The Arctic Summer Cloud-Ocean Study (ASCOS): overview and experimental design

Abstract

The climate in the Arctic is changing faster than anywhere else on Earth. Poorly understood feedback processes relating to Arctic clouds and aerosol-cloud interactions contribute to a poor understanding of the present changes in the Arctic climate system, and also to a large spread in projections of future climate in the Arctic. The problem is exacerbated by the paucity of research-quality observations in the central Arctic. Improved formulations in climate models require such observations, which can only come from measurements in-situ in this difficult to reach region with logistically demanding environmental conditions. <br><br> The Arctic Summer Cloud-Ocean Study (ASCOS) was the most extensive central Arctic Ocean expedition with an atmospheric focus during the International Polar Year (IPY) 2007–2008. ASCOS focused on the study of the formation and life cycle of low-level Arctic clouds. ASCOS departed from Longyearbyen on Svalbard on 2 August and returned on 9 September 2008. In transit into and out of the pack ice, four short research stations were undertaken in the Fram Strait; two in open water and two in the marginal ice zone. After traversing the pack-ice northward an ice camp was set up on 12 August at 87&deg;21' N 01&deg;29' W and remained in operation through 1 September, drifting with the ice. During this time extensive measurements were taken of atmospheric gas and particle chemistry and physics, mesoscale and boundary-layer meteorology, marine biology and chemistry, and upper ocean physics. <br><br> ASCOS provides a unique interdisciplinary data set for development and testing of new hypotheses on cloud processes, their interactions with the sea ice and ocean and associated physical, chemical, and biological processes and interactions. For example, the first ever quantitative observation of bubbles in Arctic leads, combined with the unique discovery of marine organic material, polymer gels with an origin in the ocean, inside cloud droplets suggest the possibility of primary marine organically derived cloud condensation nuclei in Arctic stratocumulus clouds. Direct observations of surface fluxes of aerosols could, however, not explain observed variability in aerosol concentrations and the balance between local and remote aerosols sources remains open. Lack of CCN was at times a controlling factor in low-level cloud formation, and hence for the impact of clouds on the surface energy budget. ASCOS provided detailed measurements of the surface energy balance from late summer melt into the initial autumn freeze-up, and documented the effects of clouds and storms on the surface energy balance during this transition. In addition to such process-level studies, the unique, independent ASCOS data set can and is being used for validation of satellite retrievals, operational models, and reanalysis data sets

Vertical profiling of aerosol particles and trace gases over the central Arctic Ocean during summer

“…Subsequently, Leck and Bigg (1999) used the PBL structure and the vertical distribution of particles during nucleation events to show that, if the newly formed particles did come from above, they had to come from a level less than 140 m above the surface, and not from the free troposphere. Particles with sizes below 50 nm in diameter during four Arctic icebreaker expeditions, in the summers of 1991 , 1996 , 2001 Tjernström et al, 2004) and 2008 (Paatero et al, 2009;Tjernström et al, 2013), have been shown to be produced by in situ fine-particle sources over the high Arctic…”

Size-resolved atmospheric particulate polysaccharides in the high summer Arctic

“…The quality of the samples was additionally monitored using various combustion tracers and wind speed -and direction -thresholds: provided that the wind was within ±70 • of the direction of the bow and stronger than 2 m s −1 , no ship pollution reached the sample inlets. Further details of the location and properties of air intakes and instruments, position on the ship, pumping arrangements and precautions to exclude contaminated periods can be found in Leck et al (2001) and in Tjernström et al (2013).…”

“…The Sun was continuously above the horizon of the expedition. For further cruise details see Paatero et al (2009) and Tjernström et al (2013).…”

Near-surface meteorology during the Arctic Summer Cloud Ocean Study (ASCOS): evaluation of reanalyses and global climate models

“…During August of 2008, the Swedish ice breaker Oden served as a drifting base camp for the Arctic Summer Cloud Ocean Study (ASCOS, Tjernström et al, 2013). The vessel headed north from Svalbard with a goal to spend as much time as possible in the central Arctic ice pack.…”

“…In the current work, we evaluate some of the tools described above using measurements obtained during the Arctic Summer Cloud Ocean Study (ASCOS, Tjernström et al, 2013). The ASCOS data set provides us with a unique opportunity to assess the performance of models at high latitudes.…”