Showing papers on "Radiative transfer published in 1996"

Clouds and the Earth's Radiant Energy System (CERES): An Earth Observing System Experiment

Abstract: Clouds and the Earth's Radiant Energy System (CERES) is an investigation to examine the role of cloud/radiation feedback in the Earth's climate system. The CERES broadband scanning radiometers are an improved version of the Earth Radiation Budget Experiment (ERBE) radiometers. The CERES instruments will fly on several National Aeronautics and Space Administration Earth Observing System (EOS) satellites starting in 1998 and extending over at least 15 years. The CERES science investigations will provide data to extend the ERBE climate record of top-of-atmosphere shortwave (SW) and longwave (LW) radiative fluxes CERES will also combine simultaneous cloud property data derived using EOS narrowband imagers to provide a consistent set of cloud/radiation data, including SW and LW radiative fluxes at the surface and at several selected levels within the atmosphere. CERES data are expected to provide top-of-atmosphere radiative fluxes with a factor of 2 to 3 less error than the ERBE data Estimates of radiative fluxes at the surface and especially within the atmosphere will be a much greater challenge but should also show significant improvements over current capabilities.

A generalized split-window algorithm for retrieving land-surface temperature from space

Abstract: Proposes a generalized split-window method for retrieving land-surface temperature (LST) from AVHRR and MODIS data. Accurate radiative transfer simulations show that the coefficients in the split-window algorithm for LST must vary with the viewing angle, if the authors are to achieve a LST accuracy of about 1 K for the whole scan swath range (/spl plusmn/55/spl deg/ from nadir) and for the ranges of surface temperature and atmospheric conditions over land, which are much wider than those over oceans. The authors obtain these coefficients from regression analysis of radiative transfer simulations, and they analyze sensitivity and error over wide ranges of surface temperature and emissivity and atmospheric water vapor abundance and temperature. Simulations show that when atmospheric water vapor increases and viewing angle is larger than 45/spl deg/, it is necessary to optimize the split-window method by separating the ranges of the atmospheric water vapor, lower boundary temperature, and the surface temperature into tractable subranges. The atmospheric lower boundary temperature and (vertical) column water vapor values retrieved from HIRS/2 or MODIS atmospheric sounding channels can be used to determine the range for the optimum coefficients of the split-window method. This new algorithm not only retrieves land-surface temperature more accurately, but is also less sensitive to uncertainty in emissivity and to instrument quantization error.

Studies with a flexible new radiation code. I: Choosing a configuration for a large-scale model

Abstract: A comprehensive new radiation code based on the two-stream equations in both the long-wave and short-wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well-suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line-by-line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations. The performance of the code is examined both at high spectral resolution and in a lower-resolution configuration designed for the UK Meteorological Office Unified Forecast/Climate Model (UM). Particularly for use in the UM, the code must be shown to perform satisfactorily across the whole range of atmospheric conditions. Comparisons are therefore made with reference calculations in both the long-wave and the short-wave, in clear and cloudy skies, and the accuracy with which various processes may be represented is studied. For the cloudy calculations in the short-wave, a new method is presented for deriving the single-scattering properties in broad bands, based on the analytic expression for the reflectivity of an optically thick cloud. This minimizes the errors in calculating the short-wave radiative properties of water clouds when the spectral resolution is reduced to that designed for the UM. In contrast, for ice clouds the errors are minimized by deriving the single-scattering properties using linear averaging, as appropriate for optically thin clouds. In the long-wave, the vertical distribution of the radiative heating in cirrus clouds is examined at high spectral resolution. The effect of scattering of long-wave radiation, usually ignored in large-scale models, is examined in some detail and is explained using a simple model. Taking all these studies into account, it is concluded that the configuration designed for the UM retains the generality of the code, without significantly compromising the overall accuracy.

Overview of Arctic Cloud and Radiation Characteristics

Abstract: To provide a background for ARM's activities at the North Slope of Alaska/Adjacent Arctic Ocean sites, an overview is given of our current state of knowledge of Arctic cloud and radiation properties and processes. The authors describe the Arctic temperature and humidity characteristics, cloud properties and processes, radiative characteristics of the atmosphere and surface, direct and indirect radiative effects of aerosols, and the modeling and satellite remote sensing of cloud and radiative characteristics. An assessment is given of the current performance of satellite remote sensing and climate modeling in the Arctic as related to cloud and radiation issues. Radiation-climate feedback processes are discussed, and estimates are made of the sign and magnitude of the individual feedback components. Future plans to address these issues are described.

Modeling of particle size distribution and its influence on the radiative properties of mineral dust aerosol

Abstract: The radiative parameters of mineral aerosols are strongly dependent on particle size. Therefore explicit modeling of particle size distribution is needed to calculate the radiative effects and the climate impact of mineral dust. We describe a parameterization of the global mineral aerosol size distribution in a transport model using eight size classes between 0.1 and 10 μm. The model prescribes the initial size distribution using soil texture data and aerosol size measurements close to the ground. During transport, the size distribution changes as larger particles settle out faster than smaller particles. Results of Mie scattering calculations of radiative parameters (extinction efficiency, single scattering albedo, asymmetry parameter) of mineral dust are shown at wavelengths between 0.3 and 30 μm for effective particle radii between 0.1 and 10 μm. Also included are radiative properties (reflection, absorption, transmission) calculated for a dust optical thickness of 0.1. Preliminary studies with the Goddard Institute for Space Studies (GISS) general circulation model (GCM), using two particle size modes, show regional changes in radiative flux at the top of the atmosphere as large as +15 W m -2 at solar and +5 W m -2 at thermal wavelengths in the annual mean, indicating that dust forcing is an important factor in the global radiation budget.

An Accurate Parameterization of the Infrared Radiative Properties of Cirrus Clouds for Climate Models

Abstract: An accurate parameterization is presented for the infrared radiative properties of cirrus clouds. For the singlescattering calculations, a composite scheme is developed for randomly oriented hexagonal ice crystals by comparing results from Mie theory, anomalous diffraction theory (ADT), the geometric optics method (GOM), and the finite-difference time domain technique. This scheme employs a linear combination of single-scattering properties from the Mie theory, ADT, and GOM, which is accurate for a wide range of size parameters. Following the approach of Q. Fu, the extinction coefficient, absorption coefficient, and asymmetry factor are parameterized as functions of the cloud ice water content and generalized effective size ( Dge). The present parameterization of the single-scattering properties of cirrus clouds is validated by examining the bulk radiative properties for a wide range of atmospheric conditions. Compared with reference results, the typical relative error in emissivity due to the parameterization is ;2.2%. The accuracy of this parameterization guarantees its reliability in applications to climate models. The present parameterization complements the scheme for the solar radiative properties of cirrus clouds developed by Q. Fu for use in numerical models.

Use of sky brightness measurements from ground for remote sensing of particulate polydispersions

Abstract: The software code SKYEAD.pack for retrieval of aerosol size distribution and optical thickness from data of direct and diffuse solar radiation is described; measurements are carried out with sky radiometers in the wavelength range 0.369-1.048 µm. The treatment of the radiative transfer problem concerning the optical quantities is mainly based on the IMS (improved multiple and single scattering) method, which uses the delta-M approximation for the truncation of the aerosol phase function and corrects the solution for the first- and second-order scattering. Both linear and nonlinear inversion methods can be used for retrieving the size distribution. Improved calibration methods for both direct and diffuse radiation, the data-analysis procedure, the results from the proposed code, and several connected problems are discussed. The results can be summarized as follows: (a) the SKYRAD.pack code can retrieve the columnar aerosol features with accuracy and efficiency in several environmental situations, provided the input parameters are correctly given; (b) when data of both direct and diffuse solar radiation are used, the detectable radius interval for aerosol particles is approximately from 0.03 to 10 µm; (c) besides the retrieval of the aerosol features, the data-analysis procedure also permits the determination of average values for three input parameters (real and imaginary aerosol refractive index, ground albedo) from the optical data; (d) absolute calibrations for the sky radiometer are not needed, and calibrations for direct and diffuse radiation can be carried out with field data; (e) the nonlinear inversion gives satisfactory results in a larger radius interval, without the unrealistic humps that occur with the linear inversion, but the results strongly depend on the first-guess spectrum; (f) aerosol features retrieved from simulated data showed a better agreement with the given data for the linear inversion than for the nonlinear inversion.

A new long‐wave formula for estimating downward clear‐sky radiation at the surface

Abstract: A new formula is proposed for estimating the long-wave radiation from clear skies. The formula depends on screen-level air temperature and water vapour pressure. The formula has been extensively tested using long-wave measurements covering a large range of environmental temperatures (-40 to +40 °C), and by using radiosonde profiles and an accurate radiative-transfer model (Lowtran-7). It is shown that the new formula out-performs five other widely used formulas.

The Two-Phase Pair Corona Model for Active Galactic Nuclei and X-Ray Binaries: How to Obtain Exact Solutions

Abstract: We consider two phase accretion disk-corona models for active galactic nuclei and some X-ray binaries. We describe in detail how one can exactly solve the polarized radiative transfer and Comptonization using the iterative scattering method, while simultaneously solving the energy and pair balance equation for both the cold and hot phases. We take into account Compton scattering, photon-photon pair production, pair annihilation, bremsstrahlung, and double Compton scattering, as well as exact reflection from the cold disk. We consider coronae having slab geometry as well as coronae consisting of one or more well separated active regions of cylinder or hemisphere geometry. The method is useful for determining the spectral intensity and the polarization emerging in different directions from disk-corona systems. The code is tested against a Monte-Carlo code. We also compare with earlier, less accurate, work. The method is more than an order of magnitude faster than applying Monte Carlo methods to the same problem and has the potential of being used in spectral fitting software such as XSPEC.

Ground-based near-infrared observations of the Venus nightside: The thermal structure and water abundance near the surface

Abstract: We used ground-based near-infrared (NIR) observations of thermal emission from the Venus nightside to determine the temperature structure and water vapor distribution between the surface and the 6-km level. We show that emission from spectral windows near 1.0, 1.1, and 1.18 μm originates primarily from the surface and lowest scale height (∼16 km). These windows include absorption by weak H 2 O and CO 2 lines and by the far wings of lines in strong nearby CO 2 bands. Rayleigh scattering by the 90-bar CO 2 atmosphere and Mie scattering by the H 2 SO 4 clouds attenuate this emission, but add little to its spectral dependence. Surface topography also modulates this NIR thermal emission because high-elevation regions are substantially cooler and emit less thermal radiation than the surrounding plains. These contributions to the emission are clearly resolved in moderate-resolution (λ/Δλ ∼ 400) spectral image cubes of the Venus nightside acquired with the infrared imaging spectrometer (IRIS) on the Anglo-Australian Telescope (AAT) in 1991. To analyze these observations, we used a radiative transfer model that includes all of the radiative processes listed above. Synthetic spectra for several topographic elevations were combined with Pioneer Venus altimetry data to generate spatially resolved maps of the NIR thermal emission. Comparisons between these synthetic radiance maps and the IRIS observations indicate no near-infrared signature of the surface emissivity differences seen at microwave wavelengths by the Magellan orbiter. Assuming constant surface emissivity in the near-infrared, we derive nightside averaged temperature lapse rates of -7 to -7.5 K/km in the lowest 6 km. These lapse rates are smaller and indicate much greater static stability than those inferred from earlier measurements and greenhouse models (-8 to -8.5 K/km) [Seiff 1983]. An acceptable fit to the data was obtained with an H 2 O mixing ratio profile which increases from 20 ppmv at the cloud base to 45 ppmv at 30 km, and then remains constant between that altitude and the surface. There is no evidence for H 2 O mixing ratios that decrease with altitude, like those inferred from the Pioneer Venus large probe mass spectrometer [Donahue and Hodges, 1992a] or the Venera 11 and 12 Lander spectrophotometers [Moroz, 1983].

Radiative Torques on Interstellar Grains: I. Superthermal Spinup

Abstract: Irregular dust grains are subject to radiative torques when irradiated by interstellar starlight It is shown how these radiative torques may be calculated using the discrete dipole approximation Calculations are carried out for one irregular grain geometry, and three different grain sizes It is shown that radiative torques can play an important dynamical role in spinup of interstellar dust grains, resulting in rotation rates which may exceed even those expected from H_2 formation on the grain surface Because the radiative torque on an interstellar grain is determined by the overall grain geometry rather than merely the state of the grain surface, the resulting superthermal rotation is expected to be long-lived By itself, long-lived superthermal rotation would permit grain alignment by normal paramagnetic dissipation on the "Davis-Greenstein" timescale However, radiative torques arising from anisotropy of the starlight background can act directly to alter the grain alignment on much shorter timescales, and are therefore central to the process of interstellar grain alignment Radiative torques depend strongly on the grain size, measured by a_eff, the radius of a sphere of equal volume In diffuse clouds, radiative torques dominate the torques due to H2 formation for a_eff=02micron grains, but are relatively unimportant for a_eff 01 micron grains in diffuse clouds are aligned, while there is little alignment of a_eff < 005 micron grains We show that radiative torques are ineffective at producing superthermal rotation within quiescent dark clouds, but can be very effective in star-forming regions such as the M17 molecular cloud

Evolutionary sequences of stellar models with new radiative opacities. VI. $Z=0.0001$

Abstract: We present a large grid of stellar evolutionary models with the initial chemical composition (Z = 0:0001;Y =0 :23). These tracks are conceived to extend the grid of stellar models described in the previous papers of this series, and are computed with the new radiative opacities by Iglesias et al. (1992) and convective overshoot. The tracks span the range of initial masses from 0:6 M to 100 M, and extend from the zero age main sequence (ZAMS) till very advanced evolutionary phases. Specically, low- and intermediate-mass stars are followed till the beginning of the thermally pulsing regime of the asymptotic red giant branch phase (TP-AGB), while massive stars are followed till the core C-ignition. With respect to previous papers of this series, these models incorporate a number of small modications in the input physics, particularly on the equation of state, which now incorporates the eect of Coulomb interactions between charged particles. The eect of these modications is discussed. The corresponding theoretical isochrones are presented.

The Two-Phase Pair Corona Model for Active Galactic Nuclei and X-ray Binaries: How to Obtain Exact Solutions

Abstract: We consider two phase accretion disk-corona models for active galactic nuclei and some X-ray binaries. We describe in detail how one can exactly solve the polarized radiative transfer and Comptonization using the iterative scattering method, while simultaneously solving the energy and pair balance equation for both the cold and hot phases. We take into account Compton scattering, photon-photon pair production, pair annihilation, bremsstrahlung, and double Compton scattering, as well as exact reflection from the cold disk. We consider coronae having slab geometry as well as coronae consisting of one or more well separated active regions of cylinder or hemisphere geometry. The method is useful for determining the spectral intensity and the polarization emerging in different directions from disk-corona systems. The code is tested against a Monte-Carlo code. We also compare with earlier, less accurate, work. The method is more than an order of magnitude faster than applying Monte Carlo methods to the same problem and has the potential of being used in spectral fitting software such as XSPEC.

A Simple Model of Spectral-Line Profiles from Contracting Clouds

Abstract: A simple analytic model of radiative transfer in two parts of a contracting cloud matches a wide range of line profiles in candidate infall regions and provides a sensitive estimate of Vin, the characteristic inward speed of the gas forming the line. The model assumes two uniform regions of equal temperature and velocity dispersion σ, whose density and velocity are attenuation-weighted means over the front and rear halves of a centrally condensed, contracting cloud. The model predicts two-peak profiles for "slow" infall, Vin σ, and red-shoulder profiles for "fast" infall, Vin ~ σ. A simple formula expresses Vin solely in terms of σ and of observable parameters of a two-peak line. We apply the model to fit profiles of high and low optical depth lines observed in a dense core with no star (L1544, Vin = 0.006 km s-1), with an isolated protostar (L1527, 0.025 km s-1), and with a small group of stars (L1251B, 0.35 km s-1). The mass infall rate obtained from Vin and the map size varies from (2-40) × 10-6 M☉ yr-1 and agrees within a factor ~2 in each core with the independently determined rate ~σ3 G-1 for a gravitationally collapsing isothermal sphere. This agreement suggests that the inward motions derived from the line profiles are gravitational in origin.

The Twelve-Line 1.682 eV Luminescence Center in Diamond and the Vacancy-Silicon Complex

Abstract: Ab initio cluster methods are used to investigate vacancy-impurity complexes in diamond. We assign the 1.682 eV, twelve-line optical band to a vacancy-Si complex which has a very unusual, possibly unique structure with a Si atom at the center of a split vacancy. The method also successfully accounts for the 1.945, 2.156, and 2.985 eV optical transitions in trigonal vacancy-N defects and estimates of radiative lifetimes are given.

Cooling Timescales and Temporal Structure of Gamma-Ray Bursts

Abstract: A leading mechanism for producing cosmological gamma-ray bursts (GRBs) is via ultrarelativistic particles in an expanding fireball. The kinetic energy of the particles is converted into thermal energy in two shocks, a forward shock and a reverse shock, when the outward flowing particles encounter the interstellar medium. The thermal energy is then radiated via synchrotron emission and Comptonization. We estimate the synchrotron cooling timescale of the shocked material in the forward and reverse shocks for electrons of various Lorentz factors, focusing in particular on those electrons whose radiation falls within the energy detection range of the BATSE detectors. We find that in order to produce the rapid variability observed in most bursts, the energy density of the magnetic field in the shocked material must be greater than about 1% of the thermal energy density. In addition, the electrons must be nearly in equipartition with the protons, since otherwise we do not have reasonable radiative efficiencies of GRBs. Inverse Compton scattering can increase the cooling rate of the relevant electrons, but the Comptonized emission itself is never within the BATSE range. These arguments allow us to pinpoint the conditions within the radiating regions in GRBs and to determine the important radiation processes. In addition, they provide a plausible explanation for several observations. The model predicts that the duty cycle of intensity variations in GRB light curves should be nearly independent of burst duration and should scale inversely as the square root of the observed photon energy. Both correlations are in agreement with observations. The model also provides a plausible explanation for the bimodal distribution of burst durations. There is no explanation, however, for the presence of a characteristic break energy in GRB spectra.

Faraday Rotation of Microwave Background Polarization by a Primordial Magnetic Field

Abstract: The existence of a primordial magnetic field at the last scattering surface may induce a measurable Faraday rotation in the polarization of the cosmic microwave background. We calculate the magnitude of this effect by evolving the radiative transfer equations for the microwave background polarization through the epoch of last scatter, in the presence of a magnetic field. For a primordial field amplitude corresponding to a present value of $10^{-9}{\rm G}$ (which would account for the observed galactic field if it were frozen in the pre-galactic plasma), we find a rotation angle of around $1^\circ$ at a frequency of 30 GHz. The statistical detection of this signal is feasible with future maps of the microwave background.

Mechanisms of Cloud-Radiation Interaction in the Tropics and Midlatitudes

Abstract: Radiative forcing and latent heat associated with precipitation are the two most important diabatic processes that drive the circulation of the atmosphere. Clouds can affect radiation and vice versa. It is known that longwave radiative processes can enhance precipitation in cloud systems. This paper concentrates on determining the relative importance of three specific longwave radiative mechanisms by comparing cloud-resolving models with and without one or more of these processes. Three of the ways that longwave radiation is thought to interact with clouds are as follows: 1) cloud-top cooling and cloud-base warming may alter the thermal stratification of cloud layers, 2) differential cooling between clear and cloudy regions might enhance convergence into the cloud system, and 3) large-scale cooling could change the environment. A two-dimensional version of the Goddard Cumulus Ensemble model has been used to perform a series of sensitivity tests to identify which is the dominant cloud-radiative fo...

Display of clouds taking into account multiple anisotropic scattering and sky light

Abstract: Methods to display realistic clouds are proposed To display realistic images, a precise shading model is required: two components should be considered One is multiple scattering due to particles in clouds, and the other factor to be considered is sky light For the former, the calculation of cloud intensities has been assumed to be complex due to strong forward scattering However, this paper proposes an efficient calculation method using these scattering characteristics in a positive way The latter is a very significant factor when sky light is rather stronger than direct sunlight, such as at sunset/sunrise, even though sky light has been ignored in previous methods This paper describes an efficient calculation method for light scattering due to clouds taking into account both multiple scattering and sky light, and the modeling of clouds CR Categories and Subject Descriptors:

Optical emission in periodic dielectrics

Abstract: We show rigorously that the coefficient for spontaneous emission of an atom placed in a dielectric is proportional to the local radiative density of states—that is only a part of the local density of the eigenmodes of the Maxwell equations. Spontaneous emission is inhibited if the atom is located at a position where this local radiative density is small, even if the total density of states is not vanishing. This radiative density of states can be obtained without having to perform a full quantum calculation of the radiation-matter system. We demonstrate this principle by solving numerically a scalar model for a dielectric that consists of a lattice of resonating dipoles.

Henyey–Greenstein and Mie phase functions in Monte Carlo radiative transfer computations

Abstract: Monte Carlo radiative transfer simulation of light scattering in planetary atmospheres is not a simple problem, especially the study of angular distribution of light intensity. Approximate phase functions such as Henyey-Greenstein, modified Henyey-Greenstein, or Legendre polynomial decomposition are often used to simulate the Mie phase function. An alternative solution using an exact calculation alleviates these approximations.

Absorption/Scattering Coefficients and Scattering Phase Functions in Reticulated Porous Ceramics

Abstract: Spectral absorption and scattering coefficients and spectral scattering phase functions have been derived for partially stabilized zirconia (PS ZrO 2 ) and oxide-bonded silicon carbide (OB SiC) reticulated porous ceramics (RPCs) across the wavelength range 0.4-5.0 μm. These spectral radiative properties were investigated and quantified for 10 ppi (pores/inch), 20 ppi, and 65 ppi materials. Radiative properties were recovered from spectral hemispherical reflectance and transmittance measurements using inverse analysis techniques based upon discrete ordinates radiative models. Two dual-parameter phase functions were investigated for these materials : one based on the physical structure of reticulated porous ceramics and the other a modified Henyey-Greenstein phase function. The modified Henyey- Greenstein phase function provided the most consistent spectral radiative properties. PS ZrO 2 radiative properties exhibited strongly spectrally dependent behavior across the wavelength range studied. OB SiC radiative properties exhibited radiative behavior that was relatively independent of wavelength across the wavelength spectrum studied. OB SiC also demonstrated consistently higher absorption coefficients than PS ZrO 2 at all wavelengths. Spectral scattering albedos of PS ZrO 2 were discovered to be in the range 0.81-0.999 and increased as ppi rating increased, while those for OB SiC were lower in the range 0.55-0.888 and decreased as ppi rating increased. The average extinction efficiencies for 0.4-5.0 μm were discovered to be 1.45 for PS ZrO 2 and 1.70 for OB SiC. Extinction coefficients were discovered to correlate well with geometric optics theoretical models and electromagnetic wave/fiber interaction models based on independent scattering and absorption mechanisms.

Fundamental optical transitions in GaN

Abstract: A coherent picture for the band structure near the Γ point and the associated fundamental optical transitions in wurtzite (WZ) GaN, including the electron and hole effective masses and the binding energies of the free excitons associated with different valence bands, has been derived from time‐resolved photoluminescence measurements and a theoretical calculation based on the local density approximation. We also determine the radiative recombination lifetimes of the free excitons and neutral impurity (donor and acceptor) bound excitons in WZ GaN and compare ratios of the radiative lifetimes with calculated values of the ratios obtained with existing theories of free and bound excitons.

Modeling cirrus clouds. Part II: Treatment of radiative properties

Abstract: A new radiation scheme, suitable for two-stream radiation transfer models, was developed for cirrus clouds. Analytical expressions were derived for the extinction and absorption coefficients and the asymmetry parameter. These are functions of the ice particle size distribution parameters, ice particle shapes, and wavelength. The ice particle shapes considered were hexagonal plates and columns, bullet rosettes, and planar polycrystals. These appear to be the principal crystal types found in cirrus clouds. The formulation of radiative properties accounts for the size distribution projected area and the distance radiation travels through ice particles. For absorption, refraction and internal reflection of radiation were parameterized. By assuming an idealized cirrus cloud, the dependence of the single scatter albedo, reflectance, and emissivity on wavelength, ice particle shape, and size distribution was demonstrated. Reflectance and emissivity exhibited a strong dependence on ice particle shape, wi...