Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics
Influence of particle size and shape on the backscattering linear depolarisation ratio of small ice crystals – cloud chamber measurements in the context of contrail and cirrus microphysics
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DOI：
10.5194/acp-12-10465-2012
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发表时间：
2012-11
影响因子：
6.3
通讯作者：
M. Schnaiter;S. Büttner;O. Möhler;J. Skrotzki;M. Vragel;R. Wagner
中科院分区：
文献类型：
--
作者：
M. Schnaiter;S. Büttner;O. Möhler;J. Skrotzki;M. Vragel;R. Wagner
抽象的。本文介绍了安装在卡尔斯鲁厄理工学院大型气溶胶和云室设施 AIDA 上的激光散射和去偏振仪器 SIMONE。 SIMONE 使用 488 nm 连续激光器通过测量来自 1.8° 和 178.2° 方向的散射光来探测模拟大气云。在 178.2° 处，分析散射光的线偏振状态，以推导出粒子线退偏振比 δp，这是大气激光雷达应用的常见测量参数。给出了去偏振检测概念的光学设置和数学形式。 SIMONE 球状赤铁矿气溶胶和过冷液体云中的去极化测量用于验证仪器。根据冰粒子微物理对 δp 的影响，分析了在 195 至 225 K 温度之间进行的一系列 AIDA 冰成核实验的 SIMONE 数据。我们发现，在体积当量直径仅为几微米的小生长和升华冰粒的情况下，去极化值高达 0.4。使用 T 矩阵方法进行的建模表明，假设球形和圆柱形颗粒的尺寸分布受到红外消光光谱的限制，则可以准确地再现测量的去偏振比。基于 T 矩阵建模运行，我们证明，在小冰晶的情况下，SIMONE 去偏振结果可以代表激光雷达去偏振比，该比是在 180° 的精确反向散射方向上测量的。讨论了我们的结果与解释最近卷云和尾迹激光雷达观测结果的相关性。根据我们的结果，星载激光雷达 CALIOP 在热带对流层上层观测到的高去极化率可能暗示深对流系统的流出物中存在小（升华）冰颗粒。
Abstract. The article presents the laser scattering and depolarisation instrument SIMONE that is installed at the large aerosol and cloud chamber facility AIDA of the Karlsruhe Institute of Technology. SIMONE uses a 488 nm cw laser to probe simulated atmospheric clouds by measuring the scattered light from the 1.8° and 178.2° directions. At 178.2°, the scattered light is analysed for the linear polarisation state to deduce the particle linear depolarisation ratio δp which is a common measurement parameter of atmospheric lidar applications. The optical setup and the mathematical formalism of the depolarisation detection concept are given. SIMONE depolarisation measurements in spheroidal hematite aerosol and supercooled liquid clouds are used to validate the instrument. SIMONE data from a series of AIDA ice nucleation experiments at temperatures between 195 and 225 K were analysed in terms of the impact of the ice particle microphysics on δp. We found strong depolarisation values of up to 0.4 in case of small growing and sublimating ice particles with volume equivalent diameters of only a few micrometers. Modelling runs with the T-matrix method showed that the measured depolarisation ratios can be accurately reproduced assuming spheroidal and cylindrical particles with a size distribution that has been constrained by IR extinction spectroscopy. Based on the T-matrix modelling runs, we demonstrate that in case of small ice crystals the SIMONE depolarisation results are representative for the lidar depolarisation ratio which is measured at exact backscattering direction of 180°. The relevance of our results for the interpretation of recent lidar observations in cirrus and contrails is discussed. In view of our results, the high depolarisation ratios observed by the spaceborne lidar CALIOP in the tropical upper troposphere might be a hint for the presence of small (sublimating) ice particles in the outflows of deep convective systems.