DB 2172© British Crown Owned Copyright 2011/MOD
1. AWE Blacknest, Brimpton, United Kingdom RG7 4RS ([email protected])2. CEA/DAM/DIF, F-91297 Arpajon, France
Acoustic Observations of Stratospheric Solar Tides: Examples from the Eruption of Eyjafjallajökull, Iceland, April-May 2010
David N. Green1, Julien Vergoz2, Robin S. Matoza2, Alexis Le Pichon2
200°
180° 160°
140°
100°120°
70°
60°
50°
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1. Signal Detection• TheApril-May2010summiteruptionofEyjafjallajökullvolcano,Iceland,wasrecordedacross14infrasoundarrays,including4arraysoftheInternationalMonitoringSystem(IMS)network(Figure1).
• Arrayprocessingtechniquescansuccessfullydiscriminatebetweenvolcanicinfrasoundand ambient coherent and incoherent noise (Figure 2).
• Quasi-continuoustimeseriesofdetectionsoveranumberofdaysprovideanopportunitytostudyvariationsindetectionparameters(backazimuth,apparentvelocity,signalfrequencycontent,andsignalamplitude)ondiurnaltimescales.
• Timeseriesrecordedatstationslocatedalongdistinctpropagationpathsexhibitclearlyvaryingdetectionparametersondiurnaltimescales(Figures2and3,Section2).
Figure1.LocationofinfrasonicstationsrecordingtheApril–May2010summiteruptionofEyjafjallajökull.14remoteinfrasonicarrays(greeninverted triangles) recorded the summiteruptionofEyjafjallajökull(blackdot,‘Eyjaf’),atdistancesfrom~1,745km(BKNI,UK)to~3,666km(IS48,Tunisia).BlueinvertedtrianglesshowotherIMSstationsthatdidnot record the eruption. Colorscale representsnumberofintersectingmeaninfrasonicsignalbackazimuths±3°registeredateachstationandassociatedwithEyjafjallajökull[fromMatozaetal.,(2011)].
Figure2.Eyjafjallajökullassociateddetections:identificationandtimeseries.TheupperplotsshowresultsforBKNI,withtherighthandpanelshowinga2Dhistogramofdetectionsinfrequency-azimuthspaceovertheperiod2010/4/10to2010/6/15inclusive.TheareaenclosedwithinthewhitedashedlineindicatesthedetectionsidentifiedasassociatedwiththeEyjafjallajökulleruption.Thelargertimeseriesplotshows10daysofEyjafjallajökullassociatedarrivalamplitudesbetween18/04/2010and28/04/2010,withthemedianamplitudein30minutebinsprovidedasaredline.TheinsettimeseriesshowstheextentofthedetectionsacrossAprilandMay2010,withthegreyshadedareaindicatingaperiodofdataloss.ThelowerthreepanelsshowresultsforIS18.Themapshowstherelativearraylocationsandconfigurations.
Figure3.Identifyingperiodicitiesintheunevenlysampled detection timeseries at BKNI, using the CLEANalgorithm(aniterativedeconvolutionofthesamplingfunction,e.g.,HeslopandDekkers,2002).Methodologyallowsthesignificanceofthespectralpeaks(left-handpanels)tobedetermined.Ontheright-handsidereconstructedsignalsfromtheCLEANspectraatthe80%and95%confidencelevelsareoverlainondetectiondensityplotsfor(top to bottom) signal RMS amplitude, backazimuth, apparentvelocity(Vapp)andcentralsignalfrequency.Darker shades represent higher detection densities. At BKNI, signal amplitudes and backazimuth estimates exhibit clear diurnal variations.
2. Identifying Periodicities• Toidentifythefrequencyofvariationsindetectionparameters,atechniquethatisapplicabletonon-evenlysampledtimeseriesisrequired(Figure3).
• SignificantdiurnalvariationsinsignalamplitudeandbackazimuthareobservedatBKNI,andinamplitude,apparentspeedandsignalfrequencyobservationsatIS18.
• AtBKNIvariationsinthesignalamplitudeareinphasewithstratosphericalong-pathwindvariations,whereasbackazimuthvariationsareapproximatelyinphasewithcross-windvariations(Figure4).
3. Cause of Variations - Stratospheric Tides?• Significantdiurnalvariationsindetectioncharacteristicsareobserved–whatgeneratesthesevariations?Twoplausiblesources:
1.Diurnalstratosphericwindvariationscausedbythesolardiurnaltideleadingtoperiodicvariationsinacousticwaveguidecharacteristics(e.g.,Donnetal.,1975).
2.Variationsinnear-receiver(boundarylayer)atmosphericstabilityleadingtodifferencesin detection characteristics and pressure noise levels.
• Observeddiurnalvariationsinbackazimuthareindicativeofaprocessthatmustactoverasignificantpropagationpathlength–suggestingtidalmotionsinstratosphereassourceofobservedvariations(Figure5).
4. Propagation Modelling• 3Dray-tracinghasbeenusedtoidentifyacousticpropagationpathsthroughECMWFmeteorologicalprofilesbetween2010/04/18and04/30(Figure6).
• OnpathfromEyjafjallajökulltoBKNIthestudyofhowstratospherictidesinfluencethepropagationiscomplicatedbythepresenceofstrongtroposphericducts.Fortimeperiodstudied,33%ofrayspropagatedthroughonlyastratosphericwaveguide.
• OnpathfromEyjafjallajökulltoIS1893%ofrayspropagatewithinthestratosphericduct.Theraydensityreachingthestationexhibitsadiurnalvariation(Figure6)withthecorrectphasetoexplaintheobservedsignalamplitudeanddetectiondensityvariations.
5. Conclusions• InfrasoundsignalsclearlyrecordedfromEyjafjallajökulleruptionatrangesofupto3600km.
• Detectionsexhibitdiurnalvariationsinsignalcharacteristics–andthesearecorrelatedwithdiurnalvariationsinECMWF-modelstratosphericwindspeeds.
• Identifyingdiurnalcomponentsinresultsofray-tracingisdifficult,duetocomplicationsarisingfromspatialandtemporalmeteorologicalvariations.
ReferencesDonn,W.L.,N.K.Balachandran,andD.Rind(1975),TidalWindControlofLong-RangeRocketInfrasound,J.Geophys.Res.,80(12),1662–1664.
Gudmundsson,M.T.,etal.(2010),EruptionsofEyjafjallajökullVolcano,Iceland,Eos,91(21),190–191.
Heslop,D.,andM.J.Dekkers(2002),SpectralAnalysisofUnevenlySpacedClimaticTimeSeriesusingCLEAN:SignalRecoveryandDerivationofSignificanceLevelsusingaMonteCarloSimulation,Phys.EarthPlanet.Int.,130,103–116.
Matoza,R.S.,etal.(2011b),Long-rangeAcousticObservationsoftheEyjafjallajökullEruption,Iceland,April-May2010,Geophys.Res.Lett.,38(L06308),doi:10.1029/2011GL047019.
Figure6.Effectsoftemporaland spatial variations in the effectivesoundspeedonacousticpropagationpredictions,forpathsfromEyjafjallajökulltoa)BKNIand b) IS18.
Thelargeleft-handpanelsshowthe temporal variation in along-patheffectivesoundspeed,whiletherighthandpanelsshowalong-pathspatialvariabilityineffectivesoundspeedbetweenthesource(range=0km)andthestation(whitetriangle)attimesshownontheleft-handpanel.ThetimeschosenforBKNI highlight the complications generatedbytroposphericwaveguides(timeB)andelevatedstratosphericwaveguides(timeC).ThetimeschosenforIS18showthestrengtheningofthestratosphericwaveguideacrossoneday,reflectedinthenumberofraysreachingthestation.Here,thechangesinwaveguidestrength are related to the stratospheric tidal variations.
Amp. Azi.
Data
ECMWF modelwind speeds at45km altitude
12:00 18:00 00:00 06:00 12:00Hour of Day (U.T.)
12:00 18:00 00:00 06:00 12:00Hour of Day (U.T.)
12:00 00:00 12:00Hour of Day (U.T.)
12:00 00:00 12:00Hour of Day (U.T.)
6
4
2
0
–2
4
2
0
–2
x 10–3
Amp.
-mea
n[Am
p]da
y (Pa
)
Azi.-
mea
n[Az
i] day
(°)
Along-Path Wind Cross-Path Wind
Win
d sp
eed
- mea
n[W
ind
Spee
d] (m
/s)
–1.*
Win
d sp
eed
- mea
n[W
ind
Spee
d] (m
/s)
BKNI
IS18
0.05
0.04
0.03
0.02
0.01
0
0.04
0.03
0.02
0.01
0
RMS
Amp
(Pa)
RMS
Amp
(Pa)
RMS
Amp
(Pa)
04/18 04/20 04/22 04/24 04/26 04/28
04/10 04/30 05/20
Date 2010. (mm/dd)
04/18 04/20 04/22 04/24 04/26 04/28Date 2010. (mm/dd)
Date 2010. (mm/dd)
0.02
0
RMS
Amp
(Pa)
04/10 04/30 05/20Date 2010. (mm/dd)
0.05
0
1
0.5
0
–0.5
–1
Log 1
0 Fre
q. (H
z)
1
0.5
0
–0.5
–1
Log 1
0 Fre
q. (H
z)
270 315 0 45Azimuth (°)
45 90 135 180Azimuth (°)
40
30
20
10
0
No.
of D
etec
tions
Mod
ulus
(x10
–3) 1
0.5
00 1 2 3 4 5
Mod
ulus
(x10
–3)
0.6
0.4
0.2
00 1 2 3 4 5
Mod
ulus
(x10
–3) 2
1
0
3
2
1
0 1 2 3 4 5
Mod
ulus
(x10
–3) 0.1
0.05
0
0.02
0.01
0
0 1 2 3 4 5Freq (cpd)
Freq
(Hz)
Amp
(Pa)
Azim
uth
(°)
04/18 04/19 04/20 04/21 04/22
04/18 04/19 04/20 04/21 04/22
04/18 04/19 04/20 04/21 04/22
04/18 04/19 04/20 04/21 04/22Date 2010. (mm/dd)
336
334
332
330
328
0.38
0.36
0.34
0.32
V app
(km
/s)
95
80
95
80
95
80
95
80
a) BKNI
b) IS18
120
100
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60
40
20
0
120
100
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0
Altit
ude
(km
)
Altit
ude
(km
)
Altit
ude
(km
)
A B C
04/17 04/22 04/27Date 2010. (mm/dd)
Eff. Sound Speed (km/s)0.29 0.32 0.35
A B C
04/17 04/22 04/27Date 2010. (mm/dd)
Eff. Sound Speed (km/s)0.29 0.32 0.35
80
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20
0 0 500 1000 150080
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Range (km)
Altit
ude
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)
80
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00 500 1000 1500
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00 500 1000 1500
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00 500 1000 1500
Range (km)
A :2010-04-17 21:00
B :2010-04-20 06:00
C :2010-04-25 12:00
A :2010-04-21 00:00
B :2010-04-21 09:00
C :2010-04-21 18:00
120
100
80
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20
0
Altit
ude
(km
)
0 20 40 60 80 100Max. Ceff Occillation (m/s)
120
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0
Altit
ude
(km
)
0 5 10 15 20Time of Ceff Maximum (Hour)
a) b)
Figure4.Superimposedepochanalysisofamplitude and backazimuth estimates at BKNI for2010/04/16-05/07,showingthediurnalcycleinbothparameters(upperpanels).Forcomparison,superimposedepochanalysesofECMWFmeteorologicalmodelwindsaregivenforthesameperiodataltitudesof45km(lowerpanels).Thealong-pathstratosphericwinds(left-handpanel)varyinphasewiththeamplitudevariationsandthecross-pathwinds(right-handpanel)varyapproximatelyinphasewiththebackazimuthvariations.
Amp. Azi.
Data
ECMWF modelwind speeds at45km altitude
12:00 18:00 00:00 06:00 12:00Hour of Day (U.T.)
12:00 18:00 00:00 06:00 12:00Hour of Day (U.T.)
12:00 00:00 12:00Hour of Day (U.T.)
12:00 00:00 12:00Hour of Day (U.T.)
6
4
2
0
–2
4
2
0
–2
x 10–3
Amp.
-mea
n[Am
p]da
y (Pa
)
Azi.-
mea
n[Az
i] day
(°)
Along-Path Wind Cross-Path Wind
Win
d sp
eed
- mea
n[W
ind
Spee
d] (m
/s)
–1.*
Win
d sp
eed
- mea
n[W
ind
Spee
d] (m
/s)
Figure5.The(a)amplitudeand(b)phaseofthedailyoscillationsintheeffectiveacousticspeedalongtheEyjafjallajökulltoBKNIpath,takenfromtheECMWFmeteorologicalmodelfor15/04-30/04/2010.Thediurnaloscillationsaregeneratedbysolartides:global-scaleatmosphericoscillationsexcitedbysolarinsolation, observed as periodic variations in temperature,densityandwindspeed.
