Benchmark Quality Open Source Models for Spectral or Broadband 3D Radiative Transfer in Cloudy Atmospheres

The Intercomparison of 3D Radiation Codes (I3RC) community monochromatic 3D Monte Carlo model has been extended to include a source of photon emission from the surface and atmosphere, thereby making it capable of simulating scalar radiative transfer in a 3D scattering, absorbing and emitting domain with either internal or external sources. It was then further extended to allow for broadband calculations using a Monte Carlo spectral integration approach that is statistically unbiased, but costs marginally more than than a monochromatic calculation. The theoretical basis, computational implementation, verification, and computational performance of the resulting models, the “IMC+emission” and “MCBRaT-3D”, are presented. Thorough verification includes fundamental tests of reciprocity, energy conservation, and comparison to analytical solutions. All comparisons to fundamental tests and analytical solutions are accurate to within the precision of the simulations—typically better than 0.05%. Finally, the models are applied to the established I3RC Case 4 cumulus cloud field to provide a benchmark result. Computational performance and strong and weak scaling metrics are presented. The outcome is a thoroughly vetted, publicly available, open source, benchmark tool to study 3D radiative transfer from either solar or internal sources of radiation at wavelengths for which scattering, emission, and absorption are important.