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Brussels, 12 March 1999

Tropical atmosphere much cooler and drier than expected:
the results of a large EU-supported experiment in the Seychelles

Keywords: climate, ozone layer

Two European research aircraft have probed the changing composition of air as it shoots upwards in tropical thunderstorms, affects Earth's climate, and begins its long journey through the ozone layer in the stratosphere. The aircraft have found surprisingly low temperatures at the boundary between the lower atmosphere and the stratosphere. At this boundary, called the tropopause, the aircraft detected some of the thinnest clouds on Earth, which, although thin, play nevertheless a critical role in climate change. Moreover, because nearly all of the air entering the stratosphere does so in the tropics, tropical atmospheric processes have an impact on stratospheric ozone at global scale.

Clouds affect climate by trapping heat emitted from the Earth and by reflecting sunlight. The size of these two effects on climate is uncertain for all cloud types, but most uncertain for high clouds in the tropics because detailed measurements are missing. The Intergovernmental Panel on Climate Change, the forum set up to provide the scientific advice on climate change to governments, recently identified the lack of knowledge about high tropical clouds as one of the largest obstacles in the way of a definitive statement on how climate is changing.

Using two aircraft flying together, the APE-THESEO mission has studied these very high clouds. APE-THESEO, funded mainly through the EC Environment and Climate Research Programme, is part of THESEO, the biggest ever experiment organised on ozone research. One aircraft, a Falcon 20 operated by Deutsches Zentrum für Luft und Raumfahrt, used a laser beam to probe the clouds from below. The second aircraft, a Russian M-55 Geophysica operated by Myasischev Design Bureau, which flies at about twice the height of commercial airliners, made measurements inside the clouds. The equipment on the Falcon can provide rapid information on cloud position, and this was relayed to the Geophysica. This allowed the two aircraft to make combined measurements in very thin layers of cirrus clouds, which are visible to the laser equipment but not to the human eye, or to satellites. Some of cloud layers probed were only 100 m thick, and at a height of 17 km above the Earth's surface. They contained less than a thousandth of the water found in clouds closer to the ground, making them amongst the most tenuous large-scale cloud layers on Earth.

The first surprise from the flights was the extremely low temperature of the tropopause. Two flights have recorded tropopause temperatures below -89°C, which is about 10 degrees colder than the regional average and a massive 120 degrees colder than the surface. The clouds were found in a thin layer around the temperature minimum.

Early indications from APE-THESEO are that Indian Ocean thunderstorms cannot easily explain the formation of these high clouds, except perhaps by pushing the air above the storms up and so causing it to cool, leading to cloud formation in much the same way as clouds can form above mountains. Alternatively, the clouds could be aged remnants of clouds formed many thousands of miles away. APE-THESEO coordinator, Dr. Leopoldo Stefanutti of the Consiglio Nazionale delle Ricerche in Florence, explains: "We expected to find a rather warm tropopause and high values of water vapour. Instead we have found some very cold, dry, layers at the tropopause. Perhaps these are local events, or the effect of drying thousands of miles east of the Seychelles. In either case we have a valuable dataset for studying the connection between climate and the ozone layer."

CFCs, which are the source of the reactive compounds that destroy ozone, and ozone itself, are also being measured. Long flights through the southern tropics have shown occasional reversals in the expected distribution of these compounds, hinting that large-scale weather patterns are disturbing the atmosphere even deep in the tropics. This mixing determines the impact that the tropical air will have on stratospheric ozone amounts over the rest of the World, including the densely-populated regions of Europe, America and Asia. By combining the data with computer modelling of the global atmosphere, better predictions can be made of, for example, the impact of increased air traffic on the ozone layer and the climate.

In its final week, the project turned its attention to tropical cyclone Davina. The APE-THESEO aircraft were able to make measurements of clouds and chemicals right into, and above, the cyclone. Senior project member Dr. Thomas Peter from the ETH Technical University of Zurich said: "The data collected within the top of the storm's clouds give us a rare chance to determine what impact cyclones have on the composition of air being pumped into the ozone layer."

For further information, please contact:

Dr Georgios Amanatidis
Scientific officer, DG XII/DI
Fax: + 32-2-296.30.24
E-mail :

Mr Michel Claessens
Communication Unit, DG XII
Fax: + 32-2-295.82.20

Dr Leopoldo Stefanutti
APE-THESEO field office c/o Directorate General of Civil Aviation
Mahé International Airport, Seychelles
Telephone: +248 38 4178
Fax: +248 375 376

Notes for editors

THESEO (Third European Experiment on Stratospheric Ozone) is an ambitious programme of coordinated field experiments supported by the European Commission and national funding agencies. The emphasis in THESEO is the study of the processes controlling ozone loss over the populated latitudes of the Northern Hemisphere middle latitudes and their connections with other regions. The connection between the middle latitudes and the less intensely studied tropical lower stratosphere is another THESEO theme. It consists of a core of 14 major projects within the EC Environment and Climate research programme , with an EC contribution reaching 14 million EURO. THESEO involves over 400 scientists from the EU, together with colleagues from Canada, Iceland, Japan, Norway, Poland, Russia, South Africa, Switzerland See also:

APE (Airborne Platform for Earth observation) is the major tropical component of THESEO. APE-THESEO comprises 20 research groups from the EU, Switzerland and the Russian Federation. The project is mainly funded by DG-XII of the European Commission (2,200,00 EURO), the Italian Space Agency, the European Science Foundation, and the Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie, to a total cost exceeding 4,000,000 EURO. See

Research on the atmospheric composition changes, stratospheric ozone depletion and climate will be reinforced within the Fifth Framework Programme 1999-2002, under the "Global change, climate and biodiversity" key action, with a total budget of 301 million EURO. These European efforts are directly related to the Montreal and Kyoto Protocols of which the European Union is a signatory.

The Seychelles is at 5°S 55°E, in the south-western Indian Ocean. APE-THESEO is working out of the international airport, which is on the main island, Mahé.

The M-55 Geophysica is a high-altitude aircraft that has been converted from military to scientific use. The aircraft can reach 21 km altitude, carrying a payload of 1.5 metric tonnes. It has only a single seat, so the instruments must operate automatically.

The DLR Falcon is a converted executive jet, capable of carrying 4 scientists along with a two-man crew. The scientists carry out real-time analysis of results, which allows them to guide the Geophysica to invisible cirrus and other regions of interest.

The tropopause is the boundary between the lower atmosphere, the troposphere, and the upper atmosphere, including the stratosphere and its ozone layer. Air temperatures are generally lowest at the tropopause, increasing above and below. A typical tropopause temperature over Europe or North America is -55 °C.

Cirrus clouds are high, thin clouds. In the tropics they can be formed at the top of convective systems, including cyclones, or by internal waves in the atmosphere causing local cooling. Sub-visible cirrus, that is, those not visible to an upward-looking observer or downward looking satellite, may be significant in the tropical dehydration process. Both the Falcon and the Geophysica are equipped with instruments to measure the particles in sub-visible cirrus.

Convection is the rapid vertical transport of buoyant air, which usually leads to cloud formation. If convection is sufficiently energetic, the air and cloud can reach the tropopause, where it can either deposit or remove water, depending on details of the cloud structure.

Stratospheric pollution: air entering the stratosphere in the tropics contains unreactive compounds that eventually break down into reactive compounds that contribute to ozone chemistry. Man-made CFCs are mixed thoroughly throughout the troposphere before being transported upwards in the tropics. Nitrous oxide (N2O) is a naturally-occurring compound that behaves similarly to CFCs, breaking down in the stratosphere to for ozone-depleting compounds. CFCs and N2O are measured on board the Geophysica. Ozone-depletion chemistry due to nitrogen compounds and hydrogen compounds (from water) provide the natural balance to ozone production from sunshine. CFCs and related man-made compounds perturb this balance, and reduce the amount of ozone in the stratosphere. This allows more damaging ultraviolet radiation to reach the surface. Changes to the input of water vapour into the stratosphere, due to man-made climate change, will also contribute to ozone depletion.

APE-THESEO partners and contact e-mail addresses

Consiglio Nazionale delle Ricerche
Istituto di Ricerca sulle Onde Elettromagnetiche (IROE-CNR)
Istituto di Elettronica Quantistica (IEQ-CNR)
Istituto di Fisica dell'Atmosfera (IFA-CNR)A.
Istituto per lo Studio dell'Atmosfera ed degli Oceani (ISAO-CNR) G.
APE-Comitato di GestioneS.
Universita' dell'AquilaG.
Universita' di RomaG.
Istituto Nazionale di OtticaP.
Deutsches Zentrum für Luft und Raumfahrt W. Renger
H. Finkenzeller
Max-Planck-Institut für ChemieTh.
Johannes Gutenberg Universitaet MainzS.
Universitaet FrankfurtM.
Forschungszentrum JülichC.
Myasischev Design BureauL.
Aviaecocentre Ltd.A.
The Central Aerological ObservatoryV. Khattatov
Cambridge UniversityA. R.
The Met. Office P. Salter
Stockholm UniversityK.
Observatoire de Neuchatel V.
Eidgenössische Technische HochschuleTh.
Directorate General of Civil Aviation G.

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