Progressively Complex Numerical Studies of Infrasound Generated by Atmospheric Convection

最近在美国高平原进行的现场实验表明，强雷暴发出的次声频率在0.1至10赫兹之间，比非恶劣天气系统发出的次声要强烈得多。发展中或成熟的龙卷风的不稳定运动可能是相对较强信号的一个来源。这一假设促使美国国家海洋和大气管理局进行了一项实地研究，以评估次声探测在龙卷风警报中的使用。结果是有希望的，但为了提高区分涡旋信号和外来噪声的技能，有必要提高对对流风暴中产生次声的各种机制的现有知识。虽然涡旋声学的一般理论已经非常发达，但目前对正在发展的龙卷风的结构和非定常运动的了解是有限的。因此，龙卷风次声还没有明确的理论。此外，对对流风暴中其他流动结构或非绝热云过程产生的次声也缺乏足够的了解。在缺乏详细观测的情况下，数值模拟提供了获取缺失知识的最佳方法。该项目将涉及对大气对流形式逐渐复杂的次声的产生进行系统的计算研究。所考虑的形式将包括干热、非降水积云、高耸的积雨云和非超级单体龙卷风。将确定0.1-10赫兹次声的主要来源。将研究声功率(强度)和峰值发射频率随每个对流系统的控制参数的变化。将检查对修改微物理和次网格湍流参数的敏感性。将使用区域大气模拟系统的完全可压缩版本进行主要研究。还将继续采用NCAR天气研究和预报模型(WRF)来研究次声。智能优点：这项研究将探索大气模拟的新前沿：模拟对流风暴中湍流、微物理过程和涡旋产生的次声。这个项目的学术价值在于努力阐明产生可检测到的次声的物理过程，并澄清声发射的确切结构。这一结果将为改进当前的方法奠定基础，以将观测到的排放与不断演变的风暴中的特定事件或物体联系起来。更广泛的影响：这项研究的主要更广泛的影响是，通过改进旨在补充雷达系统的次声探测方法，有可能改进龙卷风警报。此外，该项目将进一步发展大气科学和航空声学工程界之间的协同作用。

Recent field experiments in the High Plains of the United States indicate that severe thunderstorms emit infrasound at frequencies between 0.1 and 10 Hz much more intensely than non-severe weather systems. The unsteady motion of a developing or mature tornado is one likely source of the relatively strong signal. This hypothesis motivated a field study by the National Oceanic and Atmospheric Administration to evaluate the use of infrasound detection for tornado warning. The results were promising, but in order to improve the skill in distinguishing vortex signals from extraneous noise, it is essential to advance current knowledge of the various mechanisms that produce infrasound in a convective storm. Although the general theory of vortex acoustics is highly developed, current understanding of the structure and unsteady motions of a developing tornado is limited. Consequently, there is no definitive theory for tornado infrasound. Moreover, there is insufficient understanding of the infrasound that is produced by other flow structures or diabatic cloud processes within a convective storm. In the absence of detailed observations, numerical modeling provides the best method for obtaining the missing knowledge. This project will involve a systematic computational study of the production of infrasound by progressively complex forms of atmospheric convection. The forms considered will include a dry thermal, a non-precipitating cumulus, a towering cumulonimbus and a non-supercell tornado. The dominant sources of 0.1-10 Hz infrasound will be identified. The scaling of acoustic power (intensity) and peak emission frequencies with the control parameters of each convective system will be investigated. Sensitivity to modifications of microphysics and subgrid turbulence parameterizations will be examined. The principal studies will be carried out with a fully compressible version of the Regional Atmospheric Modeling System. Adaptation of the NCAR Weather Research and Forecasting model (WRF) for the purpose of studying infrasound will also be pursued. Intellectual Merit: This research will explore a new frontier of atmospheric modeling: the simulation of infrasound generated by turbulence, microphysical processes and vortices in convective storms. The intellectual merit of this project lies in the effort to elucidate the physical processes that are responsible for generating detectable infrasound, and to clarify the exact structure of the acoustic emissions. The results will build a foundation for improving current methods to connect observed emissions to specific events or objects within an evolving storm. Broader Impacts: The principal broader impact of this study is the potential for improvement of tornado warning by contributing to the refinement of infrasonic detection methods that are intended to compliment radar systems. In addition, this project will further develop a synergy between the atmospheric science and aeroacoustical engineering communities.