School of Earth and Environment

The Arctic Cloud Summer Experiment (ACSE)

A research team from ICAS is participating in a three month long research cruise to study Arctic clouds over the Arctic Ocean. Prof Ian Brooks and post-doctoral researcher Dominic Salisbury sailed on the Swedish icebreaker Oden in early July for the first leg of the cruise from Tromsø,  in northern Norway, to Barrow, Alaska. At the half-way point in late August Ian and Dom will swap places with Drs Barbara Brooks and John Prytherch, who will undertake the return leg, ending up back in Tromsø in mid-October.

Map of the Oden’s track since leaving Tromsø in early July up till the 28th July. The colour scale shows the surface temperature.
Map of the Oden’s track since leaving Tromsø in early July up till the 28th July. The colour scale shows the surface temperature.

The climate of the Arctic is changing rapidly, warming at about twice the global average rate. The dramatic reduction in the extent of summer sea ice over the last decade is the most visible manifestation of this warming. Climate model predictions are consistent with observed trends, but show much greater variability in the Arctic than elsewhere in the world. A large part of this uncertainty results from poor representation of Arctic clouds within the models. Low level clouds are the single most important factor controlling the surface energy budget over the Arctic Ocean, and hence the amount of heat entering or leaving the ocean surface waters and sea ice.

Oden at anchor off Tromsø before setting sail to Barrow.
Oden at anchor off Tromsø before setting sail to Barrow.
The Leeds Flux kit at the top of the foremast on Oden.
The Leeds Flux kit at the top of the foremast on Oden.

The ACSE project aims to improve our understanding of the properties of and processes controlling Arctic stratus clouds. ACSE is a collaboration between ICAS and groups at the Meteorological Institute, University of Stockholm and the University of Colorado/NOAA-CIRES, and follows a previous highly productive collaboration by the same team on the ASCOS project – also conducted on board the Oden in 2008. They have jointly installed a wide array of measurement systems on the Oden.

The Leeds instrumentation includes a turbulence flux system installed at the top of the foremast to measure the exchange of momentum, heat, water vapour; the addition of the CLASP aerosol spectrometer (designed within ICAS) allows aerosol fluxes to be measured. Infra red temperature sensors measure the surface temperature of the ice or sea surface, and several cameras record images of the surface from which local ice conditions can later be estimated. Several instruments from the NCAS Atmospheric Measurement Facility (AMF) are installed: a scanning microwave radiometer retrieves vertical profiles of temperature and humidity and a Doppler lidar provides wind profiles, information on boundary layer structure and turbulence dynamics into cloud, and through the depolarisation ratio provides information on cloud particle shape – distinguishing droplets and ice crystals. The lidar and microwave radiometer sit within motion stabilised platforms – designed and built within the school – to keep them horizontal as the ship pitches and rolls. An inertial motion unit records the residual 3-dimensional linear velocity in order to correct the Doppler velocity measurements for the ship motion. Other instrumentation includes a radar wind profiler (also on a motion stabilised platform) and W-band Doppler cloud radar from NOAA, and surface weather sensors and a second turbulent flux system from Stockholm.

Radiosondes (weather balloons) are launched every six hours throughout the three month cruise. Data from the radiosondes, retrieved using one of NCAS – AMF sounding stations, is sent to the UK Met Office immediately after the sounding finishes (when the balloon bursts at around 20-25km), and is then distributed world-wide for inclusion in the pool of observations used to initialise weather forecast models. The ACSE measurements thus provide and immediate benefit to weather forecasts for the Arctic region, including those we rely upon for planning.

The measurements will be used to study to what extent the properties of Arctic stratus respond to changes in surface conditions: local sea ice conditions, surface fluxes of heat and water vapour, and the thermodynamic structure of the lower atmosphere; and to the large scale synoptic meteorological conditions that control the origin of the air mass in which the cloud forms. A secondary objective is to study the impact of synoptic weather systems on sea ice conditions – the breakup of ice, and its movement, combining the field measurements with satellite retrievals of ice conditions.

The photo shows the location of a range of instruments mounted on the foredeck of Oden.
The photo shows the location of a range of instruments mounted on the foredeck of Oden.

ACSE is piggy-backed on a much larger oceanographic project – the Swedish-Russian-US Arctic Ocean Investigation of Carbon-Climate-Cryosphere Interactions (SWERUS-C3) – which is studying methane emissions from the sea bed and carbon cycling in the Arctic Ocean. The SWERUS activities include physical oceanography, acoustic measurements of sea bed topography, plumes of methane bubbles seeping from the sea bed, and geophysical structure of the sediments below the sea bed. Air and water chemistry and biochemistry measurements are made continuously from underway sampling systems, while at discrete station sampling of water chemistry at different depths is undertaken via Niskin bottle samples from CTD casts. Shallow sediment core samples are obtained for analysis of current sea-bed processes and deep cores, many metres long, provide information on climate conditions and carbon chemistry going back tens or hundreds of thousands of years. Although ACSE and SWERUS are largely independent projects the ICAS team is collaborating with a geochemistry group from Stockholm to make measurements of the turbulent flux of methane between the ocean and atmosphere. This provides an important estimate of how much of the methane observed seeping from the sea bed into the ocean makes it into the atmosphere.

The ACSE project has direct links with, and is complementary to, the Aerosol-Cloud Coupling And Climate Interaction in the Arctic (ACCACIA) project, a NERC Arctic Research Programme consortium also led by Prof Brooks. ACCACIA conducted two field campaigns in 2013 with both ship and aircraft based measurements in the Fram Strait and around Svalbard to study aerosol and cloud microphysical processes. While ACCACIA’s in-cloud measurements were, of necessity, snapshots of conditions over short periods, ACSE will provide continuous, though less detailed, information on cloud properties and dynamics over a long period, providing a more representative perspective over a wider range of conditions.

The ACSE cruise is the tenth research cruise undertaken by Prof Brooks’s group since 2006.

The LIDAR is sat within the motion stabilised platform on top of a shipping container. At the bottom right of the photo the weather-proof cover to the cloud-radar can be seen in the roof of the NOAA container.
The LIDAR is sat within the motion stabilised platform on top of a shipping container. At the bottom right of the photo the weather-proof cover to the  cloud-radar can be seen in the roof of the NOAA container.

The ICAS team are blogging about the cruise at http://frictionvelocity.wordpress.com, Barbara is also blogging through NCAS – AMF at http://ncasamf.wordpress.com, and the NOAA-CIRES team at http://ciresblogs.colorado.edu/icebreaker. Information and updates on the SWERUS-C3 project cruise can be found at: www.polar.se/en/expedition/swerus-c3 and http://swerus-c3.geo.su.se/. The current position of the Oden can be seen at http://www.smhi.se/klimatdata/2.1090 along with meteorological data from the ship. In spite of having no proper internet connection on the ship, Ian is also managing to tweet occasional cruise updates at @IanMBrooks while Barbara will be tweeting through @ncasamf. The SWERUS project tweets at @SWERUSC3.

 

Acknowledgements:

The ICAS contribution to ACSE is funded by NERC grant NE/K011820/1. The Oden ship-time costs are funded by the Swedish Polar Research Secretariat and the Knut & Alice Wallenberg Foundation, with additional support from the Bolin Centre for Climate Research at Stockholm University.