Showing papers on "Radiative transfer published in 1993"

Radiative heat transfer

Abstract: 1. Fundamentals of Thermal Radiation 2. Radiative Property Predictions from Electromagnetic Wave Theory 3. Radiative Properties of Real Surfaces 4. View Factors 5. Radiative Exchange Between Gray, Diffuse Surfaces 6. Radiative Exchange Between Partially-Specular Gray Surfaces 7. Radiative Exchange Between Nonideal Surfaces 8. Surface Radiative Exchange in the Presence of Conduction and Convection 9. The Equation of Radiative Transfer in Participating Media 10. Radiative Properties of Molecular Gases 11. Radiative Properties of Particulate Media 12. Radiative Properties of Semitransparent Media 13. Exact Solutions for One-Dimensional Gray Media 14. Approximate Solution Methods for One-Dimensional Media 15. The Method of Spherical Harmonics (PN-Approximation) 16. The Method of Discrete Ordinates (SN-Approximation) 17. The Zonal Method 18. The Treatment of Collimated Irradiation 19. The Treatment of Nongray Extinction Coefficients 20. The Monte Carlo Method for Thermal Radiation 21. Radiation Combined with Conduction and Convection 22. Inverse Radiative Heat Transfer A. Constants and Conversion Factors B. Tables for Radiative Properties of Opaque Surfaces C. Blackbody Emissive Power Table D. View Factor Catalogue E. Exponential Integral Functions F. Computer Codes Author Index Subject Index

Cooling Functions for Low-Density Astrophysical Plasmas

Abstract: The cooling functions for a plasma slab are investigated under equilibrium and nonequilibrium conditions, over a range of 10 4 -10 85 K and for a range of abundances Radiative transfer and diffuse field are calculated in the isobaric nonequilibrium models using a one-dimensional cooling flow model, and the plasma is not assumed to be optically thin to all radiation Limiting cases of the plasma diffuse field coupling are calculated, and the resulting cooling functions are presented Some functions are terminated before reaching 10 4 K when the internal photoionization halts the cooling The functions represent a self-consistent set of curves covering a wide grid of temperature and metallicities using recently published atomic data and processes

Spectroscopic constraints on the properties of dust in active galactic nuclei

Abstract: The optical properties of graphite, silicate, and SiC grains over the wavelength range 1000 microns - 1 A are calculated for grains in the 0.005-10-micron size range. Both graphite + silicate (MRN) and graphite + SiC grain mixtures are considered, with various grain size distributions. A detailed radiative transfer calculation is performed to obtain constraints on the emission properties of dust in AGN in either 'optically thin' or optically thick configurations. Warm graphite + silicate or graphite + SiC dust (T is greater than 200 K) with the MRN size distribution and a column density less than about 10 exp 23/sq cm produces a strong silicate or SiC emission feature. Such dust cannot be responsible for the observed IR emission from AGN. MRN dust with a high optical depth at 10 microns produces an emission feature with an amplitude of about 57 percent, in excess of the typical observational limit for most objects. Reddening of the broad emission lines and continuum is unlikely to be common.

Parameterization of the Radiative Properties of Cirrus Clouds

Abstract: A new approach for parameterization of the broadband solar and infrared radiative properties of ice clouds has been developed. This parameterization scheme integrates in a coherent manner the δ-four-stream approximation for radiative transfer, the correlated k-distribution method for nongray gaseous absorption, and the scattering and absorption properties of hexagonal ice crystals. A mean effective size is used, representing an area-weighted mean crystal width, to account for the ice crystal size distribution with respect to radiative calculation. Based on physical principles, the basic single-scattering properties of ice crystals, including the extinction coefficient divided by ice water content single-scattering albedo, and expansion coefficients of the phase function, can be parameterized using third-degree polynomials in terms of the mean effective size. In the development of this parameterization the results computed from a light scattering program that includes a Geometric ray-tracing progr...

Theoretical aspects of the luminescence of porous silicon.

Abstract: The luminescence in the visible range of porous silicon is analyzed in the hypothesis of quantum confinement. We calculate the electronic and optical properties of silicon crystallites and wires with sizes between 0 and 4.5 nm. The band-gap energies of such confined systems are in agreement with the photon energies observed in luminescence. We calculate the radiative recombination times of the confined excitons. We conclude that experimental nonradiative processes in porous silicon are more efficient than calculated radiative ones at T=300 K. The high photoluminescence efficiency of porous silicon is due to the small probability of finding a nonradiative recombination center in silicon nanocrystallites. Recently, it has been proposed that the low-temperature dependence of the experimental radiative decay time of the luminescence of porous silicon could be explained by the exchange splitting in the fundamental exciton. We show that the influence of the valley-orbit splitting cannot be excluded. The sharp optical-absorption edge above 3.0 eV is not proof of the molecular origin of the properties of porous silicon because silicon nanostructures present a similar absorption spectrum. We calculate the nonradiative capture of electrons or holes on silicon dangling bonds and show that it is very dependent on the confinement. We find that the presence of one dangling bond at the surface of a crystallite in porous silicon must destroy its luminescent properties above 1.1 eV but can produce a luminescence below 1.1 eV due to a radiative capture on the dangling bond.

Microscale Heat Conduction in Dielectric Thin Films

Abstract: Heat conduction in dielectric thin films is a critical issue in the design of electronic devices and packages. Depending on the material properties, there exists a range of film thickness where the Fourier law, used for macroscale heat conduction, cannot be applied. In this microscale regime, heat transport by lattice vibrations or phonons can be analyzed as a radiative transfer problem. Based on Boltzmann transport theory, an equation of phonon radiative transfer (EPRT) is developed. In the acoustically thick limit, ξ L >>1, or the macroscale regime, where the film thickness is much larger than the phonon-scattering mean free path, the EPRT reduces to the Fourier law

Development of a Second-Generation Regional Climate Model (RegCM2). Part I: Boundary-Layer and Radiative Transfer Processes

Abstract: During the last few years the development of a second-generation regional climate modeling system (RegCM2) has been completed at the National Center for Atmospheric Research (NCAR). Based upon the National Center for Atmospheric Research-Pennsylvania State University Mesoscale Model (MM4), RegCM2 includes improved formulations of boundary layer, radiative transfer, surface physics, cumulus convection, and time integration technique, which make it more physically comprehensive and more computationally efficient than the previous regional climate model version. This paper discusses a number of month-long simulations over the European region that were conducted to test the new RegCM2 boundary-layer parameterization (the scheme developed by Holtsag et al.) and radiative transfer formulation [the package developed for the NCAR Community Climate Model 2 (CCM 2)]. Both schemes significantly affect the model precipitation, temperature, moisture, and cloudiness climatology, leading to overall more realist...

A finite element approach for modeling photon transport in tissue.

Abstract: The use of optical radiation in medical physics is important in several fields for both treatment and diagnosis. In all cases an analytic and computable model of the propagation of radiation in tissue is essential for a meaningful interpretation of the procedures. A finite element method (FEM) for deriving photon density inside an object, and photon flux at its boundary, assuming that the photon transport model is the diffusion approximation to the radiative transfer equation, is introduced herein. Results from the model for a particular case are given: the calculation of the boundary flux as a function of time resulting from a delta-function input to a two-dimensional circle (equivalent to a line source in an infinite cylinder) with homogeneous scattering and absorption properties. This models the temporal point spread function of interest in near infrared spectroscopy and imaging. The convergence of the FEM results are demonstrated, as the resolution of the mesh is increased, to the analytical expression for the Green's function for this system. The diffusion approximation is very commonly adopted as appropriate for cases which are scattering dominated, i.e., where mu(s) much greater than mu(a), and results from other workers have compared it to alternative models. In this article a high degree of agreement with a Monte Carlo method is demonstrated. The principle advantage of the FE method is its speed. It is in all ways as flexible as Monte Carlo methods and in addition can produce photon density everywhere, as well as flux on the boundary. One disadvantage is that there is no means of deriving individual photon histories.

Double-integrating-sphere system for measuring the optical properties of tissue

Abstract: A system is described and evaluated for the simultaneous measurement of the intrinsic optical properties of tissue: the scattering coefficient, the absorption coefficient, and the anisotropy factor. This system synthesizes the theory of two integrating spheres and an intervening scattering sample with the inverse adding–doubling algorithm, which employs the adding–doubling solution of the radiative transfer equation to determine the optical properties from the measurement of the light flux within each sphere and of the unscattered transmission. The optical properties may be determined simultaneously, which allows for measurements to be made while the sample undergoes heating, chemical change, or some other external stimulus. An experimental validation of the system with tissue phantoms resulted in the determination of the optical properties with a <5% deviation when the optical density was between 1 and 10 and the albedo was between 0.4 and 0.95.

Comparison of numerical models for computing underwater light fields

Abstract: Seven models for computing underwater radiances and irradiances by numerical solution of the radiative transfer equation are compared. The models are applied to the solution of several problems drawn from optical oceanography. The problems include highly absorbing and highly scattering waters, scattering by molecules and by particulates, stratified water, atmospheric effects, surface-wave effects, bottom effects, and Raman scattering. The models provide consistent output, with errors (resulting from Monte Carlo statistical fluctuations) in computed irradiances that are seldom larger, and are usually smaller, than the experimental errors made in measuring irradiances when using current oceanographic instrumentation. Computed radiances display somewhat larger errors.

An Accurate Parameterization of the Radiative Properties of Water Clouds Suitable for Use in Climate Models

Abstract: A new parameterization of the radiative properties of water clouds is presented. Cloud optical properties for both solar and terrestrial spectra and for cloud equivalent radii in the range 2.5–60 µm are calculated from Mie theory. It is found that cloud optical properties depend mainly on equivalent radius throughout the solar and terrestrial spectrum and are insensitive to the details of the droplet size distribution, such as shape, skewness, width, and modality (single or bimodal). This suggests that in cloud models, aimed at predicting the evolution of cloud microphysics with climate change, it is sufficient to determine the third and the second moments of the size distribution (the ratio of which determines the equivalent radius). It also implies that measurements of the cloud liquid water content and the extinction coefficient are sufficient to determine cloud optical properties experimentally (i.e., measuring the complete droplet size distribution is not required). Based on the detailed cal...

A Spectral Line-Based Weighted-Sum-of-Gray-Gases Model for Arbitrary RTE Solvers

Abstract: This paper presents an approach for generating weighted-sum-of-gray gases (WSGG) models directly from the line-by-line spectra of H[sub 2]O. Emphasis is placed on obtaining detailed spectral division among the gray gases. Thus, for a given model spectrum, the gray gas weights are determined as blackbody fractional functions for specific subline spectral regions at all temperatures. The model allows the absorption coefficient to be the basic radiative property rather than a transmissivity or band absorptance, etc., and can be used with any arbitrary solution method for the Radiative Transfer Equation (RTE). A single absorption cross section spectrum is assumed over the entire spatial domain in order to fix the subline spectral regions associated with a single spectral calculation. The error associated with this assumption is evaluated by comparison with line-by-line benchmarks for problems of nonisothermal and nonhomogeneous media. 28 refs., 7 figs.

Radiative Climate Forcing by the Mount Pinatubo Eruption

Abstract: Radiative flux anomalies derived from the National Aeronautics and Space Administration (NASA) spaceborne Earth Radiation Budget Experiment were used to determine the volcanic radiative forcing that followed the eruption of Mount Pinatubo in June 1991. They are the first unambiguous, direct measurements of large-scale volcanic forcing. The volcanic aerosols caused a strong cooling effect immediately; the amount of cooling increased through September 1991 as shortwave forcing increased relative to the longwave forcing. The primary effects of the aerosols were a direct increase in albedo over mostly clear areas and both direct and indirect increases in the albedo of cloudy areas.

Radiative and nonradiative electron transfer in contact radical-ion pairs

Abstract: The relationship between radiative and nonradiative electron transfer is explored for return electron transfer processes in the contact radical-ion pairs formed by excitation of ground state CT complexes. Using a conventional nonadiabatic theory of electron transfer, absolute rate constants for nonradiative return electron transfer, varying over more than two orders of magnitude, can be predicted from information obtained from analyses of the corresponding radiative processes. The effects of solvent polarity, driving force and molecular dimension on the rates of nonradiative return electron transfer are studied.

Radiative-convective equilibrium with explicit two-dimensional moist convection

Abstract: Radiative-convective statistical equilibria are obtained using a two-dimensional model in which radiative transfer is interactive with the predicted moisture and cloud fields. The domain is periodic in x, with a width of 640 km, and extends from the ground to 26 km. The lower boundary is a fixed-temperature water-saturated surface. The model produces a temperature profile resembling the mean profile observed in the tropics. A number of integrations of several months' duration are described in this preliminary examination of the model's qualitative behavior. The model generates a QBO-like oscillation in the x-averaged winds with an apparent period of ∼60 days. This oscillation extends into the troposphere and influences the convective organization. In order to avoid the associated large vertical wind shears, calculations are also performed in which the x-averaged winds are constrained to vanish. The convection then evolves into a pattern in which rain falls only within a small part of the domain. ...

Ray effect and false scattering in the discrete ordinates method

Abstract: A discussion on the ray effect and false scattering occurring in discrete ordinates solution of the radiative transfer equation is presented in this article. Ray effect arises from the approximation of a continuously varying angular nature of radiation by a specified set of discrete angular directions. It is independent of the spatial discretization practice. False scattering, on the other hand, is a consequence of the spatial discretization practice and is independent of the angular discretization practice. In multidimensional computations, when a beam is not aligned with the grid line, false scattering smears the radiative intensity field. It reduces the appearance of unwanted bumps, but does not eliminate ray effect. An inappropriate view of false scattering is also presented. Four sample problems are used to explain these two effects.

Aerosol-Cloud-Climate Interactions

Abstract: R Jaenicke, Tropospheric Aerosol P V Hobbs, Aerosol-Cloud Interactions Dr Harshvardhan, Aerosol-Climate Interactions A Heymsfield, Structures of Clouds M King, Radiative Properties of Clouds DL Hartmann, Radiative Effects of Clouds on Earth's Climate H Sundquist, Parameterizafion of Clouds in Large-Scale Models MP McCormick, Stratospheric Aerosol and Clouds Subject index

On the generation of the large-scale and turbulent magnetic fields in solar-type stars

Abstract: It is thought that the large-scale solar-cycle magnetic field is generated in a thin region at the interface of the radiative core (RC) and solar convection zone (SCZ). We show that the bulk of the SCZ virogoursly generates a small-scale turbulent magnetic field. Rotation, while not essential, increases the generation rate of this field.

The embedded young stars in the Taurus-Auriga molecular cloud. I - Models for spectral energy distributions

Abstract: We describe radiative transfer calculations of infalling, dusty envelopes surrounding pre-main-sequence stars and use these models to derive physical properties for a sample of 21 heavily reddened young stars in the Taurus-Auriga molecular cloud. The density distributions needed to match the FIR peaks in the spectral energy distributions of these embedded sources suggest mass infall rates similar to those predicted for simple thermally supported clouds with temperatures about 10 K. Unless the dust opacities are badly in error, our models require substantial departures from spherical symmetry in the envelopes of all sources. These flattened envelopes may be produced by a combination of rotation and cavities excavated by bipolar flows. The rotating infall models of Terebey et al. (1984) models indicate a centrifugal radius of about 70 AU for many objects if rotation is the only important physical effect, and this radius is reasonably consistent with typical estimates for the sizes of circumstellar disks around T Tauri stars.

Land surface temperatures derived from the advanced very high resolution radiometer and the along‐track scanning radiometer: 1. Theory

Abstract: The problem of deriving accurate land surface temperatures (LSTs) using infrared satellite measurements is considered. The radiative transfer equation over the land is solved by linearization of the Planck function in temperature and wavenumber in the same manner as is done for obtaining the so-called split-window algorithm for sea surface temperature estimation. The effects of the atmosphere and of surface emissivity are both considered in the derivation. By making further assumptions, an approximate split-window algorithm which shows explicitly how atmospheric absorption by water vapor and surface emissivity both affect the accuracy of LSTs is derived. A new algorithm has been formulated for use with National Oceanic and Atmospheric Administration advanced very high resolution radiometer data and is compared with previously published algorithms. It has a global form with coefficients that depend upon local conditions. Dual angle algorithms are also derived. These may be used to estimate LST using data from the along-track scanning radiometer on board the first European remote-sensing satellite.

Spin effects in the inspiral of coalescing compact binaries.

Abstract: We derive the contributions of spin-orbit and spin-spin coupling to the gravitational radiation from coalescing binary systems of spinning compact objects. We calculate spin effects in the symmetric, trace-free radiative multipoles that determine the gravitational wave form, and the rate of energy loss. Assuming a balance between energy radiated and orbital energy lost, we determine the spin effects in the evolution of the orbital frequency and orbital radius. Assuming that a laser interferometric gravitational observatory can track the gravitational-wave frequency (twice the orbital frequency) as it sweeps through its sensitive bandwidth between about 10 Hz and 1 kHz, we estimate the accuracy with which the spins of the component bodies can be determined from the gravitational-wave signal.

Estimation of SW Flux Absorbed at the Surface from TOA Reflected Flux

Abstract: Measurements of radiation budgets, both at the top of the atmosphere (TOA) and at the surface, are essential to understanding the earth's climate. The TOA budgets can, in principle, be measured directly from satellites, while on a global scale surface budgets need to be deduced from TOA measurements. Most methods of inferring surface solar-radiation budgets from satellite measurements are applicable to particular scene types or geographic locations, and none is valid over highly reflective surfaces such as ice or snow. In addition, the majority of models require inputs such as cloud-optical thickness that are usually not known. Extensive radiative transfer modeling for different surface, atmospheric, and cloud conditions suggests a linear relationship between the TOA-reflected flux and the flux absorbed at the surface for a fixed solar zenith angle (SZA). The linear relationship is independent of cloud-optical thickness and surface albedo. Sensitivity tests show that the relationship depends stro...

Aerosol optical thickness and atmospheric path radiance

Abstract: Simultaneous measurements from the ground of the spectral optical thickness and the atmospheric path radiance from over 30 sites located in many parts of the world and affected by several different aerosol types are reported. These measurements are used to derive the relationship between the optical thickness and the path radiance for a single viewing and illumination geometry and to discuss its implications on remote sensing observations. It is shown that simple measurements performed from the ground can yield empirical relationships that can be used to check some of the common but not validated assumptions about the particle homogeneity, sphericity, composition, and size distribution used in remote sensing models and in estimates of the radiative effects of aerosol. The results are used to test concepts of atmospheric corrections and remote sensing of aerosol from space.

On the accuracy of the Eddington approximation for radiative transfer in the microwave frequencies

Abstract: The paper examines how well an Eddington approximation can reproduce brightness temperatures obtained from a more complete, N-stream discrete ordinate solution in the microwave regime. Radiation propagation through a plane parallel medium is considered. Although model discrepancies are complicated functions of the cloud constituents, the differences between an eight-stream discrete ordinate solution and an analytical Eddington solution were found to be generally small, ranging from 0 to 6 K when only one uniform layer of hydrometeors was considered. When realistic multilayered cloud hydrometeor profiles were used, the differences between these two models never exceeded 3 K over the entire range of microwave frequencies considered (6.6-183 GHz). The models agreed to within 0.2 K in the absence of scattering constituents.

Angular Momentum Transport in Magnetized Stellar Radiative Zones. II. The Solar Spin-down

Abstract: We present a large set of numerical calculations describing the rotational evolution of a solar-type star, in response to the torque exerted on it by a magnetically coupled wind emanating from its surface. We consider a situation where the internal redistribution of angular momentum in the radiative part of the envelope is dominated by magnetic stresses arising from the shearing of a preexisting, large-scale, poloidal magnetic field. By assuming a time-independent poloidal magnetic field, neglecting fluid motions in meridional planes, and restricting our attention to axisymmetric systems, we reduce the spin-down problem to solving the (coupled) Φ-components of the momentum and induction equations

Spectrum formation in supernovae : numerical techniques

Abstract: The study combines several novel techniques for spectrum simulation in the Eddington computer program which solves the comoving frame equation of transfer coupled with the statistical and radiative equilibrium equations. One of these is a generalization of the accelerated lambda iteration (ALI) scheme to include an approximate frequency-derivative operator. This greatly enhances the convergence rate of ALI in optically thick, high-velocity shear flows. Another is a partial linearization technique which is capable of efficiently solving a very large number of rate equations on a moderately sized computer. An expansion opacity and emissivity approximation is derived which makes it possible to determine the effect on the transfer and statistical equilibrium of a very large number of lines not explicitly represented in the frequency grid and additionally to treat line-blanketing from species not explicitly included in the rate equations. The utility of these techniques is illustrated with models of two supernovae.

A new interpretation of the redshift observed in optically thin transition region lines

Abstract: We propose that the pervasive redshift observed in transition region spectral lines is caused by downward propagating acoustic waves. The waves are assumed to be generated in the corona as a result of nanoflares or some other form of episodic heating. The dynamic response of a coronal loop to energy released as heat near the loop apex is studied by solving the hydrodynamic equations numerically, consistently including the effects of nonequilibrium ionization on the radiative losses and on the internal energy.