改进的单次散射相函数解析表达式

Modified analytic expression for the single-scattering phase function

单次散射相函数对电磁辐射传输模拟过程的准确性和计算效率有重要的影响.基于电磁散射与辐射传输中的基本理论,对单次散射相函数的解析表达式进行了研究,提出了一种新的单次散射相函数解析表达式.比较了单个粒子的Henyey-Greenstein相函数、Henyey-Greenstein*相函数与新的相函数随角度的分布,发现新的散射相函数提高了后向散射峰值,可以更合理地描述单个粒子的散射特性.按三种气溶胶粒子谱分布模式计算了Henyey-Greenstein*相函数和新的相函数对应的数值结果,并与多分散系Mie散射相函数进行对比,发现新的相函数提高了与多分散系Mie散射相函数的符合程度.研究表明,对于大角度(大于90°)后向散射,新的相函数与Mie散射相函数均方根差较小的占73.3%,高于Henyey-Greenstein*相函数的26.7%,证明了新的相函数可以显著提高后向散射峰值.新的相函数对准确模拟辐射传输过程具有重要意义.

In electromagnetic radiative transfer calculation, the accuracy and the computation time are usually determined by the representation of single-scattering phase function. Accurate calculation is time consuming even for spherical particle,thus, an analytic representation is commonly adopted to approximate the exact phase function and then accelerate the calculation. Most widely used single-scattering phase functions are the Henyey-Greenstein phase function and modified Henyey-Greenstein phase function（Henyey-Greenstein＊）. Although the Henyey-Greenstein phase function and the Henyey-Greenstein＊ phase function can represent the forward-scattering peak of Mie-scattering phase function well,they fail to reproduce the backscattering behavior, limiting the accuracy of the calculation. In order to better fit exact calculations and simulate the backward-scattering peak, we develop a new analytic expression based on the fundamental theory of electromagnetic scattering and radiation transmission. This phase function is an algebraic expression with one single free parameter（asymmetry factor）, and can be expanded into Legendre polynomials. The new phase function converges to the Rayleigh phase function when the asymmetry factor approximates to 0, and it can approach to the Henyey-Greenstein phase function as the asymmetry factor is about 1. We compare the Henyey-Greenstein phase function, the Henyey-Greenstein＊ phase function, and the new phase function for different asymmetry factors, and find that the new phase function provides a more realistic description for the unpolarized light scattering from small particles. Furthermore, the calculated value for the ratio of the scattering intensity at 90 degree to that in the backward direction is more reasonable. We also investigate the effectiveness by approximating the scattering from polydispersed particles through comparing the new phase function, the Henyey-Greenstein＊ phase function, and the Mie-scattering phase function for three types of Derimendjian＇s polydispersions. Results show that the new phase function fits the Mie-scattering phase function much better than the Henyey-Greenstein＊ phase function. For the new phase function,the root-mean-square error is small for 73.3% data. By contrast, only 26.7% data fit the Mie-scattering phase function well for the Henyey-Greenstein＊ phase function. Similarly, the effectiveness of new function is most significant when calculating the ratio of the scattering intensity at 90 degree to that in the backward direction. In summary, the new Henyey-Greenstein＊ phase function provides a more accurate calculation for the scattering intensity in the backward direction, and is conducive to electromagnetic radiative transfer calculation. Furthermore, because the proposed phase function has the same basic form as the Heny-Greenstein phase function, reformatting radiative transfer model in terms of the new phase function should require relatively little effort.