Incorporation of physical optics effects and computation of the Legendre expansion for ray‐tracing phase functions involving δ‐function transmission

The standard geometric optics (GO) technique predicts that the phase function for large nonspherical particles with parallel plane facets (e.g., hexagonal ice crystals) should have an infinitesimally narrow δ‐function transmission peak caused by rays twice transmitted (refracted) in exactly the forward scattering direction. However, exact T ‐matrix computations and physical considerations based on the Kirchhoff approximation suggest that this peak is an artifact of GO completely ignoring physical optics effects and must be convolved with the Fraunhofer pattern, thereby producing a phase function component with an angular profile similar to the standard diffraction component. This convolution can be performed with a simple procedure which supplements the standard ray‐tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more accurate.