What route might one expect a polluting cloud to take over a distance of several thousand kilometres? How best to warn the populations concerned in the event of a nuclear or chemical accident? Thanks to the ETEX project, an unusual simulation has made it possible to refine the computer models used in disasters of this kind. A world first.
23 October 1994, 5 p.m. With winds blowing in from the west,
a gigantic cloud of gas rises into the sky over Rennes (France)
and begins a journey of nearly 2,000 km over Europe, passing over
the United Kingdom, reaching Bulgaria, Poland, Finland. Non-toxic,
it was triggered deliberately, for the purposes of a scientific
study on pollutant dispersion. The experiment was then repeated
a few weeks later on 14 November, using a slightly different gas.
Objective: civil protection
This dual simulation was part of the ETEX project (European Tracer Experiment), launched in 1992 by the European Community's Joint Research Centre (JRC), the World Meteorological Organisation (WMO) and the International Atomic Energy Agency (IAEA). "The aim of ETEX," explains Dr. Francesco Girardi, researcher at the Ispra JRC (Italy) and coordinator of the project, "was to evaluate the capacity and accuracy of the atmospheric models for predicting the course of a polluting cloud over distances of more than a thousand kilometres, in real time. This is important for testing civil emergency plans and warning systems in the event of major nuclear or chemical accidents."
When conducting these experiments, the researchers used PFC gases
(perfluorocarbon) because of their stability and their immunity
to the chemical constituents of the atmosphere. Eight to ten grammes
of gas were released, every second, for 12 hours. A highly sensitive
technique, capable of detecting quantities of gas of around one
billionth of a millilitre, was used for control.
Weather stations and flying labs
170 weather stations, dotted all over Europe, spent 3 days collecting a total of 9,000 air samples, which were then swiftly analysed by the JRC team in Ispra.
In order to obtain information about the vertical structure of the cloud, samples were also collected at altitudes of between 300 and 1200 metres, by three aeroplanes equipped as laboratories: a Hercules provided by the British Met Office, a Stemme Motorglider from Swiss MetAir and a DO228 from the German Weather Office. The role of the Hercules was to measure the concentration of gas in real time, thanks to a special technology borrowed from an American laboratory.
As in the case of a real accident, the computer centres participating in ETEX, who were each testing their own model, didn't know where or when the gas would be released. Details about the characteristics of the substance and its origin were withheld until the moment of release into the atmosphere. The computer centres then had to predict what would happen to the cloud over the next 60 hours.
Both experiments showed that the vast communications network between these centres and all the weather stations involved works smoothly and efficiently. Once the alarm was raised, the experts were able to exchange information and relay their forecasting models concerning changes in the levels of gas in the atmosphere in under six hours.
Overall, the correlation between the cloud dispersion calculated by the models and the results observed was fairly satisfactory for the first 24 hours, particularly in the first experiment, where the parameters were more favourable. Thereafter, the number of models which managed to reproduce the actual position of the cloud at the end of 48 hours was much lower. One of the problems highlighted is the important role played by convection, which affects short- and medium-range cloud dispersion.
"ETEX is the first experiment of its kind to be conducted anywhere in the world. A far greater number of European research centres than expected wanted to take part: 24 agencies thus managed to test a total of 28 forecasting models," comments Francesco Girardi. "The transnational collaboration generated during the project was remarkable." National meteorological agencies from 22 countries took part in ETEX. The National Laboratory of Brookhaven (USA) offered its services to the European researchers, while other Canadian, Israeli, Japanese and American labs took a close interest in the experiment. The researchers can thus now draw on an extremely detailed experimental base, to help them refine their computer models.