T‐Matrix computations of zenith‐enhanced lidar backscatter from horizontally oriented ice plates
T‐Matrix computations of zenith‐enhanced lidar backscatter from horizontally oriented ice plates
复制标题
水平定向冰板天顶增强激光雷达后向散射的 T 矩阵计算
DOI：
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发表时间：
1997
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影响因子：
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通讯作者：
Barbara E. Carlson
中科院分区：
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作者：
Michael I. Mishchenko;D. J. Wielaard;Barbara E. Carlson
激光雷达光束通过卷云的天顶增强后向散射（ZEB）是一种显著的现象，通常用水平方向冰板的大平面镜面反射来解释。由于标准的几何光学近似（GO）在许多情况下可能不适用，特别是在分析红外测量时，并且忽略了物理光学效应，因此我们使用最近改进的精确T矩阵方法来计算可见光和红外波长下冰板的光散射。对尺寸参数高达50的水平和随机取向的薄圆盘和扁球体的计算表明，虽然所有颗粒都产生以正向散射方向为中心的强夫琅和费衍射峰，但强而窄的ZEB峰只能由水平取向的圆盘产生，而不能由水平取向的球体或随机取向的颗粒产生。这一发现表明，ZEB甚至可以由不在GO尺寸参数域中的颗粒产生，并支持ZEB的传统解释。此外，我们已经发现，ZEB峰的角宽度为水平取向的磁盘是等于一半的夫琅和费衍射峰的宽度。这一结果可用于在实践中推导出一个较低的估计冰颗粒尺寸从高角分辨率测量ZEB。我们表明，我们精确的T矩阵计算可以解释Platt等人[1978]在可见光中观察到的去偏振的特殊天顶角依赖性，并且可以根据修改的基尔霍夫近似定性地解释。
Zenith‐enhanced backscattering (ZEB) of a lidar beam by cirrus clouds is a remarkable phenomenon usually explained in terms of specular reflection from large plane facets of horizontally oriented ice plates. Since the standard geometric optics approximation (GO) may be inapplicable in many cases, especially in analyzing infrared measurements, and ignores physical optics effects, we use the recently improved exact T‐matrix method to compute the scattering of light by ice plates at visible and infrared wavelengths. Computations for horizontally and randomly oriented thin disks and oblate spheroids with size parameters up to 50 show that while all particles produce a strong Fraunhofer diffraction peak centered at exactly the forward‐scattering direction, a strong and narrow ZEB peak can be produced only by horizontally oriented disks but not by horizontally oriented spheroids or particles in random orientation. This finding demonstrates that ZEB can be produced even by particles which are not in the GO domain of size parameters and supports the traditional interpretation of ZEB. Also, we have found that the angular width of the ZEB peak for horizontally oriented disks is equal to half the width of the Fraunhofer diffraction peak. This result can be used in practice to derive a lower estimate of ice particle sizes from high angular resolution measurements of ZEB. We show that our exact T‐matrix computations can explain the peculiar zenith‐angle dependence of depolarization observed by Platt et al. [1978] in the visible and can be interpreted qualitatively in terms of the modified Kirchhoff approximation.