Showing papers on "Radiative transfer published in 1985"

Canopy reflectance, photosynthesis and transpiration

Abstract: A two-stream approximation model of radiative transfer is used to calculate values of hemispheric canopy reflectance in the visible and near-infrared wavelength intervals. Simple leaf models of photosynthesis and stomatal resistance are integrated over leaf orientation and canopy depth to obtain estimates of canopy photosynthesis and bulk stomatal or canopy resistance. The ratio of near-infrared and visible reflectances is predicted to be a near linear indicator of minimum canopy resistance and photosynthetic capacity but a poor predictor of leaf area index or biomass.

Foundations of Radiation Hydrodynamics

Abstract: Exposes the great foundation-stones of research on radiating flows in astrophysics. Upon them are built the walls of methodology (some understandably incomplete). Concentration is on fundamentals but with only few applications. Coverage broadly involves non-radiating fluids, physics of radiation, radiation transport, and dynamics of radiating fluids, and finally the elements of sensor calculus as used in this volume. Contents, abridged: Microphysics of gases. Dynamics of ideal fluids. Relativistic fluid flow. Radiation and radiative transfer. Radiating flows. Glossary of physical symbols. Index.

Theory of microwave remote sensing

Abstract: Active and passive microwave remote sensing of earth terrains is studied. Electromagnetic wave scattering and emission from stratified media and rough surfaces are considered with particular application to the remote sensing of soil moisture. Radiative transfer theory for both the random and discrete scatterer models is examined. Vector radiative transfer equations for nonspherical particles are developed for both active and passive remote sensing. Single and multiple scattering solutions are illustrated with applications to remote sensing problems. Analytical wave theory using the Dyson and Bethe-Salpeter equations is employed to treat scattering by random media. The backscattering enhancement effects, strong permittivity fluctuation theory, and modified radiative transfer equations are addressed. The electromagnetic wave scattering from a dense distribution of discrete scatterers is studied. The effective propagation constants and backscattering coefficients are calculated and illustrated for dense media.

Flare Loop Radiative Hydrodynamics - Part Six - Chromospheric Evaporation due to Heating by Nonthermal Electrons

Abstract: The response of the solar chromosphere to flare heating by nonthermalelectrons is examined. A number of interesting phenomena appear in our numericalsolutions of the equations of hydrodynamics and radiative transfer. Here wediscuss one aspect of these results: the phenomenon of chromosphericevaporation. We present results for a range of heating fluxes and show how thesemay be understood in simple terms. Our major conclusions are as follows: (1)There is an energy flux threshold for ''explosive'' evaporation. Explosiveevaporation occurs when the upper chromosphere is unable to radiate the flareenergy deposited there, and is therefore heated rapidly to coronal temperatures.Energy fluxes less than this threshold produce ''gentle'' evaporation, in whichthe chromosphere is eaten away by conduction at a much slower rate. (2) Theexpansion velocity of explosively evaporated plasma cannot exceedapprox.2.35c/sub s/, where c/sub s/ is the sound speed in the evaporated material. (3) We derive a simple analytic model for the temporal variation of velocity in explosively evaporated plasma. This ''gasbag'' model, based on isothermal expansion of an impulsively heated mass of plasma, is used successfully to reproduce our own numerical results, as well as those of MacNeice et al. (1984). (4) The lower transition region, in both gentle and explosive evaporation, quickly reachesmore » a quasi-steady balance between conduction and radiation, so that the conductive flux at 10/sup 5/ K is given by 3.42 x 10/sup 5/P ergs cm/sup -2/ s/sup -1/, where P(dyn cm/sup -2/) is the pressure in the flare transition region. In the case of explosive evaporation, a short powerful pulse of EUV radiation is emitted from plasma with temperatures near 10/sup 5/ K during the adjustment to this equilibrium.« less

The Relative Importance of Aerosol Scattering and Absorption in Remote Sensing

Abstract: Previous attempts to explain the effect of aerosols on satellite measurements of surface properties for the visible and near-infrared spectrum have emphasized the amount of aerosols without consideration of their absorption properties. In order to estimate the importance of absorption, the radiances of the sunlight scattered from models of the Earth-atmosphere system are computed as functions of the aerosol optical thickness and absorption. The absorption effect is small where the surface reflectance is weak, but is important for strong reflectance. These effects on classification of surface features, measuring vegetation index, and measuring surface reflectance are presented.

Observation of inhibited spontaneous emission.

Abstract: The radiative decay of the cyclotron motion of a single electron is significantly inhibited when the electron is located within microwave cavity (formed by the electrodes of a Penning trap) rather than in free space. This is the first observation of such inhibited spontaneous emission and the first use of a promising new system for radiative physics. Implications for precision measurements are mentioned.

Multiline transfer and the dynamics of stellar winds.

Abstract: A Monte Carlo technique for treating multiline transfer in stellar winds is described. With a line list containing many thousands of transitions and with fairly realistic treatments of ionization, excitation and line formation, the resulting code allows the dynamic effects of overlapping lines the investigation of and provides the means to directly synthesize the complete spectrum of a star and its wind. It is found that the computed mass loss rate for data Puppis agrees with the observed rate. The synthesized spectrum of zeta Puppis also agrees with observational data. This confirms that line driving is the dominant acceleration mechanism in this star's wind.

A method of calculating radiative heat diffusion in particle simulations

Abstract: A new method for solving heat diffusion in three dimensional particle simulations is described. The difficulties encounted by other authors is discussed, in particular the difficulty of including boundary conditions in particle simulations. One and three dimensional tests of the method are described.

Enhanced fields on rough surfaces: dipolar interactions among particles of sizes exceeding the Rayleigh limit

Abstract: Amplified electromagnetic fields generated by a surface of finitely sized metal or dielectric particles are calculated. Regular arrays of particles produced by lithographic techniques and stochastic particle distributions that occur, e.g., in island films, are discussed. Retarded dipolar interactions between the particles are explicitly taken into account. Particles of finite size are considered for which dynamic depolarization and radiation damping effects are important. Limits of validity of the present approach are indicated. The total electromagnetic field from the surface is calculated by superposition: Scalar potentials characterizing a single particle are convoluted with a distribution function describing the particle positions. The surface Hertz vector is obtained from the single-particle Hertz vector by convolution with a two-dimensional Shah function representing the array or with the autocorrelation function of the stochastic surface. A plane-wave description of the dipolar fields is used, whereby the convolution is transformed into a simple multiplication in Fourier space. Cylindrical, general spheroidal, and spherical shapes are considered for the individual particle. Particle dipole moments are obtained by a self-consistent procedure. Dipolar interactions result in shifts and broadening of the particle plasmon resonances, which are responsible for the local intensity enhancement. A set of universal curves is given from which shift and broadening can be calculated for particles of all sizes and shapes. Extrema in the dipolar interactions arise when grating orders change from radiative to evanescent character. The strong variation of the Raman enhancement with angle and wavelength in the vicinity of these extrema is clearly predicted from the Hertz-vector calculation. The formalism described permits one to calculate electromagnetic properties of the surface and enhancement factors for any electromagnetic process occurring at or near the surface. As examples, the calculation of reflectivities for s-and p-polarized excitation and surface-enhanced Raman-scattering cross sections are discussed.

Annual Heat Balance of Martian Polar Caps: Viking Observations

Abstract: The Infrared Thermal Mappers aboard the two Viking orbiters obtained solar reflectance and infrared emission measurements of the Martian north and south polar regions during an entire Mars year. The observations were used to determine annual radiation budgets, infer annual carbon dioxide frost budgets, and constrain spring season surface and atmospheric properties with the aid of a polar radiative model. The results provide further confirmation of the presence of permanent CO_2 frost deposits near the south pole and show that the stability of these deposits can be explained by their high reflectivities. In the north, the observed absence of solid CO_2 during summer was primarily the result of enhanced CO_2 sublimation rates due to lower frost reflectivities during spring. The results suggest that the present asymmetric behavior of CO_2 frost at the Martian poles is caused by preferential contamination of the north seasonal polar cap by atmospheric dust.

On ‘‘Radiative transfer in three-dimensional rectangular enclosures containing inhomogeneous,anisotropically scattering media’’

Abstract: Our 1985 paper (JQSRT 1985; 33: 533–549) reported the result of the research we conducted back then to better understand heat transfer processes in large-scale combustion chambers, especially in pulverized coal-fired furnaces. It was one of the first works exploring radiative transfer in three-dimensional enclosures where absorption and scattering coefficients due to combustion particles and gases were allowed to vary within the medium. This flexibility of the mathematical model made it useful for applications to realistic furnaces and different types of high-temperature systems. This note briefly discusses the motivation behind the paper and the immediate extension of the idea to different systems.

Recombination mechanisms in p-type HgCdTe: Freezeout and background flux effects

Abstract: The recombination mechanisms in HgCdTe are analyzed. Detailed expressions for the radiative lifetime are presented, taking into account recent measurements of the absorption coefficient. In p‐type material, carrier freezeout is shown to increase the lifetime exponentially for the radiative and the Auger processes at low temperatures. The effect of background flux is introduced, taking into account its variation with temperature due to the change in energy gap. Lifetime measurements on p‐type samples are in good agreement with combined Auger 7 and radiative mechanisms, where the Auger process is more effective for low x values. Highly compensated materials are dominated by the Shockley–Read recombination. For all samples, the intrinsic region is controlled entirely by the Auger process.

Preionization-dependent families of radiative shock waves

Abstract: Radiation shock waves are shown to be organizable into families within which the spectra are very similar. A high-velocity shock into neutral material is spectrally similar to a somewhat slower shock in fully ionized material. The similarities should allow the identification of the family to which an observed spectrum belongs, while spectral details permit subsequent identification of the separate values of shock velocity and preionization. By calculating shock spectra for the various families at constant recombination zone density, rather than constant preshock density, it is possible to separate the effects of 'velocity' (by which is meant postionization enthalpy per atom) from those of 'density' (by which is meant the degree of collisional deexcitation in the optical lines).

Cirrus Clouds. Part I: A Cirrus Cloud Model

Abstract: A two-dimensional (x, z), time-dependent, numerical cloud model is developed for the purpose of investigating the role of various physical processes involved in the maintenance of cirriform clouds. In addition to accounting for dynamic and thermodynamic processes including phase changes of water, effects due to microphysical composition and radiative processes are also explicitly incorporated into the model. Diagnostic parameterizations for the local radiative properties of cloudy volumes and the gravity induced relative fall speed of the contained ice water are presented. Results of a simulation of a thin cirrostratus cloud are given. Features of the simulated cloud structure are quite realistic. Quantitative agreement is found between the simulated ice water contents and vertical motions and comparable observations. It is shown that radiative effect may be very significant in the maintenance of cirrus. The effects of the gravity-induced relative fall speed of ice crystals are found to be of cri...

A note on Komar's anomalous factor

Abstract: Discrepancies between Komar's integrals for energy and angular momentum and those given by Einstein are simply explained. In particular the origin of the radiative anomaly is found.

Optical transitions of Pr3+ ions in fluorozirconate glass

Abstract: Optical absorption and emission spectra are presented for Pr 3+ ions in fluorozirconate (ZBLA) glass. The measured oscillator strengths and radiative rates for several transitions are compared with calculated values. Radiative transition rates for the excited states are determined by using the Judd-Ofelt theory. Energy transfer effects account for the nonradiative transitions. Although the Judd-Ofelt analysis does not yield the same excellent consistency for oscillator strengths and branching ratios for Pr 3+ as for Er 3+ and Ho 3+ , the experimental observations are consistent with the Judd-Ofelt calculations and the multiphonon emission rates predicted by the phenomenological energy-gap law. The crystallization of ZBLA glass also was studied, using Pr 3+ ions as probes.

Cirrus Clouds. Part II: Numerical Experiments on the Formation and Maintenance of Cirrus

Abstract: The numerical cirrus cloud model of Starr and Cox is used to investigate the role of various physical processes in the formation and maintenance of cirrus. Effects due to microphysical composition, i.e., crystal habit and size distribution, are found to be quite important in determining the overall cloud water budget. Radiative processes are also shown to affect the organization and bulk properties of the cloud. Substantial differences between simulations of thin cirrus under midday and nighttime conditions are found with the cloud being less dense overall (∼20%) but more persistently cellular during the day with all other environmental factors being the same. Cloud-scale interactions and feedbacks between dynamic, thermodynamic and radiative processes and the microphysical composition are significant and strongly modulate the properties of the simulated clouds. A comparison is made between simulations of weakly forced cirrostratus and nonprecipitating altostratus (liquid phase) under comparable ...

Validity of band-model calculations for CO2 and H2O applied to radiative properties and conductive-radiative transfer

Abstract: A previously presented line-by-line (LBL) calculation is used to test the validity of various approximate models. Narrow-band model parameters are generated from the lines used by the LBL approach in the 150–8000 cm -1 and 300–1500 K ranges. The accuracy of narrow-band models and of approximations for nonuniform paths applied to transmissivities and intensities of columns is studied. Random-statistical models give good results for the transmissivities but inaccurate results for the emitted intensities. Applications to combined conductive-radiative transfer in one-dimensional media are then presented. The temperature and flux distributions predicted by wide-band models are accurate when radiative transfer is preponderant; narrow-band models must be used in the case of optically thin media (e.g. in boundary layers).