Showing papers on "Radiative transfer published in 2001"

Strong radiative heating due to the mixing state of black carbon in atmospheric aerosols

Abstract: Aerosols affect the Earth's temperature and climate by altering the radiative properties of the atmosphere. A large positive component of this radiative forcing from aerosols is due to black carbon--soot--that is released from the burning of fossil fuel and biomass, and, to a lesser extent, natural fires, but the exact forcing is affected by how black carbon is mixed with other aerosol constituents. From studies of aerosol radiative forcing, it is known that black carbon can exist in one of several possible mixing states; distinct from other aerosol particles (externally mixed) or incorporated within them (internally mixed), or a black-carbon core could be surrounded by a well mixed shell. But so far it has been assumed that aerosols exist predominantly as an external mixture. Here I simulate the evolution of the chemical composition of aerosols, finding that the mixing state and direct forcing of the black-carbon component approach those of an internal mixture, largely due to coagulation and growth of aerosol particles. This finding implies a higher positive forcing from black carbon than previously thought, suggesting that the warming effect from black carbon may nearly balance the net cooling effect of other anthropogenic aerosol constituents. The magnitude of the direct radiative forcing from black carbon itself exceeds that due to CH4, suggesting that black carbon may be the second most important component of global warming after CO2 in terms of direct forcing.

A methodology for surface soil moisture and vegetation optical depth retrieval using the microwave polarization difference index

Abstract: A methodology for retrieving surface soil moisture and vegetation optical depth from satellite microwave radiometer data is presented. The procedure is tested with historical 6.6 GHz H and V polarized brightness temperature observations from the scanning multichannel microwave radiometer (SMMR) over several test sites in Illinois. Results using only nighttime data are presented at this time due to the greater stability of nighttime surface temperature estimation. The methodology uses a radiative transfer model to solve for surface soil moisture and vegetation optical depth simultaneously using a nonlinear iterative optimization procedure. It assumes known constant values for the scattering albedo and roughness, and that vegetation optical depth for H-polarization is the same as for V-polarization. Surface temperature is derived by a procedure using high frequency V-polarized brightness temperatures. The methodology does not require any field observations of soil moisture or canopy biophysical properties for calibration purposes and may be applied to other wavelengths. Results compare well with field observations of soil moisture and satellite-derived vegetation index data from optical sensors.

Infrared Emission from Interstellar Dust. I. Stochastic Heating of Small Grains

Abstract: We present a method for calculating the infrared emission from a population of dust grains heated by starlight, including very small grains for which stochastic heating by starlight photons results in high- temperature transients. Because state-to-state transition rates are generally unavailable for complex mol ecules, we consider model polycyclic aromatic hydrocarbon (PAH), graphitic, and silicate grains with realistic vibrational mode spectra and realistic radiative properties. The vibrational density of states is used in a statistical-mechanical description of the emission process. Unlike previous treatments, our approach fully incorporates multiphoton heating effects, important for large grains or strong radiation fields. We discuss how the "temperature" of the grain is related to its vibrational energy. By comparing with an "exact" statistical calculation of the emission process, we determine the conditions under which the "thermal" and the "continuous cooling" approximations can be used to calculate the emission spec trum. We present results for the infrared emission spectra of PAH grains of various sizes heated by star light. We show how the relative strengths of the 6.2, 7.7, and 11.3 μm features depend on grain size, starlight spectrum and intensity, and grain charging conditions. We show results for grains in the "cold neutral medium" and "warm ionized medium" and representative conditions in photodissociation regions. Our model results are compared to observed ratios of emission features for the Milky Way and other galaxies and for the M17 and NGC 7023 photodissociation regions.

Scaling-up and model inversion methods with narrowband optical indices for chlorophyll content estimation in closed forest canopies with hyperspectral data

Abstract: Radiative transfer theory and modeling assumptions were applied at laboratory and field scales in order to study the link between leaf reflectance and transmittance and canopy hyper-spectral data for chlorophyll content estimation. This study was focused on 12 sites of Acer saccharum M. (sugar maple) in the Algoma Region, Canada, where field measurements, laboratory-simulation experiments, and hyper-spectral compact airborne spectrographic imager (CASI) imagery of 72 channels in the visible and near-infrared region and up to 1-m spatial resolution data were acquired in the 1997, 1998, and 1999 campaigns. A different set of 14 sites of the same species were used in 2000 for validation of methodologies. Infinite reflectance and canopy reflectance models were used to link leaf to canopy levels through radiative transfer simulation. The closed and dense (LAI>4) forest canopies of Acer saccharum M. used for this study, and the high spatial resolution reflectance data targeting crowns, allowed the use of optically thick simulation formulae and turbid-medium SAILH and MCRM canopy reflectance models for chlorophyll content estimation by scaling-up and by numerical model inversion approaches through coupling to the PROSPECT leaf radiative transfer model. Study of the merit function in the numerical inversion showed that red edge optical indices used in the minimizing function such as R/sub 750//R/sub 710/ perform better than when all single spectral reflectance channels from hyper-spectral airborne CASI data are used, and in addition, the effect of shadows and LAI variation are minimized.

Introduction to special section: Outstanding problems in quantifying the radiative impacts of mineral dust

Abstract: This paper provides an introduction to the special section of the Journal of Geophysical Research on mineral dust. We briefly review the current experimental and theoretical approaches used to quantify the dust radiative impacts, highlight the outstanding issues, and discuss possible strategies to overcome the emerging problems. We also introduce the contributing papers of this special section. Despite the recent notable advances in dust studies, we demonstrate that the radiative effects of dust remain poorly quantified due to both limited data and incomplete understanding of relative physical and chemical processes. The foremost needs are (1) to quantify the spatial and temporal variations of dust burden in the atmosphere and develop a predictive capability for the size- and composition-resolved dust particle distribution; (2) to develop a quantitative description of the processes that control the spatial and temporal variabilities of dust physical and chemical properties and radiative effects; (3) to develop new instrumentation (especially to measure the dust particle size distribution in a wide range from about 0.01 μm to 100 μm, scattering phase function and light absorption by dust particles); and (4) to develop new techniques for interpreting and merging the diverse information from satellite remote sensing, in situ and ground-based measurements, laboratory studies, and model simulations. Because dust distribution and effects are heterogeneous, both spatially and temporally, a promising strategy to advance our knowledge is to perform comprehensive studies at the targeted regions affected by mineral dust of both natural and anthropogenic origin.

Radiation-driven winds of hot luminous stars - XIII. A description of NLTE line blocking and blanketing towards realistic models for expanding atmospheres

Abstract: Spectral analysis of hot luminous stars requires adequate model atmospheres which take into account the effects of NLTE and radiation driven winds properly. Here we present significant improvements of our approach in constructing detailed atmospheric models and synthetic spectra for hot luminous stars. Moreover, as we regard our solution method in its present stage already as a standard procedure, we make our program package WM- basic available to the community (download is possible from the URL given below). The most important model improvements towards a realistic description of stationary wind models concern: (i) A sophisticated and consistent description of line blocking and blanketing. Our solution concept to this problem renders the line blocking influence on the ionizing fluxes emerging from the atmospheres of hot stars - mainly the spectral ranges of the EUV and the UV are affected - in identical quality as the synthetic high resolution spectra representing the observable region. In addition, the line blanketing effect is properly accounted for in the energy balance. (ii) The atomic data archive which has been improved and enhanced considerably, providing the basis for a detailed multilevel NLTE treatment of the metal ions (from C to Zn) and an adequate representation of line blocking and the radiative line acceleration. (iii) A revised inclusion of EUV and X-ray radiation produced by cooling zones which originate from the simu- lation of shock heated matter. This new tool not only provides an easy-to-use method for O-star diagnostics, whereby physical constraints on the properties of stellar winds, stellar parameters, and abundances can be obtained via a comparison of observed and synthetic spectra, but also allows the astrophysically important information about the ionizing fluxes of hot stars to be determined automatically. Results illustrating this are discussed by means of a basic model grid calculated for O-stars with solar metallicity. To further demonstrate the astrophysical potential of our new method, we first provide a detailed spectral diagnostic determination of the stellar parameters, the wind parameters, and the abundances by an exemplary application to one of our grid-stars, the O9.5Ia O-supergiant α Cam. Our abundance determinations of the light elements indicate that these deviate considerably from the solar values.

Multilevel Radiative Transfer with Partial Frequency Redistribution

Abstract: A multilevel accelerated lambda iteration (MALI) method for radiative transfer calculations with partial frequency redistribution (PRD) is presented. The method, which is based on Rybicki & Hummer's complete frequency redistribution (CRD) formalism with full preconditioning, consistently accounts for overlapping radiative transitions. Its extension to PRD is implemented in a very natural way through the use of the Ψ operator operating on the emissivity rather than the commonly used Λ operator, which operates on the source function. Apart from requiring an additional inner computational loop to evaluate the PRD emission-line profiles with fixed population numbers, implementation of the presented method requires only a trivial addition of computer code. Since the presented method employs a diagonal operator, it is easily extended to different geometries. Currently, it has been implemented for one-, two-, and three-dimensional Cartesian grids and spherical symmetry. In all cases, the speed of convergence with PRD is very similar to that in CRD, with the former sometimes even surpassing the latter. Sample calculations exhibiting the favorable convergence behavior of the PRD code are presented in the case of the Ca II H and K lines, the Mg II h and k lines, and the hydrogen Lyα and Lyβ lines in a one-dimensional solar model and the Ca II resonance lines in a two-dimensional flux-sheet model.

HULLAC, an integrated computer package for atomic processes in plasmas

Abstract: We describe H ULLAC , an integrated code for calculating atomic structure and cross sections for collisional and radiative atomic processes. This code evolved and has been used over the years, but so far, there was no coherent, comprehensive, and in-depth presentation of it. It is based on relativistic quantum mechanical calculations including configuration interaction. The collisional cross sections are calculated in the distorted wave approximation. The theory and code are presented, emphasizing the various novel methods that has been developed to obtain accurate results very efficiently. In particular we describe the parametric potential method used for both bound and free orbitals, the factorization–interpolation method applied in the derivation of collisional rates, the phase amplitude approach for calculating the continuum orbitals and the N JGRAF graphical method used in the calculation of the angular momentum part of the matrix elements. Special effort has been made to insure the simplicity of use, which is demonstrated in an example.

Emission-Line Diagnostics of T Tauri Magnetospheric Accretion. II. Improved Model Tests and Insights into Accretion Physics

Abstract: We present new radiative transfer models of magnetospheric accretion in T Tauri stars. Hydrogen and Na I line profiles were calculated, including line damping and continuum opacity for a grid of models spanning a large range of infall rates, magnetospheric geometries, and gas temperatures. We also calculated models for rotating magnetospheres and show that for typical T Tauri rotation rates, the line profiles are not significantly affected. We show that line-damping wings can produce significant high-velocity emission at Hα, and to a lesser extent in higher Balmer lines, in much better agreement with observations than previous models. We present comparisons to specific objects spanning a wide range of accretion activity and find that in most cases the models successfully reproduce the observed emission profile features. Blueshifted absorption components cannot be explained without including a wind outside of the magnetosphere, and true P Cygni Balmer line profiles in the few objects with extreme accretion activity indicate both absorption and emission from a wind. We constrain the range of gas temperatures required to explain observational diagnostics like profile shapes, line ratios, and continuum emission. The exact heating mechanism remains unclear but is probably linked to the accretion process itself. In order to explain observed correlations between line emission and accretion luminosity, we find that the size of the emitting region must be correlated with the accretion rate. We suggest that such a correlation may manifest itself in reality via nonaxisymmetric accretion, where the number and/or width of discrete funnel flows increase with increasing accretion rate, a scenario also indicated by accretion shock models.

Radiative equilibrium and temperature correction in monte carlo radiation transfer

Abstract: We describe a general radiative equilibrium and temperature correction procedure for use in Monte Carlo radiation transfer codes with sources of temperature-independent opacity, such as astrophysical dust. The technique utilizes the fact that Monte Carlo simulations track individual photon packets, so we may easily determine where their energy is absorbed. When a packet is absorbed, it heats a particular cell within the envelope, raising its temperature. To enforce radiative equilibrium, the absorbed packet is immediately reemitted. To correct the cell temperature, the frequency of the reemitted packet is chosen so that it corrects the temperature of the spectrum previously emitted by the cell. The reemitted packet then continues being scattered, absorbed, and reemitted until it finally escapes from the envelope. As the simulation runs, the envelope heats up, and the emergent spectral energy distribution (SED) relaxes to its equilibrium value without iteration. This implies that the equilibrium temperature calculation requires no more computation time than the SED calculation of an equivalent pure scattering model with fixed temperature. In addition to avoiding iteration, our method conserves energy exactly because all injected photon packets eventually escape. Furthermore, individual packets transport energy across the entire system because they are never destroyed. This long-range communication, coupled with the lack of iteration, implies that our method does not suffer the convergence problems commonly associated with Λ-iteration. To verify our temperature correction procedure, we compare our results with standard benchmark tests, and finally we present the results of simulations for two-dimensional axisymmetric density structures.

Simulations of Pregalactic Structure Formation with Radiative Feedback

Abstract: We present results from three-dimensional hydrodynamic simulations of the high-redshift collapse of pregalactic clouds including feedback effects from a soft H2 photodissociating UV radiation field. The simulations use an Eulerian adaptive mesh refinement technique to follow the nonequilibrium chemistry of nine chemical species with cosmological initial conditions drawn from a popular Λ-dominated cold dark matter model. The results confirm that the soft UV background can delay the cooling and collapse of small halos (~106 M☉). For reasonable values of the photodissociating flux, the H2 fraction is in equilibrium throughout most of the objects we simulate. We determine the mass threshold for collapse for a range of soft-UV fluxes and also derive a simple analytic expression. Continuing the simulations beyond the point of initial collapse demonstrates that the fraction of gas which can cool depends mostly on the virial mass of the halo and the amount of soft-UV flux, with remarkably little scatter. We parameterize this relation, for use in semianalytic models.

Experimental Radiative Lifetimes, Branching Fractions, and Oscillator Strengths for La II and a New Determination of the Solar Lanthanum Abundance

Abstract: Radiative lifetimes, accurate in most cases to ±5%, from time-resolved laser-induced fluorescence measurements on a slow beam of lanthanum ions are reported for 31 odd-parity levels of La II. Experimental branching fractions for La II from emission spectra covering the near-ultraviolet to the near-infrared are also reported. The spectra were recorded using the US National Solar Observatory 1.0 m Fourier transform spectrometer. The branching fractions are combined with the radiative lifetimes to produce 84 experimentally determined transition probabilities or oscillator strengths, generally accurate to ±10%, for La II. These new experimental results are compared to older experimental and theoretical results. These data are applied to determine a new value for the solar photospheric lanthanum abundance, (La) = 1.13, with estimated internal errors of ±0.03 and external errors of ±0.03.

The Giant Flare of 1998 August 27 from SGR 1900+14. II. Radiative Mechanism and Physical Constraints on the Source

Abstract: The extraordinary 1998 August 27 giant flare places strong constraints on the physical properties of its source, SGR 1900+14. We make detailed comparisons of the published data with the magnetar model, which identifies the soft gamma repeaters as neutron stars endowed with ~1015 G magnetic fields. The giant flare evolved through three stages, whose radiative mechanisms we address in turn. The extreme peak luminosity L > 106LEdd, hard spectrum, and rapid variability of the initial ~0.5 s spike emission all point to an expanding pair fireball with very low baryon contamination. We argue that this energy must have been deposited directly through shearing and reconnection of a magnetar-strength external magnetic field. Low-order torsional oscillations of the star fail to transmit energy rapidly enough to the exterior, if the surface field is much weaker. A triggering mechanism is proposed, whereby a helical distortion of the core magnetic field induces large-scale fracturing in the crust and a twisting deformation of the crust and exterior magnetic field. After the initial spike (whose ~0.4 s duration can be related to the Alfven crossing time of the core), very hot (T 1 MeV) plasma rich in electron-positron pairs remains confined close to the star on closed magnetic field lines. The envelope of the August 27 flare can be accurately fitted, after ~40 s, by the contracting surface of such a "trapped fireball." The form of this fit gives evidence that the temperature of the trapped pair plasma decreases outward from its center. We quantify the effects of direct neutrino pair emission on the X-ray light curve, thereby deducing a lower bound μmin ~ 1 × 1032 G cm3 to the magnetic moment of the confining field, comparable to the strongest fields measured in radio pulsars. The radiative flux during the intermediate ~40 s of the burst appears to exceed the trapped fireball fit. The lack of strong rotational modulation and intermediate hardness of this smooth tail are consistent with the emission from an extended pair corona, in which O-mode photons are heated by Compton scattering. This feature could represent residual seismic activity within the star and accounts for ~10% of the total flare fluence. We consider in detail the critical luminosity, below which a stable balance can be maintained between heating and radiative cooling in a confined, magnetized pair plasma, but above which the confined plasma runs away to a trapped fireball in LTE. The emergence of large-amplitude pulsations at ~40 s probably represents a transition to a pair-depleted photosphere whose main source of opacity is electrons (and ions) ablated from the heated neutron star surface. The best-fit temperature of the blackbody component of the spectrum equilibrates at a value that agrees well with the regulating effect of photon splitting. The remarkable four-peaked substructure within each 5.16 s pulse, as well as the corresponding collimation of the X-ray flux, has a simple explanation based on the strong inequality between the scattering cross sections of the two photon polarization modes. The width of each X-ray "jet" is directly related to the amount of matter advected outward by the high cross section ordinary mode.

Spectral Energy Distributions of Passive T Tauri and Herbig Ae Disks: Grain Mineralogy, Parameter Dependences, and Comparison with Infrared Space Observatory LWS Observations

Abstract: We improve upon the radiative, hydrostatic equilibrium models of passive circumstellar disks constructed by Chiang & Goldreich. New features include (1) an account for a range of particle sizes, (2) employment of laboratory-based optical constants of representative grain materials, and (3) numerical solution of the equations of radiative and hydrostatic equilibrium within the original two-layer (disk surface plus disk interior) approximation. We systematically explore how the spectral energy distribution (SED) of a face-on disk depends on grain size distributions, disk geometries and surface densities, and stellar photospheric temperatures. Observed SEDs of three Herbig Ae and two T Tauri stars, including spectra from the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO), are fitted with our models. Silicate emission bands from optically thin, superheated disk surface layers appear in nearly all systems. Water ice emission bands appear in LWS spectra of two of the coolest stars. Infrared excesses in several sources are consistent with significant vertical settling of photospheric grains. While this work furnishes further evidence that passive reprocessing of starlight by flared disks adequately explains the origin of infrared-to-millimeter wavelength excesses of young stars, we emphasize by explicit calculations how the SED alone does not provide sufficient information to constrain particle sizes and disk masses uniquely.

Air mass factor formulation for spectroscopic measurements from satellites: Application to formaldehyde retrievals from the Global Ozone Monitoring Experiment

Abstract: We present a new formulation for the air mass factor (AMF) to convert slant column measurements of optically thin atmospheric species from space into total vertical columns. Because of atmospheric scattering, the AMF depends on the vertical distribution of the species. We formulate the AMF as the integral of the relative vertical distribution (shape factor) of the species over the depth of the atmosphere, weighted by altitude-dependent coefficients (scattering weights) computed independently from a radiative transfer model. The scattering weights are readily tabulated, and one can then obtain the AMF for any observation scene by using shape factors from a three dimensional (3-D) atmospheric chemistry model for the period of observation. This approach subsequently allows objective evaluation of the 3-D model with the observed vertical columns, since the shape factor and the vertical column in the model represent two independent pieces of information. We demonstrate the AMF method by using slant column measurements of formaldehyde at 346 nm from the Global Ozone Monitoring Experiment satellite instrument over North America during July 1996. Shape factors are cumputed with the Global Earth Observing System CHEMistry (GEOS-CHEM) global 3-D model and are checked for consistency with the few available aircraft measurements. Scattering weights increase by an order of magnitude from the surface to the upper troposphere. The AMFs are typically 20-40% less over continents than over the oceans and are approximately half the values calculated in the absence of scattering. Model-induced errors in the AMF are estimated to be approximately 10%. The GEOS-CHEM model captures 50% and 60% of the variances in the observed slant and vertical columns, respectively. Comparison of the simulated and observed vertical columns allows assessment of model bias.

Modeling the atmospheric life cycle and radiative impact of mineral dust in the Hadley Centre climate model

Abstract: A parameterization of mineral dust within the Hadley Centre atmospheric general circulation model (AGCM) is described, modeled dust distributions are compared with observations, and estimates of the radiative forcing due to the inclusion of dust in the model are obtained. The parameterization uses six particle size divisions in the range 0.3–30 μm radius, and all calculations are performed on each division independently, using the GCM's prognostic variables. The dust production scheme works within the GCM and includes dependencies on particle size distribution, soil moisture, vegetation, and friction velocity. Dust transport is carried out by the GCM's tracer advection scheme and includes vertical motion due to convection, gravitational settling, and turbulent mixing in the boundary layer. Wet and dry deposition processes are included within the GCM's precipitation schemes. Representative dust radiative parameters are incorporated into the GCM's two stream radiation code. Modeled monthly average near-surface dust concentrations are compared with observations: good agreement is seen in most locations, though the dust scheme tends to produce too little dust from China and too much from Australia in the southern spring. Global annual mean direct forcing due to the inclusion of dust in the GCM is +0.07 W m−2 at the top of the atmosphere (TOA) and −0.82 W m−2 at the surface. The geographical distributions of annual mean forcings are very inhomogeneous, with peak values exceeding the global means by a factor of approximately 2 orders of magnitude.

The heat balance of the tropical tropopause, cirrus, and stratospheric dehydration

Abstract: If tropical tropopause cirrus lie above convective anvils with tops above about 13km, then net radiative cooling from the cirrus can be produced that is large enough to offset significant subsidence heating, even at the lowest temperatures observed in the tropics. Cirrus clouds near the tropopause are strongly heated by radiation unless they lie above convective anvil clouds. Radiative relaxation in the tropical troposphere is slow above about 14km unless clouds are present. Radiative cooling of tropopause cirrus may be important in processes that dehydrate air before it enters the stratosphere.

Line ratios for helium-like ions: Applications to collision-dominated plasmas ?

Abstract: The line ratios R and G of the three main lines of He-like ions (triplet: resonance, intercombination, forbidden lines) are calculated for C v ,N vi ,O vii ,N eix ,M gxi, and Si xiii. These ratios can be used to derive electron density ne and temperatureTe of hot late-type stellar coronae and O, B stars from high-resolution spectra obtained with Chandra (LETGS, HETGS) and XMM-Newton (RGS). All excitation and radiative processes between the levels and the eect of upper-level cascades from collisional electronic excitation and from dielectronic and radiative recombination have been considered. When possible the best experimental values for radiative transition probabilities are used. For the higher-Z ions (i.e. Ne ix ,M gxi ,S ixiii) possible contributions from blended dielectronic satellite lines to each line of the triplets were included in the calculations of the line ratios R and G for four specic spectral resolutions: RGS, LETGS, HETGS-MEG, HETGS-HEG. The influence of an external stellar radiation eld on the coupling of the 2 3 S (upper level of the forbidden line) and 2 3 P levels (upper levels of the intercombination lines) is taken into account. This process is mainly important for the lower-Z ions (i.e. C v ,N vi ,O vii) at moderate radiation temperature (Trad). These improved calculations were done for plasmas in collisional ionization equilibrium, but will be later extended to photo-ionized plasmas and to transient ionization plasmas. The values for R and G are given in extensive tables, for a large range of parameters, which could be used directly to compare to the observations.

A linearized discrete ordinate radiative transfer model for atmospheric remote-sensing retrieval

Abstract: The radiative transfer forward model simulation of intensities and associated parameter derivatives (weighting functions) is a vital part of the retrieval of earth atmospheric constituent information from measurements of backscattered light. The discrete ordinate method is the most commonly used approach for the determination of solutions to the radiative transfer equation. In this paper, we carry out an internal perturbation analysis of the complete discrete ordinate solution in a plane parallel multi-layered multiple-scattering atmosphere. Perturbations in layer atmospheric quantities will translate into small changes in the single-scatter albedos and optical depth values. In addition, we consider perturbations in layer thermal emission source terms and in the surface albedo. It is shown that the solution of the boundary value problem for the perturbed intensity "eld leads in a natural way to the weighting function associated with the parameter causing the perturbation. We have developed a numerical model LIDORT (linearized discrete ordinate radiative transfer) for the simultaneous generation of backscatter intensities and weighting function output at arbitrary elevation angles, for a user-de"ned set of atmospheric variations. Results for a 5-layer test atmosphere with two scatterers and thermal emission terms are shown. Intensities are validated against benchmark discrete ordinate results, while weighting functions are checked for consistency against "nite di!erence results based on external perturbations. A second example is presented for a 60-layer terrestrial atmosphere with molecular and aerosol scattering and ozone trace gas absorption in the UV spectral range; weighting functions are shown to correspond closely with results derived from another radiative transfer model. Published by Elsevier Science Ltd.

Evaluation of aerosol indirect radiative forcing in MIRAGE

Abstract: A variety of measurements have been used to evaluate the treatment of aerosol radiative properties and radiative impacts of aerosols simulated by the Model for Integrated Research on Atmospheric Global Exchange (MIRAGE). The treatment of water uptake in MIRAGE agrees with laboratory measurements, and the growth of aerosol extinction with relative humidity in MIRAGE simulations agrees with field measurements. The simulated frequency of relative humidity near 100% is about twice that of analyzed relative humidity. When the analyzed relative humidity is used to calculate aerosol water uptake in MIRAGE, the simulated aerosol optical depth agrees with most surface measurements after cloudy conditions are filtered out and differences between model and station elevations are accounted for, but simulated optical depths are too low over Brazil and central Canada. Simulated optical depths are mostly within a factor of 2 of satellite estimates, but are too high off the east coasts of the United States and China and too low off the coast of West Africa and in the Arabian Sea. The simulated single-scatter albedo is consistent with surface measurements. MIRAGE correctly simulates a larger Angstrom exponent near regions with emissions of submicron particles and aerosol precursor gases, and a smaller exponent near regions with emissions of coarse particles. The simulated sensitivity of radiative forcing to aerosol optical depth is consistent with estimates from measurements. The simulated direct forcing is within the uncertainty of estimates from measurements in the North Atlantic.

Transport and direct radiative forcing of carbonaceous and sulfate aerosols in the GISS GCM

Abstract: We simulate the major anthropogenic aerosols, sulfate, organic carbon and black carbon, in the Goddard Institute for Space Studies General Circulation Model (GISS GCM), and examine their transport, relative abundances, and direct radiative forcing. Both present-day and projected future emissions are used, as provided by the IPCC SRES (A2) scenarios for 2030 and 2100. We consider the sensitivity of the black carbon distribution to the treatment of its solubility and allow solubility to depend upon exposure to gas phase production of sulfuric acid (case S), or time (case A), or a fixed rate (case C). We show that all three approaches can be tuned to give reasonable agreement with present-day observations. However, case S has higher black carbon in the arctic winter, owing to reduced SO2 oxidation and black carbon solubility. This improves upon the arctic deficiency in previous models, though may be somewhat excessive in this model. We also show that with a different ratio of sulfur/carbonaceous emissions, the case S mechanism can give significantly different results compared to the other mechanisms. Thus, in the 2100 simulation, with reduced sulfur and increased black carbon emissions, the black carbon burden is ∼13% higher and the direct radiative forcing is ∼40% higher in case S compared to the other cases. We consider the relative abundances of carbonaceous and sulfate aerosols in different regions. In the current simulations, carbonaceous aerosols exceed sulfate at the surface in Asia and much of Europe and throughout the column in biomass burning regions. We show that the model ratio of carbonaceous to sulfate aerosols increases with altitude over many oceanic regions, especially in summertime, as was observed during the Tropospheric Aerosol Radiative Forcing Observational Experiment campaign; however, over land and during other seasons the ratio generally decreases with altitude. The (present day) direct radiative forcings for externally mixed (case A) black carbon, organic carbon, and sulfate are calculated to be 0.35, −0.30, and −0.65 W/m2, respectively. In the 2100 simulation these forcings are 0.89, −0.64, and −0.54 W/m2, respectively. The net anthropogenic aerosol global average forcing seasonality inverts between the current and future simulations: the forcing is most negative in (Northern Hemisphere) summertime in 2000 but is least negative or even positive during (NH) summer in 2100; this inversion is more extreme in the Northern Hemisphere.

The effect of violent star formation on the state of the molecular gas in M 82

Abstract: We present the results of a high angular resolution, multi-transition analysis of the molecular gas in M 82. The analysis is based on the two lowest transitions of and the ground transition of the rare isotopes and measured with the PdBI, the BIMA array and the IRAM 30 m telescope. In order to address the question of how the intrinsic molecular cloud properties are influenced by massive star formation we have carried out radiative transfer calculations based on the observed CO line ratios. The calculations suggest that the kinetic temperature of the molecular gas is high in regions with strong star formation and drops towards the outer molecular lobes with less ongoing star formation. The location of the highest kinetic temperature is coincident with that of the mid infrared (MIR) peaks which trace emission from hot dust. The hot gas is associated with low H2 densities while the cold gas in the outer molecular lobes has high H2 densities. We find that CO intensities do not trace H2 , column densities well. Most of the molecular gas is distributed in a double-lobed distribution which surrounds the starburst. A detailed analysis of the conversion factor from CO intensity to H2 column density shows that X CO depends on the excitation conditions. We find , as expected for virialized clouds.

Nanoscale radiative heat transfer between a small particle and a plane surface

Abstract: We study the radiative heat transfer between a small dielectric particle, considered as a point-like dipole, and a surface. In the framework of electrodynamics and using the fluctuation-dissipation theorem, we can evaluate the energy exchange in the near field, which is dominated by the contribution of tunneling waves. The transfer is enhanced by several orders of magnitude if the surface or the particle can support resonant surface waves. An application to local heating is discussed.

A Module for Radiation Hydrodynamic Calculations with ZEUS-2D Using Flux-limited Diffusion

Abstract: A module for the ZEUS-2D code is described that may be used to solve the equations of radiation hydrodynamics to order unity in v/c, in the flux-limited diffusion (FLD) approximation. In this approximation, the factor Eddington tensor f, which closes the radiation moment equations, is chosen to be an empirical function of the radiation energy density. This is easier to implement and faster than full-transport techniques, in which f is computed by solving the transfer equation. However, FLD is less accurate when the flux has a component perpendicular to the gradient in radiation energy density and in optically thin regions when the radiation field depends strongly on angle. The material component of the fluid is here assumed to be in local thermodynamic equilibrium. The energy equations are operator split, with transport terms, radiation diffusion term, and other source terms evolved separately. Transport terms are applied using the same consistent transport algorithm as in ZEUS-2D. The radiation diffusion term is updated using an alternating direction-implicit method with convergence checking. Remaining source terms are advanced together implicitly using numerical root finding. However, when absorption opacity is zero, accuracy is improved by instead treating the compression and expansion source terms using a time-centered differencing scheme. Results are discussed for test problems including radiation-damped linear waves, radiation fronts propagating in optically thin media, subcritical and supercritical radiating shocks, and an optically thick shock in which radiation dominates downstream pressure.

Radiative heat transfer between nanostructures

Abstract: We use a general theory of the fluctuating electromagnetic field and a generalized Kirchhoff's law (Ref. 8) to calculate the heat transfer between macroscopic and nanoscale bodies of arbitrary shape, dispersive, and absorptive dielectric properties. We study the heat transfer between: (a) two parallel semi-infinite bodies, (b) a semi-infinite body and a spherical body, and (c) two spherical bodies. We consider the dependence of the heat transfer on the temperature T, the shape and the separation d, and discuss the role of nonlocal and retardation effects. We find that for low-resistivity material the heat transfer is dominated by retardation effects even for the very short separations.