Pieter Smets

Project researcher & Ph.D. candidate


Hello there! Welcome to PieterSmets.be, a brief version of my course of life, accounting what is most prominent in terms of education, occupations, publications and interests. Not everything is put on the world wide web, or you can't find it. In that case, feel free to contact me. No promises that I will reply.

Symphony of infrasound
Sound below the human audible range, 20 - 20k Hz, infrasound. Much much more than vibrations that travel through air, even if you can not hear it. Infrasound can travel very long distances while propagating from ground up in the atmosphere, up to the stratosphere or even the thermosphere. Some signals refract back to the earth's surface, where it reflects to start a new bounce or journey through the air (see Figure 1). The conditions for this refracting depends on the state of the atmosphere. The world record for furthest signal is for the Krakatoa eruption, 1883: 7 times around the globe in 5 days. Therefore, infrasound is used primary as monitoring technique for the Comprehensive Nuclear Test Ban Treaty (CTBT). But it can be used for much more. As infrasound propagates through the atmosphere, it is affected by temperature and wind. So, observations at the ground contain information of the (upper) atmosphere along its path. Understanding these observations reveals valuable information of a sparsely monitored upper atmosphere. These secondary applications make infasound an even more interesting and challenging technique with a bright future in probing the atmosphere!

Simulation of infrasound propagation from Liege (BE) to the infrasound arrays (white triangles) in De Bilt (NL) and Texel (NL). The effective velocity (ceff) is color coded and derived from the ECMWF analysis. The ray trajectories are plotted in black. The eigenrays, connecting source and receivers, are given as white lines.

Audible acoustics
Engineering, acoustics and (live) performance. A perfect match! Working as a PA sound technician (FOH and monitor) for small events, create light shows, or DJ'ing, it's more than a hobby: STEPS4it.
Beyond the human range
Not only focussing on your own field of interest, but opening eyes and ears to others. Most is easy is to step into the world. How I do it? By being involved as a volunteer and enthousiastic in youth pastoral activities. It not only broadens my view on the world and lightens my path, it helps improving your (people)managment and organisational skills as well.
2013-present
Ph.D. Candidate 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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2015-present
Project researcher: Atmosphere dynamics Research Infrastructure in Europe (ARISE2)
Royal Netherlands Meteorological Institute, R&D Department of Seismology and Acoustics,
De Bilt, The Netherlands.

The first ARISE desing studie project is a collaborative Research and Innovation project (2007-2013) funded by European Union FP7 program (grant number 284387). The second ARISE desing studie project is a collaborative infrastructure Design Study project (2015-2018) funded by the European Commission H2020 program (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-present
Ph.D. researcher: Infrasound for atmospheric studies
Department of Geoscience and Engineering, Delft University of Technology,
Delft, The Netherlands
The art of processing and modelling of infrasonic ambient noise to obtain valuable knowledge of the sparse observed middle atmosphere. First part studies the life cycle of a Sudden Stratospheric Warming (SSW) from infrasonic ambient noise observations, an issue in nowadays numerical weather prediction. Second part describes probabilistic infrasound propagation using realistic atmospheric perturbations, to improve the use of General Circulation Models for infrasound propagation modelling. The latter is of great importance for verification purposes of the Comprehensive Nuclear-Test-Ban Treaty and infrasonic inversion studies. Thirdly, medium-range stratospheric forecasts are validated, again, focussing on the performance during a SSW making use of continuous volcanic infrasound.

2012-2014
Project researcher: Atmosphere dynamics Research Infrastructure in Europe (ARISE)
Royal Netherlands Meteorological Institute, Department of Seismology,
De Bilt, The Netherlands.

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Peer-reviewed Articles

= open access publication

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.

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.

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.

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.

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.

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.

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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Selected Conference Presentations

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 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:

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