Showing papers in "Journal of Quantitative Spectroscopy & Radiative Transfer in 2011"

The discrete-dipole-approximation code ADDA: Capabilities and known limitations

Abstract: The open-source code ADDA is described, which implements the discrete dipole approximation (DDA), a method to simulate light scattering by finite 3D objects of arbitrary shape and composition. Besides standard sequential execution, ADDA can run on a multiprocessor distributed-memory system, parallelizing a single DDA calculation. Hence the size parameter of the scatterer is in principle limited only by total available memory and computational speed. ADDA is written in C99 and is highly portable. It provides full control over the scattering geometry (particle morphology and orientation, and incident beam) and allows one to calculate a wide variety of integral and angle-resolved scattering quantities (cross sections, the Mueller matrix, etc.). Moreover, ADDA incorporates a range of state-of-the-art DDA improvements, aimed at increasing the accuracy and computational speed of the method. We discuss both physical and computational aspects of the DDA simulations and provide a practical introduction into performing such simulations with the ADDA code. We also present several simulation results, in particular, for a sphere with size parameter 320 (100-wavelength diameter) and refractive index 1.05.

A multiple sphere T-matrix Fortran code for use on parallel computer clusters

Abstract: A general-purpose Fortran-90 code for calculation of the electromagnetic scattering and absorption properties of multiple sphere clusters is described. The code can calculate the efficiency factors and scattering matrix elements of the cluster for either fixed or random orientation with respect to the incident beam and for plane wave or localized- approximation Gaussian incident fields. In addition, the code can calculate maps of the electric field both interior and exterior to the spheres.The code is written with message passing interface instructions to enable the use on distributed memory compute clusters, and for such platforms the code can make feasible the calculation of absorption, scattering, and general EM characteristics of systems containing several thousand spheres.

The 2009 edition of the GEISA spectroscopic database

Abstract: The updated 2009 edition of the spectroscopic database GEISA (Gestion et Etude des Informations Spectroscopiques Atmospheriques; Management and Study of Atmospheric Spectroscopic Information) is described in this paper. GEISA is a computer-accessible system comprising three independent sub-databases devoted, respectively, to: line parameters, infrared and ultraviolet/visible absorption cross-sections, microphysical and optical properties of atmospheric aerosols. In this edition, 50 molecules are involved in the line parameters sub-database, including 111 isotopologues, for a total of 3,807,997 entries, in the spectral range from 10−6 to 35,877.031 cm−1. The successful performances of the new generation of hyperspectral sounders depend ultimately on the accuracy to which the spectroscopic parameters of the optically active atmospheric gases are known, since they constitute an essential input to the forward radiative transfer models that are used to interpret their observations. Currently, GEISA is involved in activities related to the assessment of the capabilities of IASI (Infrared Atmospheric Sounding Interferometer; http://smsc.cnes.fr/IASI/index.htm) on board the METOP European satellite through the GEISA/IASI database derived from GEISA. Since the Metop-A (http://www.eumetsat.int) launch (19 October 2006), GEISA is the reference spectroscopic database for the validation of the level-1 IASI data. Also, GEISA is involved in planetary research, i.e., modeling of Titan's atmosphere, in the comparison with observations performed by Voyager, or by ground-based telescopes, and by the instruments on board the Cassini–Huygens mission. GEISA, continuously developed and maintained at LMD (Laboratoire de Meteorologie Dynamique, France) since 1976, is implemented on the IPSL/CNRS (France) “Ether” Products and Services Centre WEB site (http://ether.ipsl.jussieu.fr), where all archived spectroscopic data can be handled through general and user friendly associated management software facilities. More than 350 researchers are registered for on line use of GEISA.

CDSD-4000: High-resolution, high-temperature carbon dioxide spectroscopic databank

Abstract: We present a high-resolution, high-temperature version of the Carbon Dioxide Spectroscopic Databank called CDSD-4000. The databank contains the line parameters (positions, intensities, air- and self-broadened half-widths, coefficients of temperature dependence of air- and self-broadened half-widths, and air-broadened pressure shifts) of the four most abundant isotopologues of CO 2 . A reference temperature is 296 K and an intensity cutoff is 10 −27 cm −1 /molecule cm −2 at 4000 K. The databank has 628,324,454 entries, covers the 226–8310 cm −1 spectral range and designed for the temperature range 2500–5000 K. Format of CDSD-4000 is similar to that of HITRAN-2008. The databank has been generated within the framework of the method of effective operators and based on the global fittings of spectroscopic parameters (parameters of the effective Hamiltonians and effective dipole moment operators) to observed data collected from the literature. The databank is useful for studying high-temperature radiative properties of CO 2 , including exoplanets atmospheres, aerothemal modeling for Mars entry missions, high-temperature laboratory spectra, and industrial applications. CDSD-4000 is freely accessible via the Internet site ftp://ftp.iao.ru/pub/CDSD-4000 .

ARTS, the atmospheric radiative transfer simulator, version 2

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.

Radiation transfer in photobiological carbon dioxide fixation and fuel production by microalgae

Abstract: Solar radiation is the energy source driving the metabolic activity of microorganisms able to photobiologically fixate carbon dioxide and convert solar energy into biofuels. Thus, careful radiation transfer analysis must be conducted in order to design and operate efficient photobioreactors. This review paper first introduces light harvesting mechanisms used by microorganisms as well as photosynthesis and photobiological fuel production. It then provides a thorough and critical review of both experimental and modeling efforts focusing on radiation transfer in microalgae suspension. Experimental methods to determine the radiation characteristics of microalgae are presented. Methods for solving the radiation transfer equation in photobioreactors with or without bubbles are also discussed. Sample measurements and numerical solutions are provided. Finally, novel strategies for achieving optimum light delivery and maximizing sunlight utilization in photobioreactors are discussed including genetic engineering of microorganisms with truncated chlorophyll antenna.

The Monte Carlo atmospheric radiative transfer model McArtim: Introduction and validation of Jacobians and 3D features

Abstract: A new Monte Carlo atmospheric radiative transfer model is presented which is designed to support the interpretation of UV/vis/near-IR spectroscopic measurements of scattered Sun light in the atmosphere. The integro differential equation describing the underlying transport process and its formal solution are discussed. A stochastic approach to solve the differential equation, the Monte Carlo method, is deduced and its application to the formal solution is demonstrated. It is shown how model photon trajectories of the resulting ray tracing algorithm are used to estimate functionals of the radiation field such as radiances, actinic fluxes and light path integrals. In addition, Jacobians of the former quantities with respect to optical parameters of the atmosphere are analyzed. Model output quantities are validated against measurements, by self-consistency tests and through inter comparisons with other radiative transfer models.

Generalized Lorenz–Mie theories and description of electromagnetic arbitrary shaped beams: Localized approximations and localized beam models, a review

Abstract: The description of electromagnetic arbitrary shaped beams (e.g. laser beams) under expanded forms requires the evaluation of expansion coefficients known as beam shape coefficients. Several methods have been designed to evaluate these coefficients but the most efficient one relies on the use of localization operators, leading to localized approximations and to localized beam models, whose history and features are reviewed in this paper. Localized approximations and localized beam models have been particularly useful for speeding up numerical computations in the framework of generalized Lorenz–Mie theories (GLMTs), i.e. theories dealing with the interaction between electromagnetic arbitrary shaped beams and a regular particle, allowing one to solve the problem by using the method of separation of variables. However, they can be useful in other scattering approaches, such as the extended boundary condition method (or null-field method), or more generally, when the need of an efficient description of an electromagnetic arbitrary shaped beam is required.

Scattering and absorption of light by ice particles: Solution by a new physical-geometric optics hybrid method

Abstract: A new physical-geometric optics hybrid (PGOH) method is developed to compute the scattering and absorption properties of ice particles. This method is suitable for studying the optical properties of ice particles with arbitrary orientations, complex refractive indices (i.e., particles with significant absorption), and size parameters (proportional to the ratio of particle size to incident wavelength) larger than ∼20, and includes consideration of the edge effects necessary for accurate determination of the extinction and absorption efficiencies. Light beams with polygon-shaped cross sections propagate within a particle and are traced by using a beam-splitting technique. The electric field associated with a beam is calculated using a beam-tracing process in which the amplitude and phase variations over the wavefront of the localized wave associated with the beam are considered analytically. The geometric-optics near field for each ray is obtained, and the single-scattering properties of particles are calculated from electromagnetic integral equations. The present method does not assume additional physical simplifications and approximations, except for geometric optics principles, and may be regarded as a “benchmark” within the framework of the geometric optics approach. The computational time is on the order of seconds for a single-orientation simulation and is essentially independent of the size parameter. The single-scattering properties of oriented hexagonal ice particles (ice plates and hexagons) are presented. The numerical results are compared with those computed from the discrete-dipole-approximation (DDA) method.

Luminescent studies of Dy3+ ion in alkali lead tellurofluoroborate glasses

Abstract: Dy 3+ -doped alkali lead tellurofluoroborate (RLTB) glasses (R=Li, Na and K) were prepared by melt quenching technique. Judd–Ofelt theory has been used to evaluate the three intensity parameters Ω 2 , Ω 4 and Ω 6 from the experimental oscillator strengths. The photoluminescence spectra obtained by the excitation wavelength of 385 nm show four emission bands at 454, 483, 575 and 665 nm corresponding to the 4 I 15/2 → 6 H 15/2 and 4 F 9/2 → 6 H J/2 ( J =15/2, 13/2 and 11/2) transitions, respectively. The laser characteristic parameters like fullwidth at half maxima (FWHM), stimulated emission cross-sections ( σ e ), optical gain parameters ( σ e × τ exp ) and gain bandwidth parameters ( σ e ×FWHM) were determined. From the visible emission spectra, yellow to blue (Y/B) intensity ratios and chromaticity coordinates were also estimated. The lifetimes of 4 F 9/2 metastable state were also measured and discussed.

New secondary-scattering correction in DISORT with increased efficiency for forward scattering

Abstract: We present an alternative method to calculate the directional distribution after secondary scattering of light in an atmosphere, and apply it to the correction developed by Nakajima and Tanaka (1988) [1] as implemented in the DISORT radiative transfer solver. This method employs the scattering phase functions directly, instead of expanding over their Legendre moments as in the original formulation, and hence is not compromised in cases where a prohibitive number of moments is required to maintain accuracy. The new approach is designed to be particularly efficient in the strongly forward-scattering case, which arises for example in problems involving cloud-ice or dust particles. We have implemented this in a newly rewritten C-code version of DISORT that provides additional computational efficiencies via dynamic and cache-aware memory allocation. The new version uses less memory and runs considerably faster than the original, while producing results with equal or greater accuracy.

A single-scattering approximation for infrared radiative transfer in limb geometry in the Martian atmosphere

Abstract: We present a single-scattering approximation for infrared radiative transfer in limb geometry in the Martian atmosphere. It is based on the assumption that the upwelling internal radiation field is dominated by a surface with a uniform brightness temperature. It allows the calculation of the scattering source function for individual aerosol types, mixtures of aerosol types, and mixtures of gas and aerosol. The approximation can be applied in a Curtis–Godson radiative transfer code and is used for operational retrievals from Mars Climate Sounder measurements. Radiance comparisons with a multiple scattering model show good agreement in the mid- and far-infrared although the approximate model tends to underestimate the radiances in realistic conditions of the Martian atmosphere. Relative radiance differences are found to be about 2% in the lowermost atmosphere, increasing to ∼ 10 % in the middle atmosphere of Mars. The increasing differences with altitude are mostly due to the increasing contribution to limb radiance of scattering relative to emission at the colder, higher atmospheric levels. This effect becomes smaller toward longer wavelengths at typical Martian temperatures. The relative radiance differences are expected to produce systematic errors of similar magnitude in retrieved opacity profiles.

Water vapour self-continuum and water dimers: 1. Analysis of recent work

Abstract: Recent laboratory observations and advances in theoretical quantum chemistry allow a reappraisal of the fundamental mechanisms that determine the water vapour self-continuum absorption throughout the infrared and millimetre wave spectral regions. By starting from a framework that partitions bimolecular interactions between water molecules into free-pair states, true bound and quasi-bound dimers, we present a critical review of recent observations, continuum models and theoretical predictions. In the near-infrared bands of the water monomer, we propose that spectral features in recent laboratory-derived self-continuum can be well explained as being due to a combination of true bound and quasi-bound dimers, when the spectrum of quasi-bound dimers is approximated as being double the broadened spectrum of the water monomer. Such a representation can explain both the wavenumber variation and the temperature dependence. Recent observations of the self-continuum absorption in the windows between these near-infrared bands indicate that widely used continuum models can underestimate the true strength by around an order of magnitude. An existing far-wing model does not appear able to explain the discrepancy, and although a dimer explanation is possible, currently available observations do not allow a compelling case to be made. In the 8–12 μm window, recent observations indicate that the modern continuum models either do not properly represent the temperature dependence, the wavelength variation, or both. The temperature dependence is suggestive of a transition from the dominance of true bound dimers at lower temperatures to quasi-bound dimers at higher temperatures. In the mid- and far-infrared spectral region, recent theoretical calculations indicate that true bound dimers may explain at least between 20% and 40% of the observed self-continuum. The possibility that quasi-bound dimers could cause an additional contribution of the same size is discussed. Most recent theoretical considerations agree that water dimers are likely to be the dominant contributor to the self-continuum in the mm-wave spectral range.

Light absorption and scattering by aggregates: Application to black carbon and snow grains

Abstract: A geometric-optics surface-wave approach has been developed for the computation of light absorption and scattering by nonspherical particles for application to aggregates and snow grains with external and internal mixing structures. Aggregates with closed- (internal mixing) and open-cell configurations are constructed by means of stochastic procedures using homogeneous and core-shell spheres with smooth or rough surfaces as building blocks. The complex aggregate shape and composition can be accounted for by using the hit-and-miss Monte Carlo geometric photon tracing method. We develop an integral expression for diffraction by randomly oriented aggregates based on Babinet's principle and a photon-number weighted geometric cross section. With reference to surface-wave contributions originally developed for spheres, we introduce a nonspherical correction factor using a non-dimensional volume parameter such that it is 1 for spheres and 0 for elongated particles. The extinction efficiency, single-scattering albedo, and asymmetry factor results for randomly oriented columns and plates compare reasonably well with those determined from the finite-difference time domain (FDTD) and the discrete dipole approximation (DDA) computer codes for size parameters up to about 20. The present theoretical approach covers all size ranges and is particularly attractive from the perspective of efficient light absorption and scattering calculations for complex particle shape and inhomogeneous composition. We show that under the condition of equal volume and mass, the closed-cell configuration has larger absorption than its open-cell counterpart for both ballistic and diffusion-limited aggregates. Because of stronger absorption in the closed-cell case, most of the scattered energy is confined to forward directions, leading to a larger asymmetry factor than the open-cell case. Additionally, light absorption for randomly oriented snowflakes is similar to that of their spherical counterparts under the condition of equal geometrical cross section area for both external and internal mixing states; however, nonspherical snowflakes scatter less light in forward directions than spheres, resulting in a substantial reduction of the asymmetry factor. We further demonstrate that small soot particles on the order of 1 μm internally mixed with snow grains could effectively reduce snow albedo by as much as 5–10%. Indeed, the depositions of black carbon would substantially reduce mountain-snow albedo, which would lead to surface warming and snowmelt, critical to regional climatic surface temperature amplification and feedback.

Online multi-parameter phase-curve fitting and application to a large corpus of asteroid photometric data

Abstract: We apply the new H , G 12 and H , G 1 , G 2 phase functions to a large corpus of asteroid photometric data. We make use of low-precision (generally rounded to 0.1 mag) and low-accuracy (rms magnitude uncertainties of ±0.2–0.3 mag) data obtained from the Minor Planet Center and modified at Lowell Observatory. We find (1) a correlation between G 1 , G 2 values derived for asteroid families and albedos; (2) G 12 homogeneity in families. In addition, we make available an online Java applet (titled Asteroid Phase Function Analyzer). The applet may be used to compute asteroid absolute magnitudes and slope parameters using three different phase functions: the H , G , H , G 1 , G 2 , and H , G 12 . The tool also includes non-Gaussian error analysis using Monte Carlo methods. The applet is available at http://asteroid.astro.helsinki.fi/astphase/ .

Efficient unbiased variance reduction techniques for Monte Carlo simulations of radiative transfer in cloudy atmospheres: The solution

Abstract: We present five new variance reduction techniques applicable to Monte Carlo simulations of radiative transfer in the atmosphere: detector directional importance sampling, n-tuple local estimate, prediction-based splitting and Russian roulette, and circum-solar virtual importance sampling. With this set of methods it is possible to simulate remote sensing instruments accurately and quickly. In contrast to all other known techniques used to accelerate Monte Carlo simulations in cloudy atmospheres – except for two methods limited to narrow angle lidars – the presented methods do not make any approximations, and hence do not bias the result. Nevertheless, these methods converge as quickly as any of the biasing acceleration techniques, and the probability distribution of the simulation results is almost perfectly normal. The presented variance reduction techniques have been implemented into the Monte Carlo code MYSTIC (“Monte Carlo code for the physically correct tracing of photons in cloudy atmospheres”) in order to validate the techniques.

Discrete-dipole approximation with surface interaction: Computational toolbox for MATLAB

Abstract: We describe a MATLAB toolbox that utilizes the discrete-dipole approximation (DDA) method for modelling light interaction with arbitrarily-shape scatterers in free space as well with planar surface interaction (DDA-SI). The range of applicable models range from optical micromanipulation, plamonics, nano-antennae, near-field coupling and general light interaction with scatterers ranging from a few nanometers to several microns in size.

A generalized linear Boltzmann equation for non-classical particle transport

Abstract: This paper presents a derivation and initial study of a new generalized linear Boltzmann equation (GLBE), which describes particle transport for random statistically homogeneous systems in which the distribution function for chord lengths between scattering centers is non-exponential. Such problems have recently been proposed for the description of photon transport in atmospheric clouds; this paper is a first attempt to develop a Boltzmann-like equation for these and other related applications.

Optimized implementations of rational approximations for the Voigt and complex error function

Abstract: Rational functions are frequently used as efficient yet accurate numerical approximations for real and complex valued functions. For the complex error function w(x+iy), whose real part is the Voigt function K(x,y), code optimizations of rational approximations are investigated. An assessment of requirements for atmospheric radiative transfer modeling indicates a y range over many orders of magnitude and accuracy better than 10−4. Following a brief survey of complex error function algorithms in general and rational function approximations in particular the problems associated with subdivisions of the x, y plane (i.e., conditional branches in the code) are discussed and practical aspects of Fortran and Python implementations are considered. Benchmark tests of a variety of algorithms demonstrate that programming language, compiler choice, and implementation details influence computational speed and there is no unique ranking of algorithms. A new implementation, based on subdivision of the upper half-plane in only two regions, combining Weideman's rational approximation for small |x|+y<15 and Humlicek's rational approximation otherwise is shown to be efficient and accurate for all x, y.

Validation of the community radiative transfer model

Abstract: To validate the Community Radiative Transfer Model (CRTM) developed by the U.S. Joint Center for Satellite Data Assimilation (JCSDA), the discrete ordinate radiative transfer (DISORT) model and the line-by-line radiative transfer model (LBLRTM) are combined in order to provide a reference benchmark. Compared with the benchmark, the CRTM appears quite accurate for both clear sky and ice cloud radiance simulations with RMS errors below 0.2 K, except for clouds with small ice particles. In a computer CPU run time comparison, the CRTM is faster than DISORT by approximately two orders of magnitude. Using the operational MODIS cloud products and the European Center for Medium-range Weather Forecasting (ECMWF) atmospheric profiles as an input, the CRTM is employed to simulate the Atmospheric Infrared Sounder (AIRS) radiances. The CRTM simulations are shown to be in reasonably close agreement with the AIRS measurements (the discrepancies are within 2 K in terms of brightness temperature difference). Furthermore, the impact of uncertainties in the input cloud properties and atmospheric profiles on the CRTM simulations has been assessed. The CRTM-based brightness temperatures (BTs) at the top of the atmosphere (TOA), for both thin ( τ τ >30) clouds, are highly sensitive to uncertainties in atmospheric temperature and cloud top pressure. However, for an optically thick cloud, the CRTM-based BTs are not sensitive to the uncertainties of cloud optical thickness, effective particle size, and atmospheric humidity profiles. On the contrary, the uncertainties of the CRTM-based TOA BTs resulting from effective particle size and optical thickness are not negligible in an optically thin cloud.

Toward characterization of the aerosol optical properties over Loess Plateau of Northwestern China

Abstract: Aerosol optical properties were obtained from a CIMEL sunphotometer of the Aerosol Robotic Network at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL). SACOL is located over the Loess Plateau of the Northwestern China. The observed data are analyzed for the period of August 2006-October 2008. We find that aerosol optical depths (AODs) have a pronounced annual cycle, with a maximum dust aerosol loading during the spring. The 2-year average values of AOD, ˚ Angstr ¨ om exponent ( a), and water vapor path (WVP) along with their standard deviation (in parenthesis) are 0.35 (0.21), 0.93 (0.34), and 0.77 cm (0.52 cm), respectively. The probability distributions of these quantities all have one modal value, which are 0.3, 1.1, and 0.5 cm, respectively. There is a notable feature in the relationship between daily averaged AOD and ˚ Angstr ¨ om exponent: a wide range of a corresponding to moderate to low aerosol optical depths (o0.8). There is no significant correlation between daily averaged WVP and AOD. However the daily averaged ˚ Angstr ¨ om exponent and WVP show a significant positive correlation, indicating that the smaller aerosol particles present when the WVP is large. Variations of the retrieved aerosol volume size distributions are mainly associated with the changes in the concentration of the coarse aerosol fraction. The geometric mean radii for the fine and coarse aerosols are 0.18 mm (7 0.03 mm), and 2.53 m m( 7 0.25 mm), respectively. The spectral dependences of the single scattering albedos are different between the dusty and non-dusty conditions. In the presence of dust, the SSAs increase slightly with wavelength. When dust is not a

Scattering of electromagnetic waves by ensembles of particles and discrete random media

Abstract: Current problems of the theory of multiple scattering of electromagnetic waves by discrete random media are reviewed, with an emphasis on densely packed media. All equations presented are based on the rigorous theory of electromagnetic scattering by an arbitrary system of non-spherical particles. The main relations are derived in the circular-polarization basis. By applying methods of statistical electromagnetics to a discrete random medium in the form of a plane–parallel layer, we transform these relations into equations describing the average (coherent) field and equations for the sums of ladder and cyclical diagrams in the framework of the quasi-crystalline approximation. The equation for the average field yields analytical expressions for the generalized Lorentz–Lorenz law and the generalized Ewald–Oseen extinction theorem, which are traditionally used for the calculation of the effective refractive index. By assuming that the particles are in the far-field zones of each other, we transform all equations asymptotically into the well-known equations for sparse media. Specifically, the equation for the sum of the ladder diagrams is reduced to the classical vector radiative transfer equation. We present a simple approximate solution of the equation describing the weak localization (WL) effect (i.e., the sum of cyclical diagrams) and validate it by using experimental and numerically exact theoretical data. Examples of the characteristics of WL as functions of the physical properties of a particulate medium are given. The applicability of the interference concept of WL to densely packed media is discussed using results of numerically exact computer solutions of the macroscopic Maxwell equations for large ensembles of spherical particles. These results show that theoretical predictions for spare media composed of non-absorbing or weakly absorbing particles are reasonably accurate if the particle packing density is less than ∼30%. However, a further increase of the packing density and/or absorption may cause optical effects not predicted by the low-density theory and caused by near-field effects. The origin of the near-filed effects is discussed in detail.

Trends in atmospheric halogen containing gases since 2004

Abstract: The changes in the atmospheric concentration of 16 halogenated gases in the atmosphere have been determined using measurements made by the Atmospheric Chemistry Experiment Fourier transform spectrometer (ACE-FTS). ACE-FTS has been used to measure the change in concentration between 2004 and 2010 of CCl 4 , CF 4 , CCl 3 F (CFC-11), CCl 2 F 2 (CFC-12), C 2 Cl 3 F 3 (CFC-113), CH 3 Cl, ClONO 2 , COF 2 , COCl 2 , COClF, CHF 2 Cl (HCFC-22), CH 3 CCl 2 F (HCFC-141b), CH 3 CClF 2 (HCFC-142b), HCl, HF and SF 6 between 30°N and 30°S. ACE-FTS measurements were compared to surface measurements made by the AGAGE network and output from the SLIMCAT three-dimensional (3-D) chemical transport model, which is constrained by similar surface data. ACE-FTS measurements of CFCs show declining trends which agree with both AGAGE and SLIMCAT values. There are problems with the ACE-FTS retrievals of CFC-113 and HCFCs, with work currently ongoing to correct these problems. At lower altitudes the volume mixing ratio (VMR) of these species increase with altitude. This is due to problems with the retrievals at high beta angle (the angle between the orbital plane and the Earth–Sun vector). Although some of the retrievals have problems, we are confident that the trends are generally reliable. The concentrations of HCFCs appear to be increasing with ACE-FTS, SLIMCAT and AGAGE all showing positive trends. ACE-FTS measurements of the decomposition products (COFCl and COCl 2 ) do not show any significant trends. SLIMCAT data show a negative trend for COFCl which corresponds to the decrease in CFC-11, its assumed major source, during this time. COF 2 measurements from ACE-FTS show an increasing trend, while SLIMCAT shows a decreasing trend again linked to its assumed production from CFCs. ClONO 2 is highly photosensitive, thus the ACE-FTS occultations have been divided into local morning and evening occultations. Evening measurements of ClONO 2 show a decreasing trend in VMR, while morning measurements show an increasing trend. The reason for this difference is not understood at this time. The SLIMCAT output used in this study was not saved as local sunrise and sunset: therefore, only 24 h mean fields are available for ClONO 2 . These SLIMCAT data show a decreasing trend. SLIMCAT and ACE-FTS both show an increasing trend in the VMR of HF and a decreasing trend in the VMR of HCl. These results illustrate the success of the Montreal Protocol in reducing ozone depleting substances. The reduction in anthropogenic chlorine emissions has led to a decrease in the VMR of stratospheric HCl. The replacement of CFCs with HCFCs has led to an increase in the VMR of HF in the stratosphere. As chlorine-containing compounds continue to be phased out and replaced by fluorine-containing molecules, it is likely that total atmospheric fluorine will continue increasing in the near future.

A numerical model for multigroup radiation hydrodynamics

Abstract: We present in this paper a multigroup model for radiation hydrodynamics to account for variations of the gas opacity as a function of frequency. The entropy closure model (M1) is applied to multigroup radiation transfer in a radiation hydrodynamics code. In difference from the previous grey model, we are able to reproduce the crucial effects of frequency-variable gas opacities, a situation omnipresent in physics and astrophysics. We also account for the energy exchange between neighbouring groups which is important in flows with strong velocity divergence. These terms were computed using a finite volume method in the frequency domain. The radiative transfer aspect of the method was first tested separately for global consistency (reversion to grey model) and against a well-established kinetic model through Marshak wave tests with frequency-dependent opacities. Very good agreement between the multigroup M1 and kinetic models was observed in all tests. The successful coupling of the multigroup radiative transfer to the hydrodynamics was then confirmed through a second series of tests. Finally, the model was linked to a database of opacities for a Xe gas in order to simulate realistic multigroup radiative shocks in Xe. The differences with the previous grey models are discussed.

Electromagnetic scattering by a morphologically complex object: Fundamental concepts and common misconceptions

Abstract: Following Keller(Proc Symp Appl Math 1962;13:227:46), we classify all theoretical treatments of electromagnetic scattering by a morphologically complex object into first- principle (or "honest" in Keller s terminology) and phenomenological (or "dishonest") categories. This helps us identify, analyze, and dispel several profound misconceptions widespread in the discipline of electromagnetic scattering by solitary particles and discrete random media. Our goal is not to call for a complete renunciation of phenomenological approaches but rather to encourage a critical and careful evaluation of their actual origin, virtues, and limitations. In other words, we do not intend to deter creative thinking in terms of phenomenological short-cuts, but we do want to raise awareness when we stray (often for practical reasons) from the fundamentals. The main results and conclusions are illustrated by numerically-exact data based on direct numerical solutions of the macroscopic Maxwell equations.

Canopy spectral invariants. Part 1: A new concept in remote sensing of vegetation

Abstract: The concept of canopy spectral invariants expresses the observation that simple algebraic combinations of leaf and canopy spectral reflectance become wavelength independent and determine two canopy structure specific variables – the recollision and escape probabilities. These variables specify an accurate relationship between the spectral response of a vegetation canopy to incident solar radiation at the leaf and the canopy scale. They are sensitive to important structural features of the canopy such as forest cover, tree density, leaf area index, crown geometry, forest type and stand age. This paper presents the mathematical basis of the concept which is linked to eigenvalues and eigenvectors of the three-dimensional radiative transfer equation.

Can particle shape information be retrieved from light-scattering observations using spheroidal model particles?

Abstract: We address the question if and how observations of scattered intensity and polarisation can be employed for retrieving particle shape information beyond a simple classification into spherical and nonspherical particles. To this end, we perform several numerical experiments, in which we attempt to retrieve shape information of complex particles with a simple nonspherical particle model based on homogeneous spheroids. The discrete dipole approximation is used to compute reference phase matrices for a cube, a Gaussian random sphere, and a porous oblate and prolate spheroid as a function of size parameter. Phase matrices for the model particles, homogeneous spheroids, are computed with the T-matrix method. By assuming that the refractive index and the size distribution is known, an optimal shape distribution of model particles is sought that best matches the reference phase matrix. Both the goodness of fit and the optimal shape distribution are analysed. It is found that the phase matrices of cubes and Gaussian random spheres are well reproduced by the spheroidal particle model, while the porous spheroids prove to be challenging. The “retrieved” shape distributions, however, do not correlate well with the shape of the target particle even when the phase matrix is closely reproduced. Rather, they tend to exaggerate the aspect ratio and always include multiple spheroids. A most likely explanation why spheroids succeed in mimicking phase matrices of more irregularly shaped particles, even if their shape distributions display little similarity to those of the target particles, is that by varying the spheroids’ aspect ratio one covers a large range of different phase matrices. This often makes it possible to find a shape distribution of spheroids that matches the phase matrix of more complex particles.

Laboratory measurements of single light scattering by ensembles of randomly oriented small irregular particles in air. A review

Abstract: In this paper we present an overview of light scattering experiments devoted to measure one or more elements of the scattering matrix as functions of the scattering angle of ensembles of randomly oriented small irregular particles in air. A summary of the most important findings in light scattering experiments on ensembles of randomly oriented particles in air is given. The particles of interest are relevant for studies of atmospheres of planets and satellites and also for other astronomical bodies and environments. Some applications of light scattering experiments are also presented.

Radiative effect differences between multi-layered and single-layer clouds derived from CERES, CALIPSO, and CloudSat data

Abstract: Clouds alter general circulation through modification of the radiative heating profile within the atmosphere. Their effects are complex and depend on height, vertical structure, and phase. The instantaneous cloud radiative effect (CRE) induced by multi-layered (ML) and single-layer (SL) clouds is estimated by analyzing data collected by the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO), CloudSat, and Clouds and Earth’s Radiation Energy Budget System (CERES) missions from March 2007 through February 2008. The CRE differences between ML and SL clouds at the top of the atmosphere (TOA) and at the surface were also examined. The zonal mean shortwave (SW) CRE differences between the ML and SL clouds at the TOA and surface were positive at most latitudes, peaking at 120 W m−2 in the tropics and dropping to −30 W m−2 at higher latitudes. This indicated that the ML clouds usually reflected less sunlight at the TOA and transmitted more to the surface than the SL clouds, due to their higher cloud top heights. The zonal mean longwave (LW) CRE differences between ML and SL clouds at the TOA and surface were relatively small, ranging from −30 to 30 W m−2. This showed that the ML clouds only increased the amount of thermal radiation at the TOA relative to the SL clouds in the tropics, decreasing it elsewhere. In other words, ML clouds tended to cool the atmosphere in the tropics and warm it elsewhere when compared to SL clouds. The zonal mean net CRE differences were positive at most latitudes and dominated by the SW CRE differences.