents appearance of such multiple scales within the core area.
<b>Multiscaled Solitary Waves</b><br>
Oleg G. Derzho<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-11,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 4 comments)<br>
It is analytically shown how competing nonlinearities yield new multiscaled (multi humped) structures for internal solitary waves in shallow fluids. These solitary waves only exist for large amplitudes beyond the limit of applicability of the KdV equation or its usual extensions. Multiscaling phenomenon exists or do not exist for almost identical density profiles. Trapped core inside the wave prevents appearance of such multiple scales within the core area. It is anticipated that multiscaling phenomena exist for solitary waves in various physical origins.
Copernicus Electronic Production Support Office2017-03-06T16:07:11+01:002017-03-06T16:07:11+01:00http://dx.doi.org/10.5194/npg-24-113-2017A matrix clustering method to explore patterns of land-cover transitions in satellite-derived maps of the Brazilian Amazon
<b>A matrix clustering method to explore patterns of land-cover transitions in satellite-derived maps of the Brazilian Amazon</b><br>
Finn Müller-Hansen, Manoel F. Cardoso, Eloi L. Dalla-Nora, Jonathan F. Donges, Jobst Heitzig, Jürgen Kurths, and Kirsten Thonicke<br>
Nonlin. Processes Geophys., 24, 113-123, doi:10.5194/npg-24-113-2017, 2017<br>
Deforestation and subsequent land uses in the Brazilian Amazon have huge impacts on greenhouse gas emissions, local climate and biodiversity. To better understand these land-cover changes, we apply complex systems methods uncovering spatial patterns in regional transition probabilities between land-cover types, which we estimate using maps derived from satellite imagery. The results show clusters of similar land-cover dynamics and thus complement studies at the local scale.
<b>A matrix clustering method to explore patterns of land-cover transitions in satellite-derived maps of the Brazilian Amazon</b><br>
Finn Müller-Hansen, Manoel F. Cardoso, Eloi L. Dalla-Nora, Jonathan F. Donges, Jobst Heitzig, Jürgen Kurths, and Kirsten Thonicke<br>
Nonlin. Processes Geophys., 24, 113-123, doi:10.5194/npg-24-113-2017, 2017<br>
Changes in land-use systems in tropical regions, including deforestation, are
a key challenge for global sustainability because of their huge impacts on
green-house gas emissions, local climate and biodiversity. However, the
dynamics of land-use and land-cover change in regions of frontier expansion
such as the Brazilian Amazon are not yet well understood because of the
complex interplay of ecological and socioeconomic drivers. In this paper, we
combine Markov chain analysis and complex network methods to identify regimes
of land-cover dynamics from land-cover maps (TerraClass) derived from
high-resolution (30 m) satellite imagery. We estimate regional transition
probabilities between different land-cover types and use clustering analysis
and community detection algorithms on similarity networks to explore patterns
of dominant land-cover transitions. We find that land-cover transition
probabilities in the Brazilian Amazon are heterogeneous in space, and adjacent
subregions tend to be assigned to the same clusters. When focusing on
transitions from single land-cover types, we uncover patterns that reflect
major regional differences in land-cover dynamics. Our method is able to
summarize regional patterns and thus complements studies performed at the
local scale.
Copernicus Electronic Production Support Office2017-02-28T16:07:11+01:002017-02-28T16:07:11+01:00http://dx.doi.org/10.5194/npg-24-101-2017Conditional nonlinear optimal perturbations based on the particle swarm optimization and their applications to the predictability problems
<b>Conditional nonlinear optimal perturbations based on the particle swarm optimization and their applications to the predictability problems</b><br>
Qin Zheng, Zubin Yang, Jianxin Sha, and Jun Yan<br>
Nonlin. Processes Geophys., 24, 101-112, doi:10.5194/npg-24-101-2017, 2017<br>
When the initial perturbation is large or the prediction time is long, the strong nonlinearity of the dynamical model on the prediction variable will lead to failure of the ADJ-CNOP method; when the objective function has multiple extreme values, ADJ-CNOP has a large probability of producing local CNOPs, hence making false estimations of the lower bound of maximum predictable time.
<b>Conditional nonlinear optimal perturbations based on the particle swarm optimization and their applications to the predictability problems</b><br>
Qin Zheng, Zubin Yang, Jianxin Sha, and Jun Yan<br>
Nonlin. Processes Geophys., 24, 101-112, doi:10.5194/npg-24-101-2017, 2017<br>
In predictability problem research, the conditional nonlinear
optimal perturbation (CNOP) describes the initial perturbation that satisfies
a certain constraint condition and causes the largest prediction error at the
prediction time. The CNOP has been successfully applied in estimation of the
lower bound of maximum predictable time (LBMPT). Generally, CNOPs are
calculated by a gradient descent algorithm based on the adjoint model, which
is called ADJ-CNOP. This study, through the two-dimensional Ikeda model,
investigates the impacts of the nonlinearity on ADJ-CNOP and the
corresponding precision problems when using ADJ-CNOP to estimate the LBMPT.
Our conclusions are that (1) when the initial perturbation is large
or the prediction time is long, the strong nonlinearity of the dynamical
model in the prediction variable will lead to failure of the ADJ-CNOP method,
and (2) when the objective function has multiple extreme values, ADJ-CNOP has
a large probability of producing local CNOPs, hence making a false estimation
of the LBMPT. Furthermore, the particle swarm optimization (PSO) algorithm,
one kind of intelligent algorithm, is introduced to solve this problem. The
method using PSO to compute CNOP is called PSO-CNOP. The results of numerical
experiments show that even with a large initial perturbation and long
prediction time, or when the objective function has multiple extreme values,
PSO-CNOP can always obtain the global CNOP. Since the PSO algorithm is a
heuristic search algorithm based on the population, it can overcome the
impact of nonlinearity and the disturbance from multiple extremes of the
objective function. In addition, to check the estimation accuracy of the
LBMPT presented by PSO-CNOP and ADJ-CNOP, we partition the constraint domain
of initial perturbations into sufficiently fine grid meshes and take the
LBMPT obtained by the filtering method as a benchmark. The result shows that
the estimation presented by PSO-CNOP is closer to the true value than the one
by ADJ-CNOP with the forecast time increasing.
Copernicus Electronic Production Support Office2017-02-22T16:07:11+01:002017-02-22T16:07:11+01:00http://dx.doi.org/10.5194/npg-2017-5Detecting Changes in Forced Climate Attractors with Wasserstein Distance
<b>Detecting Changes in Forced Climate Attractors with Wasserstein Distance</b><br>
Yoann Robin, Pascal Yiou, and Philippe Naveau<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-5,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 2 comments)<br>
If climate is viewed as a chaotic dynamical system, its trajectories yielding on an object called attractor. Being perturbed by an external forcing, this attractor could be modified. With Wasserstein distance, we estimate on a derived Lorenz model the impact of a forcing similar to climate change. Our approach appears to work with small data sizes. We have obtained a methodology quantifying the deformation of well known attractors, coherent with the size of data available.
<b>Detecting Changes in Forced Climate Attractors with Wasserstein Distance</b><br>
Yoann Robin, Pascal Yiou, and Philippe Naveau<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-5,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 2 comments)<br>
The climate system can been described by a dynamical system and its associated attractor. The
dynamics of this attractor depends on the external forcings that influence the climate. Such
forcings can affect the mean values or variances, but regions of the attractor that are seldom visited
can also be affected. It is an important challenge to measure how the climate attractor responds to
different forcings. Currently, the Euclidean distance or similar measures like the Mahalanobis
distance have been favoured to measure discrepancies between two climatic situations. Those
distances do not have a natural building mechanism to take into account the attractor dynamics. In
this paper, we argue that a Wasserstein distance, stemming from optimal transport theory, offers an
efficient and practical way to discriminate between dynamical systems. After treating a toy
example, we explore how the Wasserstein distance can be applied and interpreted to detect
non-autonomous dynamics from a Lorenz system driven by seasonal cycles and a warming trend.
Copernicus Electronic Production Support Office2017-02-20T16:07:11+01:002017-02-20T16:07:11+01:00http://dx.doi.org/10.5194/npg-2017-7Ensemble Kalman filter for the reconstruction of the Earth's mantle
circulation
<b>Ensemble Kalman filter for the reconstruction of the Earth's mantle
circulation</b><br>
Marie Bocher, Alexandre Fournier, and Nicolas Coltice<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-7,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 1 comment)<br>
We propose a new method to reconstruct the circulation in the Earth's Mantle for the last 300 Myrs. This method is based on the sequential assimilation of plate layouts obtained from plate tectonic reconstructions into mantle convection models. This method allows us to take into account uncertainties on plate tectonic reconstructions, and provides an estimation of the uncertainties on the final result. We test and validate this method in a controlled environment by using synthetic experiments.
<b>Ensemble Kalman filter for the reconstruction of the Earth's mantle
circulation</b><br>
Marie Bocher, Alexandre Fournier, and Nicolas Coltice<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-7,2017<br>
<b>Manuscript under review for NPG</b> (discussion: open, 1 comment)<br>
Recent advances in mantle convection modelling led to the release of a new generation of convection codes, able to generate self-consistently plate-like tectonics at their surface. Those models physically link mantle dynamics to surface tectonics. Combined with plate tectonic reconstructions, they have the potential to produce a new generation of mantle circulation models that use data assimilation methods and where uncertainties on plate tectonic reconstructions are taken into account. We recently provided a proof of this concept by applying a suboptimal Kalman Filter to the reconstruction of mantle circulation (Bocher et al., 2016). Here, we propose to go one step further and apply the ensemble Kalman filter (EnKF) to this problem. The EnKF is a sequential Monte Carlo method particularly adapted to solve high dimensional data assimilation problems with nonlinear dynamics. We tested the EnKF using synthetic observations consisting of surface velocity and heat flow measurements, on a 2D-spherical annulus model and compared it with the method developed previously. The EnKF performs on average better and is more stable than the former method. Less than 300 ensemble members are sufficient to reconstruct an evolution. We use covariance adaptive inflation and localization to correct for sampling errors. We show that the EnKF results are robust over a wide range of covariance localization parameters. The reconstruction is associated with an estimation of the error, and provides valuable information on where the reconstruction is to be trusted or not.
Copernicus Electronic Production Support Office2017-02-20T16:07:11+01:002017-02-20T16:07:11+01:00http://dx.doi.org/10.5194/npg-2017-4Network-based study of Lagrangian transport and mixing
<b>Network-based study of Lagrangian transport and mixing</b><br>
Kathrin Padberg-Gehle and Christiane Schneide<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-4,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 2 comments)<br>
Transport and mixing processes in fluid flows are crucially influenced by coherent structures, such as eddies, gyres, or jets in geophysical flows.
We propose a very simple and computationally efficient approach for analyzing coherent behavior in fluid flows. The central object is a flow network constructed directly from particle trajectories. The network's local and spectral properties are shown to give a very good indication of coherent as well as mixing regions in the underlying flow.
<b>Network-based study of Lagrangian transport and mixing</b><br>
Kathrin Padberg-Gehle and Christiane Schneide<br>
Nonlin. Processes Geophys. Discuss., doi:10.5194/npg-2017-4,2017<br>
<b>Manuscript under review for NPG</b> (discussion: final response, 2 comments)<br>
Transport and mixing processes in fluid flows are crucially influenced by coherent structures and the characterization of these Lagrangian objects is a topic of intense current research. While established mathematical approaches such as variational or transfer operator based schemes require full knowledge of the flow field or at least high resolution trajectory data, this information may not be available in applications. Recently, different computational methods have been proposed to identify coherent behavior in flows directly from Lagrangian trajectory data. In this context, spatio-temporal clustering algorithms have been proven to be very effective for the extraction of coherent sets from sparse and possibly incomplete trajectory data. Inspired by these recent approaches, we consider an unweighted, undirected network, where Lagrangian particle trajectories serve as network nodes. A link is established between two nodes if the respective trajectories come close to each other at least once in the course of time. Classical graph concepts are then employed to analyze the resulting network. In particular, local network measures such as the node degree serve as indicators of highly mixing regions, whereas spectral graph partitioning schemes allow us to extract coherent sets. The proposed methodology is very fast to run and we demonstrate its applicability in two geophysical flows – the Bickley jet as well as the antarctic stratospheric polar vortex.
Copernicus Electronic Production Support Office2017-02-17T16:07:11+01:002017-02-17T16:07:11+01:00