  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 wave-fronts, 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, 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.
