The summer 2008 central Arctic weather conditions as observed during ASCOS

This paper provides an overview of ASCOS, as a background to several other more detailed ASCOS presentations submitted to this conference. ASCOS is an interdisciplinary and international study of summer clouds in the central Arctic Ocean. The goal is to understand the life cycle of low-level Arctic clouds, from the formation of aerosols and cloud condensation nuclei (CCN) to the cloud formation and its effect on the structure of the Arctic summer boundary layer and the energy balance at the surface. An overarching goal is to provide information to improve cloud formulations in current global climate models. A specific aim is to investigate if marine biological processes in the open leads in the ice contribute to the formation of CCN and thus affect the optical processes of the clouds. ASCOS also provides data for evaluation of weather forecast and climate models for the Arctic.

To achieve these goals we brought together a science team with experts in marine biogeochemistry and physical oceanography, atmospheric gas-phase and aerosol chemistry and physics, and in meteorology. The field phase of ASCOS took place on the Swedish icebreaker Oden during 40 days in August and early September, 2008; 33 scientists from 11 countries participated in this endeavor. The target time period included the late summer melt period and the transition to the autumn freeze-up. During an almost three-week ice drift around 87°N and 5°W we observed a column extending from a few hundred meters into the ocean, through the ice and up to the top of the troposphere; several measurements were also conducted in transit.

The observation program was achieved through a combination of in-situ and surface-based remote-sensing instruments. Marine biology and chemistry was observed at a remote (~3 km away) open water station to minimize contamination from the ship, while upper ocean physical measurements was performed through the ice at a field camp on the ice deployed closer to Oden. Here eddy-correlation and mean-profile micrometeorological measurements up to 30 meters, sodar observations, surface-radiation and ice-temperature measurements, and tethered soundings through the lowest 500 meters of the atmosphere were also performed. Onboard the ship a complete aerosol and atmospheric chemistry lab was run with a sampling inlet at about ~25 meters over the surface; the helicopter was also used for some aerosol and chemistry sampling, weather permitting. Several microwave radiometers, two cloud radars (Ka- and S-band), a 449 MHz wind profiler and a micro-pulsed lidar were also operated onboard, as well as two weather stations, including visibility and cloud ceilometer instruments, while 6-hourly soundings were released from the ships helipad; a few soundings with additional instruments for ozone and radioactivity.