Radiative Transfer in the Heliosphere at Lyman α: Comparison of Numerical and Montecarlo Simulations
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Polarimetric observations of planets are providing increasing details of the three-dimensional (3D) atmospheric structure. The one-dimensional plane-parallel approximation model neglects horizontally polarized radiative transfer. Multidimensional polarized radiative transfer models, especially 3D models, are required to contain the horizontal polarization mechanism. Here, we propose a lattice Boltzmann (LB) model for multidimensional polarized radiative transfer, which enables a simple solution of the multidimensional vector radiative transfer equation (VRTE) by performing collision and streaming processes. Through the Chapman–Enskog analysis, we rigorously derive the multi-dimensional VRTE from the proposed LB model. 2D and 3D numerical tests demonstrate that the proposed LB model is effective and accurate for simulating multidimensional polarized radiative transfer. Furthermore, we apply the proposed LB model to investigate the effects of multiple scattering on radiation intensity and degree of polarization in a 3D case and find that multiple scattering enhances the radiation intensity but dampens the degree of polarization throughout almost the whole angular space in multidimensional polarized radiative transfer. This work is expected to provide a simple and effective mesoscopic tool for multidimensional polarized radiative transfer.
Degree of polarization
Lattice Boltzmann methods
Stokes parameters
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Radiative flux
Line (geometry)
Uncorrelated
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Abstract One‐dimensional radiative transfer solvers are computationally much more efficient than full three‐dimensional radiative transfer solvers but do not account for the horizontal propagation of radiation and thus produce unrealistic surface irradiance fields in models that resolve clouds. Here, we study the impact of using a 3‐D radiative transfer solver on the direct and diffuse solar irradiance beneath clouds and the subsequent effect on the surface fluxes. We couple a relatively fast 3‐D radiative transfer approximation (TenStream solver) to the Dutch Atmosphere Large‐Eddy Simulation (DALES) model and perform simulations of a convective boundary layer over grassland with either 1‐D or 3‐D radiative transfer. Based on a single case study, simulations with 3‐D radiative transfer develop larger and thicker clouds, which we attribute mainly to the displaced clouds shadows. With increasing cloud thickness, the surface fluxes decrease in cloud shadows with both radiation schemes but increase beneath clouds with 3‐D radiative transfer. We find that with 3‐D radiative transfer, the horizontal length scales dominating the spatial variability of the surface fluxes are over twice as large as with 1‐D radiative transfer. The liquid water path and vertical wind velocity in the boundary layer are also dominated by larger length scales, suggesting that 3‐D radiative transfer may lead to larger convective thermals. Our case study demonstrates that 3‐D radiative effects can significantly impact dynamic heterogeneities induced by cloud shading. This may change our view on the coupling between boundary‐layer clouds and the surface and should be further tested for generalizability in future studies.
Radiative flux
Radiative Cooling
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s bands. Absorption effects may lead to inaccuracies in infrared channels. Streamer has got two radiative transfer solvers. LibRadtran is a library of radiative transfer routines and programs. The central program can be used as a tool for the simulation of instrument signals. It has got eight radiative transfer solvers. There are four different methods given for the spectral calculations, depending on the time and the purpose of the calculation is used for. For both radiative transfer models, all infrared channels except the water vapour channels are used and the optical thickness, effective droplet radius, surface temperature and the satellite zenith angle are varied. The aim of this study is to find out which radiative transfer model is most useful for the retrieval of FLS microphysics. Output from both radiative transfer models is presented and compared. The implementation in FLS property retrieval is shown and discussed.
Zenith
Effective radius
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Abstract In the Earth Sciences, the 3D radiative transfer equation is often solved for by Monte Carlo (MC) methods. They can, however, be computationally taxing, and that can narrow their range of application and limit their use in explorations of model parameter spaces. A novel family of MC algorithms is investigated here in which single simulations provide estimates of both radiative quantities A for a set of parameters , as usual, as well as the overarching functional ( x ) that can be evaluated, extremely efficiently, at any x . One such algorithm is developed and demonstrated for horizontally averaged broadband solar radiative fluxes as functions of surface albedo for uniform Lambertian surfaces beneath inhomogeneous cloudy atmospheres. Simulations for a high‐resolution synthetic cloud field, at various solar zenith angles, illustrate the potential of the method to gain insights into the nature of 3D radiative effects for complicated atmosphere‐surface conditions using information specially derived from the MC simulation. For simulations performed with a single surface albedo it is found that as surface albedo increases, 3D radiative effects increase, too, with maxima occurring at middling to large values, and then decrease. By utilizing the derived coefficients that describe it was established that these 3D effects stem from differences in fractions of radiation entrapped at successive orders of internal multiple reflections for 1D and 3D transfer.
Albedo (alchemy)
Zenith
Solar zenith angle
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A fast radiative transfer model has been developed for simulating high‐resolution absorption bands. The first scattering radiance is calculated accurately by using the higher number of layers and streams for all required wave number grids. The multiple‐scattering component is extrapolated and/or interpolated from a finite set of calculations in the space of two integrated gaseous absorption optical depths to the wave number grids: a double‐ k approach. The double‐ k approach substantially reduces the error due to the uncorrelated nature of overlapped absorption lines. More importantly, these finite multiple‐scattering radiances at specific k (λ i ) values are computed with a reduced number of layers and/or streams in the forward radiative transfer model. To simulate an oxygen A‐band spectrum, 28 calculations of radiative transfer are needed to achieve an accuracy of 0.5% for most applications under all‐sky conditions and 1.5% for the most challenging multiple‐layer cloud systems (99% of spectrum below 0.5%). This represents a thousandfold time reduction in the standard forward radiative transfer calculation.
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Radiative transfer problems may be solved “adjointly” from an observed excident radiance flux distribution backward to the incident fluxes. We describe an adjoint formulation based on the discrete ordinate radiative transfer method, with application to atmospheric radiative transfer, including effects of the surface albedo. We compare this adjoint approach with forward radiative transfer solutions for a set of synthetic cases and also with observed surface irradiance data from a multifilter rotating shadow band radiometer (MFRSR). We compute the irradiances and mean intensities at arbitrary altitudes for fixed sky conditions but varying solar zenith angles. For these cases the adjoint method is comparably accurate and markedly faster.
Radiative flux
Albedo (alchemy)
Solar zenith angle
Zenith
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A dynamic region Monte Carlo method (DRMC) is proposed to simulate radiative heat transfer in participating medium. The basic principle and solution procedure of this method is described; radiative heat transfer in a two-dimensional rectangular region of absorbing, emitting, and/or scattering gray medium is analyzed. A comparison between DRMC and the traditional Monte Carlo method (TMC) is investigated by analyzing the simulated temperature distribution, the computing time, and the number of the sampling bundles. The investigation results show that, to compare with TMC, the DRMC can obviously reduce the computing time and storage capacity under the same solution precision for radiative transfer in optically thick medium; the DRMC allows bypassing the difficulties encountered by TMC in the limit of optically thick extinction.
Thermal Radiation
Extinction (optical mineralogy)
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Abstract. The radiative effect of anthropogenic aerosols over Europe during the 2008 EUCAARI-LONGREX campaign has been calculated using measurements collected by the FAAM BAe-146 aircraft and radiative transfer modelling. The aircraft sampled anthropogenically perturbed air masses across north-western Europe under anticyclonic conditions with aerosol optical depths ranging from 0.047 to 0.357. For one specially designed "radiative closure" flight, simulated irradiances have been compared to radiation measurements in a case of aged European aerosol in order to explore the validity of model assumptions and the degree of "radiative closure" that can be attained for the spatial and temporal variability and measurement uncertainties. Secondly, the diurnally averaged aerosol radiative effect throughout EUCAARI-LONGREX has been calculated. Surface radiative effect ranged between −3.9 and −22.8 Wm−2 (mean −11 ± 5 Wm−2) whilst top of the atmosphere (TOA) values were between −2.1 and −12.0 Wm−2 (mean −5 ± 3 Wm−2). We have quantified the uncertainties in our calculations due to the way in which aerosols and other parameters are represented in a radiative transfer model. The largest uncertainty in the aerosol radiative effect at both the surface and the TOA comes from the spectral resolution of the information used in the radiative transfer model (~ 17 %) and the aerosol description (composition and size distribution) used in the Mie calculations of the aerosol optical properties included in the radiative transfer model (~ 7 %). The aerosol radiative effect at the TOA is also highly sensitive to the surface albedo (~ 12 %).
Radiative flux
Albedo (alchemy)
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Stokes parameters
Trace gas
Linearization
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