Modelling light scattering by rough particles and particulate layers using RTDF

Computations of light-scattering properties for non-axisymmetric particles based on exact methods like the T-matrix [1] and discrete dipole approximation (DDA) [2] have upper size parameter limits of applicability, depending on particle shape and complex refractive index. For moderate values of the sizeparameter 2πa/λ, where a is a characteristic length of the particle and λ the wavelength, the finite difference time domain (FDTD) method can be used [3], but it places too severe demands on computational resources. Thus, despite its limitations, geometric optics and/or physical optics [4] are still the most widely used models for moderate to large size parameters. Improved methods to combine ray-tracing and diffraction have been presented [5-7].
RTDF [7] combines ray-tracing with diffraction by individual facets. It can be applied to arbitrary shapes, as long as they are approximated by planar facets. Implementation of a bounding box method [8] reduces the computational cost. This makes the method suitable for modelling light scattering by rough particles of intermediate size such as ice crystals and dust grains. Recently, it has been found that ice crystal roughness affects important scattering parameters like the asymmetry parameter [9]. In this contribution, RTDF results for rough hexagonal ice crystals [10] will be presented. Furthermore, RTDF has been applied to model ice crystal layers in order to study backcattering. Since, unlike in radiative transfer, the amplitudes of the reflected as well as the refracted ray are traced for each ray-surface interaction, and diffraction of light leaving the layer is taken account of, it should be possible to model coherent backscattering.