Infrasound Monitoring for Atmospheric Studies

The International Monitoring System (IMS) comprises 337 globally distributed facilities for seismic, hydroacoustic, infrasound, and radionuclide monitoring. This chapter focuses on the infrasound component of the IMS, often referred to as the IMS infrasound network. The chapter begins with an overview of the network and of the main challenges associated with its establishment, sustainability, and detection capability. It follows with a general description of IMS stations as well as with a review of the latest advances in array geometry, wind-noise reduction systems, infrasound sensors, calibration, meteorological data, data acquisition systems, and station infrastructure. This chapter is intended for researchers and engineers who are interested in the specifications, design, status, and overall capabilities of the IMS infrasound network or in the construction of state-of-the-art infrasound stations.

This chapter explains the principles of functioning of sensors dedicated to record infrasound data. For various sensor models, the types of infrasound pressure transducers are described, and the performances are given, in terms of self-noise, sensitivity, and passband. The way to calibrate these infrasound sensors and infrasound stations on field is also reported. An in situ calibration method using additional reference sensors to an infrasound station has been developed to recover the overall response of the station element including the wind noise reducing system.

Wind noise is a significant problem for infrasound detection and localization systems. Pipe arrays are commonly used for suppressing wind noise by area averaging the relatively incoherent wind noise. The area averaging and physical construction of the pipe arrays limit the ability of the array to measure infrasound pulses with waveform fidelity. The need for waveform fidelity is motivated by the recent increase in the ability to predict waveforms theoretically from the meteorology data. This chapter investigates large cylindrical and hemispherical porous windscreens, which employ single-point sensors with little or no waveform distortion. The theory of wind noise generationWind noise generation is briefly outlined to provide a basis for understanding the windscreen research. Next, four recent experiments measuring the wind noise reductionWind noise reduction of porous cylindrical screens with respect to bare sensors mounted flush with the ground, the wind noise reductionWind noise reduction of porous fabric domesFabric domes with respect to a sensor sitting on the ground surface, the wind noise reductionWind noise reduction of porous metal domesMetal domes with respect to other sensors, and the wind noise reductionWind noise reduction of porous cylinders and fabric domesFabric domes with respect to flush-mounted sensors and each other. The second and third experiments also demonstrate the ability of the windscreens to record impulses with waveform fidelity. The largest screens provide up to 20 dB of wind noise reductionWind noise reduction down to wavenumbers on the order of the inverse of the height of the windscreen. A theory of wind noise reductionWind noise reduction is developed and leads to a better understanding of the relative contribution of wind noise generated at the surface of the screen and wind noise generated by flow through the screen. It is concluded that construction of domes large enough to provide signal enhancement down to 0.1 Hz is feasible and would provide high fidelity time waveforms for comparison with theoretical predictions.

Infrasound microphones on free flying balloonsBalloon experience very little wind noise, can cross regions that lack ground station coverage, and may capture signals that seldom reach the Earth’s surface. Despite the promise of this technique, until recently very few studies had been performed on balloon-borne acoustic sensors. We summarize the history of free flying infrasound stations from the late 1940s to 2014 and report on results from a series of studies spanning 2014–2016. These include the first efforts to record infrasound in the stratosphere in half a century, the presence of a persistent ocean microbarom peak that is not always visible on the ground, and the detection of distant ground explosions. We discuss the unique operational aspects of deploying infrasound sensors on free flying balloons, the types of signals detected at altitude, and the changes to sensor response with height. Finally, we outline the applications of free flying infrasound sensing systems, including treaty verification, bolide detection, upper atmosphere monitoring, and seismoacoustic exploration of the planet Venus.

Worldwide infrasound coverage is obtained using fixed, land-based sensing networks. However, two-thirds of the earth’s surface is composed of oceans, and while islands in the ocean already host sensing stations, no capability yet exists to monitor infrasound from sensors fielded directly in the maritime environment. Deployment in the maritime would greatly enhance the ability to monitor natural and anthropogenic sources of infrasound around the world through improved event detection, localization, and classification. The additional sensing may also facilitate improved knowledge of global atmospheric environmental conditions. The advantages and challenges of infrasound sensing in the maritime environment are described, as are potential host platforms for fielding them. Some technical challenges for this concept include sensor motion, wind noise, composing arrays of sensors and survivability in the ocean environment. An in-depth analysis of one of these, the negative impact of ocean heave on performance, is described along with a potential solution for its mitigation.

TheInternational Data Centre (IDC) International Data Centre (IDC) of theComprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) Preparatory Commission receives and processes in near-real-time data from the International Monitoring System (IMS), a globally distributed network ofSeismic seismic, Hydroacoustic hydroacoustic, Infrasound infrasound and radionuclide stations. Once completed, the IMS network will comprise 60Infrasound infrasound stations of which 49 have been installed and certified as of beginning of 2017 (Fig. 6.1). TheInfrasound infrasound stations are arrays of measurement systems that are sensitive to acoustic pressure variations in the atmosphere in the IMS frequency band between 0.02 and 4 Hz. The array configurations include 4–15 elements, with typical designs of 4–8 elements, and with apertures between 1 and 3 km following IMS requirements (Marty 2018; Christie and Campus 2010). After a design and development phase of more than 10 years, the IDCAutomatic processing automatic processing system andInteractive analysis interactive analysis are fully operational forInfrasound infrasound technology since February 2010. After reception, storage and referencing in the IDC database, the station data are automatically processed individually (e.g. theStation processing station processing stage) (Brachet et al. 2010). Based on the results of the station processing theNetwork processing network processing is initiated to form events with all three waveform technologies. The event information is then reported in IDC products (or bulletins) referred to as Standard Event Lists (SELs). Since 2010, theBulletin bulletin production deadlines have been revised and accommodate late arriving data and the signalPropagation propagation times for all waveform technologies (Coyne et al. 2012). The final automatic bulletin containingInfrasound signals infrasound signals associated to waveform events is the SEL3, which is reviewed by IDC analysts. The result of the interactive review process is the Late Event Bulletin (LEB) on whichEvent definition criteria event definition criteria are applied to produce the Reviewed Event Bulletin (REB). The REB is the final waveform product of the IDC and currently, during provisional operations, the target timeline for publishing the REB is within 10 days of real time. After Entry Into Force (EIF) of the Treaty, the target timeline is reduced to 48 h. Specialized software has been developed for every processing stage at the IDC in order to improve signal-to-noise ratio, detectInfrasound signals infrasound signals, categorize and identify relevant detections, form automatic events and perform interactive review analysis. For the period 2010–2017, thousands of waveform events containing infrasoundAssociation associations appear in the IDC bulletins, and in particular in the REB and the LEB (Late EventBulletin Bulletin). This demonstrates the sensitivity of the IMSInfrasound infrasound component and the IDC ability to globally monitor theInfrasound infrasound activity. The unique information gathered by the IMS systems have been widely used for civil and scientific studies and have resulted in numerous publications onMeteor meteor impacts such as the largest everInfrasound infrasound recorded event that is the ChelyabinskMeteor meteor in February 2013 (Brown 2013; Pilger et al. 2015; Le Pichon et al. 2013; Pilger et al. 2019) as well as other observed fireballs andMeteor meteors (Marcos et al. 2016; Caudron et al. 2016; Silber and Brown 2019), on powerful volcanic eruptions (Matoza et al. 2017, 2019), on controlled explosions (Fee et al. 2013), on announced underground nuclear test by the Democratic People’s Republic of Korea (DPRK) (CTBTO 2013b, 2017b; Che et al. 2009, 2014) or on atmospheric dynamic research (Le Pichon et al. 2015; Blanc et al. 2019), on characterizing theInfrasound infrasound global wavefield (Matoza et al. 2013; Ceranna et al. 2019), or on gravity waves study (Marty et al. 2010; Chunchuzov and Kulichkov 2019; Marlton et al. 2019) that could lead to deriving a space and time-varying gravity wave climatology (Drob 2019).

Detecting a Signal Of Interest (SOI) is the first step in many applications of infrasound monitoring. This intuitively simple task is defined as separating out signals from background noise on the basis of the characteristics of observed data; it is, however, deceptively complex. The problem of detecting signals requires multiple processes that are divisible at their highest level into several fundamental tasks. These tasks include (1) defining models for SOIs and noise that properly fit the observations, (2) finding SOIs amongst noise, and (3) estimating parameters of the SOI (e.g., Direction Of Arrival (DOA), Signal-to-Noise Ratio (SNR) and confidence intervals) that can be used for signal characterization. Each of these components involves multiple subcomponents. Here, we explore these three components by examining current infrasound detection algorithms and the assumptions that are made for their operation and exploring and discussing alternative approaches to advance the performance and efficiency of detection operations. This chapter does not address new statistical methods but does offer some insights into the detection problem that may motivate further research.

Explosive detonationDetonations produce shocked transients with highly nonlinear pressure signatures in the near field. This chapter presents the properties and defining characteristics of a suite of theoretical source pressure functions representative of detonationDetonations and deflagrationDeflagrations, and constructs criteria for defining reference blast pulses. Both the primary positive overpressureOverpressure and the negative underpressureUnderpressure phases contribute to the temporal and spectral features of a blast pulse.

On October 28, 2014, the launch of the Antares 130 rocket failed just after liftoff from Wallops Flight Facility, Virginia. In addition to one infrasound station of the International Monitoring Network (IMS), the explosion was largely recorded by the Transportable USArray (TA) up to distances of 1000 km. Overall, 180 infrasound arrivals were identified as tropospheric, stratospheric or thermospheric phases on 74 low-frequency sensors of the TA. The range of celerity for those phases is exceptionally broad, from 360 m/s for some tropospheric arrivals, down to 160 m/s for some thermospheric arrivals. Ray tracing simulations provide a consistent description of infrasound propagation. Using phase-dependent propagation tables, the source location is found 2 km east of ground truth information with a difference in origin time of 2 s. The detection capability of the TA at the time of the event is quantified using a frequency-dependent semiempirical attenuation. By accounting for geometrical spreading and dissipation, an accurate picture of the ground return footprint of stratospheric arrivals as well as the wave attenuation are recovered. The high-quality data and unprecedented amount and variety of observed infrasound phases represents a unique dataset for statistically evaluating atmospheric models, numerical propagation modeling, and localization methods which are used as effective verification tools for the nuclear explosion monitoring regime.

Infrasound can provide unique data on extreme atmospheric events such as meteor impacts, severe weather systems, man-made explosions, and volcanic eruptions. Use of infrasound for remote event detection and location requires high-quality temporal and spatial atmospheric models, and infrasound generated by so-called Ground Truth events (for which the time and location are known) are necessary to evaluate atmospheric models and assess network performance. Large industrial blasts and military explosions are tightly constrained in time and space using seismic data and can generate infrasound recorded both regionally and at great distances. The most useful seismo-acoustic sources are repeating sources at which explosions take place relatively frequently. Over time, these may provide records of up to many hundreds of events from the same location from which characteristics and variability of the infrasonic wavefield and atmospheric conditions can be assessed on a broad range of timescales. Over the past 20 years or so, numerous databases of repeating explosions have been compiled in various parts of the world. Events are associated confidently with known sources, with accurately determined origin times, usually by applying waveform correlation or similar techniques to the characteristic seismic signals generated by each explosive source. The sets of sources and stations ideally result in atmospheric propagation paths covering a wide range of distances and directions, and the databases ideally include events covering all seasons. For selected repeating sources and infrasound arrays, we have assessed the variability of infrasonic observation: including the documentation of lack of observed infrasound. These observations provide empirical celerity, back azimuth deviation, and apparent velocity probability distributions. Such empirical distributions have been demonstrated in numerous recent studies to provide infrasonic event location estimates with significantly improved uncertainty estimates. Tropospheric, stratospheric, and thermospheric returns have been observed, even at distances below 200 km. This information is now providing essential input data for studies of the middle and upper atmosphere.

A dense network of eight, seismo-acoustic arrays operates in the southern Korean Peninsula, and since the first array installation in 1999, has provided data for monitoring local and regional seismic and infrasound signals from natural and anthropogenic phenomena. The main operational purpose of the network is to discriminate man-made seismic events from natural earthquakes to produce a clean earthquake catalog, and to ensure that seismic and infrasonic data are appropriately used for analyzing and characterizing various sources using the seismo-acoustic wave fields. This chapter summarizes results of several studies that used the network dataset to; (i) Compare seasonal variations in infrasound detections with local surface weather measurements and stratospheric wind dynamics, (ii) Develop seismic and acoustic data fusion methods that enhance source discrimination synergy, (iii) Understand the characteristic of local and regional infrasound propagation using repetitive surface explosion sources, and (iv) Review infrasound observations from earthquakes and underground nuclear tests. Finally, this chapter illustrates the usefulness of dense regional networks to characterize various seismo-acoustic sources and enhance detection capability in regions of interest in the context of future verification of the Comprehensive Nuclear-Test-Ban Treaty.

The explosive fragmentation of large meteoroids entering the Earth’s atmosphere is one of the strongest sources of infrasoundInfrasound and can be detected at distances of thousands of kilometers by arrays all over the world. Influence parameters on the detection capability are quantified for a single infrasoundInfrasound station and for the complete infrasound network of the International Monitoring System (IMSInternational Monitoring Network) operated by the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). They are applied to a number of strong bolidesBolides (Sulawesi Bolide, North Pacific Bolide, Chelyabinsk Bolide) of the past 15 years including the 2013 Chelyabinsk, 2010 Sulawesi, and 2009 North Pacific events. Long-range infrasound propagation modeling and realistic atmosphericAtmospheric explosions background conditions are used to identify propagation paths that connect the sources and globally distributed receivers, highlighting usual as well as unusual propagation pattern, to stations detecting and stations not detecting a meteorite event. Potential influences on infrasound detection capability are due to the directivity of the acoustic source energy emission, the long-range ducting via stratosphere and thermosphere and the diurnal change of meteorological parameters and noise conditions at the stations during the signal arrivals. Since infrasound of large bolidesBolides (Sulawesi Bolide, North Pacific Bolide, Chelyabinsk Bolide) has probably the most similar characteristics to an atmosphericAtmospheric explosions nuclear explosion, it can be utilized as reference event for studies on the global performance of the CTBTO infrasound network. Detections and non-detections of bolide infrasound at the more than 40 operational IMSInternational Monitoring Network infrasoundInfrasound stations are studied for the estimation of station and network performance and thus verification of nuclear test ban.

The ability of the International Monitoring System (IMS) global infrasound network to detect atmospheric explosions and other events of interest depends strongly on station-specific ambient incoherent noise and clutter (real but unwanted infrasound waves, coherent on an infrasound array). Characterization of coherent infrasoundInfrasound is important for quantifying the recording environment at each station and for assessing the detection probability of specific signals of interest. We systematically characterize coherent infrasoundInfrasound recorded by the IMSInternational Monitoring Network (IMS) network over 10 years on 41 stations over a broad frequency range (0.01–5 Hz). This multiyear processing emphasizes continuous signals such as mountain associated wavesMountain associated waves (MAW) and microbaroms, as well as persistent transient signals such as repetitive volcanic, surf, thunder, or anthropogenic activity. We estimate the primary source regions of continuous coherent infrasound using a global cross-bearings approach. For most IMSInternational Monitoring Network (IMS) arrays, the detection of persistent sources is controlled by the dynamics of the stratospheric wind circulation from daily to seasonal scales. Systematic and continuous characterization of multiyear array detections helps to refine knowledge of the source of ambient ocean noise and provides additional constraints on the dynamics of the middle atmosphere where data coverage is sparse.

Over the last decade, there have been improvements in global data assimilation capabilities of the lower, middle, and upper atmosphere. This includes mesoscale specification capabilities for the troposphere. This chapter provides an overview of both operational and basic scientific research specifications of the atmosphere from the ground to the thermosphere that are available for the calculation of infrasound propagation characteristics. This review is intended for scientific experts, nonexperts, researchers, educators, and policy makers alike. As atmospheric specifications for the lower and middle atmosphere are now readily available, less uncertain, and also described in other chapters of this book, some additional emphasis is placed on the challenges associated with upper atmospheric specifications for modeling thermospherically ducted infrasound propagation. Otherwise, no particular emphasis is placed on any one atmospheric specification system or institutional data provider; nor anyone particular infrasound propagation application, i.e., local, regional, global, man-made, or natural.

In this chapter, an overview of infrasound propagation modeling is presented. The atmosphere as a propagation medium is discussed with an emphasis on the various propagation paradigms. Some of the more commonly used propagation models are discussed and compared and repositories for open-source propagation model software are indicated.

The model of shaping of the 3-D and 1-D wavenumber spectra for the wind velocity and temperature fluctuations induced by atmospheric gravity waves is described here. Using the 3-D spectrum of gravity wave perturbationsGravity wave perturbations, the variances of the fluctuations of sound travel time along refracting ray paths and the azimuth of arrival of acoustic signals are estimated. These variances define the errors in localization of infrasound sources caused by gravity wave perturbations. The results of theory and numerical modeling of infrasound scatteringInfrasound scattering from gravity wave perturbationsGravity wave perturbations are presented. With a recently developed infrasound probing method the vertical profiles of the horizontal wind velocity fluctuations in the upper stratosphere (height range is 30–52 km) and lower thermosphere (90–140 km) are retrieved. The method is based on analytic relation between scattered infrasound field in the shadow zoneShadow zone and the vertical profile of the layered inhomogeneities of the effective sound speed. The obtained results show a capability of the probing method in the retrieval of the detailed wind-layered structure in the stratosphere, mesosphere and lower thermosphere. The vertical wavenumber spectra of the retrieved vertical profiles of the wind velocity fluctuations in the upper stratosphere and their coherence functions are analyzed.

The International Monitoring System (IMS) infrasound network is being deployed to ensure compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Recent global scale observations recorded by this network confirm that its detection capabilityDetection capability is highly variable in space and time. Previous studies estimated the radiated source energy from remote observations using empirical yield-scaling relationsYield-scaling relation which account for the along-path stratospheric winds. Although these relations reduce the variance in the explosive energy yield estimates, large error remains. Today, numerical modeling techniques provide a basis to better predict the effects of the source and middle atmospheric dynamicMiddle atmosperic dynamics parameters on propagation. In order to account for a realistic description of the dynamic structure of the atmosphere, model predictions are further enhanced by wind and temperature error distributions as measured by high-resolution middle atmospheric sounding techniques. In the context of the future verification of the CTBT, these predictions quantify uncertaintiesUncertainties of the IMS infrasound network performanceNetwork performance in higher resolution, and are helpful for the design and prioritizing maintenance of any arbitrary infrasound monitoringInfrasound monitoring network.

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 inversionsInversions 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.

Observations of windWind profiles in the upper stratosphereStratosphere /lower mesosphereMesosphere are challenging as the established measurement techniques based on in situ methods, radars or airglow spectrometers cannot cover this altitude range. Nevertheless, wind information from these altitudes is important for the assessment of middle-atmospheric dynamics in general and as basis for planetary wave or infrasound propagation estimates. Benefitting from recent developments in spectrometers and low-noise amplifiers, microwave radiometryMicrowave radiometry now offers the opportunity to directly and continuously measure horizontal windWind profiles at altitudes between 35 and 70 km. This is achieved by retrieving the wind-induced Doppler shifts from pressure broadened atmospheric emission spectra. The typical measurement uncertainties and vertical resolutions of daily average windWind profiles lie between 10–20 m/s and 10–16 km, respectively. In this chapter, comparisons of the measured wind profiles to different ECMWFECMWF model versions and MERRAMERRA re-analysis data are shown. Moreover, the oscillatory behaviour of ECMWFECMWF winds is investigated. It appears that the longer period waveWave activities agree well with the observations, but that the model shows less variability on timescales shorter than 10 days.

The atmospheric winds, density and temperature of the region between 80 and 100 km, known as the mesosphere and lower thermosphere (MLT), are subject to the effects of solar and particle precipitation from above as well as to tidal and gravity-wave forcing from below (Fritts and Alexander 2003). Additionally, the solar heating of ozone and chemical heating due to oxygen recombination chemistry in this region compete with long-term cooling of the upper atmosphere caused by increases in greenhouse gases (Robel and Dickenson 1989; Akmaev et al. 2006; Hervig et al. 2016). However, naturally occurring fluctuations associated with variations in ozone, solar or wave forcing can mask, or even mimic, the evidence of secular change in measurements of the temperature, density and winds of the MLT. Thus, these naturally occurring variations, their mechanisms and their seasonal and solar cycle behaviour must be quantified along with the driving forces associated with small-scale wave activity that governs the general circulation of the upper atmosphere. This is only possible using long-term observations with high time resolution so that the underlying secular trends that may be associated with human activity can be assessed. However, long-term, semi-continuous measurements of MLT parameters such as wind and temperature are difficult to obtain. In this article, we discuss two complementary techniques for monitoring the MLT region with a particular focus on the influence of small-scale gravity-wave processes. In the first section, we discuss the use of meteor radars to quantify gravity-wave momentum flux from observations of the Doppler drift velocities of meteor trails. In the second section, we outline how spectroscopic measurements of the nightglow emission, resulting from the recombination of oxygen atoms produced during the daytime, have evolved into an important tool for gravity-wave studies.

This new era of massive datasets gives us the opportunity to examine Earth structure and geophysical phenomena in more detail than previously possible. Large datasets hold much promise for transformative research but require new analytical methods that are both efficient and capable of extracting useful information from faint signals immersed in noise. With these needs in mind, we developed the AELUMA (Automated Event LocationAutomated Event Location Using a Mesh of Arrays) method that recasts any dense network of sensors as a distributed mesh of small triangular arrays (triads). Each array provides a local estimate of signal properties. Information from arrays distributed across the footprint of the network is combined to estimate the source origin time and location. The process is repeated without oversight to catalog events that have occurred over a period of time. We have analyzed ground-coupled airwavesGround-coupled airwaves recorded on vertical component broadband seismometers of the USArray Transportable ArrayUSArray Transportable Array (TA). We estimate the accuracy of the AELUMA algorithm using ground truth events at the Utah Test and Training Range (UTTR). In a study of 23 surface explosions, the mean AELUMA source location estimate is 8.6 km northwest of the ground truth location. The origin time estimates were late for most events. The mean time misfit is 19 s with a standard deviation of 39 s. We attribute the positive bias in origin time estimates to signal dispersion, as the AELUMA method estimates the time of the signal’s peak amplitude, not its onset. A comparison of AELUMA and a reverse time migration method indicates that AELUMA is more sensitive to faint signals from weak events and the event locations are more accurate in space and time. A catalog of acoustic activity from across the continental United States in the band from 0.7 to 4.0 Hz includes 7935 events that were detected by 10 or more triads. Most events were clustered into hotspots and are likely anthropogenic.

Infrasound arrays are sensitive enough to be able to detect the subtle pressure changes that occur as an overhead atmospheric gravity wave passes. The array can then provide information regarding the back azimuth, amplitude, frequency and pressure perturbation of the gravity wave. It is shown that by combining this data with meteorological data recorded at the array, further gravity wave parameters can be calculated. Some examples of time series analysis are shown for an infrasound station in the Ivory Coast illustrating how seasonal and daily variations in the weather can change the properties of gravity waves being detected. Ultimately, the parameters calculated using this method can be used by the meteorological community to improve the parametrisation of gravity waves in their models and increase understanding of the diurnal and seasonal variability in gravity wave parameters.

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.

The vertical structure of the middle atmosphereMiddle atmosphere (stratosphere and mesosphere) is mainly driven by the absorption of solar radiation by ozone, which is maximum at the stratopause defining the limit between the two layers. However, the meridional structure of the temperature field is far from the radiative equilibrium, especially in the upper mesosphere where the coldest temperatures are reached at the summer pole. This structure can be only explained if we consider the vertical and meridional circulation driven by planetary and gravity wave propagation and breaking. Rayleigh lidars providing time-resolved accurate temperature profiles from the middle stratosphere to the top of mesosphere are very efficient tools to study the characteristics of these waves and their impact on the mean temperature and wind fields. Together with other types of instrument setup in the frame of the European Design Study projects ARISE and -ARISE2, Doppler wind lidars, Mesosphere–Stratosphere–Troposphere (MST) and meteor radars, the IMS (International Monitoring System) infrasound network, airglow imagers and ionospheric sounders, they will contribute to a better knowledge and a better representation of middle atmospheric processes in numerical weather prediction and climate models.

Infrasonic waves excited at surface or in the troposphere propagate to longer distances via reflections from the middle and upper stratosphere or the lower thermosphere. These two atmospheric regions are affected by large-scale and transient disturbances, by long-term changes and trends. A brief review is given with particular emphasis on the stratosphere and lower thermosphere. The impact of such disturbances and long-term trends on the propagation of infrasonic waves is qualitatively estimated. Two dominant disturbances of solar origin, which substantially affect the atmosphere, and particularly the ionosphere, are solar flares and geomagnetic storms. Atmospheric waves, namely gravity waves, planetary waves, and tidal waves, affect both regions of infrasound reflections. The major midwinter stratospheric warming has pronounced effect on the height profile of temperature, thus they are capable to significantly affect propagation of infrasonic waves. There are also sporadic effects like earthquakes, which excite infrasound and gravity waves, but their overall impact on infrasound propagation is small. The impact of atmospheric waves is smaller than that of some sporadic effects like the major stratospheric warmings but atmospheric waves are continuously present in the atmosphere. Both the stratosphere and thermosphere experience also long-term changes and trends, in recent decades of predominantly anthropogenic origin (greenhouse effect, ozone depletion). These long-term changes are small but continuous, so they do not affect behavior of infrasonic waves on short-term scales but might have some effect on long-term scales like changes from decade to decade.

Temperature trends in the middle atmosphereAtmosphere were derived from several temperatureTemperature datasets including radiosondes, rocketsondes, lidars, and from successive radiometers onboard satellites like the Stratospheric Sounder Unit (SSU) and the Advanced Microwave Sounding Unit (AMSU). All exhibits limitation for deriving accurate trends, while none of them were set up for climateClimate issues. While ground-based suffer discontinuities, satellite measurements have atmospheric tides interferences depending on their orbits and sometimes drifting. For the future, ground-based continuity will be improved concerning their quality check for climateClimate issues and need to continue at some reference locations and operational meteorological radiometers will be onboard satellite having stable orbits. GPS occultation technique will offer a much better coverage of the lower stratosphere with fewer risks of bias compared to in situ sensors. Meteorological analyses will require improved models of the mesospheric dynamical processes including gravity waves. In this regard, assimilated wind information will be highly strategic.

Long-range infrasound propagation is controlled by atmospheric waveguides that extend up to the mesosphere and lower thermosphere and whose efficiency is affected by gravity waves (GWs). These GWs are not explicitly represented in the global models often used to calculate infrasound propagation because their spatial scales are well below the models’ resolution. These unresolved GWs also transport momentum and control in good part the large-scale circulation in the middle atmosphereMiddle atmosphere dynamics. These two issues make that the GWs need to be parameterized to improve the datasets used to calculate infrasound propagation as well as in the atmospheric general circulation model (AGCMs) Atmospheric general circulation models that are used to make weather forecasts and climate predictions. These two issues gain in being treated in conjunction. From this, improved infrasound calculations could be made by using a realistic amount of GWs. In return, using infrasound records could help specifying important characteristics of the GWs that are parameterizedGravity wave parameterization in the climate models. The paper presents a research framework developed to address these issues. It first presents a non-orographic GWs parameterization used and tested in a well-established AGCM, emphasizing the most recent developments, like the introduction of stochastic techniquesStochastic parameterization and a better specification of the GWs sources. The significance of GWs on the global climate is then illustrated by making sensitivity tests where the frontalFrontal gravity waves and convective GWsConvective gravity waves parameters are moderately changed. These changes impact the structure of the jets in the midlatitude stratosphere and the intensity of the sudden stratospheric warmingsSudden stratospheric warmings. The paper also presents a method to calculate long-range infrasound propagation, and to incorporate the contribution of the GWs that are parameterized in the AGCM. We then show that the changes in GW parameters tested in the model also impact infrasound propagation. This makes infrasound detection a potential tool to tune GWs parameterization in large-scale models.

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 atmosphereMiddle 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.

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 observationsObservations 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 stratosphereStratosphere and the upper stratosphere. At weather forecasting timescales (up to forecast day 30), better knowledge of the stratosphereStratosphere, 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.

In the context of the infrasound network of the International Monitoring System of CTBT, infrasounds and low- frequency sounds are reviewed as a method to characterize lightning flashes in a complementary way to electromagnetic observations. The physics of lightning discharges is briefly recalled, in relation with thunder characteristics and mechanisms of generation. The possibilities and limitations of following storms at various distances by means of remote acoustic detection of lightning flashes is discussed. Influence of distance, wind and ambient noise is examined. The three-dimensional reconstruction of lightning flashes is illustrated by several examples from a recent 2012 observation campaign in the French Mediterranean region. Comparison with outputs from a high-resolution electromagnetic lightning mapping array delineates the performances of acoustical reconstruction of individual lightning flashes for both intra-cloud or cloud-to-ground discharges. Analysis of a significant number of discharges allows to perform a statistical comparison of the two approaches. Special attention is brought to the lower parts of cloud-to-ground discharges. Opportunities for further investigations are finally outlined.

Infrasound, the low-frequency sound lying below the human hearing range, has the capability to propagate over very long distances in the atmosphere due to its low attenuation. Thus, infrasound can serve as a tool for monitoring explosive sources, including extraterrestrial bodies impacting the Earth’s atmosphere. This chapter describes the theoretical background on meteor physics and bolide infrasound, as well as applications of infrasound in Near-Earth Objects (NEOs) monitoring and characterization. In addition to presenting a comprehensive list of empirical relations to estimate bolide energy release, this chapter summarizes recent case studies where infrasound served as an instrumental tool in characterizing the source.

Infrasound monitoring is employed to enhance understanding of eruption dynamics and to track eruptive activity over time. Local infrasound, where sensors are positioned near to and/or on the flanks of volcanoes, implies that sound transmission from vent to receiver is approximately line-of-sight. Typical assumptions are that pressure decays as the inverse of distance and that attenuation and dispersion effects are minimal. Locally recorded signals thus represent relatively undistorted source time motions and can be used to effectively characterize volcanic activity. Local monitoring can entail a spectrum of installation topologies including single isolated sensors on a volcano, networks of sensors, a single array, or networks of sensor arrays. Stations may be installed as standalone or in telemetered configurations depending upon available resources and research objectives, which may range from simple explosion counting to degassing detection, mapping of vents, or identification of moving sources. Some of these objectives are only possible using arrays. Quantifying volcano infrasound, in terms of both spectral shape, power, and signal envelope, is relatively straightforward using data recorded at local distances. Despite inter-network variability, influenced by potential source directivity, propagation effects, and/or site response, the local infrasound records permit good estimation of total acoustic energy, identification of dominant spectral tones, and signal morphology. These parameters may be robustly compared at a volcano over time and/or compared to other volcanoes. Ultimately, infrasound analysis permits remote surveillance of activity occurring at the vent (or on the slopes) of a volcano that may otherwise be difficult to observe.

Volcanoes generate a wide variety of low-frequency (~0.01–20 Hz) acoustic signals, and infrasoundInfrasound technology is part of an expanding suite of geophysical tools available to characterize, understand, and monitor volcanic processes. We reviewReview recent advances in the field of volcano acoustics with an emphasis on scientific and potential civil application gains from the International Monitoring SystemInternational Monitoring System (IMS) infrasound network. Energetic infrasound from explosive volcanismExplosive volcanism can propagate hundreds to thousands of kilometers in atmospheric waveguides and large explosive eruptions (which represent significant societal and economic hazards) are routinely recorded by the IMS infrasound network. Significant progress in understanding volcano infrasound has been made through dedicated local deployments (within <15 km of the source) in tandem with other observation systems. This research has identified diverse source mechanisms of volcanically generated infrasound, and elucidated the influence of near-source topography and local atmospheric conditions on acoustic propagationPropagation and recordings. Similarly, advances are being achieved in inferring volcanic source processes from signals recorded at the longer ranges typically associated with IMS detections. However, practical challenges remain in the optimization of remote volcano infrasound signal detection, discrimination, association, and location. Many of these challenges are the result of strong signal variability associated with long-range acoustic propagation through the temporally and spatially varying atmosphere. We reviewReview the state of knowledge on infrasound generation by explosive volcanism, and assess progress toward the development of infrasonic eruption early warning and notification systems at regional and global scales.

Violent volcanic eruptionsVolcanic eruption, common especially in Southeast AsiaSoutheast Asia, pose an ongoing serious threat to aviation and local communities. However, the physical conditions at the eruptive vent are difficult to estimate. In order to tackle this problem, satellite imagerySatellite imagery and infrasoundInfrasound can rapidly provide information about strong eruptions of volcanoesVolcanoes not closely monitored by on-site instruments. For example, the recent infrasonic array at Singapore, installed to support the coverage of the International Monitoring System, allows identification of nearby erupting volcanoes based on the characteristics of the recorded signal. But, due to its location close to the equator, seasonal changes in the wind velocity structure of the atmosphere strongly affect its potential to detect small volcanic eruptionsVolcanic eruption at certain azimuths. To overcome this limit, infrasoundInfrasound could be augmented with satelliteSatellite data. Yet, with the high average cloud coverCloud cover in Southeast AsiaSoutheast Asia, there are also challenges to identify weak volcanic plumesPlume using satelliteSatellite-based monitoring techniques. In this chapter, we aim to examine the relative strengths and weaknesses of the two technologies to better understand the possibility to improve overall detection capabilityDetection capability by combining infrasoundInfrasound with satellite imagerySatellite imagery.

In the last 20 years, infrasound has increased significantly the potentials of volcano monitoring, with direct impact on risk evaluation for civil protectionCivil protection. Automatic systems based on infrasound are nowadays used operationally, and future improvements will reinforce this technique especially when integrated with other ground-based or satellite observations. We show how by using dedicated array processing, infrasound can be used to detect and notify, automatically and in real time, the onset of explosive eruptions and the run-out of density currents based on the apparent velocity, propagation back-azimuth, and frequency change. Such procedures have been tested and tuned for several years and are currently being applied to early warningEarly warning of explosive eruption at Etna volcano and to avalanche analysis and risk forecasting in several sites in Europe.

In the current society, volcanic eruptions can have a great impact due to the escalation in communications and transport starting from 1950. With the advent of civil aviation and the exponential growth in the air traffic, the problem of a volcanic ash encounter has become an issue of paramount importance, which needs to be addressed in real time. This chapter describes the status of the art in volcanoVolcano monitoring using infrasoundInfrasound technology at global, regional and local scale, the contribution of the ARISE project to volcano monitoring and to Volcanic Ash Advisory Centers (VAACs), and highlights the need for an integration of the CTBT IMS infrasound network with local and regional infrasound arrays capable of providing a timely early warning to VAACs.