Radiative transfer

About: Radiative transfer is a research topic. Over the lifetime, 43287 publications have been published within this topic receiving 1176539 citations.

An atmospheric radiation model for Cerro Paranal - I. The optical spectral range

Abstract: Aims. The Earth’s atmosphere affects ground-based astronomical observations. Scattering, absorption, and radiation processes deteriorate the signal-to-noise ratio of the data received. For scheduling astronomical observations it is, therefore, important to accurately estimate the wavelength-dependent effect of the Earth’s atmosphere on the observed flux.Methods. In order to increase the accuracy of the exposure time calculator of the European Southern Observatory’s (ESO) Very Large Telescope (VLT) at Cerro Paranal, an atmospheric model was developed as part of the Austrian ESO In-Kind contribution. It includes all relevant components, such as scattered moonlight, scattered starlight, zodiacal light, atmospheric thermal radiation and absorption, and non-thermal airglow emission. This paper focuses on atmospheric scattering processes that mostly affect the blue ( m) wavelength regime, and airglow emission lines and continuum that dominate the red (>0.55 μ m) wavelength regime. While the former is mainly investigated by means of radiative transfer models, the intensity and variability of the latter is studied with a sample of 1186 VLT FORS 1 spectra.Results. For a set of parameters such as the object altitude angle, Moon-object angular distance, ecliptic latitude, bimonthly period, and solar radio flux, our model predicts atmospheric radiation and transmission at a requested resolution. A comparison of our model with the FORS 1 spectra and photometric data for the night-sky brightness from the literature, suggest a model accuracy of about 20%. This is a significant improvement with respect to existing predictive atmospheric models for astronomical exposure time calculators.

Systematic long‐term comparison of spectral UV measurements and UVSPEC modeling results

Abstract: For the evaluation of radiative transfer models and for investigations on the influence of parameters like aerosols or clouds on ground level UV irradiance, a combination of spectral measurements and model calculations is required. We show an efficient method for such a combination and present a systematic comparison of the freely available UVSPEC radiative transfer model package with two years of spectrally resolved measurements made at Garmisch-Partenkirchen, Germany (47.48°N, 11.07°E, 730 m above sea level) for cloudless sky and low albedo. More than 1200 spectra have been used for the comparison, covering a wide range of ozone and aerosol conditions. Applying the PSEUDO-SPHERICAL model type, a discrete ordinate model with correction for the sphericity of the Earth, the systematic differences between measurement and model were found to range between −11 and +2% for wavelengths between 295 and 400 nm and solar zenith angles up to 80°. The small observed statistical differences of 2–3% can mostly be explained by the random error of the measurement system. Only two input parameters, total ozone column and aerosol optical depth, the latter parameterized by the Angstrom formula, are required to reach this level of agreement. It was further found that knowledge of the aerosol optical depth is essential for obtaining such a good agreement. The evaluated UVSPEC model package, together with the presented interface SDMODEL, provides an efficient tool for the investigation of the processes that control surface UV irradiance.

An efficient method for online calculations of photolysis and heating rates

Abstract: The authors present a computationally highly efficient method for the online calculation of photolysis and heating rates, which is especially suited for coupled transport–chemistry models. For this purpose, the spectral range 178.6 nm ≤ λ ≤ 752.5 nm, which is important for photochemistry in the troposphere and middle atmosphere, is divided into eight wavelength bands. For each band a parameterization of its contribution to photolysis and heating rates for a purely absorbing atmosphere is proposed, taking into consideration only absorption by O2 and O3. Scattering by molecules, aerosols, and clouds are taken into account by a correction factor, which is calculated online with a radiative transfer code at only one wavelength within each interval. The method yields photolysis and heating rates with less than 10% error for solar zenith angles less than 80° in the troposphere and even beyond in the stratosphere.

Optical emission in periodic dielectrics

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