RESEARCH ARTICLE
10.1002/2013JD021062
Key Points:
• Bidirectional atmospheric duct-ing conditions may occur durduct-ing/ after SSWs
• The duct is observed using micro-barom signals
• Microbarom source modeling reveals a previously unidentified source region Supporting Information: • Readme • Figure S1 • Figure S2 • Animation S1 • Animation S2 Correspondence to: J. D. Assink, jelle.assink@cea.fr Citation:
Assink, J. D., R. Waxler, P. Smets, and L. G. Evers (2014), Bidirectional infrasonic ducts associated with sudden stratospheric warming events, J. Geophys. Res. Atmos., 119, doi:10.1002/2013JD021062.
Received 18 OCT 2013 Accepted 6 JAN 2014
Accepted article online 10 JAN 2014
Bidirectional infrasonic ducts associated with sudden
stratospheric warming events
J. D. Assink1, R. Waxler2, P. Smets3,4, and L. G. Evers3,4
1CEA, DAM, DIF, Arpajon, France,2National Center for Physical Acoustics, University of Mississippi, University, Mississippi,
USA,3Royal Netherlands Meteorological Institute, De Bilt, Netherlands,4Department of Geoscience and Engineering,
Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, Netherlands
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 bidirectional duct for infrasound propagation developed in the middle atmosphere that persisted for 2 weeks. The ducts were due to two zonal wind jets, one between 30 and 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 are then presented in which the above mentioned bidirectional duct is observed in microbarom signals recorded at the International Monitoring System 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.1. Introduction
With respect to infrasound propagation, several ducts (i.e., low-velocity layers) exist in the atmosphere allowing for ground-to-ground propagation over long distances. The stratospheric duct, which typically forms between the ground and the region around the stratopause, is predominant for long-range infra-sound propagation [Drob et al., 2003] as the absorption is very small [Sutherland and Bass, 2004]. However, a sufficiently strong stratospheric polar vortex in the direction of propagation is typically necessary to ensure ducting. The stratospheric polar vortex changes direction throughout the year, due to the seasonal vari-ations in the heating of the stratosphere. An eastward polar vortex is present during normal wintertime conditions on the Northern Hemisphere, while a westward polar vortex is present during summertime con-ditions. The summertime polar vortex is generally a much more stable but weaker feature compared to the winter vortex. The directivity of the vortex implies that long-range infrasound propagation is anisotropic. This is reflected by the infrasound detections during recent infrasound calibration experiments on the Northern Hemisphere [Fee et al., 2013]. Long-range propagation through the mesosphere and lower ther-mosphere is expected to be limited, due to the large acoustical absorption in this region [Sutherland and Bass, 2004].
The variability of the Northern Hemisphere winter stratospheric vortex is well established [e.g., Andrews et al., 1987; Hauchecorne et al., 2010]. Due to eastward zonal winds in the winter hemisphere, planetary waves may propagate upward from the troposphere, depositing momentum in the polar stratosphere. These plan-etary waves are generated in the troposphere because of orography and land-sea temperature contrasts. The momentum deposition is accompanied by a sudden increase in stratospheric temperature, as well as a deceleration and possible reversal of the eastward polar vortex. If the temperature increase, ΔT, exceeds 25 K over a 7 day period [McInturff, 1978], we speak of a sudden stratospheric warming (SSW) event. If the temperature increase is accompanied by a reversal of the vortex, the SSW is classified as a major event; oth-erwise, the SSW is only minor. A recent study by Charlton and Polvani [2007] shows an average of about six major warnings per decade on the Northern Hemisphere. In contrast, several minor events typically occur during the Northern Hemisphere winter. The polar vortex may be displaced from the polar region, divided into smaller vortices or be disintegrated completely, depending on the planetary wave interaction with the
vortex [Mitchel et al., 2011]. SSWs are important features of the winter atmosphere, as the strongest forc-ing of the stratosphere on the troposphere is observed durforc-ing such events. Consequently, these events are also important for numerical weather prediction [Gerber et al., 2009]. The rare occurrence of SSWs in the Southern Hemisphere is interpreted to be due to the lack of landmass and, consequentially, planetary wave generation in the troposphere.
Earlier studies have focused on the effects of SSWs on infrasound propagation, including the change in direction of observed infrasound [Donn and Rind, 1972; Evers and Siegmund, 2009], change in amplitude of ambient coherent infrasound noise [Rind and Donn, 1978], and the existence of small stratospheric shadow zones during SSW events [Evers et al., 2012]. In this paper, we will focus on the development of a bidirec-tional stratospheric duct that persisted for 2 weeks during and after the minor SSW of January 2011. The development of a bidirectional duct has significant effects on infrasound propagation in the middle atmo-sphere, which is typically expected to be unidirectional. The existence of bidirectional ducting during SSW events has not been considered in the past. Such ducting significantly improves infrasound detection capa-bility during these periods, since the anisotropy is strongly reduced. We show in this paper that bidirectional ducting is observed in microbarom detections during a minor SSW event in January 2011.
Longuet-Higgins [1950] showed that ocean surface wave interactions can cause second-order pressure oscil-lations at double the surface wave frequency. The interaction of these signals with the ocean floor results in seismic signals, microseisms. The strength of the signal is modulated by the bathymetry, as certain depths are more favorable for resonance. In a recent study by Kedar et al. [2008], the authors showed a strong agreement between observed and predicted microseism amplitudes, using the theory of Longuet-Higgins [1950] with hindcast ocean wave spectra and a bathymetry model of the ocean to account for finite ocean depth. Brekhovskikh et al. [1973] showed that there is also a significant part of the energy that radiates into the atmosphere from the water column to generate microbaroms. The atmospheric counterpart is much smaller, due to the large impedance contrast between water and air. Waxler and Gilbert [2006] further elab-orated on the microbarom generation theory and showed that a second-order contribution, due to the compressibility of air, is to be accounted for. The theoretical treatment of Waxler and Gilbert [2006] was generalized to take account of the finite depth of the ocean in Waxler [2007]. The Waxler and Gilbert [2006] microbarom source model has recently been validated by Walker [2012] and Stopa et al. [2012] for micro-barom radiation from the Pacific Ocean. While Walker [2012] focused on ambient swell, Stopa et al. [2012] studied microbarom generation during hurricane conditions.
Microbaroms (and microseisms) have been observed worldwide; the associated frequency distribution is typically centered around a dominant frequency of 0.2 Hz [Baird and Banwell, 1940; Benioff and Gutenberg, 1939; Donn and Posmentier, 1967; Hetzer et al., 2010]. Large water bodies that support high-amplitude long-period swell, such as the Atlantic and Pacific Oceans, are efficient microbarom/microseism source regions. The dominant wave period is about 8–12 s, at longer periods the waves start to break [Komen et al., 1996].
In this paper, we will make use of the Waxler and Gilbert [2006] model to predict microbarom source loca-tions from 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. The measured directions correspond well with microbarom source locations that are pre-dicted using ocean wave models. 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 ocean wave models. As the Mediterranean signal is typically much weaker than the Atlantic signal, such events are often only detected when the beamforming steering vector is specifically directed toward the east.
The remainder of this paper is organized as follows. The next section describes the various measurement networks and specifications that are used in this paper: infrasound data, ocean wave models, and atmo-spheric analysis data. In section 3, we discuss the January 2011 atmoatmo-spheric wind and temperature fields and the implications for infrasound propagation, leading to the identification of the bidirectional duct. Thereafter, the infrasonic signature of the bidirectional duct is described, using microbarom signals. We conclude section 3 by the modeling of the microbarom sources in order to explain the detections at the infrasound arrays. Section 4 focuses on the (bidirectional) propagation over the Euro-Mediterranean region, which bridges the gap between the atmospheric state, the microbarom source regions, and the unique infrasound observations described in this paper. The detectability of infrasound during bidirectional ducting
Table 1. CTBT IMS Infrasound Station Coordinates Used in This Study
Name Latitude (Deg) Longitude (Deg) No. of Sensors Aperture (m)
I18DK 77.46 −69.29 8 1170
I26DE 48.85 13.71 5 2565
I43RU 56.72 37.21 6 1560
I48TN 35.81 9.32 7 1850
conditions during the winter of 2010–2011 is presented in section 5. Finally, section 6 discusses and summarizes our findings.
2. Measurements
2.1. Infrasound
In this study, we make use of infrasound recordings at infrasound stations that are part of the International Monitoring System (IMS). The IMS is a global network of infrasound, seismic, hydroacoustic, and radionuclide stations for the verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) [Dahlman et al., 2009]. Currently, 45 out of 60 infrasound stations have been installed and verified, providing continuous recordings of infrasound worldwide. Every infrasound station consists of at least four microbarometers that measure small pressure fluctuations on the order of millipascal up to tens of pascals. At most IMS infrasound stations wind noise filters are used to reduce noise levels over the infrasonic frequency band, by spatially averaging the pressure field in the vicinity of an infrasound sensor. All station elements are designed to have a flat response over the frequency band spanning from 0.08 to 4 Hz, which comprises the CTBT verification band. IMS infrasound data are sampled at 20 samples per second.
The microbarometers are installed in a spatial configuration allowing for the detection of coherent infra-sound and the estimation of the slowness vector using standard array processing techniques [Krim and Viberg, 1996]. The design of the arrays, with apertures ranging from 1 to 3 km, allow for accurate estimates in the microbarom band of 0.1 to 1.0 Hz. Small-sized nuclear tests, of around 1 kT trinitrotolueen (TNT) equivalent, are expected to generate acoustic frequencies within this range [Evers and Haak, 2001]. For the purpose of this study, we primarily make use of infrasound recordings at IMS station I48TN. Additional infra-sound observations are provided by stations I18DK, I26DE, and I43RU in Greenland, Germany, and Russia, respectively. The station coordinates, number of sensors used, and array aperture are provided in Table 1. The arrays are designed such that the array response function has a circular main lobe and low-amplitude sidelobes, far from the main lobe. The latter is done in order to avoid spatial aliasing.
To process the infrasound data, we band-pass filter the raw waveforms in between 0.1 and 1.0 Hz and apply sliding window time-domain [Melton and Bailey, 1957] and frequency-domain Fisher detectors [Smart and Flinn, 1971]. Every time bin consists of either 512 or 4096 samples of infrasound data, corresponding to 25.6 or 204.8 s, for the time-domain and frequency-domain processing, respectively. Successive windows have a 50% overlap. For every time bin, we beamform over a grid of slowness values𝐬 that are determined by back azimuth and trace velocity values of interest. For long-range propagation, relevant values of the slowness vector are found within a back azimuth range of 0–360◦and a trace velocity range of 300 to
600 m s−1. In our convention, 0◦indicates a source located to the north of the array. The grid points are
spaced 2◦and 10 m s−1apart. It is of course also possible to limit the number of beams to a certain range,
when interested in specific sources. For every beam in the grid, the Fisher ratio F is computed to estimate whether a coherent signal is present. Due to its statistical properties, the Fisher ratio provides an estimate of the probability of detection and the signal-to-noise ratio (SNR) on the traces: F = 1 + C⋅ SNR2. Here C is the number of array elements [Melton and Bailey, 1957]. It should be noted that, in the derivation of this relation, it is assumed that the signal is identical for all array elements, while the noise component is mod-eled as a white Gaussian process, which is uncorrelated over the array: violations of these assumptions have an impact on the estimated SNR value. While the time-domain detector computes the Fisher ratio for broad-band signals, the frequency-domain detector evaluates the Fisher ratio F(f ) as a function of frequency f [Smart and Flinn, 1971; Evers and Haak, 2001]. However, this extra information typically comes at a slightly higher computational cost, depending on the frequency resolution.
2.2. Ocean Wave Models
We utilize the two-dimensional wave energy spectrum (2DFD) product from the European Center for Medium-Range Weather Forecast (ECMWF) Wave Atmosphere Model (WAM), coupled to the high-resolution (HRES) atmospheric model, to compute the microbarom source strength. Observations from satellites (altimetry and synthetic aperture radar) have been assimilated in the WAM. The boundary conditions of this model involve no-energy flux into the grid and free advection of energy out of the grid at the coastline, which implies that no coastal reflections are considered [ECMWF, 2013]. Cook [1969] obtained order of mag-nitude estimates for the radiation of infrasound in the microbarom frequency band, caused by the abrupt stopping of ocean waves on straight shorelines. Evidence for this mechanism was presented in Gossard and Hooke [1975]. More recently, Ardhuin et al. [2011] showed that weak microseisms can be generated due to interactions of incident waves and coastal reflections. This mechanism is most efficient if the shoreline has a sufficiently steep slope.
The two-dimensional spectrum describes how the mean sea-surface elevation variance due to ocean waves and surface winds is distributed as a function of frequency and propagation direction. The directional wave energy spectrum consists of 36 directions and 36 frequencies, from 5◦to 355◦and from 0.0345 to 0.9695 Hz,
respectively. The 4 times daily analysis (at the synoptic times 00, 06, 12, and 18 UTC) fields range up to 85◦
latitude with a spatial resolution of 0.25◦.
2.3. Atmospheric Specifications
In this study, we use two distinct atmospheric specification products. One product is developed and maintained by ECMWF, the other by the U.S. Naval Research Lab (NRL).
The ECMWF atmospheric specifications are obtained from the HRES Model (formerly Ensemble Prediction System), T1279L91 IFS cycle 36r4, coupled to the Ensemble of Data Assimilations and the 4D-Var data assim-ilation system. The HRES analysis fields are available 4 times daily have a horizontal resolution of 16 km and exist at 91 mean pressure levels up to 0.01 hPa (approximately 78 km altitude) [ECMWF, 2013].
The NRL Ground-To-Space (G2S) specifications that are used in this study are based on the 4 times daily NOAA operational Global Forecast System (GFS) analysis products from 0 to 45 km (1 hPa) [Kalnay et al., 1990], the 4 times daily stratospheric analysis from 35 to 75 km (10 to 0.01 hPa) from the NASA Goddard Space Flight Center, Modern Era Retrospective analysis for Research and Applications system [Rienecker et al., 2008], and above 75 km the Mass Spectrometer and Incoherent Scatter Radar (MSIS00) [Picone et al., 2002] and Horizontal Wind Model (HWM07) empirical models [Drob et al., 2008]. The G2S model produces global specifications and forecasts that are self consistent from ground to space [Drob et al., 2003].
3. Observations
3.1. Atmospheric State
Essential in the description of the bidirectional ducting effect is a description of the state of the atmosphere in January 2011. Figures 1a and 1b show temperature and wind specifications from ECMWF analysis at a pressure level of 2.0 hPa (approximately 42 km altitude) on 6 January 2011 and 17 January 2011 at 00 UTC, respectively. Around 6 January 2011 the polar vortex is displaced and nearly split due to the occurrence of a minor SSW event. In the wake of the SSW, a westward stratospheric flow remains present over Europe, while the polar vortex has returned to its more common winter state.
To qualitatively relate variations of infrasound observations to variations in the atmospheric state, we con-sider variations of the effective sound speed ratio as a function of propagation direction ̂k, altitude z, and time t. The effective sound speed at a location is defined in terms of temperature T and horizontal wind𝐯0,H:
ceff(̂k, z, t) = √
𝛾RT(z, t) + 𝐯0,H(z, t) ⋅ ̂k (1) and the effective sound speed ratio, ceff,ratio, is the ratio between the effective sound speeds at altitude z and receiver altitude zrcv,
ceff,ratio(̂k, z, t) =
ceff(̂k, z, t)
ceff(̂k, zrcv, t)
(2)
In equation (1),𝛾 and R refer to the ratio of specific heats and the specific gas constant for dry air. In this paper, we assume the standard values𝛾 = 1.4 and R = 286.9 J kg−1K−1, respectively. In order for signals
b.
−30 −20 −10 0 10 Temp (deg C) 25 50 75 100 Wind (m/s) Altitude [km] January 2011 0 10 20 30 40 50 60 70 80 01Jan Jan08 Jan15 Jan22 Jan29
6 January 2011 January 2011 0 10 20 30 40 50 60 70 80 01
Jan Jan08 Jan15 Jan22 Jan29 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1
Eff. sound speed ratio
d.
c.
a.
17 January 2011
Figure 1. Temperature and wind specifications from ECMWF analysis at a pressure level of 2.0 hPa (approximately 42 km altitude) on
(a) 6 January 2011 and (b) 17 January 2011 at 00 UTC. Around 6 January 2011, the polar vortex is displaced and nearly split, due to the occurrence of a minor stratospheric warming (SSW) event. In the wake of the SSW, a westward stratospheric flow remains present over Europe, while the polar vortex returns to the more common winter situation. (c and d) The effective sound speed ratio as a function of time and altitude above I48TN, for eastward and westward propagation, respectively. These specifications suggest that during 2 to 3 weeks in January 2011, anomalous bidirectional ducting conditions exist over the Euro-Mediterranean region.
to be refracted from an altitude z to the receiver at altitude zrcv, ceff,ratio(̂k, z, t) must be near to or greater than 1.0. Figures 1c and 1d show the effective sound speed ratio as a function of time and altitude above I48TN, for eastward and westward propagation, respectively. The atmospheric specifications presented here suggest that during 2 to 3 weeks in January 2011, bidirectional ducting conditions existed over the Euro-Mediterranean region. The middle atmospheric ducts are due to westward wind flow in the strato-sphere between 30 and 50 km and eastward wind flow in the lower mesostrato-sphere at about 70 km altitude. These winds sometimes reach strengths of 70 m s−1.
This analysis illustrates that the classic paradigm of a unidirectional stratospheric duct for infrasound propagation can be broken during a minor SSW event. The bidirectional ducting condition may per-sist for weeks, which is a significant duration of time. As minor warming events occur commonly in a boreal winter atmosphere, an analysis of such periods is of large interest for infrasound detectability and propagation studies.
In the next subsection we present infrasound data in which the described effect is captured with microbarom signals at IMS station I48TN in Tunisia.
3.2. Infrasound Detections
Figure 2 shows beamforming results at I48TN in the microbarom band following the processing steps described in section 2.1. Figures 2a and 2b show back azimuths obtained using the time-domain analysis, for coherent infrasound detections during January 2011. The color scale corresponds to the signal-to-noise
0 90 180 270 360 01 08 15 22 29
Back azimuth [deg]
Days in January 2011 2 3 SNR power
a.
0 90 180 270 360 01 08 15 22 29Back azimuth [deg]
Days in January 2011 2 3 SNR power
b.
0 0.2 0.4 0.6 0.8 1 0 4 8 12 16 20 24 Frequency [Hz] Time [hrs on 17 Jan 2011] 4 5 6 SNR powerc.
0 0.2 0.4 0.6 0.8 1 0 4 8 12 16 20 24 Frequency [Hz] Time [hrs on 17 Jan 2011] 4 5 6 SNR powerd.
Figure 2. Infrasound detections in the microbarom band (0.1–1.0 Hz) at I48TN during January 2011. (a, b) The broadband beamforming
results reveal that coherent signals arrive from the northwest and the east. While the northwestern detections are much more prevalent and have higher signal-to-noise ratio’s (SNR), the eastward source is only sporadically detected. The most coherent detections for the eastward direction occur around 17 January 2011. During this time, the SNR of the detections from both directions are of the same order. The results shown in Figure 2a have been obtained by selecting the most coherent detection in one time bin. Specific beamforming toward the eastern and northwestern sources, shown in Figure 2b, reveals many more detections from the eastern direction. (c and d) Frequency-domain beamforming results for 17 January 2011, for the northwestern and eastern directions, respectively. This analysis shows that the coherency of the northwestern detections is tightly centered around 0.2 Hz, while coherency in between 0.2 and 0.6 Hz is found for the eastward direction.
power ratio SNR2, estimated from the Fisher ratio. The difference between these two plots is that Figure 2a
only shows detections with the largest coherence for a time bin in any direction, while the results from Figure 2b include all coherent detections in two directions of interest: (1) northwest (270◦–360◦) and (2) east
(45◦–135◦) for each time bin.
The beamforming results show, in both cases, that during the month two continuous sources are simultane-ously detected in the microbarom band. In general, the detectability is determined by the source strength, distance to the source, atmospheric wind and temperature, and the ambient noise conditions near the receiving array [Evers and Siegmund, 2009].
The first, and most prominent source, originates from the northwest and probably corresponds to the large microbarom source region in the Atlantic Ocean [Evers and Haak, 2001; Posmentier, 1967; Kedar et al., 2008], which is likely to be observed at I48TN during the wintertime. The second, weaker source originates from the east, a direction from which sources are more typically observed during summertime conditions. Occa-sionally, the signals from the east are coherent enough to overpower those from the much more powerful northwestern source, as shown in Figure 2a. However, Figure 2b shows that the eastward source is
Table 2. Event Directions𝜙and Associated SNR to be Associated With the Atlantic (A) and the Eastern Mediterranean (M) Regions on 17 January 2011
Array 𝜙A(Deg) SNRA 𝜙M(Deg) SNRM
I18DK 128 2.0
I26DE 295 1.9 140 0.9
I43RU 313 2.0
I48TN 305 1.6 88 1.7
persistent and that arrivals with lower coherency than those from the northwest are in fact detected throughout a much larger portion of the month. We observe an inter-val of particularly strong eastward arriinter-vals from 16 to 18 January 2011.
The 17 January 2011 data are further analyzed using the frequency-domain Fisher detector. The beamforming is performed in the two distinct directions from which the infrasound is detected. Figures 2c and 2d show the results for the northwestern and eastern directions, respectively. This analysis shows that the coherency of the northwestern detections is tightly centered around 0.2 Hz, corresponding to the classical microbarom peak frequency [Posmentier, 1967]. For the east-ward detections, coherency in a wider frequency band, ranging from 0.2 to 1.0 Hz, is found. The most coherent energy is found below 0.6 Hz. Note that the SNR power in the frequency-domain analysis is higher than in the time-domain case, as the SNR power for a time-domain analysis is the integrated value over the considered frequency band.
The comparison illustrates a well-known added value of beamforming over the use of time delay of arrival (TDOA) to detect signals [Olson, 2004]. With TDOA methods, slowness values are estimated from time-lags between sensor pairs. As these pairwise time-lags are determined from the maximum of the cross-correlation function, this implies that the method only reveals the strongest signal in a time-window while more signals may be present. Of course, the beamforming comes at a computational cost due to the necessity of performing a grid search.
The continuous character and the frequency range suggests that both sources are due to microbarom radiation. While the northwestern detections are likely to be associated with microbarom generation in the northern Atlantic Ocean, the origin of the eastward detections is not well understood. For now, it will be hypothesized that these detections are due to microbarom activity in the eastern Mediterranean Sea. Regardless of the origin of the source, the simultaneous detection of signals from two opposite directions confirms that the atmosphere indeed supported bidirectional propagation to I48TN during a significant interval in January 2011.
Apart from the detections at I48TN, we also consider detections in the same frequency band for stations I18DK, I26DE, and I43RU in order to localize the infrasonic energy measured at I48TN. The measured event directions𝜙Aand𝜙M, associated with the northern Atlantic and eastern Mediterranean regions, are listed in Table 2. The direction corresponding to the maximum SNR is given, along with the SNR values. At all sta-tions, a strong Atlantic microbarom signal is detected with a high SNR value. At I48TN, the Atlantic signal is relatively weak and of the same order of magnitude as the Mediterranean signal. It is for this reason that the Mediterranean signals appear in Figure 2a. In addition, a weak signal (SNRmax0.9) from the southeast is
detected at I26DE, which could potentially be related to the eastward detections at I48TN. The Atlantic sig-nal, however, is much stronger and dominates the detection list. We have included FK spectra [Smart and Flinn, 1971], computed for a sample of the I48TN and I26DE data, as supporting information.
Before we consider the acoustic propagation in greater detail, we will proceed to a discussion of the modeling of microbarom sources.
3.3. Modeling of Microbarom Source Regions
In order to model the microbarom source regions in the Euro-Mediterranean region, we apply the micro-barom source model as described by Waxler and Gilbert [2006]:
(f) = 4𝜌 2 ag 2π4f3 c2 a ( 9g2 4π2c2 af 2+ c2 a c2 w ) (f) (3)
The source strength density,(f), is proportional to the Hasselmann integral [Hasselmann, 1963]: (f) = ∫ 2π 0 F ( f 2, 𝜃 ) F ( f 2, 𝜃 + π ) d𝜃 (4)
which gives the spectral density of counter propagating waves of frequency f
2. F ( f 2, 𝜃 ) are the
40˚W 20˚W 0˚ 20˚E 40˚E 40˚N 60˚N 55 60 65 70 75 Source strength [dB] 0.1−0.3 Hz 40˚W 20˚W 0˚ 20˚E 40˚E 40˚N 60˚N 55 60 65 70 75 Source strength [dB] 0.3−0.6 Hz
a.
b.
Figure 3. Microbarom source region predictions for two distinct frequency bands, computed using the source model described by Waxler and Gilbert [2006] and ECMWF 2-D ocean
wave spectra, for 17 January 2011 00 UTC. The computations for the (a) 0.1–0.3 Hz and (b) 0.3–0.6 Hz frequency bands are shown, respectively.
g, f , ca, and cwcorrespond to ocean wave angle of propagation, atmospheric density, gravitational constant, frequency, atmospheric, and ocean sound speed, respectively.
The problem of finding the source strength of the radiation corresponds to the determination of the Hassel-mann integral term. As discussed in section 1, the Waxler and Gilbert [2006] microbarom source model has recently been validated by Walker [2012] and Stopa et al. [2012] for microbarom radiation from the Pacific Ocean. We note, however, that the Waxler and Gilbert [2006] formula applies to a model in which the depth of the ocean is infinite. The finite depth of the ocean does not influence the Hasselmann integral but modi-fies the overall amplitude through resonances between the ocean surface and floor [Longuet-Higgins, 1950; Waxler, 2007]. In this paper the focus is on the Hasselmann integral and the regions of the ocean that radi-ate microbaroms so that, for simplicity, the effects of finite ocean depth and bathymetry are not considered. In order to model(f), we make use of the two-dimensional wave energy spectrum that is obtained from the ECMWF ocean wave model, as described in section 2.2. The source strength is computed over a discrete set of frequencies and is integrated over two spectral bands: (1) 0.1–0.3 Hz and (2) 0.3–0.6 Hz. These ranges correspond to the infrasound observations that are described in the previous section.
Figures 3a and 3b show the microbarom source region predictions for these frequency bands using ECMWF 2-D ocean wave spectra for 17 January 2011 at 00 UTC. The source strength is computed as sound pres-sure levels in decibel (referenced to 20 μPa), the strength is given by the color scale that is shown above the figure. Clearly, most of the microbarom source energy in the 0.1–0.3 Hz band is located in the northern Atlantic Basin, directly to the northwest of Ireland. Several high-intensity source regions can be identified. This region has been previously identified in many other studies [Evers and Haak, 2001; Kedar et al., 2008]. Association of a particular microbarom source region for a specific array can be troublesome due to the abundance of potential microbarom source regions and the sparse design of the IMS infrasound network. This is particularly true for the Atlantic source region. More specifically for 17 January 2011, high intensity source regions to the south of Greenland and Iceland can be identified, in addition to a high-intensity region around 20◦W, 52◦N. While back azimuths of infrasonic detections at the I43RU and I18DK seem to
corre-spond well to the former region, microbarom detections at stations at lower latitudes, I26DE and I48TN, seem to associate with the latter region. Considering the atmospheric state, shown in Figure 1b, we hypoth-esize that the microbarom detection at I43RU and I18DK is facilitated by the strong eastward polar vortex that exists at higher latitudes and whose influence is weaker at midlatitudes (Figure 1).
The systematic analysis of microbarom source peaks and detections required to understand microbarom detections over Europe goes beyond the scope of the current paper and will not be performed here. For the purpose of the current study and the interpretation of the infrasound data, it suffices that Atlantic microbarom source regions are indeed present in the correct frequency and azimuthal band.
Most of the powerful source regions in the Atlantic are significantly reduced in the 0.3 to 0.6 Hz frequency band (see Figure 3.3b). However, the predictions reveal a previously unidentified source region in the eastern Mediterranean, in between Crete and Cyprus. No other source regions are apparent in the
0 20 40 60 80 100 120 200 250 300 350 400 450 Altitude [km] E W 40˚W 20˚W 0˚ 20˚E 40˚E 40˚N 60˚N 40˚W 20˚W 0˚ 20˚E 40˚E 40˚N 60˚N −70 −60 −50 −40 TLoss at 0.2 Hz [dB re 1 km]
a.
West East -2000 -1000 0 1000 2000 Range [km] 0 20 40 60 80 100 120 Altitude [km] -60 -55 -50 -45 -40 -35 TLoss [dB re 1 km] 40˚W 20˚W 0˚ 20˚E 40˚E 40˚N 60˚N 40˚W 20˚W 0˚ 20˚E 40˚E 40˚N 60˚N −60 −50 −40 TLoss at 0.35 Hz [dB re 1 km]b.
d.
c.
Eff. sound speed [m/s]
Figure 4. Propagation in the Euro-Mediterranean region. (a) Effective sound speed profiles at 35.00◦N; 30.00◦E on 17 January 2011 00
UTC, obtained from both G2S (solid line) and ECMWF (dashed line) databases. The profiles indicate the presence of a bidirectional duct, allowing for efficient stratospheric and mesospheric propagation to both eastward (red) and westward (blue) directions. Note that the top of the westward duct is around 40 km altitude, whereas the top of the eastward duct is around 65 km altitude. This is further illus-trated with (b) PE computations at 0.35 Hz, using range-dependent G2S specifications. (c and d) Transmission loss (TLoss) maps, showing infrasound propagation from Mediterranean and Atlantic source regions, respectively. The computations from the Mediterranean and Atlantic sources are at 0.35 Hz and 0.2 Hz, respectively. The relatively small transmission loss provides an explanation for the detection of the two distinct microbarom signals at I48TN (black triangle).
Mediterranean. The back azimuth to this source from I48TN and I26DE corresponds well with the infrasound detections, as listed in Table 2.
The microbarom source region modeling in this section, based on observational ECMWF ocean wave spec-tra and the microbarom source model of Waxler and Gilbert [2006], is consistent with our hypothesis that the detected infrasound signals at I48TN are both due to microbarom generation in two entirely different basins. Moreover, the back azimuths of the observed microbarom signals correlate well with the locations of the modeled microbarom sources, as listed in Table 2. Furthermore, the microbarom source modeling pro-vides an explanation of the difference in the observed frequency bands of the two different microbarom sources whose signals are detected at I48TN. The discrepancy in the frequency band is related to differences in the two-dimensional ocean wave spectra F(f, 𝜃) for the two basins. In the following section, we will treat the propagation modeling from the predicted source regions over Europe.
4. Propagation Modeling
Figure 4 shows propagation in the Euro-Mediterranean region using the Parabolic Equation (PE) method described in Lingevitch et al. [2002]. Figure 4a shows effective sound speed profiles at 35.0◦N, 30.0◦E on
17 January 2011 00 UTC, obtained from both G2S (solid line) and ECMWF analysis data (dashed line). This location approximately corresponds to the location of the Mediterranean microbarom source region, as described in the previous section.
Both ECMWF and G2S specifications indicate the presence of a bidirectional duct, allowing for efficient stratospheric-mesospheric propagation in both eastward (Figure 4, red) and westward (Figure 4, blue) direc-tions. Note that the top of the westward duct is around 40 km altitude, whereas the top of the eastward duct
is around 65 km altitude. Further, note that differences between ECMWF and G2S are most significant above 50 km. A similar observation was made by Fee et al. [2013].
The offset between these specific model runs can be explained by a combination of several effects. First of all, a sponge layer is applied in the ECMWF model at the two uppermost levels (here approximately above 70 km). Moreover, there are differences in the gravity wave schemes and mesospheric ozone absorption that may contribute to the differences [ECMWF, 2013].
The G2S specifications at these altitudes are due to interpolating the available data sets at these altitudes. Above the stratopause, the G2S specifications include the semiempirical HWM and MSIS models [Drob et al., 2003]. We also note that neither G2S nor ECMWF includes small-scale variability due to subgrid-scale gravity waves in their models. It is known, however, that these small-scale features have an influence on infrasound propagation [Kulichkov, 2010]. Recent studies have focused on the development of mod-els to compute gravity wave realizations consistent with the background state [Chunchuzov et al., 2011; Drob et al., 2013].
Figure 4b shows PE computations at 0.35 Hz using range-dependent G2S specifications to the west and east of 35.0◦N, 30.0◦E. Indeed, efficient ducts are predicted toward both directions. The westward duct weakens
significantly over the range considered. This is in accord with the atmospheric specifications presented in Figure 1b. The location of I48TN is indicated by the black triangle. As the purpose of this figure is to show the general structure of the bidirectional duct, we have not included small-scale fluctuations in the atmospheric specifications. Note that propagation through the mesosphere and lower thermosphere is expected to be limited, due to the large acoustical absorption in this region [Sutherland and Bass, 2004].
Figures 4c and 4d show transmission loss maps, showing infrasound propagation from Mediterranean and Atlantic source regions, respectively. In these computations, fine-scale structure due to gravity wave effects have been included [Norris et al., 2010]. As mentioned, the fine-scale structure is currently not resolved in atmospheric models but it plays an important role in long-range infrasound propagation [Kulichkov, 2010]. The computations from the Mediterranean and Atlantic sources are done at 0.35 Hz and 0.2 Hz, respec-tively. These frequencies are determined based on the analysis presented in Figures 2c and 2d. Characteristic microbarom source locations, 52.0◦N, 22.0◦W and 35.0◦N, 30.0◦E, for the Atlantic and Mediterranean, respec-tively, are chosen and the transmission loss from these locations is estimated. This provides a qualitative understanding of the propagation from microbarom source region to receiver. The relatively small trans-mission loss explains the detection of the two distinct microbarom signals at I48TN. Note that we have considered a larger dynamic range of the transmission loss for the Atlantic simulations, which is justified given the relative magnitude of the Atlantic versus the Mediterranean microbarom sources. While a sim-ilar transmission loss is computed for propagation from the eastern Mediterranean to I26DE, very few detections were made that could be associated with the eastern Mediterranean. At I26DE, we could iden-tify 72 detections with SNR> 0.75 and trace velocities in between 310 and 450 m s−1, whereas over 1400
events were detected at I48TN using similar detection criteria on 17 January 2011. Detection histograms are provided as supporting information.
The low number of detections at I26DE could be explained by higher ambient noise conditions at the sta-tion and the array configurasta-tion. In particular, the smaller number of array elements (five compared to seven at I48TN) and the larger aperture (2565 m compared to 1865 m at I48TN) plays a role [Evers and Haak, 2005]. However, an inspection of the noise levels at stations I26DE and I48TN shows that both stations had noise levels of the same order of magnitude on 17 January 2011. Potentially, the large SNR of the Atlantic micro-baroms at I26DE, in combination with the low number of array elements could have had an adverse effect on the detectability of microbarom signals from the Eastern Mediterranean, as both signals have similar waveform characteristics. Alternatively, the low number of detections from the Mediterranean could be explained by propagation losses that are not included in the propagation model used. The current model assumes a flat, rigid ground boundary condition which is appropriate for propagation over the sea. How-ever, this model does not account for losses due to scattering from topography (e.g., The Alps). While the interaction with the ground around 0.3–0.6 Hz is not well understood, transmission of infrasound into the ground might have caused additional losses.
0 10 20 30 40 50 60 70 80 Altitude [km] 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1 a. 0 90 180 270 360 01 Dec 01 Jan 01 Feb 2 3 SNR power c. b. 0 90 180 270 01 Dec 01 Jan 01 Feb 60 70 80 Source SPL [dB re 20e-6 Pa]
Back azimuth [deg]
0 90 180 270 360 60 70 80 Source SPL [dB re 20e-6 Pa]
Back azimuth [deg]
10 20 30 40 50 60 70 80 01
Dec Jan01 Feb01
Altitude [km] 0.96 0.98 1 1.02 1.04 1.06 1.08 1.1
Eff. sound speed ratio
Eff. sound speed ratio
Figure 5. (a) Microbarom source activity in the Atlantic and Mediterranean Basins, modeled using ECMWF specifications and the Waxler
and Gilbert [2006] source model. The back azimuths with respect to I48TN and the source intensity is given as a function of time. While a continuous, broadband of energetic microbarom sources are present in the northwest, the Mediterranean activity is more sporadic. The top frame shows all sources, the bottom frame shows the source activity for when local winds are less than 1 m s−1
, leading to low wind noise conditions and increased infrasound detectability. Thus, the remaining microbarom sources are effective sources. (b) Effec-tive sound speed ratio values as a function of altitude and time for (top) westward and (bottom) eastward propagation. (c) Infrasound detections during the winter of 2010–2011 similar to Figure 2b. Intervals of bidirectional ducting conditions are indicated with dashed rectangles; the eastward duct nearly always exists. During favorable conditions, microbarom signals from two opposite directions are detected at I48TN.
5. Infrasound Detectability During Bidirectional Ducting Conditions
So far, we have focused specifically on bidirectional ducting conditions during January 2011 that devel-oped in the wake of a minor SSW event earlier that month. The existence of the bidirectional duct has been demonstrated at I48TN using microbarom signals that are present in nearly opposite directions. The microbarom sources are explained based on microbarom source modeling, using operational 2-D ocean wave spectra. In this section, we report on the detectability of infrasound during the onset and offset of bidirectional ducting conditions throughout December 2010 to February 2011.
Figure 5a shows microbarom source activity in the northern Atlantic and eastern Mediterranean Basins. The back azimuths with respect to I48TN and the source intensity are given as a function of time. While a continuous, broadband of energetic microbarom sources is present in the northern Atlantic Basin, the eastern Mediterranean activity is more sporadic and weaker. As mentioned earlier, infrasound detectability is
determined by the source strength, distance to the source, atmospheric wind and temperature, and the ambient noise conditions near the receiving array [Evers and Siegmund, 2009]. Assuming efficient propa-gation conditions and a relatively nearby microbarom source, the detectability will be mostly constrained by the local noise conditions. In order to show the masking effect of the local winds on the detectability, Figure 5a (bottom frame) shows the microbarom source activity for when local winds are less than 1 m s−1,
which is taken as a practical value of low wind noise conditions, similar to Le Pichon et al. [2005]. Thus, the remaining microbarom sources are considered “effective” sources.
Figure 5b shows effective sound speed ratio values as a function of altitude and time for westward (top frame) and eastward (bottom frame) propagation. Clearly, several intervals of bidirectional ducting condi-tions exist throughout the winter of 2010–2011, the most significant interval is during January 2011. These intervals are indicated in Figure 5 (dashed rectangles). Note that the eastward duct nearly always exists. Note that this would not have been the case if a major SSW had occurred.
Lastly, Figure 5c shows infrasound detections during the winter of 2010–2011. This frame is similar to Figure 2b but includes December 2010 and the first half of February 2011 as well. First of all, note the first order agreement between intervals of northern Atlantic microbarom detections and the presence of north-ern Atlantic microbarom sources when the local winds are sufficiently low. In general, signals from both directions are detected during intervals when the bidirectional duct exists, sufficiently strong microbarom sources are present and when the local noise conditions are low. The more significant eastward detections, occurring around 10 December and—as discussed earlier—17 January 2011, are consistent with this. On the other hand, very few detections from the east are made during the second anomalous period in December 2010, likely due to the presence of strong local winds that masked the microbarom sources in the eastern Mediterranean (see Figure 5a).
6. Discussion and Conclusions
In this study, we have demonstrated that bidirectional middle atmospheric ducting of infrasound is possible during the boreal winter. In the wake of a minor warming, a bidirectional duct developed over the Mediter-ranean region that persisted for 2 weeks. This situation occurred just prior to the 2011 Sayarim experiments [Fee et al., 2013], which took place on 24 and 26 January 2011. The state of the atmosphere was of particu-lar interest in the framework of these experiments, as propagation and detectability are strongly influenced. We believe that studies like these allow for an improved understanding of infrasound propagation under variable stratospheric conditions, which is of importance for the CTBT. The results of the Sayarim experi-ments, designed to test the IMS, would have been significantly different if the explosions had occurred a week earlier.
The bidirectional ducting effect is clearly captured at I48TN, due to its favorable location with respect to microbarom sources at both sides of the station. Two microbarom signals, from nearly opposite directions are observed in January 2011. A strong and persistent signal from the northwest, with frequencies around 0.2 Hz is detected with a weaker signal from the east, in the 0.2–0.6 Hz band. Around 17 January, the signal intensity of both signals is about equal. Microbarom source predictions using ECMWF ocean wave models show that various high-intensity source regions are present in the Atlantic Ocean in the classic microbarom band 0.1–0.3 Hz. At higher frequencies, the Atlantic source regions weaken and source regions appear in the eastern Mediterranean Sea, in between Crete and Cyprus. These frequencies correspond to the microbarom frequencies observed in the infrasonic signal.
To our knowledge, this is the first description in literature of a bidirectional middle atmospheric duct asso-ciated with the dynamics of the boreal winter stratosphere. In addition, we have been able to explain microbarom detections in two significantly different frequency bands with source regions predicted using ECMWF ocean wave models. We note that this work is unlike the analyses by Donn and Rind [1972] and Evers and Siegmund [2009], who reported on significant azimuthal reversal due to the reversal of the zonal jet as a result of a major warming event.
We have treated the detectability of infrasound due to the onset and offset of bidirectional ducting con-ditions throughout the winter of 2010–2011. It is shown that the detectability depends on the source activity, propagation conditions and receiver conditions. While the presented results allow us to explain the detectability qualitatively for the period December 2010 to February 2011, we believe that more
comprehensive studies are necessary to further quantify infrasound detectability on a regional and global level. The advent of the global IMS infrasound network, the availability of global atmospheric specifications and 2-D ocean wave spectra and developments in infrasound propagation and signal processing algorithms will be helpful in attaining this goal.
Thus, further studies, similar to these, are necessary in order to further improve our understanding of long-range infrasound propagation and detectability, in particular from microbarom sources. Such knowl-edge is for example of great interest for the application of infrasound as a passive remote sensing technique of the upper atmosphere [Le Pichon et al., 2005; Kulichkov, 2010; Assink et al., 2012, 2013], for which there is renewed interest. Moreover, such knowledge is of importance for a successful verification of the Comprehensive Nuclear-Test-Ban Treaty, in which infrasound is used as a verification technique.
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Acknowledgments
This document was prepared under award NA08NWS4680044 from the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. The statements, findings, conclusions, and recommendations are those of the authors and do not neces-sarily reflect the views of the National Oceanic and Atmospheric Admin-istration or the U.S. Department of Commerce. This work was partly per-formed during the course of the ARISE collaborative project of the Seventh Framework Programme, funded by the European Union (http://arise-project. eu/). The authors would like to thank Alexis Le Pichon and Elisabeth Blanc for their interest in this study and for providing the small-scale perturbation models. We are also grateful to the CTBTO and IMS station operators for guaranteeing the high-quality infra-sound data. The GEOS5 analysis fields utilized in conjunction with other data sources within the Naval Research Lab (NRL) G2S atmospheric specifications were provided by the Global Modeling and Assimilation (GMAO) at the NASA Goddard Space Flight Center (GSFC) through the online data portal in the NASA Center for Climate Simulation. The NOAA GFS analysis fields also uti-lized in the G2S specifications were obtained from NOAA’s National Opera-tional Model Archive and Distribution System (NOMADS), which is main-tained at NOAA’s National Climatic Data Center (NCDC).
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