CrystalTrace: a Monte Carlo raytracing algorithm for radiative transfer in cirrus clouds with oriented ice crystals

Abstract. We present CrystalTrace, a Monte Carlo raytracing algorithm designed for radiative transfer computations involving ice crystals with both random and preferred orientations. Integrated into the three-dimensional radiative transfer solver MYSTIC, which is part of libRadtran, CrystalTrace enables multiple-scattering simulations of absolute radiances for realistic atmospheric scenes, including clouds with different mixtures of ice crystals and water droplets, aerosols, molecules, and surface properties. It seamlessly extends MYSTIC's macroscopic raytracing down to the microscopic scale of individual ice crystals. By computing single-scattering properties online, CrystalTrace removes the need for precomputed look-up tables, which are also subject to limited angular resolution. The current version of CrystalTrace is implemented for the visible spectral range and supports individual hexagonal prisms with different sizes and aspect ratios. It currently does not account for diffraction and ice absorption. CrystalTrace, integrated with MYSTIC, fills a critical gap by providing a computationally efficient forward radiative transfer simulator for ice clouds containing oriented crystals, thereby enabling retrievals of ice crystal properties from ground-based, airborne, and satellite imaging observations. CrystalTrace is a Monte-Carlo raytracing algorithm for radiative transfer simulations of oriented ice crystals.