Field Studies of Raindrop Axis Ratio and Orientation Distributions Using an Improved 2D-Video Disdrometer and Dual-Polarized Radar

极化雷达对降水的测量包含了关于降水粒子形状的信息。 雨滴的形状可能会随时间而振荡，但平均形状近似为球形，并且雨滴越大，偏离球形的程度越大。 因此，对雨水的偏振测量包含了关于雨滴大小的信息--除了由雷达反射率单独提供的信息之外的信息。 这是改进使用偏振测量法估计降雨率的基础。 该项目通过直接测量地面上单个液滴的大小、形状和倾斜角度，为雷达偏振测量做出了贡献。 它采用了一个由奥地利Joanneum研究公司制造的视频光盘测量仪的升级版。 该仪器记录每个单独液滴在其福尔斯通过侧面约10 cm的正方形水平采样区域时的前视图和侧视图。 它可以测量颗粒的大小、形状、倾斜角度和下落速度。 这些数据使得能够计算与偏振测量相关的参数，称为“有效β”，这取决于大小，形状和倾斜角之间的关系。 最近的一个理论表明，这个参数也可以间接推断从雷达观测的三个偏振量，在水平偏振的反射率，在水平和垂直偏振的反射率的比率，以及两个偏振分量之间的差分传播相位。 此外，知道有效贝塔值可以更准确地估计降雨率。 实验的目的是通过比较雷达估计的β值与直接从液滴的disdrometer测量特性确定的该参数的值来测试理论。 实验将在两个地点进行：在科罗拉多，使用CSU-CHILL雷达，在澳大利亚的达尔文，与气象局研究中心合作。 使用这些网站将使理论在两个气候制度可能不同的降雨特性的评估。 这项工作将提高雷达估测降雨量的准确性。

Polarimetric radar measurements of precipitation contain information about the shape of the precipitation particles. For raindrops, the shape may oscillate with time, but the average shape is approximately spheroidal, and the larger the drop the greater the deviation from spherical. Hence polarimetric measurements of rain contain information about drop size - information in addition to what is provided by the radar reflectivity alone. This is the basis for improved estimates of rainfall rate using polarimetry. This project contributes to radar polarimetry by directly measuring the sizes, shapes, and canting angles of individual drops at the ground. It employs an upgraded version of a video disdrometer manufactured by an Austrian firm, Joanneum Research. This instrument records the front and side view of each individual drop as it falls through a square, horizontal sampling area about 10 cm on the side. It measures the particle size, shape, canting angle, and fall speed. The data enable the calculation of a parameter relevant to polarimetric measurements, called "effective beta," which depends on the relation between size, shape, and canting angle. A recent theory shows that this parameter can also be inferred indirectly from radar observations of three polarimetric quantities, the reflectivity at horizontal polarization, the ratio of the reflectivities at horizontal and vertical polarization, and the differential propagation phase between the two polarization components. Furthermore, knowing the effective beta enables more accurate estimation of the rainfall rate. The purpose of the experiments is to test the theory by comparing radar estimates of beta with the values of this parameter determined directly from the disdrometer-measured properties of the drops. Experiments will be conducted at two sites: in Colorado, using the CSU-CHILL radar, and in Darwin, Australia, in collaboration with the Bureau of Meteorology Research Center. Using these sites will enable the evaluation of the theory in two climatic regimes with possibly different rainfall characteristics. The work should improve the accuracy of rainfall estimation by radar.