Showing papers on "Radiative transfer published in 1991"

A description of the correlated k distribution method for modeling nongray gaseous absorption, thermal emission, and multiple scattering in vertically inhomogeneous atmospheres

Abstract: A radiative transfer method for treating nongray gaseous absorption and thermal emission in vertically inhomogeneous multiple scattering atmospheres is described. Probability density distributions of absorption coefficient strength are derived from line-by-line calculations to construct line-by-line and band model based k distributions. The monotonic ordering of absorption coefficient strengths in these k distributions implicitly preserves the monochromatic structure of the atmosphere at different pressure levels, thus simulating monochromatic spectral integration at a fraction of the line-by-line computing cost. The k distribution approach also permits accurate modeling of overlapping absorption by different atmospheric gases and accurate treatment of nongray absorption in multiple scattering media. It is shown that the correlated k distribution method is capable of achieving numerical accuracy to within 1 percent of cooling rates obtained with line-by-line calculations throughout the troposphere and most of the stratosphere.

Perturbation of the northern hemisphere radiative balance by backscattering from anthropogenic sulfate aerosols

Abstract: Anthropogenic sulfate (SO 4 = ) aerosol particles play two potential roles in the radiative climate of the earth. In cloud-free air, SO 4 = particles scatter sunlight, some of which is lost to space, thereby reducing solar irradiance at the ground. The same particles can act as cloud condensation nuclei (CCN), the number concentration of which is an important determinant of cloud albedo. This albedo effect, in turn, also influences incoming short-wave solar radiation. Development of a three-dimensional global model for estimating the SO 4 = aerosol mass concentration, along with previously-acquired information on the scattering and back-scattering coefficients per unit mass concentration allow calculation of the effects of anthropogenic SO 4 = aerosol on clear-sky optical depth. Subsequently, this can be used to estimate the change in hemispheric and global average reflected solar radiation. The conclusion is that the change of reflected solar flux due to anthropogenic SO 4 = averaged over the Northern Hemisphere is ca. − 1.1 Wm -2 , which is comparable but opposite in sign to the present-day radiative forcing by anthropogenic CO 2 , + 1.5 Wm -2 . Because of the spatial variability of the anthropogenic SO 4 = distribution, its meteorological effects must be studied regionally. That is, global models with regional resolution and regional data are required. Unlike the direct effect on solar irradiance, the relationship of CCN number concentration to mass concentration is not known. Thus it is not yet possible to make quantitatively reliable statements about anthropogenic forcing of cloud albedo, although there is qualitative evidence that the CCN effect may also be substantial. DOI: 10.1034/j.1600-0870.1991.00013.x

Radiation and cloud radiative properties in the European Centre for Medium Range Weather Forecasts forecasting system

Abstract: The successive versions (EC1, EC2, and EC3) of the radiation scheme used in the European Centre for Medium Range Weather Forecasts (ECMWF) operational model, including the version which became operational on May 2, 1989, are reviewed and their results are compared to results of more detailed radiation models made available thanks to the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM) program. For clear-sky conditions, the shortwave H2O absorptivity is overestimated in EC1-EC2, which leads to too large shortwave atmospheric absorption (by up to 20%) and too small downward shortwave radiation at the surface (by 5–10%). EC1 and EC2 both significantly underestimate the longwave radiative cooling, with main errors in the lower troposphere with EC1, and between 700 and 300 hPa with EC2. Therefore EC1 and, to a smaller extent, EC2 underestimate the outgoing longwave radiation at the top of the atmosphere. In cloudy conditions, EC1 shows an exaggerated sensitivity to small amounts of scatterer, a problem corrected in EC2. Due an unrealistic model cloud embedded in EC1-EC2, these schemes cannot properly represent both the shortwave planetary albedo and outgoing longwave radiation for realistic cloud liquid water contents. EC3 corrects most of these deficiencies and gives results in better agreement with those of more detailed models.

ITS: the integrated TIGER series of electron/photon transport codes-Version 3.0

Abstract: The ITS system is a powerful and user-friendly software package permitting state-of-the-art Monte Carlo solution of linear time-independent coupled electron/photon radiation transport problems, with or without the presence of macroscopic electric and magnetic fields of arbitrary spatial dependence. Version 3.0 is a major upgrade of the system with important improvements in the physical model, variance reduction, I/O, and user friendliness. Improvements to the cross-section generator include the replacement of Born-approximation bremsstrahlung cross section with the results of numerical phase-shift calculations, the addition of coherent scattering and binding effects in incoherent scattering, an upgrade of collisional and radiative stopping powers, and a complete rewrite to Fortran 77 standards emphasizing Block-IF structure. Improvements in the Monte Carlo codes are also described. >

Stellar Atmospheres: Beyond Classical Models

Abstract: Section 1..- Recent Advances in Computational Methods.- Acceleration of Convergence.- Fast Solution of Radiative Transfer Problems with a Multi-Grid Method.- Line Blanketing without LTE: Simple and Complex Spectra.- Global and Local Methods for 1-D Problems Implementation Aspects and CPU-Time and Memory Scalings.- 2-D Axisymmetric Line Transport.- NLTE Spectral Line Formation in Three Dimensions.- Iteration with Approximate Lambda Operators, and its Application to the Expanding Atmospheres of WR Stars.- Analytical Methods of Line Formation Theory: Are They Still Alive?.- Iteration Factors in the Solution of the NLTE Line Transfer Problem.- Analysis of Ultraviolet P Cygni Profiles in the Spectra of O-Type Stars.- Computer Codes for Stellar Atmospheric Modeling.- Section 2..- The Quest for Physical Realism in Stellar Atmospheric Modeling.- Unified NLTE Model Atmospheres Including Spherical Extension and Stellar Winds: Euv-Fluxes and the HE II Discrepancy in Central Stars of Planetary Nebulae.- Non-LTE Model Atmosphere Calculations with Approximate Lambda Operators.- NLTE Model Atmospheres for Hot Stars.- Radiative Transfer in Expanding Atmospheres - Radiative Acceleration of Wolf-Rayet Envelopes?.- Non-LTE Analysis of Hot Stars Including Line Blanketing.- Time-Dependent Two-Dimensional Radiation Hydrodynamics of Accreting Matter onto Highly Magnetized Neutron Stars: The Evolution of Photon Bubbles.- The Origin and Development of Instabilities in Radiatively- Driven Stellar Winds.- A Smooth Source Function Method for Including Scattering in Radiatively Driven Wind Simulations.- 2-D Radiation Hydrodynamics Models of the Solar Photosphere.- Dynamics of and Radiative Transfer in Inhomogeneous Media.- Atmospheres of Late-Type Giants.- Fe II Emission Line Diagnostics of the Sun and Stars.- Chromospheric Inhomogeneities in Cool Stars: Possible Effect on Hydrogen Line Profiles.- Numerical Simulation of Photospheric Convection: Hydrodynamical Test Calculations.- Section 3..- Stellar Atmosphere Theory as a Spectroscopic Tool. The Example of Hot Stars.- The Winds of O-Stars. V: Tests of the Accuracy of the Radiation Driven Wind Models.- Diagnostics of Wolf-Rayet Atmospheres.- Central Stars of PN: Spectral Diagnostics Based on Model Atmospheres VS. Diagnostics Based on the Classical Nebular Approach.- Spectral Diagnostics of Hot Subdwarfs: Successes and Problems.- Temperatures, Gravities and Abundances of B Stars: Recent Progress and Remaining Problems.- Line Blanketing Without LTE: The Effect on Diagnostics for B-Type Stars.- Particle Transport in Magnetic Stellar Atmospheres.- Spectroscopic Tests of Late-Type Model Atmospheres of Dwarf Stars.- Dust in the Shells of Cool Giants and Supergiants.- NLTE Analysis of Massive OB Stars.- Formation of the K I 7699 A Line in Sunspots.- On the Influence of Multi-Dimensional Radiative Transfer on the Energetic Contribution of the Ca K Line.- Special Session.- The Opacity Project and the Practical Utilization of Atomic Data.- New Opacity Calculations.- 40 Years Numerical Stellar Atmospheres: Concluding Remarks and Personal Considerations.

The Escape of Lyman-Alpha Radiation from a Multiphase Interstellar Medium

Abstract: The Eddington approximation is used to model the transfer of radiation within a multiphase scattering medium. An estimate is thereby derived for the fraction of H Ly-alpha photons that escape from a galaxy with a two-phase interstellar medium in which dusty gas clouds lie embedded within an intercloud medium of negligible absorption and scattering coefficients. Under suitable conditions, Ly-alpha photons may actually suffer less attenuation than radiation which is not resonantly scattered, due to the fact that such photons spend most of their time in the intercloud medium as they bounce their way from one interstellar cloud surface to the next. This effect may lead to an enhancement in the observed line-to-continuum ratio, and to a Ly-alpha line profile which may be considerably narrower than that which would emerge from uniform medium of the same total atomic column density. 21 refs.

A new polarized atmospheric radiative transfer model

Abstract: A plane-parallel polarized radiative transfer model is described. The model is used to compute the radiance exiting a vertically inhomogeneous atmosphere containing randomly-oriented particles. Both solar and thermal sources of radiation are considered. A direct method of incorporating the polarized scattering information is combined with the doubling and adding method to produce a relatively simple formulation. Several numerical results are presented for verification and comparison.

The weighted-sum-of-gray-gases model for arbitrary solution methods in radiative transfer

Abstract: The weighted-sum-of-gray-gases approach for radiative transfer in nongray participating media, first developed by Hottel in the context of the zonal method, has been shown to be applicable to the general radiative equation of transfer Within the limits of the weighted-sum-of-gray-gases model (nonscattering media within a black-walled enclosure), any nongray radiation problem can be solved by any desired solution method after replacing the medium by an equivalent small number of gray media with constant absorption coefficients Some examples are presented for isothermal media and media at radiative equilibrium, using the exact integral equations as well as the popular P-1 approximation for the equivalent gray media solutions The results demonstrate the equivalency of the method with the quadrature of spectral results, as well as the tremendous computer times savings (by a minimum of 95 percent) that are achieved

Dissipational Galaxy Formation. I. Effects of Gasdynamics

Abstract: Numerical simulations of hierarchical galaxy formation including gasdynamics are presented. These simulations are conducted using a new, general-purpose programs for evolving self-gravitating systems in three dimensions. The gravitational forces are calculated with a hierarchical tree algorithm, while the gasdynamic properties are determined by an approach known as smoothed particle hydrodynamics. In this method the complete thermodynamic state of the gas is known everywhere, so dissipational effects can be included by allowing the gas to cool radiatively. We use standard cooling curves that include Compton, as well as radiative, cooling

Variations in the Sun's radiative output

Abstract: By how much does the Sun's radiation vary? Although the Sun has long been an object of immense fascination, fundamental information about the magnitude and variability of its radiative output, sought for over a century, is only now approaching a level satisfactory for geophysical applications. During the past decade, satellite instruments have measured, simultaneously, both the Sun's spectrally integrated radiative output and its ultraviolet spectrum. These data have been analyzed in terms of their relationships to ground-based observations that characterize different aspects of the Sun's 11-year activity cycle, allowing estimates of solar radiative output variations over times scales from days to decades and interpretations of these variations in the broader context of the variable Sun. Uncertainties still remain to be answered by the next generation of solar radiometers, which commenced observations with the launch of the Upper Atmosphere Research Satellite in September 1991, near the maximum of solar cycle 22.

Irradiation of Accretion Disks around Young Objects. I. Near-Infrared CO Bands

Abstract: The effect of irradiation by the central star on the atmospheres of optically thick, physically thin, accretion disks around young objects is calculated in an approximate way, assuming radiative equilibrium. The net effect of irradiation is to increase the temperature in the atmosphere of a viscous accretion disk relative to the case when only the viscous flux goes through it. The temperature increase at a given depth depends on the absorption and scattering coefficients characteristic of the atmosphere, and on the rate of irradiation. The effect on the near-infrared spectrum of the star-disk configuration is to decrease the strength of the absorption in the CO bands or turn them into emission, depending on the irradiation rate and on the mass accretion rate

Analysis of radiative scattering for multiple sphere configurations

Abstract: An analysis of radiative scattering for an arbitrary configuration of neighbouring spheres is presented. The analysis builds upon the previously developed superposition solution, in which the scattered field is expressed as a superposition of vector spherical harmonic expansions written about each sphere in the ensemble. The addition theorems for vector spherical harmonics, which transform harmonics from one coordinate system into another, are rederived, and simple recurrence relations for the addition coefficients are developed. The relations allow for a very efficient implementation of the 'order of scattering' solution technique for determining the scattered field coefficients for each sphere. Prediction of the radiative absorption and scattering characteristics of small particles is important to researchers in a number of fields, e.g. atmospheric modelling, analysis of radiative transfer from flames, and development of nonintrusive laser-based optical diagnostic methods. Computation of the radiative characteristics of spherical particles from Lorenz-Mie theory is practically a trivial matter due to the existence of efficient computer codes (Bohren & Huffman 1983). However, it is not unusual to encounter situations in which the individual particles, while spherical in shape, are so close together that the 'isolated sphere' assumption inherent in Lorenz-Mie theory is questionable. A common example is soot formed in combustion processes. Electron micrographs of the individual soot particles reveal them to be agglomerates of a large number of primary, spherical particles (Dobbins

Enhanced radiative recombination of free excitons in GaAs quantum wells.

Abstract: Radiative properties of free excitons in a single GaAs quantum well are studied under resonant excitation. Enhanced radiative recombination of the excitons, caused by the breakdown of the translational symmetry of the system, is evidenced by the very short lifetime as well as by the strong intensity of the signal. Dephasing mechanisms, by transferring the excitons to nonradiative states, increase the observed lifetime. We deduce a radiative lifetime of 10\ifmmode\pm\else\textpm\fi{}4 ps in the absence of dephasing mechanisms.

The propagation of optical radiation in tissue I. Models of radiation transport and their application

Abstract: This paper is the first of two reviewing the propagation of electromagnetic radiation of wavelength 0.25–10μm in tissue. After a brief discussion of light/tissue interactions, a mathematical description of light propagation in terms of radiative transfer is developed. Formal solutions of the resulting equation are outlined, but the emphasis is on approximate method of solution—namely the discrete ordinates method, the technique of functional expansion and Monte Carlo simulation. The application of the simplest of these approximate methods, namely the 2-flux and diffusion models, to tissue optics is discussed in some detail. The second paper deals with the optical properties of tissue and the salient characteristics of light fluence distributions in these tissues.

Simulation of surface urban heat islands under ‘IDEAL’ conditions at night part 1: Theory and tests against field data

Abstract: Observations show that the urban heat island in the atmospheric layer below roof level is most strongly developed during calm, cloudless conditions at night. This paper outlines two versions of a numerical model to describe the cooling of rural and street canyon surfaces under these conditions using surface thermal and radiative properties and the radiative geometry of the canyons. One version uses a full system of differential equations and the other the simpler force-restore approach. The two approaches are shown to be in general agreement and the output of the simpler model is shown to give a faithful representation of cooling of rural and urban surfaces, and therefore heat islands, when compared with field observations.

Intercomparing shortwave radiation codes for climate studies

Abstract: As a second step of the international program of Intercomparison of Radiation Codes Used in Climate Models (ICRCCM), an intercomparison of shortwave radiation models was initiated. Among the 26 codes that participated in the comparison were very detailed (line-by-line), narrow-band (high-spectral resolution), as well as highly parameterized (low-spectral resolution) models. A considerable spread was detected in the response of these models to a set of well-defined atmospheric profiles. Substantial discrepancies exist among models even for the simplest case of pure water vapor absorption with standard deviation ranging from 1% to 3% for the downward fluxes at the surface and from 6% to 11% for the total atmospheric absorption. The divergences in downward surface flux increase to nearly 4% when all absorbers and the molecular scattering are considered. In cloudy conditions the divergences range from 4% to 10%, depending on the cloud optical thickness. Another major uncertainty that has been identified is the spectral averaging of the scattering properties which can result in very significant errors for low spectral resolution codes. Since these errors appear to be systematic, they may induce unrealistic feedback mechanisms in numerical climate models. The amplitude of the differences between models is in many cases larger than the accuracy required for the achievements of several objectives of the World Climate Research Program. While reference solutions for the absorption and scattering in atmospheres can be obtained based on the state-of-the-art spectroscopic knowledge and rigorous computational techniques, the absolute tests of the validity of the radiation algorithms would be comprehensive field experiments in which the radiative and all relevant atmospheric parameters are measured to a high degree of accuracy.

Molecular-structure and temperature-dependent radiative rates in twisted intramolecular charge-transfer and exciplex systems

Abstract: Contrary to the general rules, the radiative rates in intramolecular full CT excited states are temperature-dependent: fluorescence emission is thermally activated. This is demonstrated on the TICT states and on highly polar intramolecular exciplexes. The radiative (back) electron transfer from the equilibrium structure of these excited states is highly forbidden often due to zero or minimal orbital overlap (nodal plane effects); thus, the equilibrium structures in both classes of states correspond to practically pure CT configurations. Vibrational activation (100-600 cm{sup {minus}1}) is predominantly responsible for the observed fluorescence and presents a new key to the structure of these excited species. Thermally activated emission is predicted for other flexible molecules with a strongly forbidden transition.

Li depletion in F stars by internal gravity waves

Abstract: It is argued that internal gravity waves generated by the convective envelope of a star may be effective in producing a weak mixing in its radiative interior. The spectrum of wavelengths and frequencies generated is estimated. Only the largest horizontal wavelengths and the lowest frequencies contribute significantly to the wave energy flux and the mixing at the Li-burning depth. At this layer, the combination of the dependence on spectral type, both for the wave energy flux and the shear rate induced by the waves, would produce a distribution of Li abundances similar to that of the Li gap found among F-type stars of several open clusters, and the field. Quantitative agreement with the observed location and time scale of the gap is obtained, but only by increasing the intensity of the waves generated by a factor 15 above straightforward mixing-length estimates. The ratio of mixing length to scale height needed to get the gap in the right spectral range is 1.6. The blue edge of the gap is predicted to be sharp. 47 refs.

Non-LTE models of Titan's upper atmosphere

Abstract: Models for the thermal structure of Titan's upper atmosphere, between 0.1 mbar and 0.01 nbar are presented. The calculations include non-LTE heating/cooling in the rotation-vibration bands of CH4, C2H2, and C2H6, absorption of solar IR radiation in the near-IR bands of CH4 and subsequent cascading to the nu-4 band of CH4, absorption of solar EUV and UV radiation, thermal conduction and cooling by HCN rotational lines. Unlike earlier models, the calculated exospheric temperature agrees well with observations, because of the importance of HCN cooling. The calculations predict a well-developed mesopause with a temperature of 135-140 K at an altitude of approximately 600 km and pressure of about 0.1 microbar. The mesopause is at a higher pressure than predicted by earlier calculations because non-LTE radiative transfer in the rotation-vibration bands of CH4, C2H2, and C2H6 is treated in an accurate manner. The accuracy of the LTE approximation for source functions and heating rates is discussed.

Detection of Cloud-Top Height from Backscattered Radiances within the Oxygen A Band. Part 1: Theoretical Study

Abstract: A series of radiative transfer calculations were performed to study the possibility of determining cloud-top pressure (height) from backscattered solar radiances within the oxygen A-band absorption. For the development of a cloud-top pressure algorithm, we also looked into the impact of perturbing effects, such as varying cloud properties, sun elevation, and surface albedo. The most important quantities are total cloud optical thickness δC and the vertical profile of liquid-water content. The effects of cloud optical thickness-if δC>1-are already taken into account by a cloud-top algorithm, which only considers two radiances inside and outside the oxygen absorption band. For one-layer clouds, the cloud-top heights may be derived to within an accuracy of 200 m. Multilayer clouds or varying liquid-water content profiles can only be matched with an inverse technique using radiances at up to 16 wavelengths, which, however, give cloud-top height estimates to within an error of only 50 m for all 900 cl...

Mixing layers in stellar outflows

Abstract: It has been suggested in the past that at least part of the emission observed in high-velocity outflows from young stars could be formed in turbulent mixing layers. A simplified mixing layer model based on a simple 'turbulent viscosity' description of the dissipation and transport associated with the fully developed turbulence. Predictions from these models agree very well with results from laboratory flows with Mach numbers M = 1 to 20, showing that this theoretical approach is indeed valid for the M about 10 regime that is relevant for stellar outflows. Mixing layers have a sizeable radiative luminosity, so that in principle they could substantially contribute to the total emission from stellar jets or from Herbig-Haro objects. The theoretical framework presented is quite general, and could be applied to mixing layers with detailed atomic and/or molecular processes (as would be necessary for modeling molecular outflows), or to mixing layers with different geometries (e.g., the accretion disk/stellar photosphere boundary layer). 29 refs.

Opacities for the solar radiative interior

Abstract: Opacities for the solar radiative interior are computed and compared to previous calculations. A significant enhancement of the Rosseland mean opacity near the bottom of the convection zone is reported when compared to the Los Alamos Astrophysical Opacity Library. The discrepancy is attributed mainly to the equation of state. Absorption processes are investigated and their uncertainties estimated. The new results using the OPAL opacity code are given in tabular form for the Ross-Aller, Aller, Grevesse and Anders-Grevesse solar mixtures.

Plasma points and radiative collapse in vacuum sparks

Abstract: This review discusses the intense x‐ray emitting regions, called plasma points, that appear in low‐inductance vacuum sparks and other high‐current discharges Accurate x‐ray spectroscopy indicates the existence of two types of plasma points with different plasma parameters One type is extremely small (∼microns), dense (∼1023/cm3), and hot (≳1 keV), while the second type is an order of magnitude less extreme A dynamic model (Vikhrev 1982a) based on radiation cooling with axial outflow of plasma predicts a radiative collapse that is consistent with many features of the plasma points

Axisymmetric model of the ionized gas in the Orion Nebula

Abstract: New ionization and thermal equilibrium models for the ionized gas in the Orion Nebula with an axisymmetric two-dimensional 'blister' geometry/density distribution are presented. The HII region is represented more realistically than in previous models, while the physical detail of the microphysics and radiative transfer of the earlier spherical modeling is maintained. The predicted surface brightnesses are compared with observations for a large set of lines at different positions to determine the best-fitting physical parameters. The model explains the strong singly ionized line emission along the lines of sight near the Trapezium.