1,572 research outputs found
Global dust model intercomparison in AeroCom phase I
This study presents the results of a broad intercomparison of a total of 15 global aerosol models within the AeroCom project. Each model is compared to observations related to desert dust aerosols, their direct radiative effect, and their impact on the biogeochemical cycle, i.e., aerosol optical depth (AOD) and dust deposition. Additional comparisons to Angström exponent (AE), coarse mode AOD and dust surface concentrations are included to extend the assessment of model performance and to identify common biases present in models. These data comprise a benchmark dataset that is proposed for model inspection and future dust model development. There are large differences among the global models that simulate the dust cycle and its impact on climate. In general, models simulate the climatology of vertically integrated parameters (AOD and AE) within a factor of two whereas the total deposition and surface concentration are reproduced within a factor of 10. In addition, smaller mean normalized bias and root mean square errors are obtained for the climatology of AOD and AE than for total deposition and surface concentration. Characteristics of the datasets used and their uncertainties may influence these differences. Large uncertainties still exist with respect to the deposition fluxes in the southern oceans. Further measurements and model studies are necessary to assess the general model performance to reproduce dust deposition in ocean regions sensible to iron contributions. Models overestimate the wet deposition in regions dominated by dry deposition. They generally simulate more realistic surface concentration at stations downwind of the main sources than at remote ones. Most models simulate the gradient in AOD and AE between the different dusty regions. However the seasonality and magnitude of both variables is better simulated at African stations than Middle East ones. The models simulate the offshore transport of West Africa throughout the year but they overestimate the AOD and they transport too fine particles. The models also reproduce the dust transport across the Atlantic in the summer in terms of both AOD and AE but not so well in winter-spring nor the southward displacement of the dust cloud that is responsible of the dust transport into South America. Based on the dependency of AOD on aerosol burden and size distribution we use model bias with respect to AOD and AE to infer the bias of the dust emissions in Africa and the Middle East. According to this analysis we suggest that a range of possible emissions for North Africa is 400 to 2200 Tg yr-1 and in the Middle East 26 to 526 Tg yr-1
Probing the Ginzburg-Landau potential for lasers using higher-order photon correlations
Lasing transition is known to be analogous to the second-order phase
transition. Furthermore, for some cases, it is possible to define the
Ginzburg-Landau (GL) potential, and the GL theory predicts the photon
statistical properties of lasers. However, the GL potential for lasers is
surprising, because lasers are operating in far-from-equilibrium . In this
paper, we theoretically examine the validity of the GL theory for lasers in
terms of various parameters, particularly, the ratio between photon and carrier
lifetimes. For this purpose, we use stochastic rate equations and higher-order
photon correlation functions. With higher-order photon correlation
measurements, we can check whether or not laser dynamics are described by the
GL theory. We demonstrate that, for low- lasers, the GL theory is
applicable even when the photon lifetime is comparable to the carrier lifetime.
Furthermore, this can be understand in the framework of center manifold
reduction
Longitudinal magnetic excitation in KCuCl3 studied by Raman scattering under hydrostatic pressures
We measure Raman scattering in an interacting spin-dimer system KCuCl3 under
hydrostatic pressures up to 5 GPa mediated by He gas. In the pressure-induced
quantum phase, we observe a one-magnon Raman peak, which originates from the
longitudinal magnetic excitationand is observable through the second-order
exchange interaction Raman process. We report the pressure dependence of the
frequency, halfwidth and Raman intensity of this mode.Comment: 4 pages, 3 figures, inpress in JPCS as a proceeding of LT2
Detecting Drowsy Learners at the Wheel of e-Learning Platforms with Multimodal Learning Analytics
Learners are expected to stay wakeful and focused while interacting with e-learning platforms. Although wakefulness of learners strongly relates to educational outcomes, detecting drowsy learning behaviors only from log data is not an easy task. In this study, we describe the results of our research to model learners’ wakefulness based on multimodal data generated from heart rate, seat pressure, and face recognition. We collected multimodal data from learners in a blended course of informatics and conducted two types of analysis on them. First, we clustered features based on learners’ wakefulness labels as generated by human raters and ran a statistical analysis. This analysis helped us generate insights from multimodal data that can be used to inform learner and teacher feedback in multimodal learning analytics. Second, we trained machine learning models with multiclass-Support Vector Machine (SVM), Random Forest (RF) and CatBoost Classifier (CatBoost) algorithms to recognize learners’ wakefulness states automatically. We achieved an average macro-F1 score of 0.82 in automated user-dependent models with CatBoost. We also showed that compared to unimodal data from each sensor, the multimodal sensor data can improve the accuracy of models predicting the wakefulness states of learners while they are interacting with e-learning platforms
Lattice vibrations and structural instability in Cesium near the cubic to tetragonal transition
Under pressure cesium undergoes a transition from a high-pressure fcc phase
(Cs-II) to a collapsed fcc phase (Cs-III) near 4.2GPa. At 4.4GPa there follows
a transition to the tetragonal Cs-IV phase. In order to investigate the lattice
vibrations in the fcc phase and seek a possible dynamical instability of the
lattice, the phonon spectra of fcc-Cs at volumes near the III-IV transition are
calculated using Savrasov's density functional linear-response LMTO method.
Compared with quasiharmonic model calculations including non-central
interatomic forces up to second neighbours, at the volume (
is the experimental volume of bcc-Cs with =6.048{\AA}), the
linear-response calculations show soft intermediate wavelength
phonons. Similar softening is also observed for
short wavelength and phonons and intermediate
wavelength phonons. The Born-von K\'{a}rm\'{a}n analysis of
dispersion curves indicates that the interplanar force constants exhibit
oscillating behaviours against plane spacing and the large softening of
intermediate wavelength phonons results from a
negative (110)-interplanar force-constant . The frequencies of the
phonons with around 1/3 become imaginary
and the fcc structure becomes dynamically unstable for volumes below .
It is suggested that superstructures corresponding to the
soft mode should be present as a precursor of tetragonal Cs-IV structure.Comment: 12 pages, 5 figure
Heun's equation, generalized hypergeometric function and exceptional Jacobi polynomial
We study Heun's differential equation in the case that one of the
singularities is apparent. In particular we conjecture a relationship with
generalized hypergeometric differential equation and establish it in some
cases. We apply our results to exceptional Jacobi polynomials.Comment: 15 pages; validity of the conjecture was extende
On polynomial solutions of Heun equation
By making use of a recently developed method to solve linear differential
equations of arbitrary order, we find a wide class of polynomial solutions to
the Heun equation. We construct the series solution to the Heun equation before
identifying the polynomial solutions. The Heun equation extended by the
addition of a term, - \s/x, is also amenable for polynomial solutions.Comment: 4 pages, No figur
2D Fourier Transform Spectroscopy of exciton-polaritons and their interactions
We investigate polariton-polariton interactions in a semiconductor
microcavity through two-dimensional Fourier transform (2DFT) spectroscopy. We
observe, in addition to the lower-lower and the upper-upper polariton
self-interaction, a lower-upper cross-interaction. This appears as separated
peaks in the on-diagonal and off-diagonal part of 2DFT spectra. Moreover, we
elucidate the role of the polariton dispersion through a fine structure in the
2DFT spectrum. Simulations, based on lower-upper polariton basis
Gross-Pitaevskii equations including both self and cross-interactions, result
in a 2DFT spectra in qualitative agreement with experiments
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