24-10-2012, 05:25 PM
Examination of Surface Roughness on Light Scattering by Long Ice Columns by Use of a Two-Dimensional Finite-Difference Time-Domain Algorithm
ABSTRACT
Natural particles such as ice crystals in cirrus clouds generally are not pristine but have additional micro-roughness on
their surfaces. A two-dimensional finite-difference time-domain (FDTD) program with a perfectly matched layer absorbing
boundary condition is developed to calculate the effect of surface roughness on light scattering by long ice columns. When
we use a spatial cell size of 1/120 incident wavelength for ice circular cylinders with size parameters of 6 and 24 at
wavelengths of 0.55 and 10.8 mum, respectively, the errors in the FDTD results in the extinction, scattering, and absorption
efficiencies are smaller than similar to 0.5%. The errors in the FDTD results in the asymmetry factor are smaller than similar
to 0.05%. The errors in the FDTD results in the phase-matrix elements are smaller than similar to 5%. By adding a
pseudorandom change as great as 10% of the radius of a cylinder, we calculate the scattering properties of randomly oriented
rough-surfaced ice columns. We conclude that, although the effect of small surface roughness on light scattering is negligible,
the scattering phase-matrix elements change significantly for particles with large surface roughness. The roughness on the
particle surface can make the conventional phase function smooth. The most significant effect of the surface roughness is the
decay of polarization of the scattered light.