Light scattering by nonspherical particles: remote sensing and climatic implications
Light scattering by nonspherical particles: remote sensing and climatic implications
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DOI：
10.1016/0169-8095(94)90004-3
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发表时间：
1994-04
影响因子：
5.5
通讯作者：
K. Liou;Y. Takano
中科院分区：
文献类型：
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作者：
K. Liou;Y. Takano
冰晶和气溶胶（通常是非球形的）的散射和吸收特性的计算需要特定的方法。对于非球形粒子的散射，没有唯一的理论解。实际上，所有的数值解的非球形粒子的散射，包括精确的波动方程方法，积分方程方法，和离散偶极子近似，是只适用于小于约20的尺寸参数。因此，这些方法是有用的辐射问题的研究涉及非球形气溶胶和小冰晶的热红外波长。采用几何光学近似方法研究了尺寸远大于入射波长的六角冰晶的散射、吸收和偏振特性。这种近似对于尺寸参数大于30的六边形冰晶是普遍有效的。从现有的实验数据和理论结果来看，非球形粒子比等投影面积/体积的球形粒子吸收更少，具有更小的不对称因子。特别是，我们发现六角冰晶具有许多球形粒子无法获得的晕和弧特征，并且在60° ~ 140°散射角范围内，冰晶比球形冰晶散射更多的光，利用可见光和红外通道进行卷云光学厚度和高度的卫星遥感必须使用合适的冰晶相函数。使用等效球模型将导致显着高估和低估的卷云光学深度和高度，分别。如果没有合适的冰晶模型，就无法解释从卷云反射的太阳光偏振测量结果。球形冰晶和六角冰晶的偏振模式存在较大的偏差。激光雷达后向散射和去偏振信号的解释也必须利用六角形冰晶的散射特性。等效球形模型大大低估了冰晶云的宽带太阳辐射，因为球形粒子比六角形冰晶具有更强的前向散射和更大的吸收。我们说明，净云辐射强迫在大气顶部涉及大多数卷云是积极的，这意味着红外温室效应超过太阳辐射效应。如果使用等效球体的辐射特性，云辐射强迫会显著增加。使用一维云和气候模式，我们进一步证明，有足够的模型灵敏度，在温度升高方面，在辐射计算中使用冰晶模型。
Calculations of the scattering and adsorption properties of ice crystals and aerosols, which are usually nonspherical, require specific methodologies. There is no unique theoretical solution for the scattering by nonspherical particles. Practically, all the numerical solutions for the scattering of nonspherical particles, including the exact wave equation approach, integral equation method, and discrete-dipole approximation, are applicable only to size parameters less than about 20. Thus, these methods are useful for the study of radiation problems involving nonspherical aerosols and small ice crystals in the thermal infrared wavelengths. The geometric optics approximation has been used to evaluate the scattering, absorption and polarization properties of hexagonal ice crystals whose sizes are much larger than the incident wavelength. This approximation is generally valid for hexagonal ice crystals with size parameters larger than about 30.From existing laboratory data and theoretical results, we illustrate that nonspherical particles absorb less and have a smaller asymmetry factor than the equal-projected area/volume spherical counterparts. In particular, we show that hexagonal ice crystals exhibit numerous halo and arc features that cannot be obtained from spherical particles; and that ice crystals scatter more light in the 60° to 140° scattering angle regions than the spherical counterparts.Satellite remote sensing of the optical depth and height of cirrus clouds using visible and IR channels must use appropriate phase functions for ice crystals. Use of an equivalent sphere model would lead to a significant overestimation and underestimation of the cirrus optical depth and height, respectively. Interpretation of the measurements for polarization reflected from sunlight involving cirrus clouds cannot be made without an appropriate ice crystal model. Large deviations exist for the polarization patterns between spheres and hexagonal ice crystals. Interpretation of lidar backscattering and depolarization signals must also utilize the scattering characteristics of hexagonal ice crystals.Equivalent spherical models substantially underestimate the broadband solar albedos of ice crystal clouds because of stronger forward scattering and larger absorption by spherical particles than hexagonal ice crystals. We illustrate that the net cloud radiative forcing at the top of the atmosphere involving most cirrus clouds is positive, implying that the IR greenhouse effect outweighs the solar albedo effect. If the radiative properties of equivalent spheres are used, a significant increase in cloud radiative forcing occurs. Using a one-dimensional cloud and climate model, we further demonstrate that there is sufficient model sensitivity, in terms of temperature increase, to the use of ice crystal models in radiation calculations.