PARTICLE SCATTERING IN THE RESONANCE REGIME - FULL-WAVE SOLUTION FOR AXISYMMETRICAL PARTICLES WITH LARGE ASPECT RATIOS

Reliable descriptions of the optical properties of clouds and aerosols are essential for studies of radiative transfer in planetary atmosphe res. Mie scattering algorithms provide accurate estimates of these pro perties for spherical particles with a wide range of sizes and refract ive indices, but these methods are not valid for nonspherical particle s (e.g., ice crystals, mineral dust, and smoke). Even though a host of methods exist for deriving the optical properties of nonspherical par ticles that are very small or very large compared with the wavelength, only a few methods are valid in the resonance regime, where the parti cle dimensions are comparable with the wavelength. Most such methods a re not ideal for particles with sharp edges or large axial ratios. We explore the utility of an integral equation approach for deriving the single-scattering optical properties of axisymmetric particles with la rge axial ratios. The accuracy of this technique is shown for spheres of increasing size parameters and an ensemble of randomly oriented pro late spheroids of size parameter equal to 10.079368. In this last case our results are compared with published results obtained with the T-m atrix approach. Next we derive cross sections, single-scattering albed os, and phase functions for cylinders, disks, and spheroids of ice wit h dimensions extending from the Rayleigh to the geometric optics regim e. Compared with those for a standard surface integral equation method , the storage requirement and the computer time needed by this method are reduced, thus making it attractive for generating databases to be used in multiple-scattering calculations. Our results show that water ice disks and cylinders are more strongly absorbing than equivalent vo lume spheres at most infrared wavelengths. The geometry of these parti cles also affects the angular dependence of the scattering. Disks and columns with maximum linear dimensions larger than the wavelength scat ter much more radiation in the forward and backward directions and muc h less radiation at intermediate phase angles than equivalent volume s pheres. (C) 1997 Optical Society of America [S0740-3232(97)02102-9]