Performance of an atmospheric source location algorithm at CTBTO
Monika KRYSTAand John CoynePreparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), Vienna International Centre, P.O. Box 1200, A-1400 Vienna, AustriaE-mail: [email protected]– Telephone: + 43 1 26030 6405 – Fax: + 43 1 260305932
Figure 1. PSRs constructedon the basis of the firstfictitious detection (27 Oct2010) in the consideredscenario (Tab. 1).Additionally, fourneighbouring non-detectingstations (USP72, USP74,CAP17, FRP28) are alsoincluded in the inversionalgorithm. Each plotillustrates an interval of 24hours in the periodpreceding the first detection.There are no waveformevents overlapping the PSRsimultaneously in space andtime.
Abstract27 Oct 2010 26 Oct 2010 25 Oct 2010
24 Oct 2010 23 Oct 2010 22 Oct 2010
In order to test performance of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) verification system, a realisticscenario is scrutinised by the National Data Centres (NDCs) in an annual Preparedness Exercise (NPE). Those exercisesare conceived to challenge the performance of the CTBTO’s monitoringsystem and the analysis tools by the largestpossible spectrum of national experts. Although each exercise has a different flavour, recent events have been based on aset of common assumptions: a scrutinised event is a real waveform event which is accompanied by a fictitious release ofradionuclides; the event location defines a source term for an atmospheric transport model which is then used to produce afictitious scenario of detections in the IMS radionuclide network. Usingthese fictitious detections, performance of theCTBTO source location algorithm is illustrated in the first place. Then, possible source locations resulting from thecomputations by the cooperating Regional Specialised Meteorlogical Centres (RSMC) of the World MeteorlogicalOrganisation (WMO) are shown and the discrepancies between RSMCs’ andCTBTO’s results are discussed.
Figure 5. Like Fig.1 but for NCEPmeteorologicalfields
27 Oct 2010 26 Oct 2010 25 Oct 2010
24 Oct 2010 23 Oct 2010 22 Oct 2010
T4-P26
Source attribution
The waveform event selected for the NPE2010 by the German NDC (BGR – Federal Institute for Geosciences and NaturalResources) has been disclosed during the EGU2010 conference to be an explosion at Black Thunder Mine, Wyoming, US,(105.8W, 44.1N), ID 6797924, which took place on 23 October 2010 at 21:25:21 UTC. A scenario of a fictitious release ofxenon has been provided by CTBTO. The radionuclide release was supposed tocoincide in space with the location of thewaveform event but, contrary to the previous NPEs, the fictitious release of radionuclides was not instantaneous and thetemporal characteristics of emission have not yet been unveiled. A fictitious measurement scenario created for the needs ofthe NPE2010 has been computed by the Austrian NDC (ZAMG - Central Institute for Meteorology and Geodynamics). Thedetections were predicted using the atmospheric dispersion model FLEXPART (Stohl et al., 2005). CTBTO source inversionalgorithm incorporated in WEB-Grape has been used to identify this waveform event using a scenario of fictitious detectionsof Xe-133 (Tab. 1).
jiji SMc =
Station USP75 USP75 USP74 USP75 CAP17 USP72 USP72 CAP17 USP74 USP74
Date 27 Oct 28 Oct 29 Oct 29 Oct 29 Oct 29 Oct 30 Oct 30 Oct 30 Oct 31 Oct
mBq m-3 5.67 46.60 32.80 25.60 21.90 2.37 44.10 40.50 4.43 17.30
Station CAP17 USP72 USP75 RN053 USP74 RN053 USP72 USP75 CAP17
Date 31 Oct 31 Oct 31 Oct 31 Oct 1 Nov 1 Nov 1 Nov 1 Nov 1 Nov
mBq m-3 7.29 4.69 1.14 0.867 61.60 45.40 12.80 8.16 0.311
Table 1. Part of the fictitious scenario of Xe-133 measurements which result from a fictitious release of radionuclides accompanying a waveform event occurring on 23 October 2011.,21:25:21 UTC at (105.8W, 44.1N). Only the values above the assumed minimum detectable concentration (0.1 mBq m-3) are reported. The table is organised column-wise and thecolumns are placed in two rows. Each column specifies a symbol of a detecting station (first record), the date of a detection (second record) and the activity concentration (third record)expressed in mBq m-3.
A link between a source of radionuclides S (indexed by j running over all the grid-cells which potentially contain it) andthe activity concentrations c (indexed by i running over all the grid-cells containing a radionuclide measurement station) isexpressed by a linear relationship of the form (Wotawa et al., 2003)
where j is a summation index.
For the CTBT verification regime,wherethe numberof measurementsis significantly lower thanthe numberof potential
In the framework of cooperation with the World Meteorological Organization (WMO), the CTBTO has a possibility to triggerthe requests for support in case of anomalous activity concentrations detected in the IMS radionuclide network. An NPEprovides an opportunity to test this response system as each of the fictitious detections constituting the scenario is consideredto be anomalous. Following a request for support, the cooperating meteorological centres – Regional SpecialisedMeteorological Centres (RSMCs) - provide the CTBO with the SRS fieldscomputed using their own means, within 24 hoursof a notification. The ultimate PSRs (Fig. 6) are computed by averaging those obtained independently for each of thecooperating meteorological centres (Becker et al., 2007). Furthermore, the PSRs and data fusion results based exclusively onthe supporting SRS computations are also made available to the signatories of the treaty. Due to diversity of the meteorologicalmodels used to run the atmospheric transport models, the PSRs produced ontheir basis may have larger spatial extension . Atthe same time this diversity limits the influence of the model error in the meteorological fields used at CTBTO.
Figure 2. PSRs constructed onthe basis of the first and secondfictitious detection (USP 75 on27 and 28 Oct 2010) and fournon-detecting neighbours(USP72, USP74, CAP17,FRP28). There are more than20 waveform events in theGulf of California overlappingthe PSR in space and time. ThePSR on 24 October 2011overlaps with ID 6797555,(110.3W, 43.6N), Wyoming,US. The Black Thunder Mine,Wyoming, US on 23 Oct 2010overlaps with the PSR in spacebut not in time (overlap on 26and 25 Oct but not on 23).
Examples of Possible Source Regions for NPE 2010 IIRadionuclide measurement scenario
Fig.2 illustrates 6 snapshots of the PSR built using the first and second measurement of the NPE2010 scenario (Tab. 1, USP75on 27 and 28 Oct 2010) and four neighbouring non-detections. The informative content of the second measurement is such thatit constraints the PSR to point to the wavefrom event (Black Thunder Mine explosion), which we know now to be the locationof a fictitious release. The overlap with the PSR does not take place on the day of the event actually occurred. Whether this isdue to the first inconsistent measurement, or the fact that the fictitious release has been shifted in time with respect to theoccurrence of the waveform event needs to be further investigated.
Examples of Possible Source Regions for NPE2010 IIIIt is only once the fictitious detections made on the third and further daysof the scenario are included in the inversionalgorithm, that the PSR overlaps with the Black Thunder Mine blast on 23 October 2010 (Fig. 3, plots 3-6). Nevertheless, themeasurements contributing to the inversion algorithm are not fully consistent among themselves. Indeed, the PSR for the samedate should become more and more confined as more and more consistent information is brought to the inversion algorithmlike it was in the case for NPE 2009 (compare Fig.3 for NPE2010 and Fig.4 for NPE2009). The PSRs in Fig.3 overlap with thenumerous events in the Gulf of California and with two events in Wyoming,one on 24 Oct 2010 17:43:59 UTC (110.3W, 43.6N), another one on 23 Oct 2010 21:15:21 (105.8W, 44.1N) what indicates them as possiblesources of the fictitious release ofradionuclides. Further discerningbetweenthe Treaty relevant and non-relevant waveform events indicated by the PSRs
27 Oct 2010 26 Oct 2010 25 Oct 2010
24 Oct 2010 23 Oct 2010 22 Oct 2010
27 Oct 2010 26 Oct 2010 25 Oct 2010
24 Oct 2010 23 Oct 2010 22 Oct 2010
Figure 6. PSRconstructed on the basisof the first fictitiousdetection (27 Oct 2010)and four non-detectingneighbours (USP72,USP74, CAP17,FRP28). The PSR is anaverage of the PSRsobtained for 9collaborating RSMCs.There is a noticeabledifference with respectto Fig. 1 and 5 – a PSRoverlaps in space andtime with the BlackThunder Mine blast on23 Oct 2010.
CTBTO-WMO response system
Examples of Possible Source Regions for NPE 2010 IV
Fig.5 illustrates 6 snapshots of the PSR built using the first measurementof the NPE2010 scenario, at USP75 on 27 Oct 2010and four neighbouring non-detections. The SRS fields used by the source location algorithm have been computed using theNCEP meteorological fields. The PSR is qualitatively different fromthe one obtained with the ECMWF meteorological fields(Fig. 1). In particular it has a constituent covering the Western US and overlapping in space with the Black Thunder Mineexplosion.
Figure 5. Like Fig.1 but for NCEPmeteorologicalfields
Conclusions
Disclaimer: The views expressed on this poster are those of the authors and do not necessarily reflect the views of the CTBTO Preparatory Commission
References Becker, A. et 17al., 2007: Global backtracking of anthropogenic radionuclides by means of receptor oriented ensemble dispersion modelling system in support of nuclear-test-ban treaty verification, Atmos. Env., 41, 4520-4534.Stohl, A., C. Forster, A. Frank, P. Seibert, and G. Wotawa, 2005: Technical Note: The Lagrangian particle dispersion model FLEXPART version 6.2. Atmos.Chem.Phys., 5, 2461-2474.Wotawa, G., L.-E. De Geer, P. Denier, M. Kalinowski, H. Toivonen, R. D’Amours, F. Desiato, J.-P. Issartel, M. Langer, P. Seibert, A. Frank, C. Sloan and H. Yamazawa, 2003: Atmospheric transport modelling in support of CTBT verification – overview and basic concepts. Atmos. Env., 37, 2529-2537.
Figure 4. Evolution of thePSR for a given day (26Nov 2009) preceding thefirst detection in themeasurement scenario. Thedetections integrated intothe inversion algorithmwere made up to 3, 4, 5December (top row), andup to 6, 7, and 8 December2009 (bottom row). Agradual confinement of thePSR is clearly visible asmore and more consistentmeasurements are includedin the inversion algorithm.
The WEB-Grape software has been developed by the PTS for visualisationand analysis of the atmospheric transportmodelling. NPE2010 illustrated once again its capacities to localisea source of a (fictitious in this case) release ofradionuclides by performing a fusion of the PSRs with the error ellipses of the waveform events. Once the measurementscenario became sufficiently rich (29 Oct 2010 – third detecting day - onwards), the PSRs produced by the source inversionalgorithm overlapped with a number of waveform events. Among those there was the Black Thunder Mine explosion inWyoming, US. Nevertheless, atmospheric transport modelling on its owndid not allow further discrimination of thosewaveform events and required input from seismological analysis. The most striking challenge encountered by the ATMinvestigation of NPE2010 is inconsistency of the measurement scenariowith respect to the CTBTO inversion algorithm and,in particular, its SRS fields. This may be due to a deliberately introduced modelling error in the forward run generating thefictitious measurements or an important discrepancy between the meteorological fields driving the forward and backwardrun which actually mimics such a modelling error. One should, however, bear in mind that such an inconsistency of themeasurements from the point of view of the inversion algorithm is what one hasto face in case of real detections ofradionuclides. Indeed, the measurements probe the real state while an inversion algorithm makes appeal to the SRS fieldswhich result from an imperfect model of such reality. This illustrates the limitations of the currently used tools and anecessary investment in their improvement. They need to become capable ofdealing with a complex temporal profile of anemission and of incorporating modelling error into the inversion algorithm. At the same time the added value of using twodistinct sources of meteorological fields as well as of the collaboration with the RSMCs of WMO is emphasised by theNPE2010. The broad spectrum of the meteorological models underlying the calculations of the SRS fields may alleviate theinfluence of unavoidable modelling error in the meteorological fields driving the computation of the SRS fields at CTBTO.
For the CTBT verification regime,wherethe numberof measurementsis significantly lower thanthe numberof potentialsources, the efficient way of computing the M matrix is to proceed row by row. Each row is linked to one receptor and isconstituted by a Source Receptor Sensitivity (SRS) field. It represents sensitivity of the measurement to all possiblesources. It is defined with a specified spatial accuracy (1 degree) and within a given time interval preceding themeasurement sampling (14 days). At present, CTBTO calculates those SRSfields (retroplumes) for each of theradionuclide measurements on a daily basis using FLEXPART 8.2 driven by the ECMWF and NCEP meteorological fields.The retroplumes which are associated with the radionuclide measurementsinvolved in the fictitious scenario, but also withthe neighbouring non-detecting stations, are used as input information to the source attribution algorithm. The algorithmruns over all possible source locations (currently the grid-cells of the size ofone 1 degree) and computes the value of acorrelation coefficient. The correlation coefficient is a measure of consistency between the measurements which wouldhave arisen from the source located at a specified point and the measurements constituting the fictitious scenario. Theregions characterised with non-null values of the correlation coefficient are called Possible Source Regions (PSRs).
Data fusion
up to 3 Dec 2009 up to 4 Dec 2009 up to 5 Dec 2009
up to 6 Dec 2009 up to 7 Dec 2009 up to 8 Dec 2009
Figure 3. Evolution of thePSR for a given day (23 Oct2010). The detectionsintegrated into the inversionalgorithm were made up to27, 28, 29 Oct (top row),and up to 30, 31, Oct and 1Nov 2010 (bottom row).Contrary to the results forNPE2009 (Fig. 4) thegradual confinement of thePSRs is not observed.Starting from a 3-dayscenario, the resulting PSRspoint to the Black ThunderMine explosion on 23 Oct2010.
up to 27 Oct 2010 up to 28 Oct 2010 up to 29 Oct 2010
up to 30 Oct 2010 up to 31 Oct 2010 up to 1 Nov 2010
Examples of Possible Source Regions for NPE2010 I
PSRs built with the help of an ATM have a considerable spatial extension (see, e.g. Fig. 1), even for the day immediatelypreceding the first detection. However, under the hypothesis that the detections originate necessarily from a waveformevent, the source attribution algorithm can benefit from a high accuracy ofthe location of those events. In fact, in a datafusion procedure one looks for the waveform accuracy ellipses within the PSRs. Consequently, the ATM-based sourceattribution is mainly used to discriminate between the waveform events which could have been (within the PSRs) andcould have not been (outside of the PSRs) the origin of radioactivity production and subsequent venting of the (fictitious,in case of an NPE) radioactive material detected by the IMS radionuclide network.
Fig.1 illustrates 6 snapshots of the PSR built using the first measurement(Tab.1, USP75) and four non-detections (USP72,USP74, CAP17, FRP28) on 27 Oct 2010. It is clear that the first-day scenario is inconsistent with the hypothesis that thesource of radionuclide coincides with the Black Thunder Mine explosion. The inconsistency is most likely due to asubstantial difference in the modelling of a forward scenario and the CTBTO retroplumes used by the source inversionalgorithm.
radionuclides. Further discerningbetweenthe Treaty relevant and non-relevant waveform events indicated by the PSRsrequires an in-depth investigation of waveform specialists.
Figure 7. Two PSRs constructed on the basis of the first fictitiousdetection (27 Oct 2010) and four non-detecting neighbours (USP72,USP74, CAP17, FRP28). The PSR on the LHS is an average PSRbuilt on the basis of the input by 5 RSMCs: CWAO, MEFR, RJTD,RTHO and RUOB and is consistent with the Black Thunder Mineexplosion being at origin of a fictitious release while the PSR on theRHS, built using the SRS fields from 4 meteorological centres:AMMC, ATNC, BABJ and EGRR is not consistent with the BlackThunder Mine explosion.
