Showing papers on "Radiative transfer published in 1982"

A model for microwave emission from vegetation-covered fields

Abstract: The measured brightness temperatures over vegetation-covered fields are simulated by a radiative transfer model which treats the vegetation as a uniform canopy with a constant temperature, over a moist soil which emits polarized microwave radiation. The analytic formula for the microwave emission has four parameters: roughness height, polarization mixing factor, effective canopy optical thickness, and single scattering albedo. A good representation has been obtained with the model for both the horizontally and vertically polarized brightness temperatures at 1.4 and 5 GHz frequencies, over fields covered with grass, soybean and corn. A directly proportional relation is found between effective canopy optical thickness and the amount of water present in the vegetation canopy. The effective canopy single scattering albedo depends on vegetation type.

The ionization equilibrium of astrophysically abundant elements

Abstract: The paper presents new calculations of ionization equilibrium fractions of 11 abundant elements (C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, Ni) as functions of temperature. Convenient coefficients for fitting the rates of collisional ionization, radiative recombination, and dielectronic recombination are also tabulated. Many of the ionization rates are based on recent experimental measurements of cross sections for collisional ionization and autoionization following inner-shell excitation. These rates are used elsewhere in computations of nonequilibrium ionization, radiative cooling, radiative shock models, and plasma emission diagnostics.

On the shortwave radiative properties of stratiform water clouds

Abstract: The development of a comprehensive radiation scheme for computing shortwave fluxes and heating rates in a cloudy stratified atmosphere is described. The scheme uses the Delta-Eddington multiple scattering method and treats Rayleigh scattering, absorption by water vapour and ozone, and scattering and absorption by the cloud drops. The molecular absorption data were obtained from the Lowtran 3B subroutine and incorporated using the exponential sum fitting of transmissions technique. Five versions of the scheme were used to examine the effect of spectral resolution on the results and comparisons are shown with previous work. The short-wave properties of clouds were examined using 24 spectral bands between 0.25 and 4μm. A significant dependence of the properties on the drop size distribution was found, in agreement with previous work. A simple parametrization of the single scattering properties of the cloud drops in terms of liquid water content and equivalent radius was examined and shown to produce satisfactory results, thus circumventing the need for time-consuming Mie theory computations. This allows the scheme to be used in comparisons with aircraft measurements of layer clouds, within which both liquid water content and drop size distribution are functions of height.

Vacuum fluctuations and radiation reaction : identification of their respective contributions

Abstract: It is generally considered that there exists in quantum radiation theory an indetermination in the separation of the respective effects of vacuum fluctuations and radiation reaction. We show in this paper that such an indetermination can be removed by imposing to the corresponding rates of variation to be Hermitian (this is necessary if we want them to have a physical meaning). Such a procedure is generalized to the case of a small system S interacting with a large reservoir % and allows the separation of two types of physical processes, those where % fluctuates and polarizes S (effects of reservoir fluctuations), those where it is S which polarizes % (effects of self reaction). We apply this procedure to an atomic electron interacting with the radiation field and we then identify the contribution of vacuum fluctuations and self reaction to radiative corrections and spontaneous emis- sion of radiation. The analysis of the results obtained in this way allows us to specify the physical pictures which must be associated with the various radiative processes.

Linear analysis of an oscillatory instability of radiative shock waves

Abstract: We present a linear stability analysis of a planar radiative shock with cooling function ..lambda..proportionalT/sup ..cap alpha../. We investigate an oscillatory instability similar to that found in numerical calculations of accretion onto degenerate dwarfs by Langer, Chanmugam, and Shaviv. Our analysis shows that multiple modes of oscillations are unstable for ..cap alpha..< or approx. =0.4 for the fundamental mode and ..cap alpha..< or approx. =0.8 for the first and second overtone modes. The oscillation frequency is 0.3 x/sub s/0/u/sub in/ for the fundamental mode and ranges from 0.6 x/sub s/0/u/sub in/ to 1.0 x/sub s/0/u/sub in/ for the first overtone mode for ..cap alpha.. between -1 and 2, where x/sub s/0 is the results should be applicable both to accretion onto compact objects and to radiative shock waves in the interstellar medium. The instability may explain the inability of the steady state shock wave emission models to reproduce certain observations of old supernova remnants and Herbig-Haro objects.

Electronic energy-levels in dense plasmas

Abstract: Modern inertial-confinement fusion experiments subject matter to extreme physical conditions previously studied only in theoretical astrophysics. At very high plasma density, atomic energy states are significantly altered by electric fields of neighboring ions and by free electrons; the resulting phenomena of pressure ionization and continuum lowering may be analyzed with a sequence of models, each adding new subtleties to a complex picture. In this paper, we develop a simple parameterization of pressure ionization, discuss limitations of the Debye-Huckel model for plasma perturbations, and survey an approximate description of X-ray spectra based on the WKB approximation. WKB theory leads to a simple derivation of the screened hydrogenic model for plasma ionization and radiative properties. Electron eigenvalues are obtained from the total ion energy in agreement with Koopman's theorem, and the representation of spectral terms is improved by a new set of screening coefficients.

Interpretation of airborne oceanic lidar: effects of multiple scattering

Abstract: The effects of multiple scattering on the interpretation of the time dependence of elastic backscattering of laser pulses from the ocean (lidar) are investigated through solving the radiative transfer equation by Monte Carlo techniques. In particular, after removal of the geometric loss factors, it is found that the backscat-tered power is a decaying exponential function of time, over the time interval required for photons to travel four attenuation lengths through the water. The effective attenuation coefficient of this exponential decay is found to be strongly dependent on the parameters of the lidar system and on the optical properties of the water. The significant parameter is the ratio of the radius of the spot on the sea surface viewed by the lidar receiver optics to the mean free path of photons in the water. For values of this parameter near zero, the decay is determined by the beam attenuation coefficient, while for values greater than ~5-6, the decay is given by the attenuation coefficient for downwelling irradiance, often referred to as the diffuse attenuation coefficient. Between these two extremes the interpretation of the effective attenuation coefficient requires, essentially, complete knowledge of the inherent optical properties of the water: the beam attenuation coefficient and the volume scattering function.

Radiative transfer model for heterogeneous 3-D scenes.

Abstract: A general mathematical framework for simulating processes in heterogeneous 3-D scenes is presented. Specifically, a model was designed and coded for application to radiative transfers in vegetative scenes. The model is unique in that it predicts (1) the directional spectral reflectance factors as a function of the sensor's azimuth and zenith angles and the sensor's position above the canopy, (2) the spectral absorption as a function of location within the scene, and (3) the directional spectral radiance as a function of the sensor's location within the scene. The model was shown to follow known physical principles of radiative transfer. Initial verification of the model as applied to a soybean row crop showed that the simulated directional reflectance data corresponded relatively well in gross trends to the measured data. However, the model can be greatly improved by incorporating more sophisticated and realistic anisotropic scattering algorithms

An approach to the estimation of the surface radiative properties and radiation budgets of cities

Abstract: The principles of radiation geometry and the Lambertian assumption are employed to construct a numerical model of the multiple reflection effects within an urban canyon of variable geometry and surface materials. The canyon model is used to estimate the reflection coefficients for the direct and diffuse short-wave and incoming longwave radiative fluxes and the longwave emissivity of an urban surface. The procedures described are applied to various land-use zones in Columbus, Ohio, for the solstices and equinoxes. The diurnal variation of shortwave reflection coefficients is illustrated; daily values generally increase from the CBD to the residential suburbs. Longwave radiative properties differ little between zones. Radiation budgets are synthesized for each land-use type, assuming negligible atmospheric pollution, and are compared with those for appropriate rural surfaces. Net longwave radiation varies little between zones but net shortwave and net radiation decrease from the CBD to the residential areas...

Primordial nucleosynthesis including radiative, Coulomb, and finite-temperature corrections to weak rates

Abstract: We calculate the changes in the yields of primordial nucleosynthesis which result from small corrections to rates for weak processes that connect neutrons and protons. We correct the weak rates by improved treatment of Coulomb and radiative corrections, and by inclusion of plasma effects. Our calculations lead to a systematic decrease in the predicted $^{4}\mathrm{He}$ abundance of about $\ensuremath{\Delta}Y=0.0025$. The relative changes in other primordial abundances are also 1-2%.

An Iterative Radiative Transfer Code For Ocean-Atmosphere Systems

Abstract: The details of an iterative radiative transfer code for computing the intensity and degree of polarization of diffuse radiation in models of the ocean-atmosphere system are described. The present code neglects the upwelling radiation from below the ocean surface and as such can be applied to the part of the spectrum where the absorption by water is strong. To establish the reliability of the numerical scheme and the computer code, the results are compared with those of Fraser and Walker (1968), Dave (1972), and Mullamaa (1964); they are found to be in excellent agreement. The computations also show that both the intensity and the degree of polarization of the upwelling diffuse radiation at the top of the atmosphere vary significantly when the rough ocean at the base of the atmosphere is replaced by a Lambertian surface that reflects the same energy as the rough ocean.

Radiative lifetimes and electronic quenching rate constants for single-photon-excited rotational levels of no (A2Σ+, ν′ = 0)

Abstract: A narrow-band, frequency-doubled, tunable dye laser has been used to excite fluorescence from the A2Σ+, ν′ = 0 state of NO. Collision-free lifetimes were measured for 21 different K′ levels giving a mean radiative lifetime τ = 217 ± 4 ns. Electronic quenching rate constants of NO (A2Σ+, ν′ = 0) were measured for O2, N2, H2O, CO2 and Ar. No dependence of the quenching-rate constant on the initially excited rotational level was observed.

Total Ozone Determination from the Backscattered Ultraviolet (BUV) Experiment

Abstract: The algorithm used to derive total ozone from the Nimbus 4 Backscattered Ultraviolet (BUV) experiment is described. A seven-year global data set with more than one million retrievals has been produced and archived using this algorithm. The algorithm is a physical retrieval scheme using accurate radiative transfer computations. Error sources are discussed and verified using Dobson network comparisons and the statistics of the BUV A- and B-pair derived ozone values.

A Parameterization of Scale-Dependent Radiative Damping Rates in the Middle Atmosphere

Abstract: An analysis of the radiative damping of slowly modulated sinusoidal wave disturbances is presented It is shown that it is possible to incorporate scale-dependent effects in a manner no more difficult than that used in the usual “Newtonian cooling” approximation Explicit calculations show that these effects can be very important in the middle atmosphere Detailed graphs of the scale-dependent relaxation times due to CO2 and O3 are given, as well as simple analytical parameterizations

Radiative cooling computed for model atmospheres

Abstract: Calculations of spectral radiance are reported from several model atmospheres appropriate to different climatic conditions by use of the LOWTRAN 5 computer code. From these data we evaluate the radiative cooling power and the temperature drop below ambient temperature for horizontal surfaces that radiate toward the sky. The surfaces are ideal blackbodies or have ideal infrared-selective properties with zero reflectance in the 8-13-microm range and unity reflectance elsewhere. For freely radiating surfaces, the cooling power at ambient temperature lies between 58 Wm(-2) and 113 Wm(-2) for the different surfaces and model atmospheres. The maximum temperature difference for a device with a nonradiative heat transfer coefficient of 1 Wm(-2) K(-1) is between 11 and 21 degrees C for a blackbody and between 18 and 33 degrees C for an infrared-selective surface. For radiators arranged so that they intercept only the atmospheric zenith radiance, the cooling powers and temperature differences are higher than for freely radiating surfaces, the increase being largest for humid atmospheres. The influence of altered contents of water vapor was found to affect strongly the radiative cooling, whereas changes in ozone and aerosol abundance were less important. The significance of these results to different cooling applications is briefly discussed.

A Finite-Element Model of the Atmospheric Boundary Layer Suitable for Use with Numerical Weather Prediction Models

Abstract: We give a detailed description of an atmospheric boundary layer model capable of simulating the diurnal cycles of wind, temperature and humidity. The model includes a formulation of various physical processes (radiative effects, variation of soil surface temperature and humidity, etc.) and uses a first-order closure for turbulent fluxes that relies upon a time-dependent equation for turbulent kinetic energy and on a mixing length governed by a relaxation process. The exchange processes taking place in the surface layer are dealt with in a separate micrometeorological module. The one-dimensional model uses a Galerkin technique based on linear finite elements, variable resolution in the vertical, and a time discretization of the Crank-Nicholson type. A simulation test based on day 33 of the Wangara Australian experiment indicates that the model, despite its relative simplicity, gives realistic results that compare favorably with those from higher order models while taking much less space and time o...

Radiative Width of Molecular-Cluster States

Abstract: Molecular states are characterized by enhanced electromagnetic deexcitations of many different multipolarities. The expected enhancement of $E1$, $E2$, and $E3$ transitions is examined deriving molecular sum rules for radiative deexcitation widths and via a dimensionality approach. The enhancement of the $E1$ transitions is the most striking.

Linear polarization of radio frequency lines in molecular clouds and circumstellar envelopes

Abstract: We predict that interstellar lines possess a few percent linear polarization provided that the optical depth in the source region is both anisotropic and of order unity and the radiative rates are at least comparable to the collision rates. These conditions are expected to be met in many sources which emit radio and far-infrared line radiation. Under circumstances in which the Zeeman splitting exceeds both the radiative and collisional rates the linear polarization is aligned either parallel or perpendicular to the projection of the magnetic field on the plane of the sky. This "strong magnetic field" limit is expected to apply to all radio frequency lines and to many of those far infrared lines which form between levels whose magnetic moments are comparable to the Bohr magneton. The "weak magnetic field" limit is relevant to most far-infrared lines formed between levels with magnetic moments of order the nuclear magneton. In this limit the polarization direction is determined by the orientation of the propagation direction with respect to the anisotropic optical depth.

The Saudi Arabian heat low: Aerosol distributions and thermodynamic structure

Abstract: Radiation and microphysical data are employed to deduce the bulk radiative and microphysical characteristics of the aerosol layer over the Saudi Arabian Peninsula and the adjacent Arabian Sea. Observed particle size distributions, total mass loadings, and vertical distributions of the dust are presented for five analysis periods. There is a large daily variability of the dust mass loading over the desert. The broadband radiative characteristics of the dust are inferred by comparing calculations for a dust free atmosphere with in situ broadband radiative measurements. The dust had no detectable effect on the longwave radiative fluxes, while it approximately doubled the clear sky shortwave absorption. The total shortwave absorption by the dust is dependent on the underlying surface albedo. The solar absorptance of the dust is expressed in terms of the dust mass loading and is incorporated into a broadband shortwave radiative transfer model to assess the surface and tropospheric energy budgets. For the desert cases the dust is largely responsible for a net 24 hour radiative energy convergence between, 530 and 800 mbar. A 24-hour radiative energy convergence is also found over the ocean; however, the layer depth is shallower than the desert case, being limited to the top of the dust layer. The thermodynamic structure of the heat low is presented. Calculations of temperature advection from the Saudi Arabian Peninsula to the Arabian Sea, suggest that horizontal advection aids in offsetting a net 24 hour tropospheric energy convergence over the desert. This warm air advection, along with the transport of dust over the Arabian Sea, may assist in maintaining the low level inversion and shallow moist layer observed over the western and central Arabian Sea.

A field study of nocturnal stratocumulus; III. High resolution radiative and microphysical observations

Abstract: High resolution observations from a tethered balloon of nocturnal stratocumulus on three occasions are presented. Measurements of the microphysical properties of the clouds (drop-size distributions, concentrations and liquid water contents) were obtained with an Axially Scattering Spectrometer Probe (ASSP). A ground based 95 GHz radiometer was used to infer the integrated liquid water path through the cloud. In the two thick clouds studied the drop-size data show an almost monotonic increase of mean radius from cloud base to top. The liquid water content increases with height above cloud base at slightly less than the adiabatic rate and shows considerable variability towards cloud top. The observations support the concept of the inhomogeneous mixing of the cloudy air with the dry air entrained at the cloud top. Measurements were also made of infrared net radiative fluxes, and very good agreement is found with the theoretical fluxes from a high resolution radiative transfer scheme, using the ASSP drop spectra and liquid water contents, scaled by the 95 GHz radiometer data.

Multiple scattering corrections to the Beer-Lambert law. 1: Open detector.

Abstract: Multiple scattering corrections to the Beer-Lambert law are analyzed by means of a rigorous small-angle solution to the radiative transfer equation. Transmission functions for predicting the received radiant power-a directly measured quantity in contrast to the spectral radiance in the Beer-Lambert law-are derived. Numerical algorithms and results relating to the multiple scattering effects for laser propagation in fog, cloud, and rain are presented.

Far-Infrared Emission from Population-Inverted Hot-Carrier System in p -Ge

Abstract: Experimental evidence is presented for radiative transitions of light holes accumulated in a limited area in momentum space to the heavy-hole band. Also reported is the observation of cyclotron resonance emission from the accumulated light holes. The possibility of far-infrared amplification is discussed.

Transmission, backscattering, and depolarization of waves in randomly distributed spherical particles.

Abstract: General formulations are given for the multiple scattering of a polarized wave incident upon a slab of randomly distributed spherical particles. The radiative transfer equation with Stokes vectors is decomposed into Fourier components, and they are shown for linearly and circularly polarized incident wave. For linear polarization, the copolarized and cross-polarized incoherent intensities show sinusoidal variations with the azimuthal angle. The degree of polarization is also calculated for various directions and optical thickness. The calculations are made for optical waves at 5, 10, and 15 μm in fog and compared with the first-order scattering calculations.