Numerical Investigation of Infrasound Generated by Clear-Air Turbulence

Abstract

Clear-air turbulence (CAT) represents a major hazard to the aviation industry because it occurs without visible indicators and cannot be detected by pilots or onboard instruments. This study investigates whether CAT generates infrasonic emissions that could potentially be used for remote detection. High-resolution large-eddy simulations were performed using the Weather Research and Forecasting model to produce atmospheric environments associated with two CAT encounters: a mountain-wave event over Wyoming in 2020 and a shear-driven event over Illinois in 2023. Acoustic source terms were computed from the simulated flow fields using a hybrid acoustic analogy framework to estimate the acoustic pressure at distant observer locations. When acoustic sources were computed using velocity fluctuations relative to the mean flow, the CAT region in the Wyoming case produced acoustic emissions approximately 22--29 dB stronger than the background turbulence, revealing a clear acoustic energy enhancement associated with CAT. However, further investigation incorporating the mean flow and thermodynamic sources demonstrated that the background acoustic field increased substantially due to amplification by the strong terrain-driven mean flow (Wyoming) and underlying convective processes (Illinois). These cases represent particularly energetic atmospheric conditions that reduce the apparent contrast of the CAT signal, producing only modest overall sound pressure level increases relative to the background. Importantly, the thermodynamic sources related to potential temperature remained the principal contribution within the CAT regions. This suggests that the sharp potential temperature gradients and overturning motions play a primary role in CAT-related acoustic emissions, rather than turbulence-generated sound alone. These results demonstrate that CAT can generate measurable acoustic emissions even in realistic and complex atmospheric environments, supporting further investigation of infrasound as a potential remote-detection tool.