Assimilation of atmospheric infrasound data to constrain tropospheric and stratospheric winds

Abstract

This data assimilation study exploits infrasound from explosions to probe an atmospheric wind component from the ground up to stratospheric altitudes. Planned explosions of old ammunition in Finland generate transient infrasound waves that travel through the atmosphere. These waves are partially reflected back towards the ground from stratospheric levels, and are detected at a receiver station located in northern Norway at 178 km almost due north from the explosion site. The difference between the true horizontal direction towards the source and the backazimuth direction (the horizontal direction of arrival) of the incoming infrasound wavefronts, in combination with the pulse propagation time, are exploited to provide an estimate of the average cross‐wind component in the penetrated atmosphere. We perform offline assimilation experiments with an ensemble Kalman filter and these observations, using the ERA5 ensemble reanalysis atmospheric product as background (prior) for the wind at different vertical levels. We demonstrate that information from both sources can be combined to obtain analysis (posterior) estimates of cross‐winds at different vertical levels of the atmospheric slice between the explosion site and the recording station. The assimilation makes greatest impact at the 12–60 km levels, with some changes with respect to the prior of the order of 0.1–1.0 m·s−1, which is a magnitude larger than the typical standard deviation of the ERA5 background. The reduction of background variance in the higher levels often reached 2–5%. This is the first published study demonstrating techniques to implement assimilation of infrasound data into atmospheric models. It paves the way for further exploration in the use of infrasound observations – especially natural and continuous sources – to probe the middle atmospheric dynamics and to assimilate these data into atmospheric model products.

Benchmarking microbarom radiation and propagation model against infrasound recordings: a vespagram-based approach

“…Resolving small-scale structures in atmospheric models, reanalysis, and forecasting systems remains a topic of active research. Several research efforts were made to develop methods exploiting infrasound observations to improve the representation of wind and temperature in atmospheric model products (e.g., Chunchuzov et al, 2015;Assink et al, 2019;Amezcua et al, 2020;Vera Rodriguez et al, 2020).…”

“…These limitations have been accordingly raised in the context of microbaroms (Landès et al, 2014). Therefore, in this study, the parametrization used to run the WWIII model accounts for fixed reflection coefficients of 10 % for the continents, 20 % for the islands, and 40 % for ice sheets (Ardhuin et al, 2011). Microbarom source model.…”

“…The low-frequency microbaroms can be ducted over long distances due to the weak attenuation, which is proportional to the frequency squared. Hence, there is a potential to exploit microbaroms to probe the dynamics of the stratosphere, where the representation of the atmospheric dynamics in model products is often poorly constrained (Polavarapu et al, 2005;Rienecker et al, 2011;Smith, 2012;Amezcua et al, 2020).…”

Global Microbarom Patterns: A First Confirmation of the Theory for Source and Propagation

“…As microbaroms propagate into the middle atmosphere (from around 12 to 90 km altitude), significant features of the vertical structure of the temperature and wind are reflected in the detected signal on the ground. The lack of variability found in both temperature and wind models (e.g., Charlton‐Perez et al., 2013; Lee et al., 2019) has motivated the development of atmospheric remote sensing methods for evaluating numerical weather prediction (NWP) model output (e.g., Amezcua et al., 2020; Le Pichon et al., 2015; Vanderbecken et al., 2020). With the increasing number of IMS stations complemented by dense regional networks, systematic studies using historical infrasound data sets and state‐of‐the‐art (re)analysis systems provide useful integrated information on middle atmosphere variability and biases where data coverage is sparse (Blanc et al., 2019).…”

Microbarom radiation and propagation model assessment using infrasound recordings: a vespagram-based approach

“…Because of the low frequencies, microbaroms can penetrate the middle atmosphere and return back to ground at long ranges. Hence there is potential to exploit this source to probe the dynamics of this altitude range, where the representation of the atmospheric dynamics in model products is often poorly constrained (Polavarapu et al, 2005;Rienecker et al, 2011;Smith, 2012;Amezcua et al, 2020). 1 https://doi.org/10.5194/angeo-2020-78 Preprint.…”