Scattering by Irregular Particles in Anomalous Diffraction and Discrete Dipole Approximations

The effects of irregular particles on scattering of radiation are considered using theoretical methods. Major motivation for studying scattering by irregular particles is presented on the basis of results from a simple climate model. These results indicate that uncertainties in our knowledge of the asymmetry parameter and extinction factors significantly affect the response of this model to an imposed climate change. The discrete dipole approximation (DDA) is considerably extended in this study. It is shown that this is a robust method as it includes boundary conditions in its formulation, is numerically stable, and is computationally fast. The method is validated against exact results. DDA is then applied to study scattering by particle shapes typical of ice crystal clouds. Parametrization of scattering properties of irregular particles is considered on the basis of the anomalous diffraction theory. The new approach to the ADT is introduced based on a ray tracing and compared to DDA for selected geometries. We develop the ADT for particles distributed in size. A key result is that the similarity scaling predicted by the ADT for spheres is also shown to apply for nonspherical particles. The results represent a complete parameterization of the three main optical properties: the volume extinction beta _{rm ext}, the single scattering albedo omega, and the particle asymmetry parameter g, as a function of refractive index, characteristic radius of the distribution and the distribution width.