INCORPORATION OF PHYSICAL OPTICS EFFECTS AND COMPUTATION OF THE LEGENDRE EXPANSION FOR RAY-TRACING PHASE FUNCTIONS INVOLVING DELTA-FUNCTIONTRANSMISSION

The standard geometric optics (GO) technique predicts that the phase f unction for large nonspherical particles with parallel plane facets (e .g., hexagonal ice crystals) should have an infinitesimally narrow del ta-function transmission peak caused by rays twice transmitted(refract ed) in exactly the forward scattering direction. However, exact T-matr ix computations and physical considerations based on the Kirchhoff app roximation suggest that this peak is an artifact of GO completely igno ring 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 convoluti on can be performed with a simple procedure which supplements the stan dard ray-tracing code and makes the computation of the phase function and its Legendre expansion both more physically realistic and more acc urate.