Path-tracing Monte Carlo Libraries for 3D Radiative Transfer in Cloudy Atmospheres

Interactions between clouds and radiation are at the root of many difficulties in numerically predicting future weather and climate and in retrieving the state of the atmosphere from remote sensing observations. The large range of issues related to these interactions, and in particular to three-dimensional interactions, motivated the development of accurate radiative tools able to compute all types of radiative metrics, from monochromatic, local and directional observables, to integrated energetic quantities. In the continuity of this community effort, we propose here open-source radiative tools based on Monte Carlo algorithms. These tools are designed and distributed as a set of modular, independent libraries allowing the rapid design of specific codes for each new applicative requirement. Besides the implementation of state-of-the-art data access, parallelization and ray-casting acceleration techniques, we make use of integral formulations and null-collision algorithms to handle complex heterogeneous cloudy fields. Three example algorithms are described to illustrate the diversity of questions that can be addressed with these tools: rendering of synthetic images, analysis of transmission under a cloud and its sensitivity to an optical parameter, and assessment of a parametrization of 3D radiative effects of clouds. We apply them in realistic shallow cumulus fields output from Large Eddy Simulations.