Evaluating the temporal capability of empirical climatologies for rapid long-range volcanic infrasound propagation estimates using a multidecadal data set of persistent Vanuatu volcanic eruptions
Authors: De Negri Rodrigo S , Matoza Robin S , Hupe Patrick , Le Pichon Alexis , Rose Kaelynn M , Cevuard Sandrine , Niroa John J 
Published: January 2025

SUMMARY
Powerful infrasound (acoustic waves $\lt $20 Hz) can be produced by explosive volcanic eruptions. The long-range propagation capability, over hundreds to thousands of kilometers, of atmospheric infrasound motivates the development of regional or even global scale volcano-infrasound monitoring systems. Infrasound propagation paths are subject to spatiotemporal atmospheric dynamics, which lead to deviations in the direction-of-arrival (back-azimuth) observed at sensor arrays and contribute to source location uncertainty. Here, we further investigate the utility of empirical climatologies combined with 3-D ray tracing for providing first-order estimates of infrasound propagation paths and back-azimuth deviation corrections. The intended application is in scenarios requiring rapid or pre-computed infrasound propagation calculations, such as for a volcano-infrasound monitoring system. Empirical climatologies are global observationally based function fitting models of the atmosphere, representing robust predictors of the bulk diurnal to seasonal atmospheric variability. Infrasound propagation characteristics have previously been shown to have strong seasonal and diurnal components. At the International Monitoring System infrasound station IS22, New Caledonia, quasi-continuous multiyear infrasound array detections show oscillating azimuthal variations for arrivals from volcanoes in Vanuatu, including Yasur ($\sim$400 km range), Ambrym ($\sim$670 km range) and Lopevi ($\sim$650 km range). We perform 3-D ray tracing to model infrasound propagation from the Ambrym and Yasur volcano locations to IS22 every six hours (00:00, 06:00, 12:00 and 18:00 UTC) for every day of 2004 and 2019 for Ambrym and Yasur, respectively and evaluate the results as compared to the multiyear observations. We assess a variety of models and parametrizations, including both empirical climatologies and hybrid descriptions; range-independent and range-dependent atmospheric discretizations; and unperturbed and perturbed range-independent empirical climatologies. The hybrid atmospheric descriptions are composed of fifth generation reanalysis descriptions (ERA 5) from the European Centre for Medium-Range Weather Forecasts below $\sim$80 km altitude combined with empirical climatologies above. We propose and employ simple parametric perturbations to the empirical climatologies, which are designed to enhance the stratospheric duct and compensate for missing gravity wave perturbations not included in the climatologies, and thereby better match observations. We build year-long back-azimuth deviation interpolations from the simulations and compare them with three different multiyear array detection data sets from IS22 covering from 2003 up to 2022. Through a systematic comparison, we find that the range-independent empirical climatologies can capture bulk azimuth deviation variability and could thus be useful for rapid infrasound propagation calculation scenarios, particularly during favourable sustained propagation ducting conditions. We show that the hybrid models better describe infrasound propagation during periods of weak stratospheric ducting and during transient atmospheric changes such as stratospheric wind reversals. Overall, our results support the notion that climatologies, if perturbed to compensate for missing gravity wave structure, can improve rapid low-latency and pre-computed infrasound source discrimination and location procedures.