A practical guide to writing a radiative transfer code

Using our decades-long experience in radiative transfer (RT) code development for Earth science, we endeavor to reduce the knowledge gap of bringing RT from theory quickly. Despite numerous classic and recent literature, it is still hard develop an scratch within a few weeks. It equally understand, not mention modify, existing “monster” code, which developer either located remotely or has retired. Following format “Numerical Recipes” by Press et al., collocate this paper small pieces necessary with corresponding code. These are arranged order that natural development, often opposite laying out theoretical basis. We focus on unpolarized monochromatic solar radiation plane-parallel atmosphere over reflecting surface. Both surface homogeneous (uniform) at all directions. The multiple scattering numerically solved using deterministic method Gauss-Seidel iterations. Except presented Python-Numba open-source gsit, does report any new scientific results, but rather serves as academic demonstration. If time issue reader familiar basic concepts theory, recommend proceeding directly Sec. 3 “RT development”. Program title: gsit (pronounced “jeezit”) CPC Library link program files: https://doi.org/10.17632/d3zt5zhx49.1 Developer's repository link: https://github.com/korkins/gsit Licensing provisions: MIT Programming language: Python Nature problem: present tutorial (non-stochastic) numerical simulation light bounded below problem simplified form (i.e., uniform atmosphere, no polarization, reflectance, etc.) better explain features, than physics, propagation atmosphere. Solution method: relies Fourier decomposition Radiative Transfer Equation azimuth, Gauss quadrature integration zenith iterative process height (optical depth) analytical (hence known) single approximation being starting point. relatively simple require external libraries.