大气折射对可见光波段辐射传输特性的影响

折射是影响辐射传输的重要因素. 为分析大气折射对辐射传输的影响, 基于Monte Carlo方法, 给出了考虑大气折射的矢量辐射传输模型, 实现了均匀气层和耦合面处光子随机运动过程的模拟, 实现了直射光及漫射光Stokes矢量、偏振度和辐射通量等参数的计算. 在考虑和不考虑大气折射两种条件下, 验证了模型的准确性; 在纯瑞利散射条件下, 讨论了大气折射对不同方向漫射光Stokes矢量的影响; 在不同太阳天顶角、大气廓线、气溶胶及含云大气条件下, 分析了大气折射对辐射传输过程的影响. 结果表明: 大气折射对漫射光Stokes矢量的影响主要体现在天顶角70°–110°区间, 且随着太阳入射角增大, 其影响更为显著; 不同大气廓线情形下, 大气折射对Stokes矢量的影响不一致, 其原因是不同大气廓线对应的折射率廓线存在差异. 含云及含气溶胶大气条件下, 大气折射对辐射传输的影响变弱, 沙尘型及海盐型气溶胶条件下, 折射对辐射传输的影响强于可溶型气溶胶情形; 不同形状气溶胶条件下, 大气折射对辐射传输的影响也存在显著差异; 不同云高条件下, 大气折射对漫射光Stokes矢量的影响无显著差异, 但随着云光学厚度增大, 大气折射的影响减弱.

Influence of atmospheric refraction on radiative transfer at visible light band

Refraction is an important factor influencing radiative transfer since it can change both the propagation path and polarization state of electromagnetic wave. In order to discuss the influence of atmospheric refraction on radiative transfer process, a Monte Carlo vector radiative transfer model, which takes atmospheric refraction into account, is introduced. By using this model, photon random movement in uniform atmospheric layer and at the interfaces between adjacent layers is simulated, Stokes vectors and degrees of polarizations of both directly transmitted and diffuse light, and irradiance at the specific layer is also calculated. The model is validated under two conditions: with taking atmospheric refraction into account, and comparing the simulation results with those in the literature; with taking refraction index distributed homogeneously in space, in which case the model is validated against DISORT and RT3. So, the results indicates that our model is accurate and reliable. The influences of atmospheric refraction on the Stokes vectors of diffuse light in different directions are discussed for pure molecular atmosphere, with only Rayleigh scattering considered. Simulations are performed respectively for different solar zenith angles, for different atmospheric profiles, for aerosols with different types and particle shapes, and for clouds with different base heights and optical depths, and correspondingly, the effect of atmospheric refraction on radiative transfer process is discussed as well. Simulation results show that Stokes vector of diffuse light is influenced by atmospheric refraction to a certain extent, especially for light with a zenith angle ranging from 70° to 110°, and with the increasing of solar zenith angle, the influence becomes stronger. When atmospheric profile changes, the effect of atmospheric refraction on polarized radiance field is also changed, for which the possible reason is that deference between atmospheric profiles leads to the variation of refraction index profile. When aerosol and cloud are taken into account, the influence of atmospheric refraction is reduced because of the decreasing of the ratio between side-scattering energy and backward scattering energy. Comparing the simulation results for different aerosol particles shows that the influences of atmospheric refractions in mineral and sea salt aerosol are much stronger than that in water soluble aerosol, besides, there is also great discrepancy among results for aerosols with different shapes. This phenomenon may be explained by the differences in scattering ability and spatial distribution of scattering energy among different aerosols. For cloud, there is no significant difference in result among different cloud base heights, while with the increasing of cloud optical depth, the influence of atmospheric refraction on polarized radiance is gradually weakened.