雨滴对电磁波的散射特性研究

Study on Electromagnetic Wave Scattering by Raindrops

由于重力和向上气流的作用,高速摄影显示较大的雨滴在降落过程中表现为一底部扁平、顶部圆滑的非球形对称体,已有的模型中BC模型(由Beard和Chuang提出)被认为能准确描述雨滴的形状。本文利用沿降落方向旋转对称的扁椭球体和均匀球体模拟降落中的雨滴,扁椭球体的短轴(旋转轴)与BC模型的相等,长轴通过与BC模型截面积等效法确定;利用T矩阵方法研究了水平取向的旋转扁椭球体和BC雨滴模型的散射特性,利用Mie理论计算了均匀球的散射特性,比较了半径分别为2 mm和3 mm大小的三种雨滴模型对94 GHz电磁波斜入射时的单次散射相矩阵,以及对频率为30 GHz^100 GHz电磁波的单次散射反照率,最后讨论了半径为0.5 mm^3.5 mm雨滴对35 GHz和94 GHz的后向散射函数。计算结果表明,利用均匀球模拟雨滴将带来较大误差,而利用截面积等效的旋转扁椭球体模型可以较好地模拟雨滴的散射特性。

High-speed photographs reveal that a large raindrop falling through the air exhibits a characteristic asymmetric shape having a flattened base and a smoothly rounded top for the func- tion of gravity and upward air stream. BC model （which is developed by Beard and Chuang） is re- garded as an accurate raindrop model. In this paper, we use oblate spheroid and homogeneous sphere to model the falling raindrops, and the rotational axes length of the oblate spheroid are de- termined by equal-sectional-area of BC model, and the horizontal axes length equals with the length of BC model. The scattering characteristics of oblate spheroid and BC model are studied by T matrix method, and the scattering characteristics of homogeneous sphere are studied by Mie the- ory. Single scattering phase matrix of 94 GHz electromagnetic wave which is scattered by there raindrop models having the radius of 2 mm and 3 mm are compared. The single scattering albedo are given and discussed for the 30 GHz-100 GHz electromagnetic wave, and the single backscatter- ing phase function of 35 GHz and 94 GHz electromagnetic wave scattered by three different rain- drop models with radius 0.5 mm to 3.5 mm. The result showed that spherical raindrop model would cause errors in calculating the scattering characteristics of single raindrop, and the oblate spheroid model can model the raindrop well.