Schreier, Franz, S. Gimeno-Garcia, P. Hedelt, M. Hess, J. Mendrok, M. Vasquez, J. Xu (2014)

Schreier, Franz, S. Gimeno-Garcia, P. Hedelt, M.Hess, J. Mendrok, M. Vasquez, J. Xu (2014): GARLIC - A General Purpose Atmospheric Radiative Transfer Line-by-Line Infrared Code: Implementation and Evaluation. Journal of Quantitative Spectroscopy and Radiative Transfer. Elsevier. DOI:
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A suite of programs for high resolution infrared-microwave atmospheric radiative transfer modeling has been developed with emphasis on efficient and reliable numerical algorithms and a modular approach appropriate for simulation and/or retrieval in a variety of applications. The Generic Atmospheric Radiation Line-by-line Infrared Code — GARLIC — is suitable for arbitrary observation geometry, instrumental field-of-view, and line shape. The core of GARLIC's subroutines constitutes the basis of forward models used to implement inversion codes to retrieve atmospheric state parameters from limb and nadir sounding instruments.

This paper briefly introduces the physical and mathematical basics of GARLIC and its descendants and continues with an in-depth presentation of various
implementation aspects: An optimized Voigt function algorithm combined with a two-grid approach is used to accelerate the line-by-line modeling of molecular cross sections; various quadrature methods are implemented to evaluate the Schwarzschild and Beer integrals; and Jacobians, i.e. derivatives with respect to the unknowns of the atmospheric inverse problem, are implemented by means of automatic differentiation. For an assessment of GARLIC's performance, a comparison of the quadrature methods for solution of the path integral is provided. Verification and validation are demonstrated using intercomparisons with other line-by-line codes and comparisons of synthetic spectra with spectra observed on Earth and from Venus.