August 2009

Contributing to a safer and more secure world: WMO support to the Comprehensive Nuclear-Test Ban Treaty

By Peter Chen
Chief, Data-Processing and Forecasting
Weather and Disaster Risk Reduction Services Department
World Meteorological Organization


Meteorology—beyond daily weather

The general public is growingly aware that day-to-day weather forecasts are made by the use of sophisticated numerical weather prediction (NWP) models, and that they have evolved dramatically over the last 60 years.  Those more closely involved in the developments know that there have been important improvements in NWP, especially in the last 15 years with improved observational datasets, data-assimilation systems, numerical simulation and visualization methods and dramatic increases in computing power.  The scientific and technological community has recognized the enormous potential represented by these advances in the simulation and prediction of the atmosphere in an ever-growing list of applications to many scientific and socio-economic sectors that depend on meteorological factors.   This article covers one very specialized area, to which meteorology and the World Meteorological Organization have been contributing over the last 15 years to a safer and more secure world: the Comprehensive Nuclear-Test-Ban Treaty.  


The Comprehensive Nuclear-Test-Ban Treaty (CTBT) bans nuclear test explosions in any environment.  It was negotiated and drafted at the Conference on Disarmament in Geneva and opened for signature in New York on 24 September 1996.  The Treaty builds on the work of preceding bilateral, regional and global arrangements, such as the Treaty on the Non-Proliferation of Nuclear Weapons.  The CTBT aims to eliminate nuclear weapons by constraining the development and qualitative improvement of new types of nuclear weapons.  It plays a crucial role in the prevention of nuclear proliferation and in nuclear disarmament, thus contributing to a safer and more secure world.  The CTBT Organization (CTBTO) was established in Vienna, Austria, to implement the requirements of the Treaty and to effect its earliest possible entry into force. 

CTBT-International Monitoring System (IMS)—general

The CTBTO is well advanced in implementing an International Monitoring System (IMS) of 337 monitoring facilities representing 170 seismic, 11 hydroacoustic, 60 infrasound, 80 radionuclide stations and 16 radionuclide laboratories, located all over the world.  The first three of these technologies are designed to register “waveforms”—signals of sound and energy vibrations—while radionuclide stations detect and measure radionuclides released into the atmosphere (see box). 

Atmospheric transport and dispersion modelling (ATM)

Among the different technologies designed to verify compliance with the Treaty, radionuclide monitoring is the prerequisite to detect and identify the nuclear origin of a carefully disguised, or decoupled, nuclear explosion, unambiguously.  Closely connected with this, the technology of atmospheric transport and dispersion modelling (ATM) is an advanced numerical simulation method, closely linked to numerical weather prediction (NWP), used for Treaty verification purposes to help determine the region of origin of a suspicious radionuclide following its detection.  The ATM application is also a means to provide greater confidence in detecting and identifying nuclear explosions, especially in those cases where the event is poorly detected, or even escapes detection by waveform monitoring. 


The ATM can be executed in forward mode to assess or predict the time evolution of plumes of airborne materials, hence with knowledge of the origin of release into the atmosphere, this computational model provides estimates of the atmospheric dispersion over time and space.  In the application to CTBT verification, the main capability that ATM fulfils is its execution in backward mode, to determine where a detected radionuclide in the air sample, known to be associated with a nuclear explosion, has come from.   


CTBTO-WMO agreement

WMO has been cooperating with the CTBTO under a formal agreement that was completed in May 2003, built upon many prior years of scientific and technical cooperation, motivated by a long-term vision of mutual interests and potential synergies.  The successful cooperation continues, and among the recent highlights is the implementation of a Joint ATM-atmospheric backtracking response system into operations in September 2008. This activity involves eight WMO Regional Specialized Meteorological Centres and one National Meteorological Centre with operational ATM capabilities, namely Beijing, Exeter, Melbourne, Montreal, Obninsk, Offenbach, Tokyo, Toulouse, and Vienna.  In June 2003, the CTBTO also established an agreement with the European Centre for Medium-Range Weather Forecasts (ECMWF) for obtaining routine global NWP outputs to drive its own ATM computations at the CTBTO’s International Data Centre. 

IMS data and information for civil, humanitarian and scientific applications

In addition to its actual purpose of detecting nuclear explosions, the CTBTO’s global verification regime, including its IMS, could offer cooperation on a wide range of civil, humanitarian and scientific applications, through data and information.  For example, CTBTO and WMO are open to extending their cooperation to any field where data from the IMS may be useful for scientific research and development to improve atmospheric and environmental monitoring and prediction, emergency response or disaster mitigation, in line with the provisions of the CTBTO-WMO cooperation agreement.  Examples of this are:  the development of the use of infrasound monitoring for volcanic ash plume warnings for aviation (see box); the use of natural radionuclide data for scientific purposes; and monitoring of the phenomena in the Earth-atmosphere system. 

Leading up to the Treaty: WMO assisted in specification of the Treaty’s radionuclide verification regime

As early as the mid-1990s, WMO was exchanging scientific information with a working group of the Conference on Disarmament, located in Geneva, including research and development work on the use of ATM, in particular, to “geo-locate” the possible source region of airborne material detected at the surface of the Earth.  In 1994, Ambassador Wolfgang Hoffmann (Germany), later the first Executive Secretary of the Preparatory Commission for CTBTO, contacted the director of the WMO World Weather Watch Department to explore the possibility of applying this numerical simulation technology in conjunction with the radionuclide technology of the proposed verification regime.  

During this period, ATMs were already in common use but mainly for regional scientific assessments and research studies related to long-range transport of airborne pollutants, however.  As well, triggered by the nuclear power plant accident in Chernobyl (1986), the 10th session of the Commission for Basic Systems (CBS-X, Geneva, 1992), through its then Working Group on the Global Data-processing System, was in its early stages of implementing an operational ATM-based system for environmental emergency response, including the establishment of Regional Specialized Meteorological Centres (RSMCs) with  this specialization, which provide global coverage of ATM predictions for WMO Members and for the International Atomic Energy Agency.  

WMO’s response to Ambassador Hoffmann’s request was to provide a scientific exchange, including the milestone lecture to the Conference on Disarmament (CD) on the scientific basis and possible use of ATM to “backtrack” the location of the source of a radionuclide detected at a monitoring site, thereby associating the detection with a nuclear explosion.  This lecture was given in 1994 by Janusz Pudykiewicz, a senior scientist in weather and air-quality research at Environment Canada.  The ATM technology was also used, together with historical global weather data and analyses, to carry out a series of numerical experiments to simulate the spreading of airborne radionuclides under many atmospheric explosion scenarios of varying location of explosions and seasons of the year.  The results were presented to the CD, then used to assess the performance of different possible radionuclide networks, of varying densities and detection limits, and provided an objective basis to facilitate the negotiations of the Treaty’s final configuration and design of the IMS radionuclide network. 

As part of the ongoing technical discussions, and with the increasing interest of nuclear test-ban and atmospheric scientists and policy specialists working on the final developments for the Treaty, the Informal Meeting to Discuss the Applications of Atmospheric Modelling to CTBT Verification, hosted by Canada, brought together many scientists in Montreal to exchange ideas and ongoing research that were thought at the time to contribute to, and advance, ATM applications for Treaty verification.  This took place in 1996 just when the Treaty was opened for signing at the UN, New York.  (Ref. 1)

WMO and CTBTO technical exchange and arrangements

The exploration and development of cooperation with CTBTO continued through the work of WMO’s CBS in the late 1990s.  Its then Expert Team on Nuclear Emergency Response (today’s Coordination Group on Nuclear Emergency Response Activities), started to see participation of representatives of the Provisional Technical Secretariat of CTBTO.  The International Civil Aviation Organization (ICAO) also participated, with its interests in airborne radioactivity, and learned of the potential use of infrasound measurements from CTBTO to contribute to global early detection and notification of explosive ash-producing eruptions as part of monitoring and tracking activities of its International Airways Volcano Watch programme.  Thus, ICAO, with interest in acquiring IMS infrasound information covering high volume jet aircraft traffic corridors that are poorly monitored for volcanic eruptions, established a Rapporteur on CTBT matters in 1995, a role that was filled by a CBS expert, Peter Chen, until 2004 (Ref. 2).  In 1998, a small joint task group developed a review of possible areas of technical cooperation between the two organizations, which saw mutual benefits in ATM and in potential data exchange; members included Roland Draxler (USA), Jean-Pierre Bourdette (France) and Morrison Mlaki (WMO Secretariat), representing WMO, and Lars Erik De Geer, representing the CTBTO provisional technical Secretariat (PTS). 

Soon thereafter, the CTBT Preparatory Commission’s technical working group established a Sub-Task Group on Cooperation with WMO.  The Group, chaired by Michel Jean (Canada), involved scientific and technical individuals who represented the early momentum in the development of a formal agreement between CTBTO and WMO, and many of them have continued to today contributing to the success stories of this cooperation.

The cooperation agreement of CTBTO with WMO

An early exchange of letters between the Executive Secretary of the CTBTO and Secretary General of WMO took place in the late 1990s. CTBTO and WMO concluded a formal relationship agreement in 2000 that entered into force upon approval by the CTBTO Preparatory Commission and Fourteenth World Meteorological Congress in May 2003. The provisions of the CTBTO-WMO agreement (Ref. 3) foresaw joint activities in the following three major areas

  • Cooperation and consultation
  • Reciprocal representation
  • Exchange of information and documents (including data)

In June 2003, CTBTO concluded an agreement with ECMWF on the basis of the CTBTO-WMO agreement, and paved the way to CTBTO obtaining meteorological datasets needed for its daily routine ATM workflows (Ref. 4).

During this period, CTBTO hired two atmospheric scientists on the staff of the PTS, Gerhard Wotawa and Andreas Becker, to accelerate the development of ATM for Treaty verification, including the collaborative work with WMO.  They have contributed important advances in the development and implementation of highly automated ATM-backtracking systems in cooperation with numerous WMO experts.  As well, WMO commenced to participate as an observer in the technical working groups of the Preparatory Commission. 

Since 2002, CTBTO has provided meteorological data measured at the IMS’ radionuclide stations to WMO, and made available through the GTS.  In September 2002, WMO through RSMC Montreal, provided the basic daily weather forecasts, charts and information required, in real-time, in the CTBT’s first on-site inspection field experiment conducted in Kazakhstan, to field test early ideas on this important final function of the Treaty’s verification regime.  In early 2005, CTBTO in principle agreed to release IMS data on natural radionuclides for the WMO Global Atmosphere Watch programme (Ref. 4.).


CTBTO’s radionuclide monitoring programme:

Aerosol samplers collect daily filter samples at currently more than 60 of planned 80 stations worldwide, some of them in remote areas, by filtering between 12 000 and 24 000 m³ ambient air per day under controlled conditions.

The isotope concentrations of Be-7, Pb-212 are regularly detected. Some stations also can quantify Pb-210.  These isotopes can be used as natural tracers to verify general circulation models.

Filter samples are archived in Vienna and could provide material for further investigations (dust, pollen, long-lived and stable isotopes), for example:

  • Improving the understanding on the long-range exchange of pollutants, including the climate impact of megacities by analysis of dust and pollen;
  • Monitoring potential changes in the stratosphere/troposphere exchange processes via the Be-7 or Be-7/Pb-210 ratios, serving appropriate proxies for this. Other isotopes like Al-26, Cl-36, and S-35 may be used for paleo-climatic studies and bio-geochemistry cycles of chlorine and sulphur.
  • Understanding of the Be-10-cycle will improve reconstruction of past solar activity changes

IMS radionuclide stations are either automatic or manual and typically comprise a sampling system and detector in air-conditioned housing of minimum 15 m² spacing.  There is uninterruptible power supply and also auxiliary power supply. The stations are regularly checked, most of them daily.  Some small additional equipment measuring other trace gases could be added, with certain limitations on space and power requirements.  These stations are integrated into CTBTO’s global communication system via satellite links, allowing for 24/7 connection within a dedicated telecommunications network.


The joint ATM backtracking response system

Among the main goals of the cooperative agreement between these two international organizations is the enhancement of ATM-backtracking for Treaty verification.  While the CTBTO developed its own ATM system, the inclusion of independent backtracking results from WMO’s Regional Specialized Meteorological Centres (RSMC) helps them address the uncertainties associated with the numerical modelling of the atmosphere, in particular for tracking the movement of airborne tracers.  A joint CTBTO-WMO system also provides views from authoritative modelling centres of different geographical regions and thus builds political confidence in the source region estimation results delivered by the Treaty verification regime to CTBTO Member States.  With this in mind, both organizations pursued joint activities to accomplish an ATM backtracking response system serving this purpose.

A first informal exercise between the two organizations was conducted in May 2000 to test and refine notification-reply telecommunication procedures.  This test involved seven RSMCs.  The CTBTO presented first requirements for a future CTBTO-WMO response system and an overview of envisaged products at the session of the CBS Emergency Response Activities Coordination Group (Washington DC, September 2001).  Meanwhile, the two newly recruited atmospheric scientists at the PTS organized the CTBTO-WMO, The Way Forward Workshop in 2002 in Vienna.  The workshop yielded an agreement on communication and procedures and paved the way to the first coordinated CTBTO-WMO experiment on source region estimation that took place in 2003 (Ref. 5).  The successful experiment was repeated, with a broader scope, in 2005.  

In 2006, CBS-Ext.(06)  recommended adding the CTBTO-WMO Response System to its technical regulations, into the WMO Manual on the Global Data Processing and Forecasting System (WMO-No. 485).  One final system test was carried out in December 2007, and on 1 September 2008, the joint response system was implemented, with operational commitments obtained from RSMCs Beijing, Exeter, Melbourne, Montreal, Obninsk, Offenbach, Tokyo, Toulouse, and NMC Vienna. 

The scientific basis of this system was published in 2007, with contributions from multiple authors, including CTBTO and WMO experts (Ref. 6).


Exercising the system is an important activity. The method is as follows: a CTBTO atmospheric scientist creates a fictitious radionuclide release and dispersion scenario utilizing an ATM-based simulation software, assuming a release at a pre-selected location.  The location is not chosen randomly, rather from the standard seismic event list, which is an automated report in real-time of IMS detected seismic events. Another atmospheric scientist, unaware of the selected location, is then tasked to identify the scenario’s “explosion” associated with the seismic event location, by making use of the joint response systems’ backtracking results and by applying the additional data fusion capabilities.  In the test of December 2007, although a number of seismic events occurred during this time period, with the ATM backtracking results and data fusion methods, the estimated location of the fictitious explosion was narrowed down to a few 100 km of the correct location.  

In October 2006, the underground nuclear explosion carried out by the Democratic Republic of Korea was well detected by the CTBTO’s IMS, including the ATM system which correctly pointed to the region of the explosion.  On 25 May 2009, the Democratic Republic of Korea carried out another underground test. So far no attributable radionuclide from this explosion has been detected at IMS.  The ATM was executed in forward mode to estimate possible earliest times of arrival at the monitoring stations. 

CTBT—international scientific studies

The CTBTO International Scientific Studies to evaluate the CTBT Verification Regime was launched in 2008, where the question of verifiability of the Treaty is to be assessed by the scientific community. Eight topic areas are considered, each one a technical means and critical part of the verification regime.  As part of the CTBTO-WMO Agreement, WMO took on the role as the Topic Coordinator for Atmospheric Transport Modelling (ATM), and named Richard Hogue (Canada) and Peter Chen as coordinators to organize and build exchange among scientists interested in the topic.  At the first ISS Conference in June 2009 (Ref. 8), ATM attracted much interest, with 15 papers, and a keynote address given by Andreas Stohl (Norwegian Institute for Air Research), the developer of the FLEXPART atmospheric transport model that has been implemented at CTBTO. It is envisioned that the ISS conference will be repeated every few years, to maintain the scientific critique and credibility of the technologies used in Treaty verification. 

podium   WMO participating in the CTBT International Scientific Studies Conference on the scientific panel on atmospheric explosions, Hofburg, Vienna, 10-12 June 2009.

The CTBT-ISS initiative is also a gathering place where interests in civil, humanitarian and scientific applications of CTBTO data and information continue to build.  Many papers on potential non-Treaty applications were presented at the Conference, covering seismology, hydroacoustics, infrasound, radionuclide monitoring, ATM and data exploitation. Both CTBTO and WMO are open to extend their cooperation beyond enhancing the ATM for Treaty verification, and to derive additional benefits for improved monitoring, assessment and prediction of weather, climate and the environment. 

Possible civil application: infrasound-based monitoring of volcanic eruptions

Interest emerged early in potential civil, humanitarian and scientific applications and benefits from the data and information from the evolving CTBTO monitoring and data-processing system. CTBTO and WMO, jointly with the International Civil Aviation Organization in the context of the International Airways Volcanic Watch, developed an early idea of using the Treaty’s global infrasonic data to monitor explosive, ash-producing volcanic eruptions as a potential early warning system for aircraft operations, especially along heavily travelled air routes that are crossing volcanically active but poorly monitored regions (Ref. 7). Volcanic ash represents a very serious aviation hazard. Early detection would ensure timely predictions and early warnings of possible airborne ash plumes, utilizing the established ATM technologies to track their movement, and would provide real-time guidance needed for air traffic planning and re-routing decisions. In 2003, Dieter Schiessl (WMO) and Tom Fox (ICAO) jointly presented the case of the potential use of infrasound data for enhancing the system for early warning of airborne volcanic ash for the safety and security of aircraft operations. To explore this, CTBTO entered in late 2007 into a collaborative project with the Volcanic Ash Advisory Centre (VAAC) in  Toulouse for selected volcanoes in Europe and Africa. Some initial contacts have been established with other VAACs in order to extend the investigations to additional volcanic regions in the world.



This article is adapted from two articles in CTBTO Spectrum, issues 11 (2008) and 12 (2009), entitled: “The importance of Atmospheric Transport Modelling: Over ten years of cooperation between the World Meteorological Organization and the CTBTO” (P. Chen, G. Wotawa, A. Becker).   I am grateful to A. Becker and G. Wotawa for their contributions and reviews, and for many years of fruitful collaboration. 


1. “Informal meeting to discuss the application of atmospheric modelling to CTBT verification – Group report and proceedings, Montréal, Canada, October 15 – 16, 1996”, Non-Proliferation, Arms Control and Disarmament Division, Department of Foreign Affairs and International Trade, Ottawa, Canada, January 1997.

 2. “Infrasonic detection of volcanic explosions by the CTBT International Monitoring System: Implication for aviation safety”, P. Chen and D. Christie, ICAO Volcanic Ash Warnings Study Group, VAW/2 – Study Note. 13, 30 October – 3 November 1995. 

 3. “Agreements and Working Arrangements with other international organizations”, WMO Basic Documents No. 3, WMO-No. 60 (2002), Suppl. No. 1 (I. 2004). 

4. D. Schiessl, (2005): “Potential civil and scientific applications”.  CTBTO Spectrum, 7, December 2005.

5. “CTBTO – WMO Experiment on Source Location Estimation”, CTBTO Technical Report, CTBT/PTS/TR/2004-1, July 2004.

6. A. Becker, G. Wotawa, L.-E. De Geer, P. Seibert, R. R. Draxler, C. Sloan, R. D’Amours, M. Hort, H. Glaab, P. Heinrich, V. Grillon, V. Shershakov, K. Katayama, Y. Zhang, P. Stewart, M. Hirtl, M. Jean, P. Chen (2007): “Global backtracking of anthropogenic radionuclides by means of a receptor oriented ensemble dispersion modelling system in support of Nuclear-Test-Ban Treaty verification”, Atmospheric Environment, 41, pp 4520 – 4534.

7. H. Haak, (2003): “Can CTBT infrasound technology assist in civil aviation?”, CTBTO Spectrum 3, December 2003.   

8. “International Scientific Studies Conference - Book of Abstracts, Vienna, Austria, 10 – 12 June 2009”, Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, June 2009.  

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