基于谱分辨的Raman激光雷达同时探测大气中水汽、液水和冰水的新方法

Knowledge of water phase and water content is essential for study of various terrestrial atmospheric processes. The vapor water Raman lidar is able to retrieve reasonable vaporous water profiles under clear weather conditions, except for when cloud (even though optically thin) or fluorescence from aerosol particles is present. As sharing common range in spectrum, both the Raman signals due to cloud liquid or ice water and aerosol fluorescence can interfere with the vapor Raman signal seriously. Nevertheless, the vapor water Raman lidar fails to detect and eliminate such interferences in an effective way, nor can it obtain information about atmospheric liquid or ice water simultaneously. This project proposes to utilize a spectrally resolved Raman lidar to achieve measurements of Raman signals from atmospheric three-phase water and fluorescence from aerosols together. The lidar has already been developed successfully and employs a home-made dual-grating dispersing system to expand the spectral detection range to 393-424 nm which covers the spectra of three-phase water. Based on the lidar-measured spectra, the method to detect and eliminate aerosol fluorescence as well as water phase discrimination and water content retrieval is supposed to be developed. The new lidar is capable of working under various weather conditions, providing new ways to study the atmospheric water and cloud.

精确掌握水相与水含量等信息对许多地球大气过程研究都是必需的。水汽Raman激光雷达在晴朗天气下常能获得合理的水汽剖面，但在有云（即使是薄云）或有气溶胶粒子产生荧光时失效。原因是云中液水或冰水Raman信号、气溶胶荧光在频谱上与水汽Raman 信号重叠并造成显著干扰。对此，水汽Raman激光雷达并无有效的检测与排除“干扰”手段，且不能同时获得大气中液水与冰水信息。项目基于自主研制成功的谱分辨的水Raman激光雷达，利用自制的双光栅分光系统将探测谱区拓展为393-424 nm以涵盖三相态水的Raman谱，实现对大气中三相态水和气溶胶荧光的同时探测。借助lidar实测谱，对气溶胶荧光进行检测与剔除，进一步发展水相识别与各相态水含量获取的新方法。该类型雷达可胜任更多天气状况下的工作，为研究大气水汽和云提供了新的手段。

针对传统的水汽Raman激光雷达无法应对气溶胶荧光谱信号、云区凝结水Raman谱信号干扰的难题，以及不能提供液水与冰水信息的缺陷，项目提出采用自主研制成功的新型谱分辨的水Raman激光雷达同时探测大气中水汽、液水和冰水的新方法。该谱分辨的水Raman激光雷达采用一个自主研制的双光栅分光系统，可同时分辨测量393-424 nm光谱范围内的回波信号，完全覆盖了三相态水的Raman谱（395-418 nm）。自2016年初到2018年底，运用该谱分辨的水Raman激光雷达在不同天气条件下对武汉上空大气进行了夜间探测。基于八个不同晴朗夜晚实测谱数据，国际上首次得到由雷达实测的完整的大气水汽归一化Raman谱，发现该谱谱型几乎不随时间和高度变化，与理论谱相符，可作为混合相态水Raman谱分解的背景谱，为混合相态水Raman谱分量的分离提取奠定了基础。借助探测光谱区两端的荧光谱信号，建立了荧光识别、以及在判定荧光存在时的荧光谱剔除算法。在荧光谱剔除算法及获得的水汽归一化Raman谱的基础上，发展出依次分解提取荧光谱、水汽和凝结水（液水与/或冰水）Raman谱分量的算法。运用该谱分解算法处理一次典型的冰幡事件，在依次提取出荧光谱、水汽Raman谱分量之后，发现剩余的凝结水Raman谱与理论冰水Raman谱高度相似，认为凝结水Raman谱为冰水产生，这与同步的偏振激光雷达探测结果相符，证明了发展的谱分解算法的有效性，也表明该种新型的谱分辨的水Raman激光雷达实现了对大气中水汽、凝结水（液水与/或冰水）的同时分辨测量。这种新型的谱分辨的水Raman激光雷达，彻底解决了传统的水汽Raman激光雷达遭遇的两个难题，能够胜任更多天气状况下的工作，也为研究大气气溶胶与水的相互作用、云的微物理特性等提供了全新的视角与手段。