Electromagnetic Scattering by Spheroidal Particles

Clouds are of paramount importance for the global energy balance and, thereby, our climate. Changes in cloud cover and phase (liquid water versus ice), for example, through increased greenhouse forcing, may have significant and as of yet unknown impacts on our climate. The global climate models (GCMs) designed to predict future climate, usually model the effects of clouds using the scattering and absorption properties of spherical particles at high latitudes as well as at high enough altitudes anywhere on our planet. This leads to errors of undetermined magnitude because the clouds there consist of ice crystals that are far from spherical in shape. Ice particles usually take on needle-like or flat, disk-like shapes. The GCMs therefore cannot correctly predict the evolution of our climate. We have developed a new method for calculating the single scattering solution for spheroidal particles. The single scattering solution is needed for every particle shape that we want to include in a GCM. The spheroidal particles can easily be made to closely resemble actual ice particles, and we can hence, more accurately model the scattering and absorption of radiation by polar and high altitude clouds. An important part of the single scattering solution for spheroidal particles is the calculation of the expansion coefficients that we need in the angular and radial spheroidal functions. Problems that hampered previous implementations for finding these coefficients have been overcome, and we can now handle realistic sizes and shapes, as well as particle absorption in an effective manner. In this paper, we present our new method for computation of expansion coefficients.