Abstract:
The Earth’s stratosphere is a highly dynamic region at around 15–50 km altitude in the atmosphere. Among the most extreme events that occur in this region are sudden stratospheric warmings, which can severely impact the weather conditions on the Earth’s surface. Previous works have shown that long-range numerical weather prediction will benefit from novel and enhanced upper stratospheric data. However, wind measurements at these altitudes remain limited.
Low-frequency acoustic waves – infrasound – can propagate thousands of kilometers through the stratosphere. This propagation is very sensitive to wind and temperature conditions. The infrasound research community has an ambition to provide novel and valuable measurements of upper stratospheric conditions to improve long-range numerical weather prediction systems. A specific kind of natural infrasound called microbaroms is continuously generated by opposing ocean surface waves, and are of particular interest for this purpose. When stratospherically ducted microbaroms are recorded at ground sensors, they carry information about both the source characteristics and the upper stratospheric wind conditions they have traversed.
The International Monitoring System (IMS) is a global network of sensors deployed for monitoring compliance with the Comprehensive Nuclear- Test-Ban Treaty (CTBT). The multi-decadal availability of data from the IMS infrasound stations offers a valuable dataset-of-opportunity for our atmospheric monitoring ambitions. We make use of a publicly available set of processed IMS infrasound array data provided by BGR.
To support the long-term and overarching goal of providing numerical weather prediction models with novel upper stratospheric wind measurements in real-time, the current thesis explores data-driven approaches for relating microbarom recordings to the wind pattern at altitudes up to 80 km. We have developed infrasound-based approaches to estimate diagnostics that describe the shape and strength of the stratospheric polar vortex, a key component in the upper stratospheric wind structure. We then apply detection theory frameworks to investigate how well these infrasound-based diagnostics can classify polar vortex regimes and sudden stratospheric warming events.
We find that the global microbarom infrasound data are best at estimating the polar cap eastward winds near the stratopause, at a pressure level of 1 hPa, corresponding to an altitude of approximately 50km. The IMS stations that best describe the winter variation in the polar cap average winds are found to be IS37 in Norway, IS18 on Greenland, IS44 in Russia, and IS53 in Alaska, USA. We also conclude that the polar cap eastward winds are more accurately estimated, compared to the displacement and the elliptic aspect ratio properties of the polar vortex.
As the global infrasound data also carry an imprint of the microbarom source variability, we also take the opportunity to use these data to map the spatial and temporal variation of the microbarom sources. We develop a method for enhanced microbarom source localization from infrasound data, and validate it against independent microbarom source models derived from ocean surface wind data. We demonstrate an improved match between both source estimations when comparing to the previously published method. The same methodology is also applied to the IMS data for depicting the global propagation of the massive atmospheric low-frequency waves associated with the Hunga Tonga–Hunga Ha’apai volcanic eruption in 2022.