| 1. |
- Buehler, Stefan A., et al.
(author)
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Efficient radiative transfer simulations for a broadband infrared radiometer—Combining a weighted mean of representative frequencies approach with frequency selection by simulated annealing
- 2010
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In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 111:4, s. 602-615
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Journal article (peer-reviewed)abstract
- We present a method to efficiently simulate the measurements of a broadband infrared instrument. The High Resolution Infrared Radiation Sounder (HIRS) instrument is used as example to illustrate the method. The method uses two basic ideas. Firstly, the channel radiance can be approximated by a weighted mean of the radiance at some representative frequencies, where the weights can be determined by linear regression. Secondly, a near-optimal set of representative frequencies can be found by simulated annealing.The paper does not only describe and analyze the method, it also describes how the method was used to derive optimized frequency grids for the HIRS instruments on the satellites TIROS N, NOAA 6-19, and Metop A. The grids and weights, as well as the optimization algorithm itself are openly available under a GNU public license.
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| 2. |
- Buehler, Stefan, et al.
(author)
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Absorption lookup tables in the radiative transfer model ARTS
- 2011
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In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 112:10, s. 1559-1567
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Journal article (peer-reviewed)abstract
- We describe the lookup table approach that is used to store pre-calculated absorption data in the radiative transfer model ARTS. The table stores absorption cross sections as a function of frequency, pressure, temperature, and the water vapor volume mixing ratio, where the last dimension is only included for those gas species that require it. The table is used together with an extraction strategy, which uses polynomial interpolation, with recommended interpolation orders between five and seven. We also derived recommended default settings for grid spacings and interpolation orders, and verified that the approach gives very accurate results with these default settings. The tested instrument setups were for AMSU-B, HIRS, and Odin, three well-known satellite remote sensing instruments covering a wide range of frequencies and viewing geometries. Errors introduced by the lookup table were found to be always below a few millikelvin, in terms of the simulated brightness temperature.
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| 3. |
- Dubernet, M. L., et al.
(author)
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Virtual atomic and molecular data centre
- 2010
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - 0022-4073 .- 1879-1352. ; 111:15, s. 2151-2159
-
Journal article (peer-reviewed)abstract
- The Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu) is a European Union funded collaboration between groups involved in the generation, evaluation, and use of atomic and molecular data. VAMDC aims to build a secure, documented, flexible and interoperable e-science environment-based interface to existing atomic and molecular data. The project will cover establishing the core consortium, the development and deployment of the infrastructure and the development of interfaces to the existing atomic and molecular databases. It will also provide a forum for training potential users and dissemination of expertise worldwide. This review describes the scope of the VAMDC project; it provides a survey of the atomic and molecular data sets that will be included plus a discussion of how they will be integrated. Some applications of these data are also discussed.
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| 4. |
- Eriksson, Patrick, 1964, et al.
(author)
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ARTS, the atmospheric radiative transfer simulator, Version 2
- 2011
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 112:10, s. 1551-1558
-
Journal article (peer-reviewed)abstract
- The second version of the atmospheric radiative transfer simulator. ARTS, is introduced. This is a general software package for long wavelength radiative transfer simulations, with a focus on passive microwave observations. The core part provides a workspace environment, in line with script languages. New for this version is an agenda mechanism that gives a high degree of modularity. The framework is intended to be as general as possible: the polarisation state can be fully described, the model atmosphere can be one- (1D), two- (2D) or three-dimensional (3D), a full description of geoid and surface is possible, observation geometries from the ground, from satellite, and from aeroplane or balloon are handled, and surface reflection can be treated in simple or complex manners. Remote sensing applications are supported by a comprehensive and efficient treatment of sensor characteristics. jacobians can be calculated for the most important atmospheric variables in non-scattering conditions. Finally, the most prominent feature is the rigorous treatment of scattering that has been implemented in two modules: a discrete ordinate iterative approach mainly used for 1D atmospheres, and a Monte Carlo approach which is the preferred algorithm for 3D atmospheres. ARTS is freely available, and maintained as an open-source project.
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| 5. |
- Gasteiger, J., et al.
(author)
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Representative wavelengths absorption parameterization applied to satellite channels and spectral bands
- 2014
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In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 148, s. 99-115
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Journal article (peer-reviewed)abstract
- Accurate modeling of wavelength-integrated radiative quantities, e.g. integrated over a spectral band or an instrument channel response function, requires computations for a large number of wavelengths if the radiation is affected by gas absorption which typically comprises a complex line structure. In order to increase computational speed of modeling radiation in the Earth׳s atmosphere, we parameterized wavelength-integrals as weighted means over representative wavelengths. We parameterized spectral bands of different widths (1 cm−1, 5 cm−1, and 15 cm−1) in the solar and thermal spectral range, as well as a number of instrument channels on the ADEOS, ALOS, EarthCARE, Envisat, ERS, Landsat, MSG, PARASOL, Proba, Sentinel, Seosat, and SPOT satellites. A root mean square relative deviation lower than 1% from a “training data set” was selected as the accuracy threshold for the parameterization of each band and channel. The training data set included high spectral resolution calculations of radiances at the top of atmosphere for a set of highly variable atmospheric states including clouds and aerosols. The gas absorption was calculated from the HITRAN 2004 spectroscopic data set and state-of-the-art continuum models using the ARTS radiative transfer model. Three representative wavelengths were required on average to fulfill the accuracy threshold. We implemented the parameterized spectral bands and satellite channels in the uvspec radiative transfer model which is part of the libRadtran software package. The parameterization data files, including the representative wavelengths and weights as well as lookup tables of absorption cross sections of various gases, are provided at the libRadtran webpage.In the paper we describe the parameterization approach and its application. We validate the approach by comparing modeling results of parameterized bands and channels with results from high spectral resolution calculations for atmospheric states that were not part of the training data set. Irradiances are not only compared at the top of atmosphere but also at the surface for which this parameterization approach was not optimized. It is found that the parameterized bands and channels provide a good compromise between computation time requirements and uncertainty for typical radiative transfer problems. In particular for satellite radiometer simulations the computation time requirement and the parameterization uncertainty is low. Band-integrated irradiances at any level as well as heating and cooling rates below 20 km can also be modeled with low uncertainty.
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| 6. |
- Holl, Gerrit, et al.
(author)
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Optimised frequency grids for infrared radiative transfer simulations in cloudy conditions
- 2012
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In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 113:16, s. 2124-2134
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Journal article (peer-reviewed)abstract
- This paper shows that radiometer channel radiances for cloudy atmospheric conditions can be simulated with an optimised frequency grid derived under clear-sky conditions. A new clear-sky optimised grid is derived for AVHRR channel . For HIRS channel 11 and AVHRR channel 5, radiative transfer simulations using an optimised frequency grid are compared with simulations using a reference grid, where the optimised grid has roughly 100–1000 times less frequencies than the full grid. The root mean square error between the optimised and the reference simulation is found to be less than 0.3 K for both comparisons, with the magnitude of the bias less than 0.03 K. The simulations have been carried out with the radiative transfer model Atmospheric Radiative Transfer Simulator (ARTS), version 2, using a backward Monte Carlo module for the treatment of clouds. With this module, the optimised simulations are more than 10 times faster than the reference simulations. Although the number of photons is the same, the smaller number of frequencies reduces the overhead for preparing the optical properties for each frequency. With deterministic scattering solvers, the relative decrease in runtime would be even more. The results allow for new radiative transfer applications, such as the development of new retrievals, because it becomes much quicker to carry out a large number of simulations. The conclusions are applicable to any downlooking infrared radiometer.
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| 7. |
- Larsson, Richard, et al.
(author)
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A treatment of the Zeeman effect using Stokes formalism and its implementation in the Atmospheric Radiative Transfer Simulator (ARTS)
- 2014
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 133, s. 445-453
-
Journal article (peer-reviewed)abstract
- This paper presents the practical theory that was used to implement the Zeeman effect using Stokes formalism in the Atmospheric Radiative Transfer Simulator (ARTS). ARTS now treats the Zeeman effect in a general manner for several gas species for all polarizations and takes into account variations in both magnetic and atmospheric fields along a full 3D geometry. We present how Zeeman splitting affects polarization in radiative transfer simulations and find that the effect may be large in Earth settings for polarized receivers in limb observing geometry. We find that not taking a spatially varying magnetic field into account can result in absolute errors in the measurement vector of at least 10K in Earth magnetic field settings. The paper also presents qualitative tests for O2 lines against previous models (61.15GHz line) and satellite data from Odin-SMR (487.25GHz line), and the overall consistency between previous models, satellite data, and the new ARTS Zeeman module seems encouraging.
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| 8. |
- Puschnig, Johannes, et al.
(author)
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The night sky brightness at Potsdam-Babelsberg including overcast and moonlit conditions
- 2014
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 139, s. 76-81
-
Journal article (peer-reviewed)abstract
- We analyze the results of 2 years (2011-2012) of night sky photometry performed at the Leibniz Institute for Astrophysics in Potsdam-Babelsberg. This institute is located 23 km to the southwest of the center of Berlin. Our measurements have been performed with a Sky Quality Meter. We find night sky brightness values ranging from 16.5 to 203 mag(SQM) arcsec(-2); the latter value corresponds to 4.8 times the natural zenithal night sky brightness. We focus on the influence of clouds and of the moon on the night sky brightness. It turns out that Potsdam-Babelsberg, despite its proximity to Berlin, still shows a significant correlation of the night sky brightness with the lunar phases. However, the light-pollution-enhancing effect of clouds dominates the night sky brightness by far: overcast nights (up to 16.5 mag(SQM) arcsec-2) are much brighter than clear full moon nights (18-18.5 mag(SQM),arcsec(-2)).
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| 9. |
- Radziute, Laima, et al.
(author)
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Energy level structure of Er3
- 2014
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier. - 0022-4073 .- 1879-1352. ; 152, s. 94-106
-
Journal article (peer-reviewed)abstract
- AtomicenergylevelsandE2andM1transitionratesarereportedforthe[Xe]4f11 ground configurationinEr3þ from relativisticmulticonfigurationDirac–Hartree–Fockandcon- figuration interactioncalculations.TheBreittransverseinteractionandleadingQED effectsareincludedasperturbations.Differentstrategiesfordescribingelectroncorrela- tion effectsaretestedandevaluated.Thecalculatedenergylevelsarecomparedwith experimentandresultsfromsemi-empiricalmethods.
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| 10. |
- Richard, C., et al.
(author)
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New section of the HITRAN database: Collision-induced absorption (CIA)
- 2012
-
In: Journal of Quantitative Spectroscopy & Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 113:11, s. 1276-1285
-
Journal article (peer-reviewed)abstract
- This paper describes the addition of Collision-Induced Absorption (CIA) into the HITRAN compilation. The data from different experimental and theoretical sources have been cast into a consistent format and formalism. The implementation of these new spectral data into the HITRAN database is invaluable for modeling and interpreting spectra of telluric and other planetary atmospheres as well as stellar atmospheres. In this implementation for HITRAN, CIAs of N-2, H-2, O-2, CO2, and CH4 due to various collisionally interacting atoms or molecules are presented. Some CIA spectra are given over an extended range of frequencies, including several H-2 overtone bands that are dipole-forbidden in the non-interacting molecules. Temperatures from tens to thousands of Kelvin are considered, as required, for example, in astrophysical analyses of objects, including cool white dwarfs, brown dwarfs. M dwarfs, cool main sequence stars, solar and extra-solar planets, and the formation of so-called first stars. (C) 2011 Elsevier Ltd. All rights reserved.
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| 11. |
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| 12. |
- Schreier, Franz, et al.
(author)
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GARLIC - a general purpose atmospheric radiative transfer line-by-line infrared-microwave code : Implementation and evaluation
- 2014
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 137, s. 29-50
-
Journal article (peer-reviewed)abstract
- A suite of programs for high resolution infrared-microwave atmospheric radiative transfer modeling has been developed with emphasis on efficient and reliable numerical algorithms and a modular approach appropriate for simulation and/or retrieval in a variety of applications. The Generic Atmospheric Radiation Line-by-line Infrared Code — GARLIC — is suitable for arbitrary observation geometry, instrumental field–of–view, and line shape. The core of GARLIC's subroutines constitutes the basis of forward models used to implement inversion codes to retrieve atmospheric state parameters from limb and nadir sounding instruments.This paper briefly introduces the physical and mathematical basics of GARLIC and its descendants and continues with an in-depth presentation of various implementation aspects: An optimized Voigt function algorithm combined with a two-grid approach is used to accelerate the line-by-line modeling of molecular cross sections; various quadrature methods are implemented to evaluate the Schwarzschild and Beer integrals; and Jacobians, i.e. derivatives with respect to the unknowns of the atmospheric inverse problem, are implemented by means of automatic differentiation. For an assessment of GARLIC's performance, a comparison of the quadrature methods for solution of the path integral is provided. Verification and validation are demonstrated using intercomparisons with other line-by-line codes and comparisons of synthetic spectra with spectra observed on Earth and from Venus.
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| 13. |
- Wang, Junyang, et al.
(author)
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On the accuracy of the assessment of molecular concentration and spectroscopic parameters by frequency modulation spectrometry and NICE-OHMS
- 2014
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073 .- 1879-1352. ; 136, s. 28-44
-
Journal article (peer-reviewed)abstract
- Frequency modulation spectrometry (FMS), and thereby also noise immune cavity enhanced optical heterodyne molecular spectrometry (NICE-OHMS), can detect both absorption and dispersion signals, and can therefore, by curve fitting, extract molecular parameters from both these types of signals. However, parameters evaluated from the two modes of detection have been previously shown not to be identical. Their accuracy is affected by both the type of lineshape used by the fit and the accuracy of the detection phase. A thorough study is presented of the influence of three lineshape functions [Voigt, Rautian, and speed-dependent Voigt (SDV)] and errors in the detection phase on the retrieval of various molecular parameters, in particular the signal strength, which provides information about the concentration of molecules in a gas, from reference spectra in the 10-260 Torr region. It was found that for data detected and evaluated at pure absorption or dispersion phase by a system calibrated in the Doppler limit the signal strength can be underestimated at higher pressures by up to 45% if the evaluation is made using the Voigt profile. If the detection is plagued by phase errors additional inaccuracies, often in the order of percent per degree phase error (%/deg), can occur. More reliable parameters can be obtained if an appropriate lineshape function is used and the detection phase is considered a free parameter. However, despite this, none of the evaluation procedures can retrieve the molecular parameters fully correctly; the most accurate assessments of the signal strength, obtained when the data is detected close to absorption phase and evaluated by the SDV lineshape function, are still associated with an error of a few percent. (C) 2013 Elsevier Ltd. All rights reserved.
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| 14. |
- Westberg, Jonas, et al.
(author)
-
Analytical expression for the nth Fourier coefficient of a modulated Lorentzian dispersion lineshape function
- 2011
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier. - 0022-4073 .- 1879-1352. ; 112:9, s. 1443-1449
-
Journal article (peer-reviewed)abstract
- Modulated spectroscopic detection techniques that rely on dispersion, e.g. Faraday modulation/rotation spectroscopy and wavelength-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy, are often described in terms of Fourier coefficients of a modulated dispersion lineshape function. This work derives a non-complex analytical expression for the nth Fourier coefficient of a modulated Lorentzian dispersion lineshape function. The expression is easier to implement, and orders of magnitude faster to execute, than previous approaches involving numerical calculations of integrals. The first six Fourier coefficients are explicitly given and illustrated for their optimum modulation amplitudes, which are also given
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| 15. |
- Westberg, Jonas, et al.
(author)
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Fast and non-approximate methodology for calculation of wavelength-modulated Voigt lineshape functions suitable for real-time curve fitting
- 2012
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - Oxford : Elsevier. - 0022-4073 .- 1879-1352. ; 113:16, s. 2049-2057
-
Journal article (peer-reviewed)abstract
- Wavelength modulation (WM) produces lock-in signals that are proportional to various Fourier coefficients of the modulated lineshape function of the molecular transition targeted. Unlike the case for the Lorentzian lineshape function, there is no known analytical expression for the Fourier coefficients of a modulated Voigt lineshape function; they consist of nested integrals that have to be solved numerically, which is often time-consuming and prevents real-time curve fitting. Previous attempts to overcome these limitations have so far consisted of approximations of the Voigt lineshape function, which brings in inaccuracies. In this paper we demonstrate a new means to calculate the lineshape of nf-WM absorption signals from a transition with a Voigt profile. It is shown that the signal can conveniently be expressed as a convolution of one or several Fourier coefficients of a modulated Lorentzian lineshape function, for which there are analytical expressions, and the Maxwell-Boltzmann velocity distribution for the system under study. Mathematically, the procedure involves no approximations, wherefore its accuracy is limited only by the numerical precision of the software used (in this case similar to 10(-16)) while the calculation time is reduced by roughly three orders of magnitude (10(-3)) as compared to the conventional methodology, i.e. typically from the second to the millisecond range. This makes feasible real-time curve fitting to lock-in output signals from modulated Voigt profiles. (C) 2012 Elsevier Ltd. All rights reserved.
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| 16. |
- Westberg, Jonas, et al.
(author)
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Methodology for fast curve fitting to modulated Voigt dispersion lineshape functions
- 2014
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier. - 0022-4073 .- 1879-1352. ; 133, s. 244-250
-
Journal article (peer-reviewed)abstract
- Faraday rotation spectroscopy (FAMOS) as well as other modulated techniques that rely on dispersion produce lock-in signals that are proportional to various Fourier coefficients of modulated dispersion lineshape functions of the molecular transition targeted. In order to enable real-time curve fitting to such signals a fast methodology for calculating the Fourier coefficients of modulated lineshape functions is needed. Although there exist an analytical expression for such Fourier coefficients of modulated Lorentzian absorption and dispersion lineshape functions, there is no corresponding expression for a modulated Voigt dispersion function. The conventional computational route of such Fourier coefficients has therefore so far either consisted of using various approximations to the modulated Voigt lineshape function or solving time-consuming integrals, which has precluded accurate real-time curve fitting. Here we present a new methodology to calculate Fourier coefficients of modulated Voigt dispersion lineshape functions that is significantly faster (several orders of magnitude) and more accurate than previous approximative calculation procedures, which allows for real-time curve fitting to FAMOS signals also in the Voigt regime.
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| 17. |
- Westberg, Jonas, et al.
(author)
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Quantitative description of Faraday modulation spectrometry in terms of the integrated linestrength and 1st Fourier coefficients of the modulated lineshape function
- 2010
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier. - 0022-4073 .- 1879-1352. ; 111:16, s. 2415-2433
-
Journal article (peer-reviewed)abstract
- A quantitative description of the strength and shape of Faraday modulation spectrometry (FAMOS) signals is given. It is first shown how the signal can be expressed in terms of the integrated linestrength for the targeted transition, Si,j. Secondly, since the technique relies on a periodic modulation of the transition frequency induced by an alternating magnetic field, it is explicitly shown that it is possible to express the FAMOS signal concisely in terms of 1st Fourier coefficients of a magnetic-field-modulated dispersive lineshape function for left- and right-handed circularly polarized light. Expressions for the FAMOS signal in terms of the integrated linestrength and such Fourier coefficients are given for three cases: (i) for transitions between two arbitrary types of states, (ii) for transitions between two states that both belong to Hund’s coupling case (a), as is the case for rotational–vibrational transitions of NO, and finally (iii) for the commonly used Q-transitions between such states. It is finally shown that the FAMOS signal from a Q-transition can be expressed succinctly solely in terms of one 1st Fourier coefficient. A general analysis of FAMOS addressing an arbitrary Q-transition as well as the most sensitive Q3/2(3/2) transition in NO is given. The conditions for maximum signal are specifically identified.
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| 18. |
- Kahnert, Michael, 1968, et al.
(author)
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Light scattering by particles with small-scale surface roughness: comparison of four classes of model geometries
- 2012
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 113:18, s. 2356-2367
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Journal article (peer-reviewed)abstract
- We compare four different model geometries for particles with small-scale surface roughness. The geometries are based on regular and stochastic surface perturbations, as well as on 2D- and 3D-roughness models. We further compare T-matrix and discrete dipole computations. Particle size parameters of 5 and 50 are considered, as well as refractive indices of 1.6 + 0.0005i and 3 +0.1i. The effect of small-scale surface roughness on the intensity and polarisation of the scattered light strongly depends on the size parameter and refractive index. In general, 2D surface roughness models predict stronger effects than 3D models. Stochastic surface roughness models tend to predict the strongest depolarising effects, while regular surface roughness models can have a stronger effect on the angular distribution of the scattered intensity. Computations with the discrete dipole approximation only cover a limited range of size parameters. T-matrix computations allow us to significantly extend that range, but at the price of restricting the model particles to symmetric surface perturbations with small amplitudes.
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| 19. |
- Kahnert, Michael, 1968, et al.
(author)
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Review: Model particles in atmospheric optics
- 2014
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In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 146, s. 41-58
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Journal article (peer-reviewed)abstract
- This review paper provides an overview over model geometries for computing light scattering by small particles. The emphasis is on atmospheric optics, although much of this review will also be relevant to neighbouring fields, in particular to astronomy. Various morphological particle properties are discussed, such as overall nonsphericity, pristine shapes, aggregation, and different forms of inhomogeneity, e.g. porous and compact inhomogeneous morphologies, as well as encapsulated aggregates. Models employed to reproduce the optical properties of complex particles range from strongly simplified to highly realistic and morphologically sophisticated model geometries. Besides reviewing the most recent literature, we discuss the idea behind models of varying degree of complexity with regard to the intended use of the models. Applications range from fundamental studies of light scattering processes to routine applications of particle optics look-up tables in operational modelling systems.
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| 20. |
- Kahnert, Michael, 1968
(author)
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T-matrix computations for particles with high-order finite symmetries
- 2013
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In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 123, s. 79-91
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Journal article (peer-reviewed)abstract
- The use of group theoretical methods can substantially reduce numerical ill-conditioning problems in T-matrix computations. There are specific problems related to obtaining the irreducible characters of high-order symmetry groups and to the construction of a transformation from the basis of vector spherical wave functions to the irreducible basis of high-order symmetry groups. These problems are addressed, and numerical solutions are discussed and tested. An important application of the method is non-convex particles perturbed with high-order polynomials. Such morphologies can serve as models for particles with small-scale surface roughness, such as mineral aerosols, atmospheric ice particles with rimed surfaces, and various types of cosmic dust particles. The method is tested for high-order 3D-Chebyshev particles, and the performance of the method is gauged by comparing the results to computations based on iteratively solving a Lippmann-Schwinger T-matrix equation. The latter method trades ill-conditioning problems for potential slow-convergence problems, and it is rather specific, as it is tailored to particles with small-scale surface roughness. The group theoretical method is general and not plagued by slow-convergence problems. The comparison of results shows that both methods achieve a comparable numerical stability. This suggests that for particles with high-order symmetries the group-theoretical approach is able to overcome the illconditioning problems. Remaining numerical limitations are likely to be associated with loss-of-precision problems in the numerical evaluation of the surface integrals.
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| 21. |
- Kahnert, Michael, 1968
(author)
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The T-matrix code Tsym for homogeneous dielectric particles with finite symmetries
- 2013
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 123, s. 62-78
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Journal article (peer-reviewed)abstract
- A T-matrix code tailored to non-axisymmetric particles with finite symmetries is described. The code exploits geometric symmetries of particles by use of group theoretical methods. Commutation relations of the T-matrix are implemented for reducing CPU-time requirements. Irreducible representations of finite groups are employed for alleviating ill-conditioning problems in numerical computations. Further, an iterative T-matrix method for particles with small-scale surface perturbations is implemented. The code can compute both differential and integrated optical properties of particles in,either fixed or random orientation. Methods for testing the convergence and correctness of the computational results are discussed. The package also includes a database of pre-computed group-character tables, as well as an interface to the GAP programming language for computational group theory.
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| 22. |
- Kristensson, Gerhard
(author)
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Electromagnetic scattering by a bounded obstacle in a parallel plate waveguide
- 2013
-
In: Journal of Quantitative Spectroscopy & Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 123, s. 92-102
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Journal article (peer-reviewed)abstract
- This paper concerns scattering of an electromagnetic wave by a bounded object located inside a parallel plate waveguide. The exciting field in the waveguide is either an arbitrary source located at a finite distance from the obstacle or a plane wave generated in the far zone. In the latter case, the generating field corresponds to the lowest propagating mode (TEM) in the waveguide. The analytic treatment of the problem relies on an extension of the null field approach, or the T-matrix method, originally developed by Peter Waterman, and later generalized to deal with objects close to an interface. The present paper generalizes this approach further to deal with obstacles inside a parallel plate waveguide. This problem shows features that reflect both the two-dimensional geometry and the three-dimensional scattering characteristics. The analysis is illustrated by several numerical examples. (C) 2013 Elsevier Ltd. All rights reserved.
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| 23. |
- Mackowski, D. W., et al.
(author)
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A T matrix method based upon scalar basis functions
- 2013
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 123, s. 113-121
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Journal article (peer-reviewed)abstract
- A surface integral formulation is developed for the T matrix of a homogenous and isotropic particle of arbitrary shape, which employs scalar basis functions represented by the translation matrix elements of the vector spherical wave functions. The formulation begins with the volume integral equation for scattering by the particle, which is transformed so that the vector and dyadic components in the equation are replaced with associated dipole and multipole level scalar harmonic wave functions. The approach leads to a volume integral formulation for the T matrix, which can be extended, by the use of Green's identities, to the surface integral formulation. The result is shown to be equivalent to the traditional surface integral formulas based on the VSWF basis.
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| 24. |
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| 25. |
- Takano, Y., et al.
(author)
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The single-scattering properties of black carbon aggregates determined from the geometric-optics surface-wave approach and the T-matrix method
- 2013
-
In: Journal of Quantitative Spectroscopy and Radiative Transfer. - : Elsevier BV. - 0022-4073. ; 125, s. 51-56
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Journal article (peer-reviewed)abstract
- The single-scattering properties of eight black carbon (BC, soot) fractal aggregates, composed of primary spheres from 7 to 600, computed by the geometric-optics surface-wave (GOS) approach coupled with the Rayleigh-Gans-Debye (RGD) adjustment for size parameters smaller than approximately 2, are compared with those determined from the superposition T-matrix method. We show that under the condition of random orientation, the results from GOS/RGD are in general agreement with those from T-matrix in terms of the extinction and absorption cross-sections, the single-scattering co-albedo, and the asymmetry factor. When compared with the specific absorption (m(2)/g) measured in the laboratory, we illustrate that using the observed radii of primary spheres ranging from 3.3 to 25 nm, the theoretical values determined from GOS/RGD for primary sphere numbers of 100-600 are within the range of measured values. The GOS approach can be effectively applied to aggregates composed of a large number of primary spheres (e.g., > 6000) and large size parameters (>> 2) in terms of computational efforts.
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