The main objective of the Action was to advance understanding of the state of the global upper troposphere and lower stratosphere (UTLS), and the role of the dynamic, chemical, and radiative processes in this altitude region, in order to provide an improved basis for policy advice in connection with global change. Action 723 provided value-added quality-controlled datasets of geophysical parameters (e.g. ozone and water vapour) which are important for the study of radiative, dynamic, and photochemical processes in the tropopause altitude region (5km above and below the tropopause). Hitherto, there has been little information on these geological parameters in this region. The Action contributed to ward making the best use of observations, models and assimilation algorithms, and towards the definition of new strategies for future research. Assessment of UTLS measurement capabilities showed that ozone is reasonably well covered by existing databases (NDSC, GAW and WOUDC). The most prominent open issue on the global scale was found to be the inconsistency and uncertain quality of different humidity data sets. The Action assessed the quality of the different datasets by intercomparison studies, including direct in situ comparisons, as well as comparisons using satellite instruments as a transfer standard. To improve the quality of radiosondes, a measurement campaign (LAUTLOS) was carried out. Results of the campaign were discussed at a dedicated expert meeting. The opportunity offered by the EOS Aura satellite as a transfer standard was used to compare measurements of the different existing or emerging lidars.
For an assimilated Ozone and Humidity Dataset three approaches were identified: 1) assimilation into a NWP model based on a General Circulation Model (GCM), 2) assimilation into a CTM and 3) assimilation into a coupled GCM/CTM model. The Action organized a workshop on cirrus clouds. The Action is producing two journal special issues: Atmospheric Chemistry Physics/Atmospheric Chemistry Physics (ACPD/ACP), Quarterly Journal of the Royal Meteorological Society (QJRMS). In addition, a proposal for a follow-up COST Action entitled ”Atmospheric Water Vapour in the Climate System was build” has been successful.
