Numerical Study of the Riming Growth of Graupel

结冰过程中冰粒的增长是云物理的核心问题之一，对大气中的许多过程都有很大的影响。这是形成冰雹和冰雹的主要过程。它发生在深对流云中，由于它的热力学和动力效应，它影响着这类云的进一步发展。这也是导致雷雨云带电的最重要的过程之一。对这一过程的定量知识对于准确的天气和气候预测是必要的。尽管褶皱生长过程至关重要，但现有的关于褶皱生长的野外和实验室实验数据有限，而且对该过程的理论处理也非常简单。为了加深对这一过程的了解，将从原始冰晶开始，通过吸积过冷液滴的方式对冰粒的生长进行数值研究，并跟随其生长成为砾石。将开发两个数值模型，一个用于模拟下落的冰粒在吸收液滴(从而改变其形状)时的坠落行为；另一个用于确定过冷液滴与冰粒的碰撞效率。第一种模型通过数值求解相关的N-S方程，得到下落颗粒周围的流场。利用这些流场，可以求出过冷液滴在下落冰粒周围的运动方程，从而确定它们的轨迹，从而确定它们的碰撞效率。此外，还将计算下落的边缘颗粒的通风系数。这些数值模型将被用来进行计算，以确定各种冰晶/水滴大小组合和大气条件下的碰撞效率。这将产生一个广泛的数据集，在不同的条件下，格劳佩尔的定量碰撞增长率。这项拟议工作的学术价值在于，这项研究的结果将填补云物理这一基本领域现有知识的巨大空白，并提供关于云层中边缘生长过程如何运作的必要的定量信息。这些信息可以用来设计更好的涡旋边缘参数，用于风暴尺度的数值模式，以更好地理解对流云动力学。它还可以用于更大尺度的天气/气候模式，以改进其大尺度降水参数化。本研究所获得的经验和数值流场也将为今后冰冰碰撞生长的研究提供有益的参考。研究结果的更广泛影响包括利用它们通过使用数值预测模型来增强我们预测天气和气候演变的能力。研究活动将有助于培养将为大气科学研究做出贡献的研究生和青年科学家。研究成果将在科学期刊、会议和公开讲座中传播，以便更广泛的受众了解它们。首席调查员定期将最近的研究成果纳入调查水平的课程(针对大学新生)。他们向年轻一代传达了科学发现的兴奋和他们对社会的巨大贡献。

The growth of ice particles by riming is one of the central problems in cloud physics and has great impacts on many other atmospheric processes. This is the dominant process that forms graupel and hail. It occurs in deep convective clouds and it influences the further development of such clouds due to its thermodynamic and dynamic effects. It is also one of the most important processes leading to the electrification of thunderclouds. Quantitative knowledge of this process is necessary for accurate weather and climate predictions. Despite the critical importance of the riming growth process, there are only limited field and laboratory experimental data on riming growth available, and only very simplistic theoretical treatments of the process. In order to increase understanding of the process, a numerical study will be conducted on the growth of ice particles by accreting supercooled droplets, starting from a pristine ice crystal and following its growth to become a graupel. Two numerical models will be developed, one for simulating the fall behavior of the falling ice particles while accreting droplets (hence changing its shape); the other for determining the collision efficiency of the supercooled droplet hitting the ice particle. The first model involves solving numerically the relevant Navier-Stokes equations to obtain the flow field around the falling particle. Using these flow fields, solutions will be found to the equations of motion of supercooled droplets around the falling ice particle so as to determine their trajectories and hence their collision efficiency. Computations of the ventilation coefficients of the falling rimed particles will be performed as well. These numerical models will be used to perform computations to determine the collision efficiencies for a wide range of ice crystal/droplet size combinations and atmospheric conditions. This will yield an extensive data set of quantitative collisional growth rates of graupel in different conditions. The intellectual merit of the proposed work is that the results from this study will fill a large gap in available knowledge in this fundamental area of cloud physics and provide necessary quantitative information on how the riming growth process operates in clouds. Such information can be used to design better graupel riming parameterizations for use in storm-scale numerical models to understand better the convective cloud dynamics. It can also be used by larger scale weather/climate models to improve their large scale precipitation parameterizations. The experience gained and the numerical flow fields obtained from this study also will also be useful for the future study of ice-ice collisional growth. The broader impacts of the results of the study include their use to enhance our capability to predict weather and climate evolution through the use of numerical prediction models. The research activity will help train graduate students and young scientists who will contribute to atmospheric science research. The research results will be disseminated in scientific journals, meetings, and public lectures so that wider audience becomes aware of them. Recent research results are regularly incorporated into survey-level courses (aiming at college freshmen) by the principal investigator. They convey to the younger generation the excitement of scientific discoveries and their great contributions to society.