EU experiment found tropical atmosphere dry and cool
A European Union-supported experiment in the Seychelles has shown that tropical atmosphere is much cooler and drier than expected. By using two aircraft flying together, the APE-THESEO mission studied the very high clouds between the lower atmosphere and the stratosphere. APE (Airborne Platform for Earth Observation) is the major tropical component of THESEO (Third European Experiment of Stratospheric Ozone), which is the biggest ever experiment organized on ozone research. 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. In the mission, 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, flying 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, so the two aircraft could make combined measurements in very thin layers of Cirrus clouds, which are high and thin, visible to the laser equipment but not to the human eye or to satellites. Some of cloud layers probed were only 100m thick, and at a height of 17km above the Earth's surface. They contained less than 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 degrees Celsius, 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 form above mountains. Alternatively, the clouds could be aged remnants of clouds formed many thousands of miles away, as APE-THESEO coordinator, Dr Leopoldo Stefanutti of the Consiglio Nazionale delle ricerche in Florence, explained. "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," he said.