Absorption of electromagnetic radiation by absorptive dielectric spheres such as snow grains in the near-infrared part of the solar spectrum cannot be neglected when computing radiative properties of snow. Thus, a new geometric-optics code is developed to compute scattering and absorption cross sections of large dielectric particles of arbitrary complex refractive index. The number of internal reflections and transmissions are truncated based on the ratio of the irradiance incident at the n-th interface to the irradiance incident at the first interface for a specific optical ray. Thus the truncation number is a function of the angle of incidence. Phase functions for both near- and far-field absorption and scattering of electromagnetic radiation are calculated directly at any desired scattering angle using a hybrid algorithm based on the bisection and Newton-Raphson methods. With these methods the absorption and scattering properties of light by a large sphere can be calculated for any wavelength from the ultraviolet to the microwave region. Assuming that large snow meltclusters (1 cm-order), observed ubiquitously in the snow cover during summer, can be characterized as spheres, one may compute absorption and scattering efficiencies, and the scattering phase function on the basis of this geometrical optics method. A “Geometrical Optics Method for SPHERE” code  (GOMsphere) is developed and tested against Wiscombe’s Mie scattering code (MIE0) and a Monte Carlo code for a range of size parameters. GOMsphere can be combined with MIE0 to calculate the single scattering properties of dielectric spheres of any size.

The GOMsphere code is a computer program written in ANSI 77 standard FORTRAN to calculate the absorption coefficients, scattering coefficients and phase function for both near- and far-field scattering of unpolarized radiation by large homogeneous dielectric spheres. For general-purpose applications, the input parameters required are the wavelength of the light in meters, the radius of the particle in meters, the particle refractive index, and the scattering angle in radians. If only absorption and scattering cross sections are needed, the scattering angle is irrelevant. The code and associated information can be downloaded from below. Download GOMsphere.zip, which is winzipped, then right-click it and select "Extract All...", you will get two *.f files: GOMsphere.f and GOMsphereMAIN.f.

1. GOMsphere document
2. GOMsphere codes: GOMsphere.zip