A study of optical scattering modelling for mixed-phase polar stratospheric clouds
A study of optical scattering modelling for mixed-phase polar stratospheric clouds
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DOI：
10.5194/amt-16-419-2023
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发表时间：
2023-01
影响因子：
3.8
通讯作者：
F. Cairo;T. Deshler;Luca Di Liberto;Andrea Scoccione;M. Snels
中科院分区：
文献类型：
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作者：
F. Cairo;T. Deshler;Luca Di Liberto;Andrea Scoccione;M. Snels
抽象。利用散射程序研究了极地平流层云的光学特性。粒子后向散射和退偏系数可以计算与现有的散射代码，一旦颗粒尺寸分布（PSD）是已知的，并假定一个合适的折射率。然而，PSC通常表现为过冷三元溶液（STS）液滴、固体硝酸三水合物（NAT）和可能的冰颗粒的外部混合物，使得假设单一折射率和单一形态来模拟散射体是有问题的。在这里，我们考虑一组15个符合测量的PSC以上麦默多站，南极洲，使用地基激光雷达，球载光学粒子计数器（OPC）和原位观测的激光后向散射探测仪和OPC在四个气球平流层飞行从基律纳，瑞典。这个独特的微物理和光学观察数据集使我们能够测试当存在不同成分和形状的球形和非球形散射体时光学散射模型的性能。如果粒子的半径小于某个阈值Rth，我们认为粒子是STS，如果半径大于某个阈值Rth，我们认为粒子是NAT，或者可能是冰。米氏散射用于STS，假定为球形。NAT颗粒的散射，被认为是不同纵横比（AR）的球体，在适用的情况下，用T矩阵结果处理。几何光学积分方程的方法被用来当颗粒尺寸参数太大，允许收敛的T-矩阵方法。我们的模型的参数Rth和AR分别在0.1和2 μm之间和0.3和3之间变化，并将计算的后向散射系数和去极化与观测值进行了比较。发现Rth在0.5和0.8 µm之间以及AR小于0.55和大于1.5时的最佳一致性。为了进一步限制AR在确定的区间内的变异性，我们通过逐个改变AR并通过适当选择小于0.55和大于1.5的AR以及0.5和0.8 µm区间内的Rth进一步优化一致性来寻求与实验数据的一致性。以这种方式识别的AR聚集在值0.5和2.5附近。的计算与测量的比较，并进行了讨论。这项工作的结果有助于设置限制PSC固体颗粒的尺寸和非球面的可变性，在我们的模型的T-矩阵散射理论的基础上的适用性范围内，并在一个共同的粒子形状的PSD和一个共同的阈值半径为所有的PSD的假设。
Abstract. Scattering codes are used to study the optical properties of polar stratospheric clouds (PSCs). Particle backscattering and depolarization coefficients can be computed with available scattering codes once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of supercooled ternary solution (STS) droplets, solid nitric acid trihydrate (NAT) and possibly ice particles, making the assumption of a single refractive index and a single morphology to model the scatterers questionable. Here we consider a set of 15 coincident measurements of PSCs above McMurdo Station, Antarctica, using ground-based lidar, a balloon-borne optical particle counter (OPC) and in situ observations taken by a laser backscattersonde and OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows us to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present. We consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if it is above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical. Scattering from NAT particles, considered spheroids of different aspect ratio (AR), is treated with T-matrix results where applicable. The geometric-optics–integral-equation approach is used whenever the particle size parameter is too large to allow for a convergence of the T-matrix method. The parameters Rth and AR of our model have been varied between 0.1 and 2 µm and between 0.3 and 3, respectively, and the calculated backscattering coefficient and depolarization were compared with the observed ones. The best agreement was found for Rth between 0.5 and 0.8 µm and for AR less than 0.55 and greater than 1.5. To further constrain the variability of AR within the identified intervals, we have sought an agreement with the experimental data by varying AR on a case-by-case basis and further optimizing the agreement by a proper choice of AR smaller than 0.55 and greater than 1.5 and Rth within the interval 0.5 and 0.8 µm. The ARs identified in this way cluster around the values 0.5 and 2.5. The comparison of the calculations with the measurements is presented and discussed. The results of this work help to set limits to the variability of the dimensions and asphericity of PSC solid particles, within the limits of applicability of our model based on the T-matrix theory of scattering and on assumptions on a common particle shape in a PSD and a common threshold radius for all the PSDs.