Showing papers on "Radiative transfer published in 2003"

Quantitative Remote Sensing of Land Surfaces

Abstract: Preface. Acronyms. Chapter 1. Introduction. 1.1 Quantitative Models in Optical remote Sensing. 1.2 Basic Concepts. 1.3 Remote Sensing Modeling System. 1.4 Summary. 1.5 References. Chapter 2. Atmospheric Shortwave Radiative Transfer Modeling. 2.1 Radiative Transfer Equation. 2.2 Surface Statistical BRDF Models. 2.3 Atmospheric Optical Properties. 2.4 Solving Radiative Transfer Equations. 2.5 Approximate Representation for Incorporating Surface BRDF. 2.6 Summary. 2.7 References. Chapter 3. Canopy Reflectance Modeling. 3.1 Canopy Radiative Transfer Formulation. 3.2 Leaf Optical Models. 3.3 Solving Radiative Transfer Equations. 3.4 Geometric Optical Models. 3.5 Computer Simulation Models. 3.6 Summary. 3.7 References. Chapter 4. Soil and Snow Reflectance Modeling. 4.1 Single Scattering Properties of Snow and Soil. 4.2 Multiple Scattering Solutions for Angular Reflectance from Snow and Soil. 4.3 Geome tric Optical Modeling. 4.4 Inversion of Snow Parameters. 4.5 Practical Issues. 4.6 Summary. 4.7 References. Chapter 5. Satellite Sensor Radiometric Calibration. 5.1 Background. 5.2 Post-launch Calibration Methods. 5.3 Calibration Coefficients for Landsat TM and AVHRR Reflective Bands. 5.4 Summary. 5.6 References. Chapter 6. Atmospheric Correction. 6.1 Introduction. 6.2 Methods for Correcting Single Viewing-angle Imagery. 6.3 Methods for Correcting Multiangular Observations. 6.4 Methods for Estimating Total Column Water Vapor Content. 6.5 Summary. 6.6 References. Chapter 7. Topographic Correction Methods. 7.1 Introduction. 7.2 Cosine Correction Algorithms. 7.3 IPW Method. 7.4 Shadowing Function Algorithms. 7.5 DEM Data and Generation. 7.6 Summary. 7.7 References. Chapter 8. Estimation of Land Surface Biophysical variables. 8.1 Statistical Methods. 8.2 Optimization Inversion Method. 8.3 Generic Algorithm (GA). 8.4 Table Look-up Methods. 8.5 Hybrid Inversion Methods. 8.6 Comparisons of Different Inversion Methods. 8.7 Summary. 8.8 References. Chapter 9. Estimation of Surface Radiation Budget: I. Broadband Albedo. 9.1 Introduction. 9.2 Broadband Albedo Characteristics. 9.3 Narrowband to Broadband Conversion. 9.4 Direct Estimation of Surface Broadband Albedos. 9.5 Diurnal Cycle Modeling. 9.6 Summary. 9.7 References. Chapter 10. Estimation of Surface Radiation Budget (II): Longwave. 10.1 Introduction. 10.2 Monochromatic Radiative Transfer Formulation and Solutions. 10.3 Line-by-line Methods. 10.4 Band Models. 10.5 Correlated k-Distribution Methods. 10.6 Atmospheric Correction Methods. 10.7 Split-window Algorithm for Estimating LST. 10.8 Multispectral Algorithms for Separating Temperature and Emissivity. 10.9 Computing Broadband Emissivity. 10.10 Surface Energy Balance Modeling. 10.11 Summary. 10.12 References. Chapter 11. Four-Dimensional (4D) Data Assimilation. 11.1 Introduction. 11.2 Assimilation Algorithms. 11.3 Minimization Algorithms. 11.4 Data Assimilation in Hydrology. 11.5 Data Assimilationdata with Crop Growth Models. 11.6 Summary. 11.7 References. Chapter 12. Validation and Spatial Scaling. 12.1 Rationale of Validation. 12.2 Validation Methodology. 12.3 Spatial Scaling Techniques. 12.4 Summary. 12.5 References. Chapter 13. Applications. 13.1 Methodologies for Integrating Remote Sensing with Ecological Process Models. 13.2 Agricultural Applications. 13.3 "Urban Heat Island" Effects. 13.4 Carbon Cycle Studies. 13.5 Land-atmospheric Interaction. 13.6 Summary. References. Appendix. CD-ROM Content. Data Directory Software Directory. Index.

Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere

Abstract: ranging from 0.40 to 0.78 W m 2 on a global and annual average. The lower stratosphere contributes an additional 7.5–9.3 DU to the calculated increase in the ozone column, increasing radiative forcing by 0.15–0.17 W m 2 . The modeled radiative forcing depends on the height distribution and geographical pattern of predicted ozone changes and shows a distinct seasonal variation. Despite the large variations between the 11 participating models, the calculated range for normalized radiative forcing is within 25%, indicating the ability to scale radiative forcing to global-mean ozone column change. INDEX TERMS: 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 0341 Atmospheric Composition and Structure: Middle atmosphere—constituent transport and chemistry (3334) Citation: Gauss, M., et al., Radiative forcing in the 21st century due to ozone changes in the troposphere and the lower stratosphere, J. Geophys. Res., 108(D9), 4292, doi:10.1029/2002JD002624, 2003.

Two-dimensional Radiative Transfer in Protostellar Envelopes. I. Effects of Geometry on Class I Sources

Abstract: We present two-dimensional radiation transfer models of class I protostars and show the effect of including more realistic geometries on the resulting spectral energy distributions and images. We begin with a rotationally flattened infalling envelope as our comparison model and add a flared disk and bipolar cavity. The disk affects the spectral energy distribution most strongly at edge-on inclinations, causing a broad dip at about 10 lm (independent of the silicate feature) due to high extinction and low scattering albedo in this wavelength region. The bipolar cavities allow more direct stellar+disk radiation to emerge into polar directions and more scattering radiation to emerge into all directions. The wavelength-integrated flux, often interpreted as luminosity, varies with viewing angle, with pole-on viewing angles seeing 2–4 times as much flux as edge-on, depending on geometry. Thus, observational estimates of luminosity should take into account the inclination of a source. The envelopes with cavities are significantly bluer in near-IR and mid-IR color-color plots than those without cavities. Using one-dimensional models to interpret Class I sources with bipolar cavities would lead to an underestimate of envelope mass and an overestimate of the implied evolutionary state. We compute images at near-, mid-, and far-IR wavelengths. We find that the mid-IR colors and images are sensitive to scattering albedo and that the flared disk shadows the midplane on large size scales at all wavelengths plotted. Finally, our models produce polarization spectra that can be used to diagnose dust properties, such as albedo variations due to grain growth. Our results of polarization across the 3.1 l mi ce feature agree well with observations for ice mantles covering 5% of the radius of the grains. Subject headings: circumstellar matter — dust, extinction — polarization — radiative transfer — stars: formation — stars: pre–main-sequence

2-D Radiative Transfer in Protostellar Envelopes: I. Effects of Geometry on Class I Sources

Abstract: We present 2-D radiation transfer models of Class I Protostars and show the effect of including more realistic geometries on the resulting spectral energy distributions and images. We begin with a rotationally flattened infalling envelope as our comparison model, and add a flared disk and bipolar cavity. The disk affects the spectral energy distribution most strongly at edge-on inclinations, causing a broad dip at about 10 um (independent of the silicate feature) due to high extinction and low scattering albedo in this wavelength region. The bipolar cavities allow more direct stellar+disk radiation to emerge into polar directions, and more scattering radiation to emerge into all directions. The wavelength-integrated flux, often interpreted as luminosity, varies with viewing angle, with pole-on viewing angles seeing 2-4 times as much flux as edge-on, depending on geometry. Thus, observational estimates of luminosity should take into account the inclination of a source. The envelopes with cavities are significantly bluer in near-IR and mid-IR color-color plots than those without cavities. Using 1-D models to interpret Class I sources with bipolar cavities would lead to an underestimate of envelope mass and an overestimate of the implied evolutionary state. We compute images at near-, mid-, and far-IR wavelengths. We find that the mid-IR colors and images are sensitive to scattering albedo, and that the flared disk shadows the midplane on large size scales at all wavelengths plotted. Finally, our models produce polarization spectra which can be used to diagnose dust properties, such as albedo variations due to grain growth. Our results of polarization across the 3.1 um ice feature agree well with observations for ice mantles covering 5% of the radius of the grains.

A prototype AMSR-E global snow area and snow depth algorithm

Abstract: A methodologically simple approach to estimate snow depth from spaceborne microwave instruments is described. The scattering signal observed in multifrequency passive microwave data is used to detect snow cover. Wet snow, frozen ground, precipitation, and other anomalous scattering signals are screened using established methods. The results from two different approaches (a simple time and continentwide static approach and a space and time dynamic approach) to estimating snow depth were compared. The static approach, based on radiative transfer calculations, assumes a temporally constant grain size and density. The dynamic approach assumes that snowpack properties are spatially and temporally dynamic and requires two simple empirical models of density and snowpack grain radius evolution, plus a dense media radiative transfer model based on the quasicrystalline approximation and sticky particle theory. To test the approaches, a four-year record of daily snow depth measurements at 71 meteorological stations plus passive microwave data from the Special Sensor Microwave Imager, land cover data and a digital elevation model were used. In addition, testing was performed for a global dataset of over 1000 World Meteorological Organization meteorological stations recording snow depth during the 2000-2001 winter season. When compared with the snow depth data, the new algorithm had an average error of 23 cm for the one-year dataset and 21 cm for the four-year dataset (131% and 94% relative error, respectively). More importantly, the dynamic algorithm tended to underestimate the snow depth less than the static algorithm. This approach will be developed further and implemented for use with the Advanced Microwave Scanning Radiometer-Earth Observing System aboard Aqua.

Two-dimensional radiative transfer in protostellar envelopes. ii. an evolutionary sequence

Abstract: We present model spectral energy distributions (SEDs), colors, polarization, and images for an evolutionary sequence of a low-mass protostar from the early collapse stage (Class 0) to the remnant disk stage (Class III). We find a substantial overlap in colors and SEDs between protostars embedded in envelopes (Class 0–I) and T Tauri disks (Class II), especially at mid-IR wavelengths. Edge-on Class I–II sources show double-peaked SEDs, with a short-wavelength hump due to scattered light and a long-wavelength hump due to thermal emission. These are the bluest sources in mid-IR color-color diagrams. Since Class 0 and I sources are diffuse, the size of the aperture over which fluxes are integrated has a substantial effect on the computed colors, with larger aperture results showing significantly bluer colors. Viewed through large apertures, the Class 0 colors fall in the same regions of mid-IR color-color diagrams as Class I sources and are even bluer than Class II–III sources in some colors. It is important to take this into account when comparing color-color diagrams of star formation regions at different distances or different sets of observations of the same region. However, the near-IR polarization of the Class 0 sources is much higher than the Class I–II sources, providing a means to separate these evolutionary states. We varied the grain properties in the circumstellar envelope, allowing for larger grains in the disk midplane and smaller grains in the envelope. In comparing with models with the same grain properties throughout, we find that the SED of the Class 0 source is sensitive to the grain properties of the envelope only—that is, grain growth in the disk in Class 0 sources cannot be detected from the SED. Grain growth in disks of Class I sources can be detected at wavelengths greater than 100 lm. Our image calculations predict that the diffuse emission from edge-on Class I and II sources should be detectable in the mid-IR with the Space Infrared Telescope Facility (SIRTF) in nearby star-forming regions (out to several hundred parsecs). Subject headings: circumstellar matter — dust, extinction — polarization — radiative transfer — stars: formation — stars: pre–main-sequence

Radiative properties and direct radiative effect of Saharan dust measured by the C-130 aircraft during SHADE: 1. Solar spectrum

Abstract: [1] The physical and optical properties of Saharan dust aerosol measured by the Met Office C-130 during the Saharan Dust Experiment (SHADE) are presented. Additional radiation measurements enable the determination of the aerosol optical depth, taerl, and the direct radiative effect (DRE) of the mineral dust. The results suggest that the absorption by Saharan dust is significantly overestimated in the solar spectrum if standard refractive indices are used. Our measurements suggest an imaginary part of the refractive index of 0.0015i is appropriate at a wavelength l of 0.55 mm. Different methods for determining taerl=0.55 are presented, and the accuracy of each retrieval method is assessed. The value taerl=0.55 is estimated as 1.48 ± 0.05 during the period of heaviest dust loading, which is derived from an instantaneous DRE of approximately � 129 ± 5 Wm � 2 or an enhancement of the local planetary albedo over ocean of a factor of 2.7 ± 0.1. A comparison of the DRE derived from the C-130 instrumentation and from the Clouds and the Earth’s Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite is presented; the results generally showing agreement to within a factor of 1.2. The results suggest that Saharan dust aerosol exerts the largest local and global DRE of all aerosol species and should be considered explicitly in global radiation budget studies. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0360 Atmospheric Composition and Structure: Transmission and scattering of radiation; 1640 Global Change: Remote sensing; 3359 Meteorology and Atmospheric Dynamics: Radiative processes;

A fast, flexible, approximate technique for computing radiative transfer in inhomogeneous cloud fields

Abstract: [1] Radiative transfer schemes in large-scale models tightly couple assumptions about cloud structure to methods for solving the radiative transfer equation, which makes these schemes inflexible, difficult to extend, and potentially susceptible to biases. A new technique, based on simultaneously sampling cloud state and spectral interval, provides radiative fluxes that are guaranteed to be unbiased with respect to the benchmark Independent Column Approximation and works equally well no matter how cloud structure is specified. Fluxes computed in this way are subject to random, uncorrelated errors that depend on the distribution of cloud optical properties. Seasonal forecasts, however, are not sensitive to this noise, making the method useful in weather and climate prediction models.

Multiple temperature regimes of radiative decay in CdSe nanocrystal quantum dots: Intrinsic limits to the dark-exciton lifetime

Abstract: We investigate the strongly temperature-dependent radiative lifetime of electron–hole excitations in colloidal CdSe nanocrystal quantum dots over nearly three orders of magnitude in temperature (300 K to 380 mK). These studies reveal an intrinsic, radiative upper limit of ∼1 μs for the storage of excitons below 2 K. At higher temperatures, exciton lifetimes are consistent with thermal activation from the dark-exciton ground state, but with two different activation thresholds.

An overview of the AIRS radiative transfer model

Abstract: The two main elements of the Atmospheric Infrared Sounder radiative transfer algorithm (AIRS-RTA) are described in this paper: 1) the fast parameterization of the atmospheric transmittances that are used to perform the AIRS physical retrievals and 2) the spectroscopy used to generate the parameterized transmittances. We concentrate on those aspects of the spectroscopy that are especially relevant for temperature and water vapor retrievals. The AIRS-RTA is a hybrid model in that it parameterizes most gases on a fixed grid of pressures, while the water optical depths are parameterized on a fixed grid of water amounts. Water vapor, ozone, carbon monoxide, and methane profiles can be varied, in addition to the column abundance of carbon dioxide.

On the nature of the X‐ray emission from the accreting millisecond pulsar SAX J1808.4−3658

Abstract: We study the pulse profiles of the accreting X-ray millisecond pulsar SAX J1808.4-3658 at different energies. The two main emission components, the blackbody and the Comptonized tail, which are clearly identified in the time-averaged spectrum, show strong variability with the first component lagging the second one. The observed variability can be explained if the emission is produced by Comptonization in a hot slab (radiative shock) of Thomson optical depth ∼0.3-1 at the neutron star surface. The blackbody radiation is strongly beamed along the normal to the slab (a 'knife'-like or 'pencil'-like emission pattern), while the Comptonized emission has a broader angular distribution peaking at about 50°-60° from the slab normal (a 'fan'-like pattern). We construct a detailed model of the X-ray production accounting for the Doppler boosting, relativistic aberration and gravitational light bending in the Schwarzschild space-time. We present also accurate analytical formulae for computations of the light curves from rapidly rotating neutron stars using the formalism recently developed by Beloborodov. Our model reproduces well the pulse profiles at different energies simultaneously, corresponding phase lags, as well as the time-averaged spectrum. We constrain the compact star mass to be bounded between 1.2 and 1.6 M O .. By fitting the observed profiles, we determine the radius of the compact object to be R ∼11 km if M = 1.6 M O ., while for M = 1.2 M O . the best-fitting radius is ∼6.5 km, indicating that the compact object in SAX J1808.4-3658 can be a strange star. We obtain a lower limit on the inclination of the system of 65°.

Comparison of size and morphological measurements of coarse mode dust particles from Africa

Abstract: [1] A multitude of sensitivity studies in the literature point to the importance of proper chemical and morphological characterization of particles when the radiative impacts of airborne dusts are modeled. However, the community data set is based on heterogeneous measurement methods relying on varying aerodynamic, chemical, morphological, and optical means. During the Puerto Rico Dust Experiment, size distributions of dust particles from Africa were measured using a variety of aerodynamic, optical, and geometric means. Consistent with the literature, comparisons of these size distributions showed quite dissimilar results. “Measured” volume median diameters varied from 2.5 to 9 μm for various geometric, aerodynamic, optical, and optical inversion methods. Aerodynamic systems showed mixed performance. Column integrated size distributions inverted from AERONET Sun/sky radiance data produced somewhat reasonable results in the coarse mode when given proper constraints and taken in the proper context. The largest systematic errors were found in optical particle counters due to insensitivities to particle size in the 4–10 μm region with further complications due to dust particle morphology and index of refraction issues. As these methods can produce quite dissimilar size distributions, considerable errors in calculated radiative properties can occur if incorrectly modeled into dust parameters. None of the methods compared in this study can adequately reproduce the measured mass extinction or mass scattering efficiency of the dust using spherical geometry methods. Given all of the uncertainties in the sizing methods, we promote the use of fundamental and quantifiable descriptors of particles such as mass as a function of aerodynamic diameter.

Dependence of the Yb 3+ emission cross section and lifetime on temperature and concentration in yttrium aluminum garnet

Abstract: Measurements are reported of the spectroscopic properties (absorption and emission spectra, stimulated-emission cross section, and radiative lifetime) of (YbxY1-x)3Al5O12 for nominal x values of 0.025, 0.05, 0.1, 0.2 and 0.3 at temperatures of 15–300 K. The emission cross sections of Yb:YAG with different Yb3+ concentrations were determined by use of the Fuchtbauer–Ladenburg formula and the reciprocity method. At low temperatures, the product (στ) of the effective stimulated-emission cross section and the radiative lifetime is greater than at room temperature for all concentrations. Product στ is nearly independent of Yb3+ concentration at a given temperature. These results will aid in the design of high-power thin disk lasers by use of highly doped Yb:YAG.

Comparison of size and morphological measurements of coarse mode dust particles from Africa : Puerto Rico Dust Experiment (PRIDE1)

Abstract: A multitude of sensitivity studies in the literature point to the importance of proper chemical and morphological characterization of particles when the radiative impacts of airborne dusts are modeled. However, the community data set is based on heterogeneous measurement methods relying on varying aerodynamic, chemical, morphological, and optical means. During the Puerto Rico Dust Experiment, size distributions of dust particles from Africa were measured using a variety of aerodynamic, optical, and geometric means. Consistent with the literature, comparisons of these size distributions showed quite dissimilar results. Measured volume median diameters varied from 2.5 to 9 μm for various geometric, aerodynamic, optical, and optical inversion methods. Aerodynamic systems showed mixed performance. Column integrated size distributions inverted from AERONET Sun/sky radiance data produced somewhat reasonable results in the coarse mode when given proper constraints and taken in the proper context. The largest systematic errors were found in optical particle counters due to insensitivities to particle size in the 4-10 μm region with further complications due to dust particle morphology and index of refraction issues. As these methods can produce quite dissimilar size distributions, considerable errors in calculated radiative properties can occur if incorrectly modeled into dust parameters. None of the methods compared in this study can adequately reproduce the measured mass extinction or mass scattering efficiency of the dust using spherical geometry methods. Given all of the uncertainties in the sizing methods, we promote the use of fundamental and quantifiable descriptors of particles such as mass as a function of aerodynamic diameter.

MOCASSIN: a fully three-dimensional Monte Carlo photoionization code

Abstract: The study of photoionized environments is fundamental to many astrophysical problems. Up to the present most photoionization codes have numerically solved the equations of radiative transfer by making the extreme simplifying assumption of spherical symmetry. Unfortunately very few real astronomical nebulae satisfy this requirement. To remedy these shortcomings, a self-consistent, three-dimensional radiative transfer code has been developed using Monte Carlo techniques. The code, MOCASSIN, is designed to build realistic models of photoionized nebulae having arbitrary geometry and density distributions, with both the stellar and diffuse radiation fields treated self-consistently. In addition, the code is capable of treating one or more exciting stars located at non-central locations. The gaseous region is approximated by a cuboidal Cartesian grid composed of numerous cells. The physical conditions within each grid cell are determined by solving the thermal equilibrium and ionization balance equations. This requires a knowledge of the local primary and secondary radiation fields, which are calculated self-consistently by locally simulating the individual processes of ionization and recombination. The structure and the computational methods used in the MOCASSIN code are described in this paper. MOCASSIN has been benchmarked against established one-dimensional spherically symmetric codes for a number of standard cases, as defined by the Lexington/Meudon photoionization workshops: at Meudon in 1985 and at Lexington in 1995 and 2000. The results obtained for the benchmark cases are satisfactory and are presented in this paper. A performance analysis has also been carried out and is discussed here.

Constraints on the size of Martian aerosols from Thermal Emission Spectrometer observations

Abstract: [1] We combine a robust multiple-scattering radiative transfer algorithm with the Thermal Emission Spectrometer (TES) spectral data set in order to characterize the properties of Martian aerosol particles. Because of the importance of accurate model input when performing such retrievals, we include self-consistent and physically plausible treatments of surface emissivity and atmospheric aerosol dielectric functions, as well as gaseous absorption effects. Considerable effort is expended in the identification and discussion of potential sources of error and uncertainty. Significant results stemming from this analysis are a new dust aerosol dielectric function that appears to well represent the IR spectral behavior sampled by TES for a wide range of dust loading conditions, two distinct populations of water ice particles with reff of ∼1–2 μm and ∼3–4 μm; and distinct departures in dust particle sizes during the 2001A global dust storm from the canonical 1.6–1.7 μm values. Very consistent aerosol size distributions are obtained when 9 μm dust and 12 μm ice optical depths retrieved from this analysis are compared to visible optical depths retrieved from TES solar band emission phase function sequences [Clancy et al., 2003]. Direct comparison of our optical depths to those available from the Planetary Data System (PDS) (as provided by the TES science team) reveals a systematic bias toward τ values which are 20–30% (or more) too small. Much of this offset stems from the fact that TES PDS aerosol optical depths are actually an approximation to τabsorption, which is ∼30% lower than τextinction for Mars dust aerosols. Additional biases in TES optical depths arise from assumptions of fixed surface emissivity and temperature.

Characterization of semiconductor laser gain media by the segmented contact method

Abstract: In this paper, we describe methods for analysis of edge-emitted amplified spontaneous emission spectra measured as a function of the pumped stripe length. We show that both the modal gain and the unamplified spontaneous emission spectra can be extracted from the data, and we describe a means of calibrating the spontaneous emission in real units, without requiring the carrier populations to be described by Fermi functions. The gain and emission spectra can be determined for transverse electric and transverse magnetic polarizations and by summing the recombination currents for each polarization the total radiative current can be measured. This enables the overall internal radiative quantum efficiency to be calculated. Once the calibration factor is known the internal stimulated recombination rate at the facet can also be estimated. The experiment can be configured to give a measurement of the passive modal absorption of the gain medium. The internal optical mode loss can be determined from the long-wavelength region of the gain spectrum or the modal absorption spectrum. In summary, we show that measurements of amplified spontaneous emission spectra provide a full characterization of the gain medium.

Measurement and modeling of the Saharan dust radiative impact: Overview of the Saharan Dust Experiment (SHADE)

Abstract: [1] Aerosols are known to be important in determining Earth’s radiative balance. Dust aerosols are of particular interest since, in addition to their scattering and absorbing properties that affect the solar radiation, they also perturb the terrestrial radiation. Recent studies have shown that a significant proportion of mineral dust in the atmosphere may be of anthropogenic origin, and therefore they may have an important role in climate change by exerting a significant radiative forcing. However, the optical and radiative properties of dust are not yet very well-determined, and even the sign of the resulting forcing is still questionable. The Saharan Dust Experiment (SHADE) was designed to better determine the parameters that are relevant for computing the direct radiative effect. Two aircraft combining in situ and remote sensing instruments were coordinated with satellite overpasses and ground-based observations during the experiment, which was based in the Cape Verde area during the period 19–29 September 2000. These in situ and remotely sensed data provide new valuable information on the microphysical, optical properties, and radiative effects of a large mineral dust outbreak. In addition, a global chemical transport model was used for assessing the radiative impact of these events, which are shown to be important on regional and global scales. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 3359 Meteorology and Atmospheric Dynamics: Radiative processes; 3360 Meteorology and Atmospheric Dynamics: Remote sensing; KEYWORDS: Saharan dust, physical and optical properties, dust direct radiative forcing

Ionizing photon emission rates from o- and early b-type stars and clusters

Abstract: We present new computations of the ionizing spectral energy distributions (SEDs), and Lyman continuum (Lyc) and He I continuum photon emission rates, for hot O-type and early B-type stars. We consider solar-metallicity stars, with effective temperatures ranging from 25,000 to 55,000 K and surface gravities (cm s-2) log g ranging from 3 to 4, covering the full range of spectral types and luminosity classes for hot stars. We use our updated (WM-basic) code to construct radiation-driven wind atmosphere models for hot stars. Our models include the coupled effects of hydrodynamics and non-LTE radiative transfer in spherically outflowing winds, including the detailed effects of metal line blocking and line blanketing on the radiative transfer and energy balance. Our grid of model atmospheres is available on the World Wide Web. We incorporate our hot-star models into our population synthesis code (STARS), and we compute the time-dependent SEDs and resulting Lyc and He I emission rates for evolving star clusters. We present results for continuous and impulsive star formation for a range of assumed stellar initial mass functions.

An analytical model for subsurface irradiance and remote sensing reflectance in deep and shallow case-2 waters

Abstract: Subsurface remote sensing signals, represented by the irradiance reflectance and the remote sensing reflectance, were investigated. The present study is based on simulations with the radiative transfer program Hydrolight using optical properties of Lake Constance (German: Bodensee) based on in-situ measurements of the water constituents and the bottom characteristics. Analytical equations are derived for the irradiance reflectance and remote sensing reflectance for deep and shallow water applications. The input of the parameterization are the inherent optical properties of the water-absorption a(λ) and backscattering bb(λ). Additionally, the solar zenith angle θs, the viewing angle θv, and the surface wind speed u are considered. For shallow water applications the bottom albedo RB and the bottom depth zB are included into the parameterizations. The result is a complete set of analytical equations for the remote sensing signals R and Rrs in deep and shallow waters with an accuracy better than 4%. In addition, parameterizations of apparent optical properties were derived for the upward and downward diffuse attenuation coefficients Ku and Kd.

Solar radiative forcing by biomass burning aerosol particles during SAFARI 2000: A case study based on measured aerosol and cloud properties

Abstract: [1] This study investigates the top of atmosphere (TOA) solar radiative forcing by biomass burning aerosol from the African continent. Radiative transfer calculations are based on airborne aerosol and cloud measurements made on 7 September 2000, being representative for the aerosol optical properties and the relative location of aerosol and clouds during Southern African Fire-Atmosphere Research Initiative (SAFARI) 2000. We focus on how the radiative effects of biomass aerosol are changed in the presence of clouds to the west of the southern African coastline. The typical scenario observed showed a clear separation between an elevated biomass aerosol layer (1.8–3.7 km altitude) and low-level stratiform clouds (below 1 km, liquid water path 85 g m−2). The aerosol was characterized by particle concentrations of 1800 cm−3 and a single scattering albedo at 550 nm wavelength of around 0.90. Presuming no effect by the biomass aerosol on the clouds themselves, the presence of clouds converted the negative TOA forcing by the biomass aerosol in clear skies into a positive one (−13.0 W m−2 converted to +11.5 W m−2 for average optical properties of the biomass aerosol and a solar zenith angle of 60°). As biomass aerosol was found above cloud thousands of kilometers away from the southern African coastline, positive direct TOA forcings can be expected in extended sea areas over the Namibian cloud sheet. These results suggest errors in global climate model (GCM) modeling assessments of the direct radiative forcing due to biomass aerosols. Additionally, we estimate the possibility that a potential indirect effect dominates the overall forcing, resulting generally in strong negative TOA forcings.

Ionizing Photon Emission Rates from O- and Early B-type Stars and Clusters

Abstract: We present new computations of the ionizing spectral energy distributions (SEDs) and Lyman continuum (Lyc) and HeI continuum photon emission rates, for hot O-type and early B-type stars. We consider solar metallicity stars, with effective temperatures ranging from 25,000 to 55,000 K and surface gravities (cm s^-2) logg ranging from 3 to 4, covering the full range of spectral types and luminosity classes for hot stars. We use our updated (WM-basic) code to construct radiation-driven wind atmosphere models for hot stars. Our models include the coupled effects of hydrodynamics and non-LTE radiative transfer in spherically outflowing winds, including the detailed effects of metal line blocking and line blanketing on the radiative transfer and energy balance. We incorporate our hot-star models into our population synthesis code (STARS), and we compute the time-dependent SEDs and resulting Lyc and HeI emission rates for evolving star clusters. We present results for continuous and impulsive star formation for a range of assumed stellar initial mass functions.

Effects of the Solvent Refractive Index and Its Dispersion on the Radiative Decay Rate and Extinction Coefficient of a Fluorescent Solute

Abstract: It is well known that the probabilities of radiative transitions in a medium differ from those in vacuum. Excitation of a fluorescent molecule and its radiative decay are examples of radiative transitions. The rates of these processes in solution depend on the optical characteristics of the solvent. In this article the radiative decay rate and the extinction coefficient of a fluorescent molecule in solution are expressed in terms of the intrinsic properties of the fluorescent molecule (electronic transition moments) and the optical characteristics of the solvent (refractive index, group velocity of light). It is shown that the group velocity does not enter in the final expressions for the radiative decay rate and the extinction coefficient; this means that the dispersion of the refractive index has no effect on these quantities. The expressions for both the radiative decay rate and the extinction coefficient contain the refractive index of the solvent and the local field correction factor. The latter depends on the cavity model, and, for some cavity models, on the shape of the cavity. Four types of cavity models are discussed; for each model the limits of applicability are examined. Experimental evidence in support of specific cavity models is reviewed.

Narrow-band and full-spectrum k-distributions for radiative heat transfer—correlated-k vs. scaling approximation

Abstract: The correlated-k and scaled-k distribution methods for radiative heat transfer in molecular gases are developed based on precise mathematical principles, for both narrow band and full spectrum models. Their differences and commonalities are high-lighted and discussed. Applications to narrow spectral bands of nonhomogeneous gases show both methods to be about equally accurate. For full-spectrum calculations, on the other hand, the scaled-k distribution consistently outperforms the correlated-k model.

Dynamic model atmospheres of AGB stars - III. Effects of frequency-dependent radiative transfer

Abstract: We present a new generation of model atmospheres for AGB stars which combine time-dependent dynamics and frequency-dependent radiative transfer This allows us to take both the effects of pulsation (shock waves, stellar winds) and the complex influence of molecular opacities into account In the case of C-rich stars, the models also include a self-consistent time-dependent description of dust formation We investigate the influence of frequency-dependent radiative transfer on the energy and momentum balance of the atmosphere and compare our new models to existing grey dynamical models as well as to classical hydrostatic model atmospheres We stress the importance of non-grey radiative transfer for obtaining realistic density-temperature structures even in highly dynamical models, discussing both the resulting observable properties and the wind characteristics Presenting synthetic spectra, we argue that the current dynamical models represent an important step in a process leading from a qualitative to a quantitative description of atmospheres and winds of pulsating AGB stars

The Millennium Arecibo 21 Centimeter Absorption-Line Survey. I. Techniques and Gaussian Fits

Abstract: We review the theory of measuring spectral lines in emission/absorption observations and apply it to a new survey of the 21 cm line against 79 continuum sources. We develop an observing technique and least-squares procedure to determine the opacity profile, the expected emission profile, and their uncertainty profiles. We discuss the radiative transfer for the two-component interstellar H I gas and use Gaussian components, separate ones for the warm and cold neutral media (WNM and CNM), as a practical implementation of a simple but physically correct model that successfully treats both simple and complicated profiles. Our Gaussians provide CNM spin temperatures, upper limits on kinetic temperatures for both CNM and WNM from the line widths, column densities, and velocities; we discuss these astrophysical aspects in Paper II.

Judd–Ofelt analysis of the Er3+(4f11) absorption intensities in phosphate glass: Er3+, Yb3+

Abstract: A spectroscopic Judd–Ofelt investigation has been performed on Er3+ in the doubly doped phosphate glass:Er3+, Yb3+ in order to assess its potential as a laser system. The standard Judd–Ofelt model was applied to the room-temperature absorption intensities of Er3+(4f11) transitions to determine the intensity parameters: Ω2=6.28×10−20 cm2, Ω4=1.03×10−20 cm2, and Ω6=1.39×10−20 cm2 in the phosphate glass host. The intensity parameters are used to determine the radiative decay rates (emission probabilities of transitions) and branching ratios of the Er3+ transitions from the excited-state J manifolds to the lower-lying J′ manifolds. The radiative lifetimes of these excited states are determined from the radiative decay rates. The predicted decay rates and radiative lifetimes are compared to those of Er3+ transitions in other glass hosts. The quantum efficiency of the eye-safe laser transition 4I13/2→4I15/2 (1.54 μm) of Er3+ is approximately 80% in the phosphate glass host.

Radiative transfer in disc galaxies – III. The observed kinematics of dusty disc galaxies

Abstract: We present skirt (Stellar Kinematics Including Radiative Transfer), a new Monte Carlo radiative transfer code that allows the calculation of the observed stellar kinematics of a dusty galaxy. The code incorporates the effects of both absorption and scattering by interstellar dust grains, and calculates the Doppler shift of the emerging radiation exactly by taking into account the velocities of the emitting stars and the individual scattering dust grains. The code supports arbitrary distributions of dust through a cellular approach, whereby the integration through the dust is optimized by means of a novel efficient trilinear interpolation technique. We apply our modelling technique to calculate the observed kinematics of realistic models for dusty disc galaxies. We find that the effects of dust on the mean projected velocity and projected velocity dispersion are severe for edge-on galaxies. For galaxies which deviate more than a few degrees from exactly edge-on, the effects are already strongly reduced. As a consequence, dust attenuation cannot serve as a possible way to reconcile the discrepancy between the observed shallow slopes of the inner rotation curves of low surface brightness galaxies and the predictions of cold dark matter cosmological models. For face-on galaxies, the velocity dispersion increases with increasing dust mass owing to scattering, but the effects are limited, even for extended dust distributions. Finally, we show that serious errors can be made when the individual velocities of the dust grains are neglected in the calculations.