## Examination of surface roughness on light scattering by long ice columns by use...

Natural particles such as ice crystals in cirrus clouds generally are not pristine but have additional microroughness on their surfaces. A two-dimensional finite-difference time-domain ͑FDTD͒ 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 1͞120 incident wavelength for ice circular cylinders with size parameters of 6 and 24 at wavelengths of 0.55 and 10.8 m, respectively, the errors in the FDTD results in the extinction, scattering, and absorption efficiencies are smaller than ϳ0.5%. The errors in the FDTD results in the asymmetry factor are smaller than ϳ0.05%. The errors in the FDTD results in the phase-matrix elements are smaller than ϳ5%. 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.