Critical Raindrop Characteristics: Fall Speed, Shape, and Size Distributions

雷达气象学和更广泛的降水科学中的主流模式一直是雨滴以最终速度下降的假设。然而，最近的研究表明，情况并不总是如此，揭示了雨的一个重要的新方面，需要进一步探索。在这个项目下，将进行一项实地研究，旨在揭示雨滴下降特性的新的基本物理。更具体地说，这项工作试图开发一种改进的雨滴下降速度、弦比和尺寸分布的预测能力。假设偏离更传统计算的终端速度可能会导致偏离平衡弦比，进而在推断的液滴尺寸分布中引入误差。这提出了几个新的问题，其中最重要的两个问题是：(1)实际的雨滴下降速度、弦比和大小分布是什么？(2)导致这些量偏离其平衡值的主导环境因素和过程是什么，以及如何在特定条件下(例如，降雨率)预测这种偏差？受支持的研究人员将使用最先进的高速相机和数字图像分析系统来测量雨滴下降的速度、直径和形状，以寻求这些和其他问题的答案。这项工作的智力价值在于开发和解释一种独特的微物理数据集，该数据集将用于揭示雨滴下降的特征，重点是提供与不同降雨条件的关系。这一办法应能改进各种类型和强度的降水中的水滴大小分布和弦比(即水滴扁率，偏振多样化雷达对此特别敏感)的参数化。这项工作被认为是第一次系统地研究雨滴的实际下落速度及其对雨滴几何和尺寸分布的影响。更广泛的影响将包括改进对降水下降速度、弦比和大小分布的预测能力，以及它们与通常从双极化雷达数据得出的降雨率的关系。这些努力预计将改进对降雨率和降雨量的提取，这反过来可能有利于洪水预报、水资源管理以及天气、水文和气候模型的相关投入。此外，所获得的知识将有助于改进对大量安装的Joss-Waldvogel型视盘测定仪的数据的解释，这通常需要对降速的先验假设来准确地推断降雨率。克莱姆森大学的降水研究基础设施将通过开发一个仪表化的实地站点得到加强，该站点将作为本科生和研究生的教育平台。相关结果将通过学生的论文和同行评议的文献进行传播。

The prevailing paradigm in radar meteorology, and precipitation science more generally, has been the assumption that raindrops fall at terminal velocity. However, recent research suggests this is not always the case, revealing an important new aspect of rain that requires further exploration. Under this project a field study designed to uncover new fundamental physics of raindrop fall characteristics will be conducted. More specifically this effort seeks to develop an improved predictive capability for raindrop fall velocity, chord ratio, and size distribution. It is hypothesized deviations from more traditionally-computed terminal velocities may induce deviations from equilibrium chord ratios and in turn introduce errors in inferred drop size distributions. This raises several new questions, the two most important being: (1) What are the actual raindrop fall velocities, chord ratios, and size distributions?; and (2) What are the governing environmental factors and processes responsible for deviations of these quantities away from their equilibrium values, and how can such deviations be predicted for specific conditions (e.g., rainfall rates)? Supported investigators will measure raindrop fall velocity, diameter, and shape using a state-of-the-art high-speed camera and digital image analysis system to seek answers to these and other questions.The intellectual merit of this effort rests in development and interpretation of a unique microphysical dataset that will exploited to uncover raindrop fall characteristics, with a focus on providing relationships with varying rainfall conditions. This approach should lead to improved parameterizations for the drop-size distributions and chord ratio (i.e. drop oblateness, to which polarization-diverse radars are particularly sensitive) in falling precipitation of various types and intensities. This work is thought to be the first systematic study of the actual fall velocity of raindrops and its effects on raindrop geometry and size distribution. Broader impacts will include improved predictive capability for precipitation fall speeds, chord ratios and size distributions, as well as their relationship to rain rates as commonly derived from dual-polarization radar data. These efforts are anticipated to lead to improved retrievals of rainfall rates and accumulations, which may in turn benefit flood prediction, management of water resources, and related inputs to weather, hydrologic and climatological models. Additionally, knowledge gained will serve to improve interpretation of data from a large installed base of Joss-Waldvogel type disdrometers, which typically require a priori assumption of fallspeed to accurately infer rainfall rate. Precipitation research infrastructure at Clemson University will be enhanced through development of an instrumented field site, which will serve as an educational platform for undergraduate and graduate students. Related results will be disseminated via students' dissertations and the peer-reviewed literature.