Radiative Transfer Simulations (radiative + transfer_simulation)

Distribution by Scientific Domains


Selected Abstracts


Spectrally based remote sensing of river bathymetry

EARTH SURFACE PROCESSES AND LANDFORMS, Issue 8 2009
Carl J. Legleiter
Abstract This paper evaluates the potential for remote mapping of river bathymetry by (1) examining the theoretical basis of a simple, ratio-based technique for retrieving depth information from passive optical image data; (2) performing radiative transfer simulations to quantify the effects of suspended sediment concentration, bottom reflectance, and water surface state; (3) assessing the accuracy of spectrally based depth retrieval under field conditions via ground-based reflectance measurements; and (4) producing bathymetric maps for a pair of gravel-bed rivers from hyperspectral image data. Consideration of the relative magnitudes of various radiance components allowed us to define the range of conditions under which spectrally based depth retrieval is appropriate: the remotely sensed signal must be dominated by bottom-reflected radiance. We developed a simple algorithm, called optimal band ratio analysis (OBRA), for identifying pairs of wavelengths for which this critical assumption is valid and which yield strong, linear relationships between an image-derived quantity X and flow depth d. OBRA of simulated spectra indicated that water column optical properties were accounted for by a shorter-wavelength numerator band sensitive to scattering by suspended sediment while depth information was provided by a longer-wavelength denominator band subject to strong absorption by pure water. Field spectra suggested that bottom reflectance was fairly homogeneous, isolating the effect of depth, and that radiance measured above the water surface was primarily reflected from the bottom, not the water column. OBRA of these data, 28% of which were collected during a period of high turbidity, yielded strong X versus d relations (R2 from 0·792 to 0·976), demonstrating that accurate depth retrieval is feasible under field conditions. Moreover, application of OBRA to hyperspectral image data resulted in spatially coherent, hydraulically reasonable bathymetric maps, though negative depth estimates occurred along channel margins where pixels were mixed. This study indicates that passive optical remote sensing could become a viable tool for measuring river bathymetry. Copyright © 2009 John Wiley & Sons, Ltd. [source]


Radiative transfer in disc galaxies , IV.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010
The effects of dust attenuation on bulge, disc structural parameters
ABSTRACT Combining Monte Carlo radiative transfer simulations and accurate 2D bulge/disc decompositions, we present a new study to investigate the effects of dust attenuation on the apparent structural properties of the disc and bulge of spiral galaxies. We find that dust affects the results from such decompositions in ways which cannot be identified when one studies dust effects on bulge and disc components separately. In particular, the effects of dust in galaxies hosting pseudo-bulges might be different from those in galaxies hosting classical bulges, even if their dust content is identical. Confirming previous results, we find that disc scalelengths are overestimated when dust effects are important. In addition, we also find that bulge effective radii and Sérsic indices are underestimated. Furthermore, the apparent attenuation of the integrated disc light is underestimated, whereas the corresponding attenuation of bulge light is overestimated. Dust effects are more significant for the bulge parameters, and, combined, they lead to a strong underestimation of the bulge-to-disc ratio, which can reach a factor of 2 in the V band, even at relatively low galaxy inclinations and dust opacities. Nevertheless, it never reaches factors larger than about 3, which corresponds to a factor of 2 in bulge-to-total ratio. Such effect can have an impact on studies of the black hole/bulge scaling relations. [source]


Reionization bias in high-redshift quasar near-zones

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
J. Stuart B. Wyithe
ABSTRACT Absorption spectra of high-redshift quasars exhibit an increasingly thick Ly, forest, suggesting that the fraction of neutral hydrogen in the intergalactic medium (IGM) is increasing towards z, 6. However, the interpretation of these spectra is complicated by the fact that the Ly, optical depth is already large for neutral hydrogen fractions in excess of 10,4, and also because quasars are expected to reside in dense regions of the IGM. We present a model for the evolution of the ionization state of the IGM which is applicable to the dense, biased regions around high-redshift quasars as well as more typical regions in the IGM. We employ a cold dark matter based model in which the ionizing photons for reionization are produced by star formation in dark matter haloes spanning a wide range of masses, combined with numerical radiative transfer simulations which model the resulting opacity distribution in quasar absorption spectra. With an appropriate choice for the parameter which controls the star formation efficiency, our model is able to simultaneously reproduce the observed Ly, forest opacity at 4 < z < 6, the ionizing photon mean-free-path at z, 4 and the rapid evolution of highly ionized near-zone sizes around high-redshift quasars at 5.8 < z < 6.4. In our model, reionization extends over a wide redshift range, starting at z, 10 and completing as H ii regions overlap at z, 6,7. We find that within 5 physical Mpc of a high-redshift quasar, the evolution of the ionization state of the IGM precedes that in more typical regions by around 0.3 redshift units. More importantly, when combined with the rapid increase in the ionizing photon mean-free-path expected shortly after overlap, this offset results in an ionizing background near the quasar which exceeds the value in the rest of the IGM by a factor of ,2,3. We further find that in the post-overlap phase of reionization the size of the observed quasar near-zones is not directly sensitive to the neutral hydrogen fraction of the IGM. Instead, these sizes probe the level of the background ionization rate and the temperature of the surrounding IGM. The observed rapid evolution of the quasar near-zone sizes at 5.8 < z < 6.4 can thus be explained by the rapid evolution of the ionizing background, which in our model is caused by the completion of overlap at the end of reionization by 6 ,z, 7. [source]


Fate of clumps in damped Ly, systems

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Ilian T. Iliev
ABSTRACT Recent observations have revealed that damped Ly, clouds (DLAs) host star formation activity. In order to examine if such star formation activity can be triggered by ionization fronts, we perform high-resolution hydrodynamics and radiative transfer simulations of the effect of radiative feedback from propagating ionization fronts on high-density clumps. We examine two sources of ultraviolet (UV) radiation field to which high-redshift (z, 3) galaxies could be exposed: one corresponding to the UV radiation originating from stars within the DLA, itself, and the other corresponding to the UV background radiation. We find that, for larger clouds, the propagating I-fronts created by local stellar sources can trigger cooling instability and collapse of significant part, up to 85 per cent, of the cloud, creating conditions for star formation in a time-scale of a few Myr. The passage of the I-front also triggers collapse of smaller clumps (with radii below ,4 pc), but in these cases the resulting cold and dense gas does not reach conditions conducive to star formation. Assuming that 85 per cent of the gas initially in the clump is converted into stars, we obtain a star formation rate of ,0.25 M, yr,1 kpc,2. This is somewhat higher than the value derived from recent observations. On the other hand, the background UV radiation which has harder spectrum fails to trigger cooling and collapse. Instead, the hard photons which have long mean free-path heat the dense clumps, which as a result expand and essentially dissolve in the ambient medium. Therefore, the star formation activity in DLAs is strongly regulated by the radiative feedback, both from the external UV background and internal stellar sources and we predict quiescent evolution of DLAs (not starburst-like evolution). [source]


The potential of variational retrieval of temperature and humidity profiles from Meteosat Second Generation observations

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 638 2009
F. Di Giuseppe
Abstract The quality of temperature and humidity retrievals from the infrared Spinning Enhanced Visible and Infrared Imager (SEVIRI) sensors on the geostationary Meteosat Second Generation (MSG) satellites is assessed by means of a one-dimensional variational algorithm. The study is performed with the aim of improving the spatial and temporal resolution of available observations to feed analysis systems designed for high-resolution regional-scale numerical weather prediction (NWP) models. The non-hydrostatic forecast model COSMO in the ARPA-SIMC operational configuration is used to provide background fields. Only clear-sky observations over sea are processed. An optimized one-dimensional variational set-up comprised of two water-vapour and three window channels is selected. It maximizes the reduction of errors in the model backgrounds while ensuring ease of operational implementation through accurate bias correction procedures and correct radiative transfer simulations. The 1Dvar retrieval quality is first quantified in relative terms, employing statistics to estimate the reduction in the background model errors. Additionally the absolute retrieval accuracy is assessed by comparing the analysis with independent radiosonde observations. The inclusion of satellite data brings a substantial reduction in the warm and dry biases present in the forecast model. Moreover it is shown that the use of the retrieved profiles generated by the 1Dvar in the COSMO nudging scheme can locally reduce forecast errors. Copyright © 2009 Royal Meteorological Society [source]


Mie simulations as an error source in mineral aerosol radiative forcing calculations

THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 623 2007
M. Kahnert
Abstract The role of aerosols remains a major uncertainty for climate and climate change. For the direct radiative forcing by mineral aerosols, the uncertainty in the refractive index m has been regarded as the most important error source, while the impact of aerosol non-sphericity has been considered a minor issue and is neglected in climate models. Here, the errors caused by the spherical particle approximation (SPA) are evaluated by comparing radiative fluxes based on (i) Mie simulations and (ii) laboratory measurements of aerosol optical properties. Furthermore, they are contrasted with the errors related to the uncertainty in the refractive index. These two error sources are found to be of comparable magnitude, although they are strongly dependent on optical depth, surface albedo, and particle size. Thus, our results provide evidence that, contrary to common beliefs, the use of spherical model particles in radiative transfer simulations is probably among the major sources of error in quantifying the climate forcing effect of mineral aerosols. This stems from misrepresentation of the scattering phase function and the asymmetry parameter. Aerosol single-scattering computations based on non-spherical model particles are expected to reduce the shape-related errors and thus significantly improve the accuracy of radiative forcing simulations. Copyright © 2007 Royal Meteorological Society [source]