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Single-scattering of preferentially oriented ice crystals at centimeter and millimeter wavelengths

机译：优先定向的冰晶在厘米和毫米波长处的单散射

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摘要

The scattering properties of frozen cloud and precipitation particles at centimeter and millimeter wavelengths are fundamental for active and passive microwave remote sensing. So far the scattering parameters (scattering-, absorption-, and backscattering cross sections as well as asymmetry factor) have been calculated for randomly oriented particles only. However, some particle types, especially planar crystals, exhibit a preferred horizontal orientation. In this study the scattering and absorption parameters of four different planar ice crystal types have been modeled and exemplarily calculated from 1 GHz to 300 GHz with a freely available software code implementing the Discrete Dipole Approximation (DDA). It is presented how scattering and absorption of horizontally oriented ice crystals turn out for nadir-looking and inclined satellite sensors. The results show that small deviations around the horizontal orientation, as they are reported by relevant papers on cloud microphysics, have only marginal influence. Among all scattering parameters, the backscattering cross section of oriented crystals can differ most with respect to a random orientation. Especially if the incident wave direction is normal to the basal face, the parameter was found to be up to 20 times higher. Therefore the consideration of the orientation especially can be important for active microwave sensors. The more so if horizontally oriented particles and a nadir looking active sensor are involved. Furthermore the existing compilations for the index of refraction of ice at microwave frequencies have been reviewed.
机译：冷冻云和降水颗粒在厘米和毫米波长处的散射特性对于主动和被动微波遥感至关重要。到目前为止，仅针对随机取向的粒子计算了散射参数（散射，吸收和后向散射截面以及不对称因子）。然而，某些颗粒类型，尤其是平面晶体，表现出优选的水平取向。在这项研究中，已经对四种不同的平面冰晶类型的散射和吸收参数进行了建模，并示例性地使用实现离散偶极近似（DDA）的免费软件代码从1 GHz到300 GHz进行了计算。它介绍了水平定向冰晶的散射和吸收是如何产生于最低点和倾斜卫星传感器的。结果表明，如有关云微物理学的相关论文所报道的，围绕水平方向的小偏差仅具有很小的影响。在所有散射参数中，定向晶体的后向散射截面相对于随机取向而言差异最大。特别是如果入射波方向垂直于基面，则发现该参数高出20倍。因此，对于有源微波传感器，考虑取向尤其重要。如果涉及水平定向的粒子和最低点的主动传感器，则更是如此。此外，已经审查了微波频率下冰的折射率的现有汇编。

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