扩展利用太阳/天空辐射计偏振角测量的超薄云检测方法研究

Super-thin clouds with optical depths smaller than 0.3 are usually transparent to human’s eye. Detection of these super-thin clouds is a big challenge for remote sensing. Up to now, any clouds detection methods utilizing intensity measurements are not appropriate for super-thin clouds detection. Polarized remote sensing presents more additional information except intensity. Compared with the degree of polarization (DoP), applications of the angle of polarization (AoP) containing unique information of polarization orientation have been lagged behind. In view of the requirement of super-thin clouds (with optical depths smaller than 0.3) detection, we develop the new algorithm of the super-thin clouds detection based on the saltation features of the angle of polarization. Moreover, we also present a new combined scheme for clouds detection which is based on the angle of polarization, the degree of linear polarization, and the total radiance measured by polarized channels of the sun/sky radiometers. In this study, we innovatively put forward a new polarization calibration protocol adding calibration of the angle of polarization to obtain high-precision measurements of the AoP of skylight. At the same time, we analyze the mechanism of sudden changes of the AoP under different atmospheric conditions with combination of super-thin clouds and aerosols by using 3D vector radiation transfer models. This study can make up for shortfalls in current cloud detection scheme of the sun/sky radiometer which is only based on non-polarization measurements. Meanwhile, this study will be also instructive for detection of super-thin cloud by satellite-based polarization instrument in future.

光学厚度小于0.3的超薄云对于人眼往往是透明不可见的，它们是遥感云检测面临的一大难点。目前无论哪种基于强度测量的云检测方法，对超薄云检测起来都比较困难。偏振遥感在强度探测的基础上增加额外的信息量，但除偏振度外，包含偏振方向信息的偏振角参数的应用相对滞后。本项目针对光学厚度小于0.3的超薄水云/冰云检测问题，提出利用天空光偏振角在超薄云存在情况下某些散射角度上发生突变的特性，发展基于太阳/天空辐射计偏振角测量的超薄云检测算法，并进一步建立综合利用偏振角、线偏振度和偏振通道总辐亮度测量的整体云检测方案。在项目研究中，创新性地提出增加偏振角定标的太阳/天空辐射计新偏振定标方案以定量获取高精度的偏振角测量，并基于3D矢量辐射传输模拟揭示不同超薄云和气溶胶大气组合情况下偏振角突变的机理。本项目弥补了现有地基太阳/天空辐射计云识别算法在超薄云检测中的不足，也为后续星载偏振遥感的超薄云检测积累研究基础。

新一代Cimel太阳/天空偏振辐射计CE318-DP广泛布设在全球气溶胶自动观测网（AERONET）和中国太阳/天空辐射计观测网（SONET）中，在全球和中国典型区域开展长期连续的偏振测量。虽然CE318-DP已开展十年左右的偏振观测，但一直以来从偏振通道测量数据中获取和利用的天空光信息仅有线偏振度DoLP和总辐亮度I。偏振角AoP等包含偏振方向信息的参量对大气中散射粒子形状等某些微观特性更加敏感，但获取偏振角需要解决偏振片优势透光轴方向精确定标的难题，它在大气遥感中的定量应用潜力远未得到充分挖掘。. 本项目在太阳/天空辐射计传统偏振定标流程的基础上，更加精细地考虑了仪器观测过程中偏振片转轮磨损导致的偏振片相对取向误差，提出了利用旋转参考线偏振片装置同时标定相对取向夹角误差和偏振片效率的新偏振定标方法，使线偏振度定标精度大幅提高，平均绝对偏差下降10倍。新太阳/天空辐射计偏振定标方案使用的定标设备成本低、操作自动化程度高、计算处理简单、所有步骤物理意义明晰。该成果除适用于CE318-DP型多通道偏振辐射计外，还可以在上一代Cimel的CE318-2型单通道偏振辐射计和更新一代CE318-TP型偏振辐射计、以及其他同类型仪器上推广应用。在提高偏振测量精度的基础上，本项目发展了利用天空光偏振角的突变特性进行云检测的算法并建立了综合利用太阳/天空辐射计偏振测量获得的偏振角、线偏振度和总辐亮度的偏振通道整体云检测方案，经地基激光雷达同步对比验证，对超薄云、薄云和厚云均能有效检测。弥补了现有的地基太阳/天空辐射计缺少偏振通道云检测的不足，为后续开展太阳/天空辐射计偏振观测的业务化处理和反演奠定了基础，也为后续星载偏振遥感的超薄云检测积累了研究基础。截至项目结题，各项计划任务均取得了良好进展，完成了项目全部预期成果，并超额完成了论文发表考核指标。