Progressively Complex Numerical Studies of Infrasound Generated by Atmospheric Convection

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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.