Growth of Rosette Ice Crystals and Impact on Cirrus Development

玫瑰花结冰晶的生长及其对卷云发育的影响

• 批准号: 0244505
• 负责人: Pao-Kuan Wang
• 金额: $ 31.01万
• 依托单位: University of Wisconsin-Madison
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2003
• 资助国家: 美国
• 起止时间: 2003-04-01 至 2008-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0244505&HistoricalAwards=false
• 关键词: Growth; Rosette; Ice; Crystals; Impact

Clouds are the most important atmospheric constituent for determining the earth's albedo. Many clouds are composed of ice crystals. The life of such clouds and their effect on solar and terrestrial radiation depend on the sizes, shapes, and concentration of the ice crystals, and their growth and evaporation. This project is a theoretical study of certain microphysical properties of rosette-shaped ice crystals, which are a common constituent of cirrus clouds. In particular, the work focuses on developing an accurate formulation of the growth and evaporation of rosette crystals by the condensation-diffusion process. The rate of growth of an ice crystal in a given environment is determined by the size and shape of the crystal and the excess vapor pressure over the equilibrium value for ice. The conventional theory that describes these effects makes use of an analogy between the equation for the diffusion of water vapor in air and the equation in electrostatics that describes the distribution of potential around a charged conductor. Using this analogy, the shape is described in terms of the electrical capacitance of a conductor having the same shape as the ice crystal. This provides a great simplification in the theory of crystal growth. A single parameter, the capacitance, is used instead of solving for the rate of vapor deposition point-by-point over the complex surface of the crystal. Values of the capacitance are known for simple crystal forms such as plates, columns, and needles, but not for rosettes, which are the predominant form at temperatures colder than about -40C. In the first part of this study, mathematical approximations will be formulated to describe the shapes of rosette crystals of various thickness, lengths, and numbers of branches. For those that closely approximate the crystals that have been observed in cirrus clouds, the capacitance will be computed using finite element numerical techniques. In the second part of the study, the capacitance values will be employed in an existing numerical model of cirrus clouds to establish accurate parameterizations of rosette growth rates. Simulated clouds including crystals of various forms will then be employed with existing radiative transfer codes to determine the effects of cirrus clouds through their life cycles on solar and terrestrial radiation. Sensitivity studies will produce useful information for the broad scientific community to access the influence of cirrus clouds on the global climate process.

云是决定地球反照率的最重要的大气成分。 许多云是由冰晶组成的。 这种云的寿命及其对太阳和地面辐射的影响取决于冰晶的大小、形状和浓度及其生长和蒸发。 该项目是对玫瑰花状冰晶某些微物理特性的理论研究，玫瑰花状冰晶是卷云的常见组成部分。 特别是，这项工作的重点是通过冷凝-扩散过程开发玫瑰花晶体生长和蒸发的精确公式。 给定环境中冰晶的生长速率由晶体的尺寸和形状以及超过冰平衡值的过量蒸气压决定。 描述这些效应的传统理论利用了空气中水蒸气扩散方程与描述带电导体周围电势分布的静电方程之间的类比。 使用这个类比，形状是根据具有与冰晶相同形状的导体的电容来描述的。 这极大地简化了晶体生长理论。 使用单个参数，即电容，而不是逐点求解晶体复杂表面上的气相沉积速率。 简单晶体形式（如板状、柱状和针状）的电容值是已知的，但玫瑰花结的电容值却不是已知的，玫瑰花结是温度低于约-40°C 时的主要形式。 在本研究的第一部分中，将制定数学近似来描述各种厚度、长度和分支数量的莲座晶体的形状。 对于那些非常接近在卷云中观察到的晶体的晶体，将使用有限元数值技术计算电容。 在研究的第二部分中，电容值将用于现有的卷云数值模型中，以建立玫瑰花结生长速率的准确参数化。 然后，包括各种形式晶体的模拟云将与现有的辐射传输代码一起使用，以确定卷云在其生命周期中对太阳和地面辐射的影响。 敏感性研究将为广大科学界提供有用的信息，以了解卷云对全球气候过程的影响。