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It has been demonstrated several times that when principal component analysis (PCA) is used for detection of modes of atmospheric circulation variability (teleconnections), principal components must be rotated. Despite it, unrotated PCA is still often used. Here we demonstrate on the examples of North Atlantic Oscillation (NAO), Arctic Oscillation (AO), Barents Oscillation (BO), and the summer East Atlantic (SEA) pattern that unrotated PCA results in patterns that are artifacts of the analysis method rather than true modes of variability. This claim is based on the comparison of the spatial patterns of the modes with spatial autocorrelations, on the sensitivity of the patterns to spatial and temporal subsampling, and, for the SEA pattern, on correlations with tropical sea surface temperature. Unlike NAO, which is defined by rotated PCA, the other modes, that is, AO, BO, and SEA pattern, defined by unrotated PCA, do not correspond well to underlying autocorrelation structures and are more sensitive to choices of spatial domain and time interval over which they are defined. We reiterate that a great care must be taken when interpreting outputs of PCA when applied to the detection of modes of circulation variability: a comparison with spatial autocorrelations and check for their spatial and temporal stability are necessary to distinguish true modes from statistical artifacts, which we call “ghost patterns.” Key Points North Atlantic Oscillation, Arctic Oscillation, Barents Oscillation, and the summer East Atlantic pattern are examined as for their correspondence with underlying correlation structures and spatial and temporal subsampling Arctic Oscillation, Barents Oscillation, and the summer East Atlantic pattern are not true modes of atmospheric circulation variability We reiterate that rotated principal component analysis must be used to detect modes of circulation variability; the use of unrotated principal component analysis is more likely to result in statistical artifacts

A low frequency Alfven eigenmode in the ST40 spherical tokamak (ST) is found to be resonantly excited by thermal ions from global gyrokinetic GTC simulations. The analyses of phase-space resonances and wave-particle energy exchanges show that the Beta-induced Alfvén-Acoustic Eigenmode (BAAE) is primarily driven unstable by thermal ions, but is stabilized by energetic particles (EPs). When all kinetic effects are artificially suppressed, GTC simulations in the ideal magnetohydrodynamic (MHD) limit confirm the existence of the MHD eigenmode with a frequency in the BAAEs gap, as previously found by the ideal MHD code NOVA. The kinetic effects of thermal ions induce a frequency downshift compared to the stable BAAE in the MHD simulations. These results support the verification for global gyrokinetic simulations of the BAAE in the ST. However, the simulations do not explain the experimental observations of the BAAE in the ST40, possibly owing to the limitation of the collisionless gyrokinetic Monte Carlo model for the EPs.

Catechol is an oxygenated aromatic volatile organic compound and a biogenic precursor of secondary organic aerosols. Monitoring this compound in the gas phase is desirable due to its appreciable reactivity with tropospheric ozone. From a molecular point of view, this molecule is attractive since the two adjacent hydroxy groups can interchangeably act as donor and acceptor in an intramolecular hydrogen bonding due to the tunnelling between two symmetrically equivalent structures. Using synchrotron radiation, we recorded a rotationally-resolved Fourier Transform far-infrared (IR) spectrum of the torsional modes of the free and bonded -OH groups forming the intramolecular hydrogen bond. Additionally, the room temperature, pure rotational spectrum was measured in the 70–220 GHz frequency range using a millimeter-wave spectrometer. The assignment of these molecular transitions was assisted by anharmonic high-level quantum-chemical calculations. In particular, pure rotational lines belonging to the ground and the four lowest energy, vibrationally excited states were assigned. Splitting due to the tunnelling was resolved for the free -OH torsional state. A global fit combining the far-IR and millimeter-wave data provided the spectroscopic parameters of the low-energy far-IR modes, in particular those characterizing the intramolecular hydrogen bond dynamics.

During the island divertor operation of W7-X, especially in standard magnetic configuration (with 5/5 island chain in the scrape-off-layer (SOL)), quasi-periodic electromagnetic oscillation is observed. It appears in the frequency range of 1 kHz–2 kHz, therefore it is called the low frequency mode (LFM) within this paper. It is observed by multiple diagnostics, amongst them, a poloidal correlation reflectometer allowing radial localization of the LFM. The LFM is localized in the SOL and shows an obvious modulation effect on the plasma perpendicular velocity ( V ⊥ ). Furthermore, broadband turbulence is observed in the fluctuation spectra of the electron density and the magnetic field and the perpendicular correlation length of turbulence eddies are modulated. The calculation of the poloidal flow velocity and its oscillation allows us to study the effect of the LFM on the flow. Cross correlation analysis shows that the perpendicular flow oscillation and the turbulence modulation are closely correlated. The application of external control coils to adjust the island size and position result in a strong modification of the magnetic topology at the plasma edge which affects the appearance, amplitude and frequency of the LFM. Bi-coherence analysis indicates that nonlinear interactions among turbulence components is a possible mechanism for the generation of the LFM.

In the paper High-resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data by S. Rosat et al. (Geophysical Research Letters, 32, L13304, doi:10.1029/2005GL023128, 2005), a method proposed by Dahlen was employed to compute the errors on the estimated frequencies; however, a mistake was made when implementing the method, so that the error bars were inconsistent with the signal-to-noise ratio. Indeed we used only one sample to estimate the noise spectrum instead of averaging over many samples of the demoded spectrum in the vicinity of the mode frequency. Another problem came from the fact that the estimated amplitude and quality factor of the mode used in the variance computation are correlated. We have then improved the least-squares fit by optimizing with a Levenberg-Marquardt algorithm [Marquardt, 1963]. To bring some constraints on the 3D density structure of the Earth, a reliable error estimate of the eigenfrequency is required [Häfner and Widmer-Schnidrig, 2012]. The revised error bars (Tables 1, 2 and 3) are much more robust than those published in the 2005 paper. --- Citation: Rosat, S., T. Sato, Y. Imanishi, J. Hinderer, Y. Tamura, H. McQueen, and M. Ohashi (2012), Correction to High-resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data, Geophys. Res. Lett., 39, L22601, doi:10.1029/2012GL054248. ©2012. American Geophysical Union. All Rights Reserved.

A linear stability analysis shows that the jet in crossflow is characterized by self-sustained global oscillations for a jet-to-crossflow velocity ratio of 3. A fully three-dimensional unstable steady-state solution and its associated global eigenmodes are computed by direct numerical simulations and iterative eigenvalue routines. The steady flow, obtained by means of selective frequency damping, consists mainly of a (steady) counter-rotating vortex pair (CVP) in the far field and horseshoe-shaped vortices close to the wall. High-frequency unstable global eigenmodes associated with shear-layer instabilities on the CVP and low-frequency modes associated with shedding vortices in the wake of the jet are identified. Furthermore, different spanwise symmetries of the global modes are discussed. This work constitutes the first simulation-based global stability analysis of a fully three-dimensional base flow.

This article identifies the main coherent structures driving the flow dynamics in the turbulent channel flow over anisotropic porous walls. Two different cases have been analyzed where the drag increases or decreases with respect to a channel with isotropic porous walls. Higher order dynamic mode decomposition (HODMD) is applied to analyze these data, identifying 20 and 15 high amplitude modes in the drag increasing (DI) and drag reducing (DR) cases, respectively, which well reflects the largest flow complexity in the former case. The frequency of 13 modes and the three-dimensional structure of the modes are similar in the DR and DI cases, suggesting the need of using more complex analyses to deepen our physical insight of these flows. The spatio-temporal HODMD analysis identifies a periodic solution along the spanwise direction (as imposed by the boundary conditions). The wavenumbers related to the modes with highest amplitude are β = 0 and β = 3 (Lz = 2 3π ). The rollers, groups of spanwise correlated structures, are mostly identified in the DI case near the wall, with β = 0, while the presence of the streaks, streamwise correlated structures are mostly identified in the DR case. Although, in areas far away from the wall it is possible to identify these two types of structures with β = 3 in both cases, depending on the temporal frequency of the DMD modes, the rollers and the streaks are related to high and low frequency DMD modes, respectively. Finally, a model is constructed to predict the temporal evolution of the wall shear, using the 6 most relevant DMD modes interacting near the channel wall: 6 low frequency modes for DR and 3 low and 3 high frequency modes for DI. In the DR case the wall shear is predicted for almost 300 time units with relative error ∼ 2%, however, this error is larger in the DI case, ∼ 6%, suggesting the need of using a larger number of modes to represent this more complex flow.

The presence of oscillation modes in a power system with low damping rates can compromise its proper operation and motivated the development of different damping control projects to improve these damping rates. Central damping controllers using PMU data showed to be able to mitigate oscillation modes of power systems with high damping ratios. However, time delays and vulnerability of communication channels to cyber-attacks can damage the desired operation of this type of damping controller. This paper proposes an optimization problem-based method for designing centralized controllers that are resilient to channel losses and using time delays as variables of the proposed optimization procedure. The bio-inspired algorithms Marine Predators Algorithm, Particle Swarm Optimization and Genetic Algorithms were applied and evaluated in the proposed optimization model. The IEEE 68-bus was used as a test system, and a set of case studies were carried out. The obtained results show that considering time delays as optimization variables can be beneficial to achieve optimal control objectives. Furthermore, the communication failure robustness strategy was effective as observed by the modal analyses and nonlinear time domain simulations of the test system under contingencies.

Gold-enhanced mechanical exfoliation method attracts broad interests in recent years, which has been widely used for preparing large-area and high-quality 2D single crystals. Even many calculations predict that there is strong interaction between Au film and the exfoliated 2D crystals, direct experimental evidence is still lacking. Here, we perform Raman spectroscopy measurements for few layer MoS 2 with and without Au film underneath. The main peaks of MoS 2 on Au film show no obvious change at higher frequency, however, the breathing and shear modes at low-frequency are suppressed, especially for breathing modes. In contrast, both breathing modes and shear modes can be detected on suspended MoS 2 and the samples are transferred from Au film to SiO 2 /Si. These comparison results provide direct evidence for the existence of covalent-like quasi-bonding at the interface of Au film and the exfoliated MoS 2 crystal. This MoS 2 /Au interface interaction presents a special pinning-effect for low-frequency rigid vibration. Similar pinning-effect is also discovered in WS 2 /Au system. Our work reports the suppression of low-frequency Raman modes of MoS 2 , WS 2 on Au film, which will deliver new inspiration for studying other interactions between layered materials and solid surfaces.

We applied the method of coarse-graining the intermolecular vibrations to molecular heterodimers assembled by double hydrogen bonding. This method is based on principal component analysis, by which the original atomic displacement vectors are projected onto a lower-dimensional space spanned by a basis set of translations, librations, and intramolecular vibrations of the constituent molecules. Compared with homodimers, the following points are particularly noted: (1) alignment of the constituent molecules in a non-symmetric atomic arrangement of the whole system and (2) the scheme of reordering the bases to construct an optimal coarse-grained space. We tested three schemes for reordering the intramolecular vibration vectors to determine that the best one is equivalent to size reduction based on the singular value decomposition. The coarse-graining analysis affords three parameters, Φ intra , Φ inter , and Φ app , which are relevant to the mechanical nature of the molecular assembly. The Φ intra values account for the internal stiffness of molecules, while the Φ inter values are true stiffness constants of the intermolecular force and show a good correlation with the association energies of the dimers. The Φ app values are the apparent intermolecular stiffness smaller than Φ inter , as a result of compensation for neglecting intramolecular vibrations. All these values are consistent with each other under the coupled oscillator model, showing that the present coarse-graining analysis is valid for heterodimers as well as homodimers.