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FRINATEK-Fri prosj.st. mat.,naturv.,tek

Middle Atmosphere Dynamics: Exploiting Infrasound Using a Multidisciplinary Approach at High Latitudes

Alternative title: Den midtre atmosfærens dynamikk: tverrfaglig anvendelse av infralyd ved nordlige breddegrader

Awarded: NOK 9.4 mill.

Project Number:

274377

Application Type:

Project Period:

2018 - 2023

Location:

Partner countries:

The weather is controlled by processes in the lower atmosphere - the troposphere (up to around 15 km altitude) - and observations are fed into models that are used for weather prediction. However, the lower atmosphere is also influenced by the dynamics of the middle atmosphere: the stratosphere (15-50 km altitude) and the mesosphere (50-90 km). Middle atmospheric processes have a longer memory than for the troposphere and variations in its dynamics can couple downward to influence weather over monthly to seasonal timescales. Medium-range weather forecasting will improve with a better probing and understanding of middle-atmosphere dynamics and the coupling between atmospheric layers. Because the region is less accessible to measurement techniques applied below (e.g., weather balloons), direct observations of the dynamics in the upper stratosphere and the mesosphere are much harder to make than in the troposphere, and we need to rely on more indirect methods of remote sensing to characterize this region. An alternative approach to probe the atmosphere relies on infrasound, which is sound at lower frequencies than humans can hear (below 20 Hz). Infrasound can be generated by sources such as explosions, sonic booms, volcanoes, meteors, rocket launches, and ocean swell. Infrasound travels for long distances (up to thousands of kilometers) through the middle atmosphere, and, just as high-frequency ultrasound can be used to probe the human body, low-frequency infrasound observations can be used to provide information about the atmosphere. Our project "Middle Atmosphere Dynamics: Exploiting Infrasound Using a Multidisciplinary Approach at High Latitudes" (MADEIRA) has exploited infrasound observations to feed into atmospheric models and to probe dynamical properties of the middle atmosphere, focusing on infrasound generated by controlled explosions and ocean swell (microbaroms). The following are research highlights from MADEIRA: 1) MADEIRA together with the University of Reading showed for the first time how one can assimilate infrasound data from explosions to constrain atmospheric models. There is still much research needed before we can use infrasound in everyday weather prediction systems. We also do not yet know how much it will actually improve our predictions. Despite these uncertainties, this study is an important step forward and could lead to better prediction systems in the future. 2) MADEIRA successfully created a direct connection between infrasound data and the upper stratospheric polar cap circulation. We trained a specialized neural network to map infrasound timeseries to zonally-average stratospheric winds, using five years of processed infrasound recordings from three polar infrasound stations and the ERA5 atmospheric re-analysis model. The processing was designed to detect microbarom signals, and these are greatly influenced by stratospheric winds. By validating our results with two unseen years of infrasound and ERA5 data, we showed that we can estimate the average eastward winds in the polar cap stratosphere at a specific altitude using only infrasound data. The uncertainty in our estimations is around 12 m/s. This is a promising outcome, especially considering that we used only three infrasound stations and the neural network did not have information about the microbarom source properties and locations. Since infrasound data can be obtained in near-realtime, this lays the groundwork for future routine diagnostics of the stratospheric polar vortex. 3) MADEIRA probed mesospheric gravity waves by analyzing infrasound data from a consistent set of explosions from the same site. Gravity waves are created, for example, when air moves upward over mountains, and they play an important role in transferring energy between different parts of the atmosphere. Better representing small-scale gravity waves is important since current climate and weather prediction models generally do not fully model their dynamics. In this MADEIRA study, we used infrasound recordings from 49 explosions and applied a deconvolution-based approach to examine the vertical wavenumber spectrum of fine-scale mesospheric structures related to gravity waves. We compared our findings with radar data and found good agreement with the infrasound-based analysis. This study is the first to use infrasound from a large and consistent set of explosive events to probe gravity waves using this approach. Hence, MADEIRA has explored innovative methods to exploit explosion-generated and global microbarom infrasound datasets, aiming at improving the high-top model representation of dynamics in the middle atmosphere. This project paves the way for medium-range numerical weather prediction model enhancements.

MADEIRA has developed and validated methodologies to explore the middle atmosphere with novel infrasound data assimilation techniques, propagation modeling, and data processing approaches to demonstrate a remote atmospheric sensing potential. MADEIRA has paved the way for realtime infrasound-based stratospheric polar vortex diagnostic tools. MADEIRA facilitates work towards inclusion of more realistic middle atmospheric wind into medium- and long-range prediction models. A more consistent new-generation models will be valuable to a society relying on accurate medium- and long-range range weather predictions. This includes sectors such as energy, agriculture, health, offshore, transport, food security, disaster risk reduction, and insurance. Another long-term aim is to derive the global signature of atmospheric tides and normal mode oscillations using the global infrasound network.

We will contribute to a more realistic incorporation of the middle atmosphere in weather models, providing a path towards more accurate medium-range forecasts. We use techniques having the advantage of providing continuous observations over long timescales with wide geographical coverage. Our data-driven tools to derive the dynamic properties of the middle atmosphere at 30-60 km altitudes rely on time series of infrasound (very low-frequency, inaudible sound) recorded at ground-based Arctic stations. In particular, we consider infrasonic ambient noise from ocean surface wave interaction - microbaroms - where oceanic wave-action models will be used to identify active sources. These data will provide constraints on probabilistic medium-range weather forecasts and climate models. The middle atmosphere is relatively inaccessible to continuous measurements, particularly for winds above 40 km altitude. The region is poorly studied and not well understood compared with the thermosphere aloft from which satellites and radars provide data, and with altitudes below probed by weather balloons and other technologies. Established forecast models assimilate very little middle atmospheric data, so modelling is less accurate above ~50 km altitude. Still, this region is coupled to the troposphere below, with events like Sudden Stratospheric Warmings (SSWs) known to influence the conditions at ground level. Infrasound data will also be interpreted in the context of: a) Meteor radar wind measurements at 70-100 km: We will obtain new understanding of stratosphere-mesosphere coupling, especially for regional and global events like SSWs. b) The role of middle atmosphere dynamics in probabilistic medium-range weather prediction and sub-seasonal prediction: This will be based on combined meteor radar and infrasound data. We will design diagnostic tools to interpret the data in terms of the middle atmospheric circumpolar vortex circulation.

Publications from Cristin

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Funding scheme:

FRINATEK-Fri prosj.st. mat.,naturv.,tek