Helios-k: an Ultrafast, Open-source Opacity Calculator for Radiative Transfer

We present an ultrafast opacity calculator that we name HELIOS-K. It takes a line list as an input, computes the shape of each spectral line and provides an option for grouping an enormous number of lines into a manageable number of bins. We implement a combination of Algorithm 916 and GaussHermite quadrature to compute the Voigt profile, write the code in CUDA and optimise the computation for graphics processing units (GPUs). We restate the theory of the k-distribution method and use it to reduce ∼ 105 to 108 lines to ∼ 10 to 104 wavenumber bins, which may then be used for radiative transfer, atmospheric retrieval and general circulation models. The choice of line-wing cutoff for the Voigt profile is a significant source of error and affects the value of the computed flux by ∼ 10 DOI: https://doi.org/10.1088/0004-637X/808/2/182 Posted at the Zurich Open Repository and Archive, University of Zurich ZORA URL: https://doi.org/10.5167/uzh-122015 Accepted Version Originally published at: Grimm, Simon L; Heng, Kevin (2015). HELIOS-K: an ultrafast, open-source opacity calculator for radiative transfer. The Astrophysical Journal, 808(2):182. DOI: https://doi.org/10.1088/0004-637X/808/2/182 ar X iv :1 50 3. 03 80 6v 2 [ as tr oph .E P] 1 8 Ju n 20 15 DRAFT VERSION JUNE 22, 2015 Preprint typeset using LTEX style emulateapj v. 05/12/14 HELIOS-K: AN ULTRAFAST, OPEN-SOURCE OPACITY CALCULATOR FOR RADIATIVE TRANSFER SIMON L. GRIMM & KEVIN HENG Draft version June 22, 2015 ABSTRACT We present an ultrafast opacity calculator that we name HELIOS-K. It takes a line list as an input, computes the shape of each spectral line and provides an option for grouping an enormous number of lines into a manageable number of bins. We implement a combination of Algorithm 916 and Gauss-Hermite quadrature to compute the Voigt profile, write the code in CUDA and optimise the computation for graphics processing units (GPUs). We restate the theory of the k-distribution method and use it to reduce ∼ 10–10 lines to ∼ 10–10 wavenumber bins, which may then be used for radiative transfer, atmospheric retrieval and general circulation models. The choice of line-wing cutoff for the Voigt profile is a significant source of error and affects the value of the computed flux by ∼ 10%. This is an outstanding physical (rather than computational) problem, due to our incomplete knowledge of pressure broadening of spectral lines in the far line wings. We emphasize that this problem remains regardless of whether one performs line-by-line calculations or uses the k-distribution method and affects all calculations of exoplanetary atmospheres requiring the use of wavelength-dependent opacities. We elucidate the correlated-k approximation and demonstrate that it applies equally to inhomogeneous atmospheres with a single atomic/molecular species or homogeneous atmospheres with multiple species. Using a NVIDIA K20 GPU, HELIOS-K is capable of computing an opacity function with ∼ 10 spectral lines in ∼ 1 second and is publicly available as part of the Exoclimes Simulation Platform (ESP; www.exoclime.org). Subject headings: radiative transfer — planets and satellites: atmospheres — methods: numericalWe present an ultrafast opacity calculator that we name HELIOS-K. It takes a line list as an input, computes the shape of each spectral line and provides an option for grouping an enormous number of lines into a manageable number of bins. We implement a combination of Algorithm 916 and Gauss-Hermite quadrature to compute the Voigt profile, write the code in CUDA and optimise the computation for graphics processing units (GPUs). We restate the theory of the k-distribution method and use it to reduce ∼ 10–10 lines to ∼ 10–10 wavenumber bins, which may then be used for radiative transfer, atmospheric retrieval and general circulation models. The choice of line-wing cutoff for the Voigt profile is a significant source of error and affects the value of the computed flux by ∼ 10%. This is an outstanding physical (rather than computational) problem, due to our incomplete knowledge of pressure broadening of spectral lines in the far line wings. We emphasize that this problem remains regardless of whether one performs line-by-line calculations or uses the k-distribution method and affects all calculations of exoplanetary atmospheres requiring the use of wavelength-dependent opacities. We elucidate the correlated-k approximation and demonstrate that it applies equally to inhomogeneous atmospheres with a single atomic/molecular species or homogeneous atmospheres with multiple species. Using a NVIDIA K20 GPU, HELIOS-K is capable of computing an opacity function with ∼ 10 spectral lines in ∼ 1 second and is publicly available as part of the Exoclimes Simulation Platform (ESP; www.exoclime.org). Subject headings: radiative transfer — planets and satellites: atmospheres — methods: numerical