Turbulent Collision-Coalescence of Cloud Droplets and its Impact on Warm Rain Formation

pi正在寻求量化湍流对云滴收集核的影响，以及它如何影响暖雨降水过程。空气湍流可以通过(1)由于差分加速度和剪切效应而增强相对运动，(2)由于液滴局部聚集而增强平均对密度，(3)由于湍流波动而提高碰撞效率来影响收集核。增强的程度以一种复杂的方式取决于液滴的大小（这反过来决定了响应时间和沉降速度）以及通过流动耗散率和雷诺数测量的空气湍流强度。本研究的第一个目标是通过混合直接数值模拟（HDNS）方法和理论方法相结合来参数化湍流的上述影响。HDNS方法将液滴扰动流的改进叠加方法集成到无扰动空气湍流的伪谱模拟中。这允许在DNS中直接合并流体动力相互作用，并根据与碰撞合并相关的所有统计信息的第一原理进行计算。理论方法是基于连续的对轨迹近似和对统计的概率分布函数建模，并将用于模拟雷诺数效应。结果将是一个参数化的收集核，用于不同的液滴对大小组合、流动耗散速率和雷诺数。本研究将使用来自积云观测的真实流动条件，例如小积云微物理研究（SCMS）和海洋积云降雨（RICO）实验。第二个目标是研究湍流对液滴尺寸谱展宽速率的影响，方法是使用上述开发的增强收集核对云模型中的动力学收集方程（KCE）进行数值积分。特别是，我们将研究毛毛雨从最初狭窄的尺寸分布中产生的时间尺度。我们的方法是结合我们最近开发的数值方法（高斯正交线性积分法）来处理暖降水云模型（上升包）的KCE。我们的目标是理解和量化上述湍流效应与观测到的暖降水云之间的联系。研究的广泛影响：本研究旨在解决空气湍流能否调和现有模式预测与积云观测之间的差异（大小差距问题）这一悬而未决的问题。众所周知，由暖雨过程引起的飞机结冰和冰冻降水等关键天气现象会对经济产生重大影响。这项研究还将影响大气科学和工程的其他领域：如气溶胶对天气和气候的间接影响、喷雾燃烧、粉末生产和工业排放。来自特拉华大学和国家大气研究中心的跨学科团队允许新的理论发展和先进的工程研究工具有效地应用于大气过程。这项研究为两名研究生和一名本科少数民族学生提供了独特的教育体验，因为他们利用了两所机构的资源。

The PIs are seeking to quantify the effects of turbulence on the collection kernel of cloud droplets and how this impacts the warm rain precipitation process. Air turbulence could affect the collection kernel through (1) enhanced relative motion due to differential acceleration and shear effects, (2) enhanced average pair density due to local clustering of droplets, and (3) enhanced collision efficiency due to turbulent fluctuations. The levels of enhancements depend, in a complex manner, on the size of droplets (which in turn determines the response time and settling velocity) and the strength of air turbulence as measured by the flow dissipation rate and Reynolds number.The first objective of this research is to parameterize the above effects of turbulence by combining a Hybrid Direct Numerical Simulation (HDNS) approach and a theoretical approach. The HDNS approach integrates an improved superposition method for the disturbance flows due to droplets into a pseudo-spectral simulation of undisturbed air turbulence. This allows for the direct incorporation of hydrodynamic interactions within DNS and computations from first principles of all statistical information related to collision-coalescence. The theoretical approach is based on a successive pair-trajectory approximation and probability distribution function modeling of pair statistics, and will be used to model Reynolds-number effects. The deliverable will be a parameterized collection kernel for different droplet-pair size combinations, flow dissipation rates, and Reynolds numbers. Realistic flow conditions from cumulus cloud observations, e.g., the Small Cumulus Microphysics Study (SCMS) and the Rain In Cumulus over the Ocean (RICO) experiment, will be used in this investigation.The second objective is to study the effects of turbulence on the broadening rate of droplet size spectrum by numerically integrating the kinetic collection equation (KCE) within cloud models, using the enhanced collection kernel developed above. In particular, we will study the time scale during which drizzle drops can be generated from initially narrow size distribution. Our approach is to combine our recently developed numerical method (Linear integral method with Gauss quadrature) for treating KCE with a warm precipitating cloud model (a rising parcel). The goal is to understand and quantify the connection between the above effects of turbulence with observations in warm precipitating clouds.Broader impacts of the research: The research intends to resolve the outstanding question of whether the air turbulence can reconcile the discrepancy between the existing model predictions and observations in cumulus clouds (the size-gap problem). Critical weather phenomena such as aircraft icing and freezing precipitation resulting from warm rain processes are known to have a significant economical impact. The research will also impact other areas of atmospheric science and engineering: such as indirect aerosol effects on weather and climate, spray combustion, powder production, and industrial emissions. The interdisciplinary team from the University of Delaware and the National Center for Atmospheric research allows new theoretical developments and advanced engineering research tools to be efficiently applied to atmospheric processes. The research offers two graduate students and one undergraduate minority student a unique educational experience as they take advantage of the resources at both institutions.