THE THIRD
EUROPEAN STRATOSPHERIC EXPERIMENT
ON OZONE (THESEO)
The Third European Stratospheric Experiment on Ozone (THESEO)
1998/99 is the second winter of the Third European Stratospheric Experiment on Ozone (THESEO), when the campaign enters its most intensive phase. THESEO is an ambitious programme of coordinated field experiments, with associated modelling and theory, 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. This is particularly important for studies of aircraft impact on the stratosphere.
A wide variety of measurements are planned. Observations from five research aircraft (the French Mystere and Arat, the German DLR Falcon, the Russian stratospheric aircraft Geophysika and the Swiss Learjet) should add significantly to our knowledge of stratospheric ozone, particles and water vapour. These aircraft are variously carrying out experiments in the Arctic, mid-latitudes and tropics.
A large balloon measurement campaign centred on the Swedish balloon facility of ESRANGE at Kiruna will deploy a variety of instruments to examine the composition of the Arctic atmosphere during periods of ozone destruction, as well as the composition of the aerosols and PSCs which prime the atmosphere for ozone destruction. Further springtime measurements will be made at middle latitude balloon sites.
Multiple sonde measurements around the Northern Hemisphere are being used to measure ozone loss directly in the Arctic and at mid-latitudes. This project involves co-operation between most European countries as well as scientists in Russia, the US, Canada and Japan.
Many UK scientists are involved in THESEO in a wide range of projects ranging from observations to modelling and including studies of dynamics and chemistry. Articles describing some of these activities appear elsewhere in this newsletter. The UK involvement has been augmented by recent support from the NERC UTLS programme.
One of our objectives in Cambridge is to study exchange of air between the polar vortex and middle latitudes. This will help in understanding the contribution of polar processes on middle latitude ozone decline. Early results are very interesting. The polar vortex was cold during early December, sufficient to trigger the chemical processes responsible for ozone depletion. Then, in mid December, a major warming occurred. This slowed the chemical loss but has provided a special opportunity to study mixing processes. Figure 1 from Adrian Lee and Genevieve Cahill, shows the accumulated loss on the 480K isentropic surface, due exclusively to processes occurring within the vortex by 31 December 1998 (a PV tracer included in the model allows the location of the ozone loss to be tracked). As the major dynamical disturbance perturbs the lower stratosphere, the high loss areas (essentially still high PV areas) are interleaved with areas of low loss from middle and low latitudes. We may expect significant mixing to occur. Our studies will aim to quantify the mixing process and the impact of polar chemistry on middle latitudes.
John Pyle
ACMSU, University of Cambridge
John.Pyle@atm.ch.cam.ac.uk
