云粒子偏振探测理论与实验研究

Ice crystals with sizes in the range from 2 to 50 micron play an important role in the sciences of cloud radiation and cloud microphysics. But effective observation of the size spectra of ice crystals can’t be implemented with existing forward scattering probe because it only collects the total amount of the scattered laser in fixed solid angle. The scattering properties of ice crystals with typical shapes will be analyzed using nonspherical scattering theories in this project. The depolarization threshold which can be used to discriminate ice crystals from liquid droplets is retrieved through comparisons between depolarizations of spherical particles and nonspherical particles. The experiment setup of cloud particle detection with polarization measurement will be constructed based on the analysis of the nonspherical scattering. Spherical particulates with known seizes and nonspherical particles will be used to calibrate the experiment setup. Through system calibration and investigations to invertion algorithm of size spectra, the experiment setup will have detection ability of ice crystal addition to that of liquid droplet. In this project, the depolarization is used to determine the phase of cloud particle and the effective size of ice crystal is inverted combining the depolarization and the forward scattering. The research works in this project which has broad application prospects in the field of weather modification, will provide theoretical and technical basises for polarized cloud particle probe used for both ice crystal and liquid droplet detections.

云内尺度为2-50μm的冰晶在云辐射及云微物理科学研究中具有重要作用,但现有的前向散射式云粒子探测器由于只接收一定立体角范围内的激光散射总量而无法实现小冰晶粒径谱的有效探测。本项目提出利用非球形粒子散射理论对典型形状的冰晶散射特性进行理论分析,将球形粒子退偏与非球形粒子退偏进行比较,获得用于判断云粒子相态的退偏阈值。在此基础上建立具有偏振探测功能的云粒子探测实验系统,通过对实验系统的球形、非球形粒子标定和粒径谱反演算法研究,实现实验系统的云滴和冰晶同时探测功能。项目通过检测云粒子的退偏振确定粒子相态,并使用冰晶对激光的退偏振和前向散射强度反演冰晶的有效尺寸。项目研究为建立能同时测量云滴和冰晶的偏振云粒子探测器提供理论与技术基础,在人工影响天气研究中具有广阔的应用前景。

现有的散射式机载云粒子探测器均基于云滴散射原理，具备相态识别的云粒子探测器能够减少探测过程中由于相态差异引起的误差，对深入了解云内的微物理变化过程和机制具有重要意义。本项目以散射光偏振区分云粒子相态，建立了机载云粒子探测器实验室样机，主要研究内容包括理论分析和实验研究两个方面。非球形粒子散射理论分析方面，使用T矩阵和离散偶极子近似法对非球形粒子散射特性进行分析，模拟了小尺度范围内粒子有效半径、横纵比等形状因子与不同形态粒子的相函数及等效Mie散射误差的变化规律和形状因子对粒子散射截面和退偏等散射参数的影响。实验研究方面，基于偏振探测，建立了可同时探测粒子散射的前向（4 °~14 °）散射能量和后向（176 °~146 °）散射退偏的云粒子探测系统，可用于探测50μm以内的云粒子尺寸和相态。使用固定尺寸分别为2μm，20μm和40μm的标准粒子对系统进行标定，获得探测系统对标准粒子的响应曲线，完成了云粒子探测器的定标。根据不同能量下标准粒子与云滴粒子之间的对应关系，获得系统对云滴粒子的响应曲线。使用离散偶极子近似法对典型的非球形粒子进行散射特性模拟，结合实验数据，表明该探测系统具有一定的非球形粒子探测能力，可用于云粒子的偏振探测。基于以上研究，项目团队申请了国内发明和实用新型专利，研究成果也在美国光学学会期刊applied optics 等期刊上发表。项目建立的新型与栗子探测器经工程化开发后，可用于开发新型机载云粒子探测器，对我国云微物理研究具有一定的促进作用。