Pieter Smets

Researcher in applied geophysics

professional
signal processing, data analysis, numerical modelling, programming, web applications, engineering
personal
swimming, running, live sound mixing, gardening, cookery
2013-2018
Ph.D. Applied Geophysics
Delft University of Technology, The Netherlands.
2009-2011
M.Sc. Aerospace Engineering - Earth and Planetary Observation
Delft University of Technology, The Netherlands.
2006-2010
B.Sc. Aerospace Engineering
Delft University of Technology, The Netherlands.
2000-2006
Visual Arts Education - Spatial Arts Design
Part-time Art Education (DKO), Academie Noord, Brasschaat, Belgium.
2000-2006
Grammar school
Sint-Michielscollege Brasschaat, Belgium.

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2018-present
PostDoc: Probing the changing deep oceans and upper atmosphere with seismo-acoustic waves
Department of Geoscience and Engineering, Delft University of Technology,
Delft, The Netherlands.
Funder: Netherlands Organisation for Scientific Research (NWO). Grant number: 864.14.005.

Low frequency seismo-acoustic waves and ambient noise contain a wealth of information on temperature changes in the upper atmosphere and deep oceans, where in-situ observations are very rare and satellite and other remote sensing techniques are hardly applicable.Building on my expertise in geophysics, i.e., seismology, hydro-acoustics and infrasound, I propose to probe the deep oceans and upper atmosphere using low frequency seismic- acoustic waves to monitor coupled spatial-temporal changes, which can be attributed to specific geophysical processes and climate change.Geophysical processes such as earthquakes and tsunamis, (underwater) volcanic eruptions, land slides and avalanches, exploding meteors and severe weather, all leave a signature in the earth, oceans and/or atmosphere.Low frequency waves are hardly attenuated by the media, i.e., earth, ocean and atmosphere, and can be detected over long ranges enabling both the study of the processes and the medium.Only recently, more mature networks have become available which can be used for imaging the media with low frequency seismo-acoustic waves. Therefore, the time is right for in-depth studies by combining datasets and using collocated measurements of seismo-acoustic waves. This enables an unique integrated approach towards probing the media and a better understanding of geophysical processes by simultaneously analyzing seismo-acoustic signatures.I will be the first to use an integrated interferometric approach on seismo-acoustic waves, where I will retrieve deep ocean and upper atmosphere temperature changes on an unprecedented spatial and temporal scale. For this, I will use the continuous ambient noise in the different media, which allows monitoring the media on a local, regional and global scale. An improved fundamental understanding of geophysical processes and spatial-temporal changes in the upper atmosphere and deep ocean is highly relevant and will contribute to a better assessment of the changing earth environment and the related increasing vulnerability of our society.

2015-2018
Project researcher: Atmosphere dynamics Research Infrastructure in Europe (ARISE2)
Royal Netherlands Meteorological Institute, R&D Department of Seismology and Acoustics,
De Bilt, The Netherlands.
Funder: European Commission H2020 programme. Grant number: 653980.

The first ARISE design studie project is a collaborative Research and Innovation project (2007-2013) funded by the European Commission 7th Framework programme (grant number 284387). The second ARISE design studie project is a collaborative infrastructure Design Study project (2015-2018) funded by the European Commission H2020 programme (grant number 653980).
The ARISE project aims at establishing a unique atmospheric research and data platform in Europe, combining observations with theoretical and modelling studies, to elucidate the dynamics of the middle and upper atmosphere. ARISE combines for the first time international networks with complementary technologies such as infrasound, lidar, airglow, radar systems. This joint network provides advanced data products used as benchmarks for weather forecast models. The ARISE network also allows enhanced and detailed observations of other extreme events in the Earth system, such as erupting volcanoes, magnetic storms, tornadoes and tropical thunderstorms, for a better understanding of underlying physical processes and future monitoring for civil applications. More information: arise-project.eu

2013-2018
Ph.D.: Infrasound and the Dynamical Stratosphere : A new application for operational weather and climate prediction
Department of Geoscience and Engineering, Delft University of Technology,
Delft, The Netherlands

Infrasound is low-frequency inaudible sound (< 20Hz), used as a waveform verification technique for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). In this thesis, it is shown that infrasound can provide useful additional upper atmospheric observations in a region where data coverage is sparse; beyond the middle stratosphere direct information of wind is missing in current numerical weather prediction models. Passive ambient infrasound signals can act as an atmospheric probe. Observed and simulated infrasound wavefront parameters are compared, i.e., back azimuth, apparent velocity, or travel time. Wavefront parameters are extracted from the continuous waveform recordings using signal detectors and array processing techniques and simulated by ray theory using various atmospheric conditions. The theoretical basis of this method relies on the assertion that sound propagates through a particular atmospheric state. The state that is closest to reality will then lead to simulated values that are closest to the observed values. It is demonstrated that infrasound has the potential to contribute to operational weather prediction applications by validation of atmospheric analysis and forecast products, in particular in regions above 30 km altitude. In addition, knowledge of the dynamical stratosphere is utilized for infrasound monitoring, in order to benefit CTBT verification efforts.

Three research objectives form the basis of this thesis:

  • To use passive ambient infrasound signals as an atmospheric probe. To extract relevant parameters from continuous waveforms by signal detectors and array processing techniques.
  • To develop a technique to use infrasound in current atmospheric models. To demonstrate how infrasound data, both observations and simulations, can be of use for operational weather and climate modelling in the stratosphere.
  • To apply knowledge of stratospheric variability to problems of infrasonic de- tection and propagation modelling. To utilise knowledge of the dynamical stratosphere when undertaking infrasound analysis, in order to benefit CTBT verification efforts.
2012-2014
Project researcher: Atmosphere dynamics Research Infrastructure in Europe (ARISE)
Royal Netherlands Meteorological Institute, Department of Seismology,
De Bilt, The Netherlands.
Funder: European Commission 7th Framework programme (FP7). Grant number: 284387.

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Doctoral thesis

2018

Abstract: Infrasound is low-frequency inaudible sound (< 20Hz), used as a waveform verification technique for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). In this thesis, it is shown that infrasound can provide useful additional upper atmospheric observations in a region where data coverage is sparse; beyond the middle stratosphere direct information of wind is missing in current numerical weather prediction models. Passive ambient infrasound signals can act as an atmospheric probe. Observed and simulated infrasound wavefront parameters are compared, i.e., back azimuth, apparent velocity, or travel time. Wavefront parameters are extracted from the continuous waveform recordings using signal detectors and array processing techniques and simulated by ray theory using various atmospheric conditions. The theoretical basis of this method relies on the assertion that sound propagates through a particular atmospheric state. The state that is closest to reality will then lead to simulated values that are closest to the observed values. It is demonstrated that infrasound has the potential to contribute to operational weather prediction applications by validation of atmospheric analysis and forecast products, in particular in regions above 30 km altitude. In addition, knowledge of the dynamical stratosphere is utilized for infrasound monitoring, in order to benefit CTBT verification efforts.

Peer-reviewed Articles

= open access publication

2020
G. Averbuch, R. Waxler, P. S. M. Smets, L. G. Evers
Journal of the Acoustical Society of America, 147, 1066-1077.

Abstract: In seismology, the depth of a near-surface source is hard to estimate in the absence of local stations. The depth-yield trade-off leads to significant uncertainties in the source's depth and strength estimations. Long-range infrasound propagation from an underwater or underground source is very sensitive to variations in the source's depth and strength. This characteristic is employed in an infrasound based inversion for the submerged source parameters. First, a Bayesian inversion scheme is tested under the variations of the number of stations, the signal's frequency band, and the signal-to-noise ratio (SNR). Second, an ensemble of realistic perturbed atmospheric profiles is used to investigate the effect of atmospheric uncertainties on the inversion results. Results show that long-range infrasound signals can be used to estimate the depth and strength of an underwater source. Using a broadband signal proved to be a fundamental element to obtain the real source parameters, whereas the SNR was secondary. Multiple station inversions perform better than one-station inversions; however, variations in their position can lead to source strength estimations with uncertainties up to 50%. Regardless of the number of stations, their positions, and SNRs, all of the estimated depths were within 10% from the real source depth.
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O. F. C. den Ouden, J. D. Assink, P. S. M. Smets, S. Shani-Kadmiel, G. Averbuch, L. G. Evers
Geophysical Journal International, 221, 305-317.

Abstract: The detection and characterization of signals of interest in the presence of (in)coherent ambient noise is central to the analysis of infrasound array data. Microbaroms have an extended source region and a dynamical character. From the perspective of an infrasound array, these coherent noise sources appear as interfering signals which conventional beamform methods may not correctly resolve. This limits the ability of an infrasound array to dissect the incoming wavefield into individual components. In this paper, this problem will be addressed by proposing a high-resolution beamform technique in combination with the CLEAN algorithm. CLEAN iteratively selects the maximum of the f/k spectrum (i.e., following the Bartlett or Capon method) and removes a percentage of the corresponding signal from the cross-spectral density matrix. In this procedure, the array response is deconvolved from the f/k spectral density function. The spectral peaks are retained in a "clean" spectrum. A data-driven stopping criterion for CLEAN is proposed that relies on the framework of Fisher statistics. This allows the construction of an automated algorithm that continuously extracts coherent energy until the point is reached that only incoherent noise is left in the data. CLEAN is tested on a synthetic data-set and is applied to data from multiple IMS infrasound arrays. The results show that the proposed method allows for the identification of multiple microbarom source regions in the Northern Atlantic, that would have remained unidentified if conventional methods had been applied.
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2019
M. M. E. Smink, J. D. Assink, F. C. Bosveld, P. S. M. Smets and L. G. Evers
Journal of Geophysical Research - Atmospheres, 124, 9299-9313.

Abstract: The Royal Netherlands Meteorological Institute (KNMI) operates a three‐dimensional microbarometer array at the Cabauw Experimental Site for Atmospheric Research observatory. The array consists of five microbarometers on a meteorological tower up to an altitude of 200 m. Ten ground‐based microbarometers surround the tower with an array aperture of 800 m. This unique setup allows for the study of infrasound propagation in three dimensions. The added value of the vertical dimension is the sensitivity to wind and temperature in the atmospheric boundary layer over multiple altitudes. In this study, we analyze infrasound generated by an accidental chemical explosion at the Moerdijk petrochemical plant on 3 June 2014. The recordings of the tower microbarometers show two sequential arrivals, whereas the recordings on the ground show one wavefront. This arrival structure is interpreted to be the upgoing and downgoing wavefronts. The observations are compared with propagation modeling results using global‐scale and mesoscale atmospheric models. Independent temperature and wind measurements, which are available at the Cabauw Experimental Site for Atmospheric Research, are used for comparison with model output. The modeling results explain the signal arrival times; however, the tower wavefront arrivals are not explained. This study is important for understanding the influence of the atmospheric boundary layer on infrasound detections and propagation.
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G. Averbuch, Y. Ben-Horin, P. S. M. Smets and L. G. Evers
Geophysical Journal International, 219, 1109-1117.

Abstract: Measurements of seismo-acoustic events by collocated seismic and infrasound arrays allow for studying the two wavefields that were produced by the same event. However, some of the scientific and technical constraints on the building of the two technologies are different and may be contradicting. For the case of a new station, an optimal design that will satisfy the constraints of the two technologies can be found. However, in the case of upgrading an existing array by adding the complementing technology, the situation is different. The site location, the array configuration and physical constraints are fixed and may not be optimal for the complementing technology, which may lead to rejection of the upgrade. The International Monitoring System (IMS) for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) includes 37 seismic arrays and 51 infrasound arrays. Although the CTBT verification regime is fixed in the treaty, an upgrade of the existing arrays by adding more technologies is possible. The Mount Meron seismic array (MMAI), which is part of the IMS, is composed of 16 sites. Microbarometers were installed at five MMAI sites to form the Mount Meron infrasound array. Due to regulation and physical constraints, it was not possible to relocate the sites nor to install analogue noise reduction filters (i.e. a pipe array). In this study, it is demonstrated that the installation of the MMAI infrasound array is beneficial despite the non-optimal conditions. It is shown that the noise levels of the individual array sites are between the high and median global noise levels. However, we claim that the more indicative measures are the noise levels of the beams of interest, as demonstrated by analysing the microbaroms originated from the Mediterranean Sea. Moreover, the ability to detect events relevant to the CTBT is demonstrated by analysing man-made events during 2011 from the Libya region.
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G. J. Marlton, A. J. Charlton‐Perez, R. G. Harrison, E. Blanc, L. G. Evers, A. Le‐Pichon, P. S. M. Smets
Journal of Geophysical Research - Atmospheres, 124, 4352-4364.

Abstract: Gravity waves are an important part of the momentum budget of the atmosphere. Despite this, parameterizations of gravity wave spectra in atmospheric models are poorly constrained. Gravity waves are formed by jet streams, flow over topography and convection, all of which produce pressure perturbations as they propagate over the Earth's surface, detectable by microbarometer arrays used for sensing infrasound. In this study, observations of gravity waves between 2007 and 2011 at an infrasound station in the Ivory Coast, West Africa are combined with meteorological data to calculate parameters such as intrinsic phase speed and wavenumber. Through spectral analysis, the seasonal and daily variations in all gravity wave parameters are examined. The gravity wave back azimuth varies with the migration of the Inter‐Tropical Convergence Zone, a region of intense convection, supporting previous studies. Daily variations in gravity wave arrivals at the station can be linked to two distinct convective cycles over the land and ocean. This was achieved by combining the gravity wave parameters with lightning strikes detected by the Met Office's Arrival Time Difference lightning detection system. Noise generated by turbulence in the middle of the day was found to attenuate smaller pressure amplitude gravity waves, artificially amplifying the daily variations in some gravity wave parameters. Detection of daily and seasonal variations in gravity wave parameters has the potential be used to improve the representation of gravity‐wave spectra in atmospheric models.
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K. A. M. Sambell, P. S. M. Smets, D. G. Simons, M. Snellen, L. G. Evers
Geophysical Journal International, 218, 88-99.

Abstract: Continuous long-term sound sources are recorded at hydroacoustic station H03S, a three-element hydrophone array south of Robinson Crusoe Island, between 23 April 2014 and 20 August 2017. The origin of the signal between 3-20Hz is investigated by using cross-correlation, array processing using plane wave beamforming, and spectral analysis. One-bit normalization is successfully applied as a cross-correlation preprocessing step in order to suppress undesired earthquake events in the data. Travel times retrieved from averaged cross-correlations do not yield a coherent array direction of arrival. Averaged envelopes of the cross-correlations, however, indicate a coherent signal approaching H03S from a south-southwest direction. Beamforming indicates two dominant back azimuth directions: 172°-224° (Antarctica) and 242° (Monowai Volcanic Seamount). This continuous source field creates possibilities to investigate the applicability of acoustic thermometry at hydrophones H03 S1-S2. Cross-correlation and array processing indicate significant directional variation in local modal propagation, most likely related to the steep slope in the bathymetry near H03S. In addition, it is demonstrated that the ambient noise field is not sufficiently equipartitioned. It is shown that this causes a large error in the estimated temperature, primarily due to the short receiver spacing. These large errors have not been addressed in previous studies on deep-ocean acoustic thermometry. Hence, it is shown that acoustic thermometry does not perform well on small arrays such as H03S. The power spectral density yields a strong broadband signal in January - March, most likely related to iceberg noise. A narrow banded signal around 17Hz between April and September corresponds to whale calls. The best-beam sound pressure levels towards Antarctica are compared to ERA5 climatologies for sea ice cover and normalized stress into the ocean, supporting the hypothesis of iceberg noise.
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2018
C. Pilger, L. Ceranna, J. Ole Ross, J. Vergoz, A. Le Pichon, N. Brachet, E. Blanc, J. Kero, L. Liszka, S. Gibbons, T. Kvaerna, S. P. Näsholm, E. Marchetti, M. Ripepe, P. Smets, L. Evers, D. Ghica, C. Ionescu, T. Sindelarova, Y. Ben Horin, P. Mialle
Pure and Applied Geophysics, 175, 3619–3638.

Abstract: The European Infrasound Bulletin highlights infrasound activity produced mostly by anthropogenic sources, recorded all over Europe and collected in the course of the ARISE and ARISE2 projects (Atmospheric dynamics Research InfraStructure in Europe). Data includes high-frequency (> 0.7 Hz) infrasound detections at 24 European infrasound arrays from nine different national institutions complemented with infrasound stations of the International Monitoring System for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Data were acquired during 16 years of operation (from 2000 to 2015) and processed to identify and locate ∼ 48,000 infrasound events within Europe. The source locations of these events were derived by combining at least two corresponding station detections per event. Comparisons with ground-truth sources, e.g., Scandinavian mining activity, are provided as well as comparisons with the CTBT Late Event Bulletin (LEB). Relocation is performed using ray-tracing methods to estimate celerity and back-azimuth corrections for source location based on meteorological wind and temperature values for each event derived from European Centre for Medium-range Weather Forecast (ECMWF) data. This study focuses on the analysis of repeating, man-made infrasound events (e.g., mining blasts and supersonic flights) and on the seasonal, weekly and diurnal variation of the infrasonic activity of sources in Europe. Drawing comparisons to previous studies shows that improvements in terms of detection, association and location are made within this study due to increasing the station density and thus the number of events and determined source regions. This improves the capability of the infrasound station network in Europe to more comprehensively estimate the activity of anthropogenic infrasound sources in Europe.
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J. D. Assink, G. Averbuch, S. Shani-Kadmiel, P. S. M. Smets, L. G. Evers
Seismological Research Letters, 89, 2025-2033.

Abstract: The 2017 North Korean nuclear test gave rise to seismic and low-frequency acoustic signals, that is, infrasound. The infrasonic signals are due to seismo-acoustic coupling and have been detected on microbarometer array I45RU in the Russian Federation at 401 km from the test site. I45RU is part of the International Monitoring System for the verification of the Comprehensive Nuclear-Test-Ban Treaty. We analyze the seismo-acoustic coupling by making use of array-processing and backprojection techniques. The backprojections show that infrasound radiation is not confined to the epicentral region. More distant regions are found to be consistent with locations of topography, sedimentary basins, and underwater evanescent sources. The backprojections can be used to estimate the average infrasonic propagation speed through the atmosphere. We discuss these findings in the context of infrasound propagation conditions during the sixth nuclear test. It is suggested that propagation from the test site to I45RU may have occurred along unexpected paths instead of typical stratospheric propagation. We present several scenarios that could be considered in the interpretation of the observations.
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G. Averbuch, J. D. Assink, P. S. M. Smets, L. G. Evers
Geophysics, 83, WC43-WC51.

Abstract: Low-frequency acoustic, i.e., infrasound, waves are measured by sparse arrays of microbarometers. Recorded data are processed by automatic detection algorithms based on array-processing techniques such as time-domain beam forming and f-k analysis. These algorithms use a signal-to-noise ratio (S/N) value as a detection criterion. In the case of high background noise or in the presence of multiple coinciding signals, the event’s S/N decreases and can be missed by automatic processing. In seismology, detecting low-S/N events with geophone arrays is a well-known problem. Whether it is in global earthquake monitoring or reservoir microseismic activity characterization, detecting low-S/N events is needed to better understand the sources or the medium of propagation. We use an image-processing technique as a post-processing step in the automatic detection of low S/N events. In particular, we consider the use of the Hough transform (HT) technique to detect straight lines in beam-forming results, i.e., a back azimuth (BA) time series. The presence of such lines, due to similar BA values, can be indicative of a low-S/N event. A statistical framework is developed for the HT parameterization, which includes defining a threshold value for detection as well as evaluating the false alarm rate. The method is tested on synthetic data and five years of recorded infrasound from glaciers. It is shown that the automatic detection capability is increased by detecting low-S/N events while keeping a low false-alarm rate.
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L. G. Evers, J. D. Assink, P. S. M. Smets
Geophysical Journal International, 213, 1785–1791.

Abstract: Supersonic rockets generate low-frequency acoustic waves, i.e., infrasound, during the launch and re-entry. Infrasound is routinely observed at infrasound arrays from the International Monitoring System, in place for the verification of the Comprehensive Nuclear-Test-Ban Treaty. Association and source identification are key elements of the verification system. The moving nature of a rocket is a defining criterion, in order to distinguish it from an isolated explosion. Here, it is shown how infrasound recordings can be associated, which leads to identification of the rocket. Propagation modeling is included to further constrain the source identification. Four rocket launches by the Democratic People's Republic of Korea in 2009 and 2017 are analyzed, in which multiple arrays detected the infrasound. Source identification in this region is important for verification purposes. It is concluded that with a passive monitoring technique such as infrasound, characteristics can be remotely obtained on sources of interest, i.e., infrasonic intelligence, over 4500+ km.
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S. Shani-Kadmiel, J. D. Assink, P. S. M. Smets, L. G. Evers
Geophysical Research Letters, 45, 427-435.

Abstract: In this study we analyze infrasound signals from three earthquakes in Central Italy. The MW 6.0 Amatrice, MW 5.9 Visso, and MW 6.5 Norcia earthquakes generated significant epicentral ground motions that couple to the atmosphere and produce infrasonic waves. Epicentral seismic and infrasonic signals are detected at I26DE, however, a third type of signal, which arrives after the seismic-wave train and before the epicentral infrasound signal, is also detected. This peculiar signal propagates across the array at acoustic-wave speeds but the celerity associated with it is three-times the speed-of-sound. Atmosphere independent backprojections and full 3-D ray tracing using atmospheric conditions of the European Centre for Medium-Range Weather Forecasts are used to demonstrate that this apparently fast-arriving infrasound signal originates from ground motions more than 400 km away from the epicenter. The location of the secondary infrasound patch coincides with the closest bounce-point to I26DE as depicted by ray tracing backprojections.
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E. Blanc, L. Ceranna, A. Hauchecorne, A. Charlton-Perez, E. Marchetti, L. G. Evers, T. Kvaerna, J. Lastovicka, L. Eliasson, N. B. Crosby, P. Blanc-Benon, A. Le Pichon, N. Brachet, C. Pilger, P. Keckhut, J. A. Assink, P. S. M. Smets, C. F. Lee, J. Kero, T. Sindelarova, N. Kämpfer, R. Rüfenacht, T. Farges, C. Millet, S. P. Näsholm, S. J. Gibbons, P. J. Espy, R. E. Hibbins, P. Heinrich, M. Ripepe, S. Khaykin, N. Mze, J. Chum
Surveys in Geophysics, 39, 171–225.

Abstract: This paper reviews recent progress toward understanding the dynamics of the middle atmosphere in the framework of the Atmospheric Dynamics Research InfraStructure in Europe (ARISE) initiative. The middle atmosphere, integrating the stratosphere and mesosphere, is a crucial region which influences tropospheric weather and climate. Enhancing the understanding of middle atmosphere dynamics requires improved measurement of the propagation and breaking of planetary and gravity waves originating in the lowest levels of the atmosphere. Inter-comparison studies have shown large discrepancies between observations and models, especially during unresolved disturbances such as sudden stratospheric warmings for which model accuracy is poorer due to a lack of observational constraints. Correctly predicting the variability of the middle atmosphere can lead to improvements in tropospheric weather forecasts on timescales of weeks to season. The ARISE project integrates different station networks providing observations from ground to the lower thermosphere, including the infrasound system developed for the Comprehensive Nuclear-Test-Ban Treaty verification, the Lidar Network for the Detection of Atmospheric Composition Change, complementary meteor radars, wind radiometers, ionospheric sounders and satellites. This paper presents several examples which show how multi-instrument observations can provide a better description of the vertical dynamics structure of the middle atmosphere, especially during large disturbances such as gravity waves activity and stratospheric warming events. The paper then demonstrates the interest of ARISE data in data assimilation for weather forecasting and re-analyzes the determination of dynamics evolution with climate change and the monitoring of atmospheric extreme events which have an atmospheric signature, such as thunderstorms or volcanic eruptions.
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2016
P. S. M. Smets, J. D. Assink, A. Le Pichon, and L. G. Evers
Journal of Geophysical Research - Atmospheres, 121, 4637–4650.

Abstract: Accurate prediction of Sudden Stratospheric Warming (SSW) events is important for the performance of numerical weather prediction due to significant stratosphere–troposphere coupling. In this study, for the first time middle atmospheric numerical weather forecasts are evaluated using infrasound. A year of near continuous infrasound from the volcano Mt. Tolbachik (Kamchatka, Russian Federation) is compared with simulations using high resolution deterministic forecasts of the European Centre for Medium-range Weather Forecasts (ECMWF). For the entire timespan the nowcast generally performs best, indicated by a higher continuity of the predicted wavefront characteristics with a minimal back azimuth difference. Best performance for all forecasts is obtained in summer. The difference between the infrasound observations and the predictions based on the forecasts is significantly larger during the 2013 SSW period for all forecasts. Simulations show that the SSW onset is better captured by the ten day forecast while the recovery is better captured by the nowcast.
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J. D. Assink, G. Averbuch, P. S. M. Smets, and L. G. Evers
Geophysical Research Letters, 43, 3526–3533.

Abstract: The underground nuclear tests by the Democratic People's Republic of Korea (DPRK) generated atmospheric infrasound, both in 2013 and 2016. Clear detections were made in the Russian Federation (I45RU) and Japan (I30JP) in 2013 at stations from the International Monitoring System. Both tropospheric and stratospheric refractions arrived at the stations. In 2016, only a weak return was potentially observed at I45RU. Data analysis and propagation modeling shows that the noise level at the stations and the stratospheric circumpolar vortex were different in 2016 compared to 2013. As the seismic magnitude of the 2013 and 2016 nuclear test explosions was comparable, we hypothesize that the 2016 test occurred at least 1.5 times deeper. In such a case, less seismic energy would couple through the lithosphere-atmosphere interface, leading to less observable infrasound. Since explosion depth is difficult to estimate from seismic data alone, this motivates a synergy between seismics and infrasonics.
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2015
P. S. M. Smets, L. G. Evers, S. P. Naesholm, and S. J. Gibbons
Geophysical Research Letters, 42, 6510-6517.

Abstract: This study demonstrates probabilistic infrasound propagation modelling using realistic perturbations. The ensembles of perturbed analyses, provided by the European Centre for Medium-range Weather Forecasts (ECMWF), include error variances of both model and assimilated observations. Ensemble spread profiles indicate a yearly mean effective sound speed variation of up to 8 m/s in the stratosphere, exceeding occasionally 25 m/s for a single ensemble set. It is shown that errors in point estimates of effective sound speed are dominated by variations in wind strength and direction. One year of large mining explosions in the Aitik mine, northern Sweden, observed at infrasound array IS37 in northern Norway are simulated using 3-D ray tracing. Probabilistic propagation modelling using the ensembles demonstrates that small-scale fluctuations are not always necessary to improve the match between predictions and observations.
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A. Le Pichon, J. D. Assink, P. Heinrich, E. Blanc, A. Charlton-Perez, C. F. Lee, P. Keckhut, A. Hauchecorne, R. Rufenacht, N. Kampfer, P. S. M. Smets, L. G. Evers, L. Ceranna, C. Pilger, O. Ross, C. Claud
Journal of Geophysical Research - Atmospheres, 120, 8318–8331.

Abstract: High-resolution, ground-based and independent observations including co-located wind radiometer, lidar stations, and infrasound instruments are used to evaluate the accuracy of general circulation models and data constrained assimilation systems in the middle atmosphere at northern hemisphere mid-latitudes. Systematic comparisons between observations, the Medium-Range Weather Forecasts (ECMWF) operational analyses including the recent Integrated Forecast System (IFS) cycles 38r1 and 38r2, the NASA's Modern Era Retrospective analysis for Research and Applications (MERRA) re-analyses and the free running climate Max Planck Institute Earth System Model (MPI-ESM-LR) are carried out in both temporal and spectral domains. We find that ECMWF and MERRA are broadly consistent with lidar and wind radiometer measurements up to ~40 km. For both temperature and horizontal wind components, deviations increase with altitude as the assimilated observations become sparser. Between 40 and 60 km altitude, the standard deviation of the mean difference exceeds 5 K for the temperature and 20 m/s for the zonal wind. The largest deviations are observed in winter when the variability from large-scale planetary waves dominates. Between lidar data and MPI-ESM-LR, there is an overall agreement in spectral amplitude down to 15-20 days. At shorter time-scales, the variability is lacking in the model by ~10 dB. Infrasound observations indicate a general good agreement with ECWMF wind and temperature products. As such, this study demonstrates the potential of the infrastructure of the Atmospheric Dynamics Research Infrastructure in Europe project (ARISE) that integrates various measurements and provides a quantitative understanding of stratosphere-troposphere dynamical coupling for numerical weather prediction applications.
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2014
P. S. M. Smets and L. G. Evers
Journal of Geophysical Research - Atmospheres, 119, 12,084–12,099.

Abstract: A method is presented to study the life cycle of a SSW using infrasonic ambient noise observations. The potential of infrasound is shown to provide the missing observations required by numerical weather prediction to better resolve the upper atmosphere. Microbarom observations are evaluated to identify detections that cannot be explained by the analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF). Identified differences can be related to the either the altitude limit of the analysis, not resolving thermospheric ducts, or to an actual error in the analysis. Therefore, daily normalized spectral powers are proposed to distinguish stratospheric from thermospheric return height, based on the different signature of solar tidal amplitude fluctuations. The microbarom source model of Waxler et al. [2007], including bathymetry to allow column resonances, and an atmospheric propagation model using 3-D ray tracing, coupled using a simplified simulation model, are used to verify the observations with the analysis. It is shown, that a SSW is not a smooth event as following from the analysis, but a series of abrupt changes with a period of 10 to 16 days, increasing in intensity and duration. This is in agreement with the wave period of Rossby waves, interacting with the stratospheric circumpolar vortex. The type of vortex disturbance, split or reversal, can be deduced from the combined effect of the change in back-azimuth direction, solar tidal signature type and or phase variation of the amplitude variation of the observed microbaroms.
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J. T. Fricke, L. G. Evers, P. S. M. Smets, K. Wapenaar, and D. G. Simons
Journal of Geophysical Research - Atmospheres, 119, 9654-9665.

Abstract: We present the results of infrasonic interferometry applied to microbaroms, obtained from ambient noise. For this purpose the ’Large Aperture Infrasound Array’ (LAIA) was used, which has been installed in the Netherlands. Pre-processing appeared to be an essential step in enhancing the microbarom signals from ambient noise that strongly influences the results of the interferometry. Both the state of the atmosphere and the noise characteristics are taken into account to assess the strength of the cross correlation. The delay time of the microbaroms between two stations is determined through cross correlating the recordings. By calculating the cross correlations between all 55 station pairs of LAIA we are able to find the delay time of microbaroms up to a inter-station distance of 40.6 km. Using the strength of the cross correlations we are able to show that the coherence of the microbaroms along the direction of arrival is higher than orthogonal to it. A comparison of the atmospheric state, with a cross correlation, over a period of 10 days, reveals that the infrasound propagation over the array is correlated with the tropospheric temperature and wind. Based on the cross correlations between the three closest stations, we are able to passively estimate the effective sound speed and the wind speed as a function of time.
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L. G. Evers, D. Brown, K. D. Heaney, J. D. Assink, P. S. M. Smets, and M. Snellen
Geophysical Research Letters , 41, 1644-1650.

Abstract: Atmospheric low frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here, we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1,325 km from the epicenter. Seismic waves coupled to hydro-acoustic waves at the ocean floor, after which the energy entered the SOund Fixing And Ranging (SOFAR) channel and was detected on a hydrophone array. The energy was diffracted by a sea mount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.
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J. D. Assink, R. Waxler, P. Smets, and L. G. Evers
Journal of Geophysical Research - Atmospheres, 119, 1140–1153.

Abstract: In January 2011, the state of the polar vortex in the midlatitudes changed significantly due to a minor Sudden Stratospheric Warming event. As a result, a bi-directional duct for infrasound propagation developed in the middle atmosphere that persistedfor two weeks. The ducts were due to two zonal wind jets, one between 30-50 km and the other around 70 km altitude. In this paper, using microbarom source modeling, a previously unidentified source region in the eastern Mediterranean is identified,besides the more well known microbarom source regions in the Atlantic Ocean. Infrasound data is then presented in which the above mentioned bi-directional duct is observed in microbarom signals recorded at the IMS station I48TN in Tunisia from the Mediterranean region to the east and from the Atlantic Ocean to the west. While the frequency bands of the two sources overlap, the Mediterranean signal is coherent up to about 0.6 Hz. This observation is consistent with the microbarom source modeling; the discrepancy in the frequency band is related to differences in the ocean wave spectra for the two basins considered. This work demonstrates the sensitivity of infrasound to stratospheric dynamics and illustrates that the classic paradigm of a unidirectional stratospheric duct for infrasound propagation can be broken during a Sudden Stratospheric Warming event.
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2013
P. Brown, J. Assink, L. Astiz, R. Blaauw, M. Boslough, J. Borovicka, N. Brachet, D. Brown, M. Campbell-Brown, L. Ceranna, W. Cooke, C. de Groot-Hedlin, D. Drob, W. Edwards, L. Evers, M. Garces, J. Gill, M. Hedlin, A. Kingery, G. Laske, A. Le Pichon, P. Mialle, D. Moser, A. Saffer, E. Silber, P. Smets, R. Spalding, P. Spurny, E. Tagliaferri, D. Uren, R. Weryk, R. Whitaker, Z. Krzeminski
Nature, 503, 238-241.

Abstract: Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave, but owing to lack of observations this is uncertain. Here we report an analysis of the damage from the airburst of an asteroid about 19 metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×1012 joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates5, 6 near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques. This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.
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D. N. Green, L. G. Evers, D. Fee, R. S. Matoza, M. Snellen, P. Smets, and D. Simons
Journal of Volcanology and Geothermal Research, 256, 31-43.

Abstract: Explosive submarine volcanic processes are poorly understood, due to the difficulties associated with both direct observation and continuous monitoring. In this study hydroacoustic, infrasound, and seismic signals recorded during the May 2010 submarine eruption of South Sarigan seamount, Marianas Arc, are used to construct a detailed event chronology. The signals were recorded on stations of the International Monitoring System, which is a component of the verification measures for the Comprehensive Nuclear-Test-Ban Treaty. Numerical hydroacoustic and infrasound propagation modelling confirms that viable propagation paths from the source to receivers exist, and provide traveltimes allowing signals recorded on the different technologies to be associated. The eruption occurred in three stages, separated by three-hour periods of quiescence. 1) A 46 h period during which broadband impulsive hydroacoustic signals were generated in clusters lasting between 2 and 13 min. 95% of the 7602 identified events could be classified into 4 groups based on their waveform similarity. The time interval between clusters decreased steadily from 80 to 25 min during this period. 2) A five-hour period of 10 Hz hydroacoustic tremor, interspersed with large-amplitude, broadband signals. Associated infrasound signals were also recorded at this time. 3) An hour-long period of transient broadband events culminated in two large-amplitude hydroacoustic events and one broadband infrasound signal. A speculative interpretation, consistent with the data, suggests that during phase (1) transitions between endogenous dome growth and phreatomagmatic explosions occurred with the magma ascent rate accelerating throughout the period; during phase (2) continuous venting of fragmented magma occurred, and was powerful enough to breach the sea surface. During the climactic phase (3) discrete powerful explosions occurred, and sufficient seawater was vaporised to produce the contemporaneous 12 km altitude steam plume.
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2012
L. G. Evers, A. R. J. van Geyt, P. Smets, and J. T. Fricke
Journal of Geophysical Research - Atmospheres, 117, D06,120.

Abstract: Long-range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the Earth's surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test Ban Treaty, in which infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In previous studies, attention has been paid to the strength and direction of the circumpolar vortex wind. In this study, an analysis is made of the temperature effect in the stratosphere on infrasound propagation. A case study is presented from an explosion during a sudden stratospheric warming. During such conditions, the size of the classical stratospheric shadow zone (∼200 km) appeared to be reduced by a factor of 2. The occurrence of such conditions is quantified by evaluating 10 years of atmospheric specifications. It unexpectedly appeared that the size of the shadow zone can become smaller than 100 km, which is confirmed by evaluating infrasound detections from mining blasts in southwestern Siberia, Russia. These results are valid over a latitudinal range of 20°N to 60°N, which is determined by the stratospheric surf zone.
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Books and book chapters

2019
Smets, P., J. Assink and L. Evers
In: Le Pichon A., Blanc E., Hauchecorne A. (eds) Infrasound Monitoring for Atmospheric Studies. pp 723-755. Springer, Cham.

Abstract: Infrasound has a long history of monitoring sudden stratospheric warmings. Several pioneering studies have focused on the various effects of a major warming on the propagation of infrasound. A clear transition has been made from observing anomalous signatures towards the use of these signals to study anomalies in upper atmospheric conditions. Typically, the infrasonic signature of a major warming corresponds to summer-like infrasound characteristics observed in midwinter. More subtile changes occur during a minor warming, recognisable by the presence of a bidirectional stratospheric duct or propagation through a warm stratosphere leading to small shadow zones. A combined analysis of all signal characteristics unravels the general stratospheric structure throughout the life cycle of the warming. A new methodology to evaluate the state of the atmosphere as represented by various weather and climate models is demonstrated. A case study comparing regional volcano infrasound with simulations using various forecast steps indicates significant differences in stratospheric forecast skill, associated with a data assimilation issue during the warming.

Lee, C., P. Smets, A. Charlton-Perez, L. Evers, G. Harrison, G. Marlton
In: Le Pichon A., Blanc E., Hauchecorne A. (eds) Infrasound Monitoring for Atmospheric Studies. pp 889-907. Springer, Cham.

Abstract: This chapter examines the potential improvements in tropospheric weather forecasts that might arise from an enhanced representation of the upper stratospheric state. First, the chapter reviews current operational practice regarding observation of the atmosphere and the relative paucity of observations in the altitude range 40–70 km. Then, we describe some idealised model calculations to quantify the potential gain in skill available from improved monitoring in this region. The idealised model experiments use a relaxation technique with the Hadley Centre General Environment Model, to assess the potential gain in skill from observations both of the whole stratosphere and the upper stratosphere. At weather forecasting timescales (up to forecast day 30), better knowledge of the stratosphere, close to the onset of a sudden stratospheric warming, improves forecasts of the tropospheric northern annular mode. Whole-stratosphere information significantly improved average surface temperature anomalies over northern North America, whilst upper stratosphere information improved anomalies over Central Siberia. These results suggest any new observational technique which can contribute to monitoring of the 40–70 km region would likely benefit tropospheric forecast skill during wintertime.

Assink, J., P. Smets, O. Marcillo, C. Weemstra, J.-M. Lalande, R. Waxler, and L. Evers
In: Le Pichon A., Blanc E., Hauchecorne A. (eds) Infrasound Monitoring for Atmospheric Studies. pp 605-632. Springer, Cham.

Abstract: Infrasound recordings can be used as input to inversion procedures to delineate the vertical structure of temperature and wind in a range of altitudes where ground-based or satellite measurements are rare and where fine-scale atmospheric structures are not resolved by the current atmospheric specifications. As infrasound is measured worldwide, this allows for a remote sensing technique that can be applied globally. This chapter provides an overview of recently developed infrasonic remote sensing methods. The methods range from linearized inversions to direct search methods as well as interferometric techniques for atmospheric infrasound. The evaluation of numerical weather prediction (NWP) products shows the added value of infrasound, e.g., during sudden stratospheric warming (SSW) and equinox periods. The potential transition toward assimilation of infrasound in numerical weather prediction models is discussed.

Blanc, E., K. Pol, A. Le Pichon, A. Hauchecorne, P. Keckhut, G. Baumgarten, J. Hildebrand, J. Höffner, G. Stober, R. Hibbins, P. Espy, M. Rapp, B. Kaifler, L. Ceranna, P. Hupe, J. Hagen, R. Rüfenacht, N. Kämpfer, P. Smets
In: Le Pichon A., Blanc E., Hauchecorne A. (eds) Infrasound Monitoring for Atmospheric Studies. pp 845-887. Springer, Cham.

Abstract: The middle atmosphere (from about 10–110 km altitude) is a highly variable environment at seasonal and sub-seasonal timescales. This variability influences the general atmospheric circulation through the propagation and breaking of planetary and gravity waves. Multi-instrument observations, performed in the framework of the ARISE (Atmospheric Dynamics Research InfraStructure in Europe) project, are used to quantify uncertainties in Numerical Weather Prediction (NWP) models such as the one of the European Centre for Medium-Range Weather Forecasts (ECMWF). We show the potential of routine and measurement campaigns to monitor the evolution of the middle atmosphere and demonstrate the limitations of NWP models to properly depict small-scale atmospheric disturbances. Continuous lidar and radar measurements conducted over several days at ALOMAR provide a unique high-resolution full description of solar tides and small-scale structures. Nightly averaged lidar profiles routinely performed in fair weather conditions at the Observatoire Haute-Provence (OHP) and Maïdo observatory (Reunion Island) provide a year-to-year evolution of stratosphere and mesosphere temperature profiles. Routine meteor radar observations depict the evolution of wind profiles and solar tides in the mesosphere and lower thermosphere. With the recent development of the portable Compact Rayleigh Autonomous Lidar (CORAL) which automatically measures temperature profiles at high temporal resolution, the possibility of combining different instruments at different places is now offered, promising the expansion of multi-instrument stations in the near future. Through a better description of infrasound propagation in the middle atmosphere and stratosphere–troposphere couplings, these new middle atmosphere datasets are relevant for infrasound monitoring operations, as well as for weather forecasting and other civil applications.

Selected Conference Presentations

2017
Smets, P. S. M.
NVBM Autumn Symposium 2017, Wageningen.
Oral.

Abstract:

Smets, P. S. M., J. D. Assink, L. G. Evers
Infrasound Technology Workshop 2017, Tromsø, Norway.
Oral.

Abstract: For users of ensemble weather forecasts, a key metric of forecast success is its ability to effectively predict the uncertainty of a given weather condition happening at some point in the future. Knowledge on the quality of the forecasting system is essential. Evaluation methods of forecast performance focus predominantly on the surface and troposphere. However, knowledge on the stratospheric performance is valuable. The potential of infrasound as an independent measure to evaluate the stratospheric forecast performance has been demonstrated for the 2013 sudden stratospheric warming, addressing a model bias due to a data assimilation issue. In this study, the performance of the stratospheric ensemble forecasts is evaluated to constrain ensemble predictions and detect model biases. A year of near continuous infrasound from the volcano Etna is compared with simulations using the ensemble forecast of the European Centre for Medium-range Weather Forecasts (ECMWF).

Smets, P. S. M., J. D. Assink, L. G. Evers
Acoustical Society of America Spring 2017 Meeting, Boston, USA.
Oral.

Abstract: Microbaroms are atmospheric pressure oscillations radiated from non-linear ocean surface wave interactions. Large regions of interacting high-energetic ocean waves, e.g., ocean swell and marine storms, radiate almost continuously acoustic energy. Microbaroms dominate the infrasound ambient noise field, which makes them a preferred source for passive atmospheric probing. Microbarom are simulated using a two-fluid model, representing an atmosphere over a finite-depth ocean and a coupled ocean-wave model providing the sea state. Air-sea coupling is crucial due to the two-way interaction between surface winds and ocean waves. In this study, a detailed overview is given on how global microbarom simulations are obtained, including a sensitivity analysis of the various model input data and parameterizations. Simulations are validated by infrasound array observations of the International Monitoring Systems (IMS) of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). A brief demonstration is given on the added value of global microbarom simulations for infrasound studies and how to obtain these source simulations.

Smets, P. S. M., J. D. Assink, L. G. Evers
Acoustical Society of America Spring 2017 Meeting, Boston, USA.
Oral.

Abstract: Infrasound has a long history in monitoring SSWs. Several pioneering studies have focused on the various effects of a major warming on the propagation of infrasound, described throughout this chapter. A clear transition can be denoted from observing anomalous signatures towards the use of these signals to study anomalies in upper atmospheric specifications. First studies describe the various infrasonic signatures of a major warming. In general, the significant change in observed infrasound characteristics correspond to summer-like conditions in midwinter. More subtle changes are denoted during a minor warming, recognizable by the presence of a bidirectional stratospheric duct. A combined analysis of all signal characteristic unravels the general stratospheric structure throughout the life cycle of the warming. From then on, infrasound observations are used to evaluate the state of the atmosphere as represented by various NWP models. A new methodology, comparing regional volcano infrasound with simulations using various forecast steps, indicates interesting variations in stratospheric skill.

2016
J. D. Assink, P. S. M. Smets, A. Le Pichon, L. G. Evers
Acoustical Society of America Spring 2016 Meeting, Salt Lake City, USA.
Oral.

Abstract: The middle atmosphere has gained more and more importance for the purpose of weather and climate prediction, since increasing evidence indicates that the troposphere and stratosphere are more closely coupled than assumed before. Significant effort has been made toward a more comprehensive representation of the atmosphere to better capture the stratospheric variability as well as the stratospheric-tropospheric interactions, for example, during Sudden Stratospheric Warming (SSW) events. Despite these advances, the upper layers of the atmosphere have remained a region that is difficult to monitor. Over recent years, new developments in the field of infrasound have lead to an innovative method for evaluating numerical weather predictionmodels. In this presentation, the general technique will be described and a case study will be presented in which stratospheric forecasts of the 2013 major SSW are evaluated.

P. S. M. Smets, J. D. Assink, A. Le Pichon, L. G. Evers
European Geosciences Union General Assembly 2016, Vienna.
Oral.

Abstract: A year of near continuous infrasound from the volcano Mount Tolbachik (Kamchatka, Russian Federation) is compared with simulations using high resolution deterministic forecasts of the European Centre for Medium-range Weather Forecasts (ECMWF). For the entire timespan the nowcast generally performs best. However, simulations show that the 2013 SSW onset is better captured by the ten day forecast while the recovery is better captured by the nowcast.

J. D. Assink, P. S. M. Smets, A. Le Pichon, L. G. Evers
European Geosciences Union General Assembly 2016, Vienna.
Poster.

Abstract: A year of near continuous infrasound from the volcano Mount Tolbachik (Kamchatka, Russian Federation) is compared with simulations using high resolution deterministic forecasts of the European Centre for Medium-range Weather Forecasts (ECMWF). For the entire timespan the nowcast generally performs best. However, simulations show that the 2013 SSW onset is better captured by the ten day forecast while the recovery is better captured by the nowcast.

2015
P. S. M. Smets and L. G. Evers
Acoustical Society of America Spring 2015 Meeting, Pittsburgh, USA.
Oral, invited.

Abstract: The state of the atmosphere is of utmost importance for infrasound propagation. In propagation modeling, the true state of the atmosphere is mainly represented by the analysis. The analysis is the best deterministic estimate of the atmosphere using a data assimilation system existing of a General Circulation Model (GCM). However, the analysis excludes error variances of both model and observations. In addition, the coarse resolution of GCM results in averaging of, e.g., clouds or gravity waves, over larger regions known as parameterization. Consequentially, arrivals due to fine-scale structure in wind and temperature can be missing. Therefore, infrasound propagation including the atmospheric best-estimate error variances based on an ensemble model is proposed. The ensemble system exists of model perturbations with an amplitude comparable to the analysis error estimates to obtain a probability density function. The best-estimate analysis error variances are described by a set of perturbations using the European Centre for Medium-range Weather Forecasts (ECMWF) Ensemble Data Assimilation (EDA) system. Probabilistic infrasound propagation is demonstrated by one year of mining activity, e.g., blasting, in Gällivare, northern Sweden, observed at infrasound array IS37 in Norway, part of the International Monitoring System (IMS) for verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Probabilistic infrasound propagation is compared with the standard deterministic result obtained using the analysis.

J. D. Assink, P. S. M. Smets, L. G. Evers, A. Le Pichon
Acoustical Society of America Spring 2015 Meeting, Pittsburgh, USA.
Oral.

Abstract: While the influence of the troposphere on the stratosphere is well known, recent observational and modeling studies have demonstrated that the stratosphere has an impact on the troposphere as well. The dynamical coupling between stratosphere and troposphere is particularly strong during sudden stratospheric warming (SSW) events. The correct forecasting of the onset and duration of SSW events is therefore important and is a current challenge for weather forecasting centers. As there is a lack of observations in the upper stratosphere with good temporal and spatial coverage, additional techniques may be helpful to constrain SSWs. This is illustrated using volcanic infrasound measurements. The observations are compared with nowcast and forecast models up to 10 days. While a general agreement is found during the summer period, larger discrepancies are found during the equinox and major SSW of January 2013.

L. G. Evers, D. Brown, K. Heaney, J. D. Assink, P. S. M. Smets, M. Snellen
Acoustical Society of America Spring 2015 Meeting, Pittsburgh, USA.
Oral.

Abstract: Atmospheric low-frequency sound, i.e., infrasound, from underwater events has not been considered thus far, due to the high impedance contrast of the water-air interface making it almost fully reflective. Here, we report for the first time on atmospheric infrasound from a large underwater earthquake (Mw 8.1) near the Macquarie Ridge, which was recorded at 1325 km from the epicenter. Seismic waves coupled to hydroacoustic waves at the ocean floor, after which the energy entered the Sound Fixing and Ranging channel and was detected on a hydrophone array. The energy was diffracted by a seamount and an oceanic ridge, which acted as a secondary source, into the water column followed by coupling into the atmosphere. The latter results from evanescent wave coupling and the attendant anomalous transparency of the sea surface for very low frequency acoustic waves.

P. S. M. Smets, L. G. Evers, S. P. Naeshold, S. Gibbons
European Geosciences Union General Assembly 2015, Vienna.
Oral.

Abstract:

2014
P. S. M. Smets, L. G. Evers
Infrasound Technology Workshop 2014, Vienna.
Oral.

Abstract: Long range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the earth’s surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test-Ban Treaty, where infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In this study, the size of the classical shadow zone is evaluated. It appears that shadow zones smaller than 100 km exist from modeling with ECMWF atmospheric specifications. These results are confirmed with infrasound from mining blasts observed in Siberia.

P. S. M. Smets and L. G. Evers
European Centre for Medium-Range Weather Forecats, Reading.
Poster.

Abstract: Long range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the earth’s surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test-Ban Treaty, where infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In this study, the size of the classical shadow zone is evaluated. It appears that shadow zones smaller than 100 km exist from modeling with ECMWF atmospheric specifications. These results are confirmed with infrasound from mining blasts observed in Siberia.
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P. S. M. Smets, L. G. Evers, K. Wapenaar
European Geosciences Union General Assembly 2014, Vienna.
Oral, invited.

Abstract: Long range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the earth’s surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test-Ban Treaty, where infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In this study, the size of the classical shadow zone is evaluated. It appears that shadow zones smaller than 100 km exist from modeling with ECMWF atmospheric specifications. These results are confirmed with infrasound from mining blasts observed in Siberia.

2013
J. D. Assink, R. Waxler, L. G. Evers, P. S. M. Smets, A. Le Pichon, E. Blanc
Acoustical Society of America 2013 Meeting, San Francisco, USA.
Oral.

Abstract: In this talk, we will present recent work on various infrasound remote sensing studies. We will focus on bi-directional stratospheric ducting during a Sudden Stratospheric Warming (SSW) event and the associated infrasonic signature. We present infrasound data in which the described effect is captured with microbarom signals in the Mediterranean region. Microbarom source locations are modeled using operational ocean wave models. The modeling reveals a previously unidentified microbarom source region in the Eastern Mediterranean besides the more typical microbarom source region in the Atlantic Ocean. This work illustrates that the classic paradigm of a unidirectional stratospheric duct for infrasound propagation can be broken during a SSW event. Furthermore, we will present a case study in which the influence of atmospheric dynamics on infrasound propagation is studied. We make use of over 6 years of nearly continuous volcanic infrasound recordings from Mount Etna, Italy (37 N) that are available through the Atmospheric dynamics Research InfraStructure in Europe (ARISE) network. The infrasound observables are compared to theoretical estimates obtained from propagation modeling using existing European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric databases. While a good agreement is often found, we also report on significant discrepancies around the equinox period and during intervals during which anomalous detections occur during the winter.

C. F. Lee, P. S. M. Smets, A. J. Charlton-Perez, R. G. Harrison, P. Keckhut, C. Schmidt, S. Wust, M. Bittner
European Geosciences Union General Assembly 2013, Vienna.
Poster.

Abstract: Major Sudden Stratospheric Warmings (SSWs) present a challenge for weather forecasting. ARISE measurements can provide valuable information on atmospheric dynamics before and during such events. Comparing ARISE observations of the Winter 2012/13 SSWs with forecasts reveals: 1) Cooling around the mesopause proceeded both major SSWs. 2) Difficultly in forecasting vortex positions after the vortex split. 3) Changes in polar vortex winds substantial altered infrasound propagation.
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P. S. M. Smets, L. G. Evers, K. Wapenaar
European Geosciences Union General Assembly 2013, Vienna.
Poster.

Abstract: Infrasound observations of IMS stations above 30 degrees latitude observing microbaroms are used to determine the ssw signature. Signature differ between stations and warmings, but vortex splits show a similar behaviour. Arrays in range of both Atlantic and Pacific marine storms have a clear signature of ssw with vortex split. Simulations with ECMWF specifications correlate well with observations in direction of the zonal-mean flow. Observations indicate a faster change of the atmosphere compared to simulations.
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2012
L. G. Evers and P. S. M. Smets
Acoustical Society of America 2012 Meeting, Kansas City, USA.
Oral.

Abstract: Long range infrasound propagation depends on the wind and temperature around the stratopause (alt. 50 km). There is a seasonal change in the wind direction around the equinoxes. In summer, the wind and temperature structure of the stratosphere is stable. In winter, however, planetary waves in the troposphere can travel into the stratosphere and disturb the mean flow. This mean flow is most pronounced in the stratospheric surf zone from 20N (20S) to 60N (60S). One of the most dramatic events in the stratosphere is a Sudden Stratospheric Warming (SSW) during the winter. These occur every winter on the Northern Hemisphere as minor Warmings with a major SSW each other year. SSWs have a strong influence on infrasound propagation due to the large change in temperature and possible reversal of the wind. Therefore, SSWs are important to consider in relation to, e.g., regional and global monitoring with infrasound for verification purposes or other strategic deployments. In this presentation, the detectability of infrasound will be considered as a function of the state of the stratosphere. Variations in strength of the circumpolar vortex (around the stratopause) and temperature changes will give rise to specific propagation conditions which can often not be foreseen.

P. S. M. Smets, L. G. Evers, A. R. J. van Geyt, J. T. Fricke
European Geosciences Union General Assembly 2012, Vienna.
Poster.

Abstract: Long range infrasound propagation strongly depends on the state of the stratosphere. Infrasound can be efficiently ducted between the earth’s surface and the stratopause under a favorable wind and temperature structure between 40 and 50 km altitude. Understanding infrasound propagation under variable stratospheric conditions is of importance for a successful verification of the Comprehensive Nuclear-Test-Ban Treaty, where infrasound is used as a verification technique. Inversely, infrasound observations can be used in acoustic remote sensing of the upper atmosphere. In this study, the size of the classical shadow zone is evaluated. It appears that shadow zones smaller than 100 km exist from modeling with ECMWF atmospheric specifications. These results are confirmed with infrasound from mining blasts observed in Siberia.
Download pdf