低太阳高度角条件下的天空偏振模式模拟及大气折射影响研究

Atmospheric polarization pattern simulation for small solar elevation angles and the analysis of atmospheric effect

为模拟低太阳高度角条件下的天空偏振模式,自主开发了考虑大气球形几何及大气折射效应的辐射传输模式VSPART,并将其运用于漫射光偏振特性仿真.在模式中,基于射线追踪法实现了光线传播轨迹的追踪和入射光偏振态及透过率的计算,基于矩阵算法实现了辐射传输方程的求解.将VSPART模拟结果与基准值、SPDISORT模拟值进行了比较,验证了模型的准确性.在瑞利散射大气和含气溶胶大气条件下,模拟并分析了漫射光偏振度及偏振方向的分布特征,讨论了大气球形几何及折射效应对天空偏振度的影响.结果表明,低太阳高度角条件下,随着波长增加,瑞利散射大气对应的偏振度整体随之增强,中性点向大天顶角方向移动;气溶胶的存在并不改变天空偏振度分布特征,但对偏振方向影响显著,随着光学厚度的增加,天空偏振度值迅速降低;中性点的偏移可能与低阶散射过程紧密相关;大气球形几何和折射效应的主要影响区域为地平线区域、两中性点附近及天顶区域;瑞利散射大气条件下,随着波长增加,大气球形几何及折射效应的影响逐步减弱,特别在中性点附近及天顶区域,其影响逐步消失;随着气溶胶光学厚度的增加,其影响随之增强.

To simulate the atmospheric polarization pattern for small solar elevation angle, we develop a the vector radiative transfer model VSPART（vector pseudo-spherical radiative transfer model considering refraction）, and use it to calculate the polarization state of downwelling diffuse light. In this model, the propagation trajectory, transmittance rate and polarization states of directly transmitted light are tracked by ray-tracing method for spherical refractive atmosphere.Based on the matrix algorithm, an improved method to solve the radiative transfer equation is proposed. Output of this model includes not only the Stokes vector and degree of polarization of diffuse light, but also the polarized irradiance.The precision of VSPART is validated against the benchmark results, literature results and SPDISORT, and excellent agreement is achieved. DOP（degree of polarization） and AOP（angle of polarization） are simulated for pure Rayleigh scattering atmosphere and atmosphere with aerosol, and the characteristics of their angular distributions are analyzed.In addition, the influences of atmospheric spherical geometry and refraction effect on the sky DOP are discussed as well.Simulation results show that for low solar elevation angle, with the increasing of wavelength, DOP increases gradually,and the Arago and Babinet neutral points move towards the horizon when Rayleigh scattering atmosphere is considered.Although the existence of aerosol does not change the basic distribution of DOP, it has a significant influence on AOP.With the increasing of aerosol optical depth, DOP decreases gradually, and the distribution of AOP changes dramatically.By comparing the sky distribution of DOP, it could also be concluded that the neutral points might arise from low order scattering. The area affected by atmospheric spherical geometry and atmospheric refraction effect mainly includes the area near horizontal directions, the area near the neutral points and the area perpendicular to the ground. For pure Rayleigh scattering atmosphere, the influence is reduced with the increasing of the wavelength of incident light, especially for the areas near the neutral points, where the influence gradually disappears as wavelength increases. For atmosphere with aerosol, with their optical depth increasing, the effects of atmospheric spherical geometry and atmospheric refraction are gradually enhanced.