Path-tracing Monte Carlo Libraries for 3D Radiative Transfer in Cloudy Atmospheres
Résumé
Interactions between clouds and radiation are at the root of many difficulties in numeri-
cally 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 an open-source library for general use in Monte Carlo
algorithms. This library is devoted to the acceleration of path-tracing in complex data,
typically high-resolution large-domain grounds and clouds. The main algorithmic advances
embedded in the library are those related to the construction and traversal of hierarchical
grids accelerating the tracing of paths through heterogeneous fields in null-collision (maxi-
mum cross-section) algorithms. We show that with these hierarchical grids, the computing
time is only weakly sensitivive to the refinement of the volumetric data. The library is tested
with a rendering algorithm that produces synthetic images of cloud radiances. Two other
examples are given as illustrations, that are respectively used to analyse the transmission
of solar radiation under a cloud together with its sensitivity to an optical parameter, and
to assess a parametrization of 3D radiative effects of clouds.