First coordinated CTBTO/WMO experiment on source location estimation 2003

First coordinated CTBTO/WMO experiment on source location estimation 2003

A cooperation agreement between the CTBTO Preparatory Commission and the World Meteorological Organization (WMO) has resulted in the first global experiment in reverse atmospheric transport modelling (backtracking). The experiment was carried out between 20 and 22 March 2003, with the cooperation of seven WMO Regional Specialised Meteorological Centres (RSMC) designated for dispersion modelling, three national data centres and the CTBTO Provisional Technical Secretariat (PTS).

Atmospheric transport modelling (ATM) is generally used to analyse and forecast the pathways of material set free in the atmosphere. Parties to the 1986 Convention on Early Notification of a Nuclear Accident, established after the Chernobyl disaster, use forward calculations to estimate where air movements will bring radionuclide particulates in the days and months following an accidental release. In the case of a clandestine nuclear test explosion, however, there is no notification, and the first indication of a radioactive discharge will probably be the detection of relevant radionuclides at one of the CTBTO's radionuclide monitoring facilities. In such a case, air transport modelling will have to work backwards in time to estimate the source location of the detected radionuclides. With the introduction of the new ATM software system early this year, the International Data Centre carries out ATM calculations on all 79 known radionuclide treaty locations on a routine basis. The new CTBTO/WMO agreement now allows the insertion of WMO modelling data into IDC processing to increase confidence in source location identification in cases of concern.

The joint March experiment was based on a hypothetical nuclear explosion, with a release location to the south west of Iceland selected at random by computer. Using real tracking data based on the real state of the atmosphere, the PTS made forward calculations to forecast the movement of the hypothetical release plume. Stations in the path of the plume were identified, and requests for support were sent to the WMO centres and the national data centres, giving the location of the stations and the measurement period. The exact measurement scenario was not disclosed to the WMO centres. ATM support was also requested for several stations that were not directly affected by the radioactive debris, but were neighbouring to affected stations. The experiment lasted for three days. The WMO centres and the national data centres had 24 hours within which to provide ATM data on each of the requested stations. All centres demonstrated excellent response times, in all cases within the 24 hour target and in many cases much faster.

In environmental emergency response cases, atmospheric transport modelling requests are usually issued through the medium of facsimiles, and results are received by the same means. The coordinated CTBTO/WMO experiment was the first global one to be conducted entirely electronically. Requests were issued by email, and data files were received in return. The data format was agreed upon at a preceding CTBTO/WMO workshop in October 2002.

Reverse air transport modelling calculations have traditionally been regarded as a research exercise that could take several months to perform. The joint CTBTO/WMO exercise is the first instance of a fully automated reverse calculation taking place in near real time. The resulting fields of regard - source location areas identified by each participating centre - were overlaid to identify areas with the highest level of agreement on source location among all participating centres. Agreement among the overlays allows the CTBTO to provide location data to Member States with a considerably improved degree of confidence.

It is expected that WMO data could automatically be requested for inclusion in IDC calculations whenever CTBTO radionuclide data indicating a predetermined level of concern is received. A second joint experiment will be discussed at a meeting in November, and a regular exercise schedule is expected to start in the near future.