Showing papers on "Radiative transfer published in 1987"

Linewidth dependence of radiative exciton lifetimes in quantum wells

Abstract: The fundamental relationship between radiative lifetime and spectral linewidth of free excitons is demonstrated theoretically and experimentally for quasi 2D excitons in GaAs/AlGaAs quantum wells.

Radiative Bow Shock Models of Herbig-Haro Objects

Abstract: The bow-shock models of HH objects presently constructed from a collection of 43 radiative-planar-shock models are used to predict the line ratios and line profiles expected from HH objects. It is found that a radiating-bow-shock model formed around a 'bullet' of dense gas plowing into the ambient medium accounts for existing observations sufficiently well to constitute a unifying model. For particular bow-shock orientations, double-peaked profiles are predicted in spatially unresolved spectra. A simple analytic formula is presented that can be used to estimate the shock velocity and bow-shock orientation from a single high-resolution observation of a low-excitation line. 63 references.

On the variability of desert aerosol radiative characteristics

Abstract: Size distributions of desert aerosol observed in arid regions of Senegal, Mali, and Niger under different weather conditions are classified in terms of visibility ranges into three categories and converted to lognormal distributions. The resulting curves are considered to represent average distributions encountered during fair weather, wind carrying dust, and sandstorm episodes. By means of the Mie theory, main radiative characteristics, i.e., attenuation coefficients, single scattering albedo, asymmetry factor, and phase functions, are computed for the spectral range between 0.3 and 40 μm. Regardless of the prevailing atmospheric conditions, extinction and scattering coefficients indicate a nearly wavelength independent pattern in the solar spectral range due to the large and effective particle scattering diameter. All size distributions yield strong absorption peaks at 10 μm, clogging the atmospheric window. Moreover, the temporal variation of single scattering albedo and asymmetry factor is most sensitive in the solar spectrum. For climate modeling purposes, average values are computed for seven spectral bands of the European Centre for Medium Range Weather Forecast radiation scheme. In addition, phase functions for eight characteristic wavelengths are presented and show predominantly forward scattering properties. Optical depth measurements performed in the framework of the African turbidity monitoring network between November 1980 and February 1984 indicate a high space and time fluctuation of aerosol concentration over the Sahara and surrounding areas. Daily average optical depth values (base e) between 0.03 in the Hoggar mountains and 3.5 during sandstorms in Boutilimit (Mauritania) have been observed for the mandatory wavelength 0.5 μm. Turbidity values in Zaria (Nigeria) indicate that the Sahel region is an insignificant area for mineral dust production.

The zonal method for calculating light intensities in the presence of a participating medium

Abstract: The zonal method for calculating radiative transfer in the presence of a participating medium is applied to the generation of realistic synthetic images. The method generalizes the radiosity method and allows for emission, scattering, and absorption by a participating medium. The zonal method accounts for volume/surface interactions which have not been previously included, as well as volume/volume and surface/surface interactions. In addition, new algorithms, based on the hemi-cube formulation, are introduced for calculating the geometric factors required by the zonal method.

Discrete Ordinate Methods for Radiative Heat Transfer in Isotropically and Anisotropically Scattering Media

Abstract: The objective of this note is to focus on the discrete ordinate method as a method for analyzing radiative heat transfer in isotropically and anisotropically scattering media. In particular, one dimension is chosen only for simplicity, since exact solutions are available for benchmarking the discrete ordinate method. One-dimensional radiative heat transfer has been analyzed in many previous investigations and has been extensively reviewed by Viskanta. The development of the discrete ordinate method has been described in detail by Chandrasekhar and Lathrop. The discrete ordinate method using Gauss quadrature has been compared with approximate solutions. In the cases previously analyzed solutions were obtained using ordinate sets that were larger than necessary. This note demonstrates that if ordinate sets are chosen to satisfy key moments of the radiative intensity, the resulting Nth order solution with the proposed quadrature is superior to the Nth order solution using Gauss quadrature.

The Middle Atmosphere In the Absence of Dynamical Heat Fluxes

Abstract: A new radiative transfer scheme, developed for use in a three-dimensional dynamical model of the stratosphere and mesosphere, is used to consider the hypothetical state of the middle atmosphere in the absence of dynamical heat fluxes. Most previous work in this area has considered radiative equilibrium temperatures using a fixed solar forcing. This equilibrium state is shown to be different from a state calculated using a seasonal cycle of solar forcing; in particular, the winter pole is found to be warmer and the latitudinal temperature gradient across the edge of the polar night is found to be smaller. This difference is due to two processes. Firstly, latitudes just polewards of the edge of the polar night at solstice are in darkness for only a short period. Secondly, radiative time scales lengthen greatly as temperatures decrease, so that equilibrium cannot be reached before the sun reappears. Recent ozone climatologies are used to impose a realistic annual cycle of ozone on the model. It is found that the simulation of the lower stratosphere is in considerably better agreement with observations than previous radiation-only calculations. the better simulation is shown to be, in most cases, due to the use of the new ozone climatology. It is also shown that a realistic ozone distribution is necessary for a good simulation of the polar stratopause at the summer solstice. A comparison of the observed and calculated annual cycle of temperature in the polar regions of the southern hemisphere lower stratosphere indicates that this region is under strong radiative control. This contrasts with an implied strong dynamical control for the same region in the northern hemisphere.

Observations of Saharan Aerosols: Results of ECLATS Field Experiment. Part II: Broadband Radiative Characteristics of the Aerosols and Vertical Radiative Flux Divergence

Abstract: The results presented in this paper are a part of those obtained during the ECLATS experiment The broadband radiative characteristics of the Sahelian aerosol layer and the vertical radiative flux divergence within the dust layer were determined both from in situ measurements and Mie calculations. In situ measurements of the aerosol layer's reflectances and transmittances of solar radiation led to aerosol single-scattering albedos close to ωA∼0.95. Measurements of the 8–14 μm radiances led to an optied depth by unit of volume of dust in a vertical column CA∼0.34 μm−1. Mie calculations assuming the aerosol refractive index published by Carlson and Benjamin for solar radiation and that measured by Volz for the atmospheric window, showed good agreement with observations. The ratio of infrared to visible optical thickness was δA(8–14 μm)/δA (0.55 μm)∼0.1, instead of 0.3 as calculated by Carlson and Benjamin. This discrepancy is attributable to differences in size distributions assumed. The radiative b...

Multiconfigurational Dirac-Fock studies of two-electron ions. II. Radiative corrections and comparison with experiment

Abstract: For pt.I see ibid., vol.20, p.639-49 (1987). A new method of estimating self-energy corrections in two-electron systems is presented. Multiconfiguration Dirac-Fock results for helium-like ions, including radiative corrections, are discussed and compared with recent experimental data. Magnetic interaction is shown to induce level crossings between 1s2p fine-structure levels.

Quenching rate coefficients for N2(a′ 1Σ−u)

Abstract: We have studied the kinetics of the lowest excited singlet state of molecular nitrogen, N2(a′ 1Σ−u,v=0), in a discharge flow reactor. The metastables were generated in a hollow cathode dc discharge through molecular nitrogen highly diluted in argon, and detected by VUV flourescence of the forbidden N2(a′ 1Σ−u–X 1Σ+g) band system. Observations of the equilibrium between the N2(a 1Πg) and N2(a′ 1Σ−u) states in our reactor indicate that the metastable has a radiative lifetime ≳23+11−6ms, assuming a radiative lifetime of 80+40−20μs for N2(a 1Πg). Rate coefficients for quenching by NO, CH4, CO, and N2O are approximately gas kinetic, those by H2, O2, and CO2 roughly tenth gas kinetic, and that by molecular nitrogen (1.9±0.5)×10−13 cm3 molecule−1 s−1. The interaction between N2(a′) and CO produced CO(A 1Π–X 1Σ) emission in about 21+10−8% of the quenching events.

A study of the radiative effects of enhanced atmospheric CO2 and CH4 on early Earth surface temperatures

Abstract: Large concentrations of atmospheric CO2 in the atmosphere of the early earth have been proposed as a possible explanation of the apparent absence of frozen earth in spite of a faint early sun. However, the most thorough treatments of this question, by Owen et al. (1979) and Kasting et al. (1984), apparently disagree as to the warming effects of large amounts of CO2. We recalculate the evolution of surface temperature over the last 4.25 billion year time period, using the same scenario for CO2 partial pressures and solar constant as employed by the previous authors. We find good agreement with Kasting et al. (1984) and also explain why the results of Owen et al. are at variance with our findings and those of Kasting et al. Using the concept of direct radiative forcing, we present analytic relations between the solar luminosity and CO2 mixing ratio required to maintain the troposphere close to its present thermal structure. As a by-product, we present new broadband parameterizations for the 961 and 1064 cm−1 CO2 bands that can be used in climate models. We also consider the direct radiative forcing of large amounts of CH4, or changes in model clouds, and discuss how these might reduce the CO2 mixing ratio required to balance the faint early sun.

Coastal Zone Color Scanner atmospheric correction algorithm: multiple scattering effects.

Abstract: Errors due to multiple scattering which are expected to be encountered in application of the current Coastal Zone Color Scanner (CZCS) atmospheric correction algorithm are analyzed. The analysis is based on radiative transfer computations in model atmospheres, in which the aerosols and molecules are distributed vertically in an exponential manner, with most of the aerosol scattering located below the molecular scattering. A unique feature of the analysis is that it is carried out in scan coordinates rather than typical earth-sun coordinates, making it possible to determine the errors along typical CZCS scan lines. Information provided by the analysis makes it possible to judge the efficacy of the current algorithm with the current sensor and to estimate the impact of the algorithm-induced errors on a variety of applications.

Magnetic fields and non-uniform rotation in stellar radiative zones

Abstract: Effets des instabilites dynamiques et resistives sur la redistribution magnetique du moment angulaire a l'interieur d'une etoile

A Pattern Recognition Technique for Distinguishing Surface and Cloud Types in the Polar Regions

Abstract: Measurement of polar cloud cover is important because of its strong radiative influence on the energy balance of the snow and ice surface. Conventional satellite cloud detection schemes often fail in the polar regions because the visible and thermal contrasts between cloud and surface are typically small. Nevertheless, experts looking at satellite imagery can distinguish clouds from the surface by examining the textural characteristics of the scene. This paper describes an automated pattern recognition algorithm winch identities regions of various surface and cloud types at high latitudes from visible, near-infrared, and infrared AVHRR satellite data. Five spectral features give information about the magnitude of albedos and brightness temperatures, while three textural features describe the variability and “bumpiness” in a scene. The maximum likelihood decision rule is used to classify that region into one of seven surface categories or 11 cloud categories. The algorithm was able to classify 870...

Vacuum radiative level shift and spontaneous-emission linewidth of an atom in an optical resonator.

Abstract: The center frequency and linewidth of the $^{1}S_{0}\ensuremath{-}^{1}P_{1}$ resonance line of barium atoms placed near the center of a concentric optical resonator are studied as functions of cavity tuning. Shifts in the transition center frequency, due to radiative level shifts, and changes in linewidth, due to enhanced and suppressed spontaneous emission, are observed. A QED calculation which explicitly includes the resonator mode density gives good agreement with the data.

Marfes: Radiative condensation in tokamak edge plasma

Abstract: Marfes are toroidally symmetric bands of high density radiating plasma that form at the edge of tokamak plasmas. The marfe results from a process of radiative condensation: A local increase in the plasma density increases the radiation rate and lowers the temperature, allowing the density to rise further to maintain pressure balance. It is demonstrated that the marfe onsets when the plasma density exceeds a critical threshold that is just below the density limit, in agreement with observations. This threshold results from a balance between condensation and cross‐field thermal flux from the central hot plasma. Finally, it is noted that radiative condensation is also the driving mechanism of solar prominences and other astrophysical objects.

Exploratory studies of cloud radiative forcing with a general circulation model

Abstract: Cloud radiative forcing constitutes the radiative impact of clouds upon the earth's present climate, while cloud radiative feedback is the change in this forcing associated with climatic change. The present study addresses two issues concerning cloud radiative forcing. The first is that an intercomparison of cloud radiative forcing, as predicted by six different general circulation models, shows a considerable lack of agreement, underscoring the need for an improved understanding of cloud/radiation interactions within such models. The second issue pertains to an examination of strategies by which model-predicted cloud/radiation interactions should be compared with satellite-derived data sets. DOI: 10.1111/j.1600-0870.1987.tb00321.x

Numerical radiative transfer

Abstract: Preface Introduction Part I. Operator Perturbation: 1. Survey of operator perturbation methods W. Kalkofen 2. Line formation in expanding atmospheres: multilevel calculations using approximate lambda operators W. R. Hamann 3. Stellar atmospheres in non-LTE: model construction and line formation calculations using approximate lambda operators K. Werner 4. Acceleration of convergence L. H. Auer 5. Line formation in a time-dependent atmosphere W. Kalkofen 6. Iterative solution of multilevel transfer problems Eugene H. Avrett and Rudolf Loeser 7. An algorithm for the simultaneous solution of thousands of transfer equations under global constraints Lawrence S. Anderson 8. Operator perturbation for differential equations W. Kalkofen Part II. Polarised Radiation: 9. A gentle introduction to polarised radiative transfer David E. Rees 10. Non-LTE polarised radiative transfer in special lines David E. Rees and Graham A. Murphy 11. Transfer of polarised radiation using 4x4 matrices E. Landi Degli'Innocenti 12. Radiative transfer in the presence of strong magnetic fields A. A. van Ballegooijen 13. An integral operator technique of radiative transfer in spherical symmetry A. Peraiah 14. Discrete ordinate matrix method M. Schmidt and R. Wehrse.

Dust distribution in spiral galaxies

Abstract: A method for determining the dust distribution in edge-on, late Hubble type, spiral galaxies from their surface photometry is described. The method assumes that both the stars and the dust are distributed isothermally in the direction perpendicular to the galactic plane with scale heights z(s) and z(d) respectively, and exponentially in the radial direction with corresponding scale lengths h(s) and h(d). By comparing the observed surface photometry and the model calculations in which the radiative transfer is treated properly, z(s), z(d), h(s), and h(d) can be determined, as well as the optical depth along the line of sight and the central brightness. As a demonstration, the method is applied to NGC 891. The extension of the method to early-type spiral galaxies and to inclination angles less than 90 deg is described. 33 references.

A computation of the stratospheric diabatic circulation using an accurate radiative transfer model

Abstract: The global diabatic circulation is computed for the months of January, April, July and October over the altitude region 100 to 0.1 mb using an accurate troposphere-stratosphere radiative transfer model, SBUV and SME ozone data, and NMC temperatures. There is high correlation between the level of wave activity and the local departure of the atmosphere from radiative equilibrium. An excess in the globally averaged net stratospheric heating from 40 to 50 km is computed for all months, and a deficit from 50 to 60 km is computed during solstice. A 20 percent uniform reduction in ozone from 40 to 50 km, or a temperature perturbation with an increase of 5 K at 1 mb, will bring the atmosphere into global radiative equilibrium without significant impact on the diabatic circulation. In the transitional months of April and October, the net heating in the fall hemispheres are very similar, while substantial differences exist between the spring hemispheres.

Multiple scattering lowtran and fascode models

Abstract: This paper describes the implementation of an efficient and accurate multiple scattering parameterization within the LOWTRAN and FASCODE transmittance/radiance models. The parameterization is based on a stream approximation in which the local radiance field needed to evaluate the multiple scattering source function is estimated from the local radiation fluxes. The latter are calculated based on a parameterized two-flux for individual layers and an adding method for combining layers. Because of the line-by-line nature of FASCODE, it is straightforward to implement the multiple scattering treatment. For LOWTRAN, an interface scheme was developed using the k-distribution method to match the multiple scattering approach to the band model calculation of gas absorption. The interface scheme represents the LOWTRAN band model by a sum of pseudomonochromatic calculations. The approach is valid for any band model for which k-distribution parameters can be evaluated. The accuracy of the multiple scattering parameterization has been demonstrated by comparing it with more detailed calculations for a variety of atmospheric conditions. RMS errors in radiance considering all possible viewing angles are <20%. In addition, to insure consistency between models, overlapping LOWTRAN and FASCODE spectral regions are compared. Finally, it is demonstrated that the implemented multiple scattering parameterization corrects LOWTRAN's previous underestimation of path radiance for long horizon paths where multiple scattering is significant.

Radiative gravitational fields in general relativity II. Asymptotic behaviour at future null infinity

Abstract: We prove that Penrose’s requirements for asymptotic simplicity are formally satisfied by the general metric, (1), which admits both post-Minkowskian and multipolar expansions, (2), which is stationary in the past and asymptotically Minkowskian in the past, (3), which admits harmonic coordinates, and (4), which is a solution of Einstein’s vacuum equations outside a spatially bounded region. The proof is based on the setting up, by using the method of a previous work (L. Blanchet & T. Damour ( Phil . Trans . R . Soc . Lond . A 320, 379-430 (1986))), of an improved algorithm that generates a metric equivalent to the general harmonic metric of that work but written in radiative coordinates, i. e. admitting an expansion in powers of r -1 for r → ∞ and t - r fixed. The arbitrary parameters of the construction are the radiative multipole moments in the sense of K. S. Thorne ( Rev . mod . Phys . 52, 299 (1980)).

Ground state of hydrogen as a zero-point-fluctuation-determined state

Abstract: We show here that, within the stochastic electrodynamic formulation and at the level of Bohr theory, the ground state of the hydrogen atom can be precisely defined as resulting from a dynamic equilibrium between radiation emitted due to acceleration of the electron in its ground-state orbit and radiation absorbed from zero-point fluctuations of the background vacuum electromagnetic field, thereby resolving the issue of radiative collapse of the Bohr atom.

The Absorption of Solar Radiation by Cloud Droplets: An Application of Anomalous Diffraction Theory

Abstract: In this paper we demonstrate that the anomalous diffraction theory of van de Hulst with some modifications, provides a reasonable approximation of the volume extinction and absorption coefficients. We also show how the shortwave radiative properties of a water cloud can be derived in terms of 1) cloud liquid water path, 2) the effective radius of the droplet distribution, and 3) the balk absorption coefficient of water. With the aid of the approximate diffraction theory we describe how cloud albedo and shortwave absorption depend on the droplet size. We demonstrate this dependence to be somewhat complex and show that the variation of absorption with variation of droplet size depends also on the cloud liquid water path. For “deep” or semi-infinite clouds, absorption increases monotonically with increasing effective radius, but the reverse dependence is established for thin clouds. The implications of these results to the so-called absorption paradox and to the possibility of determining droplet si...