Clouds constitute one of the major uncertainties in understanding the climate system and changes in the clouds as a consequence of global climate change is not well constrained by observations. This is particularly true in the Arctic, where clouds constitute the larges single factor affecting the surface energy balance, and therefore on melting and freezing of sea ice.

ASCOS is a highly interdisciplinary project with a major field experiment in the central Arctic Ocean during August/September 2008, approximately at 87N and 7W, deployed on the Swedish icebreaker Oden as a part of the International Polar Year (IPY). The ASCOS main target is to study the formation and life cycle of Arctic summer low-level clouds. To achieve this we deployed instruments for process level observations in a column from 0.5 km in to the ocean, through the ocean/ice surface up through the atmospheric boundary layer, and to the top of the troposphere (also see http://www.ascos.se). ASCOS measurements range from in-situ observations, to surface-based remote sensing, to airborne observations. The most intense observations were during a 3-week ice drift, starting with typical Arctic summer melt conditions and ending with the initial freeze-up of autumn. ASCOS was also coordinated with the Arctic Mechanisms of Interaction between the Surface and Atmosphere (AMISA) project, providing airborne measurements from the NASA DC8 research aircraft in the vicinity of the ASCOS column, flying in from Kiruna, Sweden.

The science team on Oden consisted of 33 researchers from 14 institutes in 11 different countries; many more are involved in analysis and associated modelling studies. This, and the experimental set-up, makes ASCOS the most extensive atmosphere-oriented experiment in the central Arctic for the entire IPY. ASCOS science cuts across several disciplines, with links to microbial life in ocean and ice, atmospheric chemistry and physics, cloud microphysics, turbulent exchange at the surfaces above and below the ice, and atmospheric circulation. A large part of ASCOS (atmospheric gas and particulate chemistry, aerosol physics, boundary-layer and synoptic meteorology) fall within the remit of ACP while physical oceanography and marine biology/chemistry fall within the remit of OS which is the incentive for a joint ACP/OS issue; only this way will it become possible to develop a special issue spanning the whole width of the science in ASCOS.