Sun Wenbo, Loeb Norman G, Videen Gorden, Fu Qiang
Center for Atmospheric Sciences, Hampton University, Hampton, Virginia 23668, USA.
Appl Opt. 2004 Mar 20;43(9):1957-64. doi: 10.1364/ao.43.001957.
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 (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 microm, respectively, the errors in the FDTD results in the extinction, scattering, and absorption efficiencies are smaller than approximately 0.5%. The errors in the FDTD results in the asymmetry factor are smaller than approximately 0.05%. The errors in the FDTD results in the phase-matrix elements are smaller than approximately 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.
诸如卷云中的冰晶之类的天然粒子通常并非纯净的，而是其表面具有额外的微观粗糙度。开发了一种具有完美匹配层吸收边界条件的二维时域有限差分（FDTD）程序，以计算表面粗糙度对长冰柱光散射的影响。当我们分别在波长为0.55和10.8微米时，对于尺寸参数为6和24的冰圆柱体使用1/120入射波长的空间网格尺寸时，FDTD结果在消光、散射和吸收效率方面的误差小于约0.5%。FDTD结果在不对称因子方面的误差小于约0.05%。FDTD结果在相位矩阵元素方面的误差小于约5%。通过添加高达圆柱体半径10%的伪随机变化，我们计算了随机取向的粗糙表面冰柱的散射特性。我们得出结论，尽管小表面粗糙度对光散射的影响可以忽略不计，但对于具有大表面粗糙度的粒子，散射相位矩阵元素会发生显著变化。粒子表面的粗糙度会使传统的相位函数变得平滑。表面粗糙度最显著的影响是散射光偏振的衰减。
