ACA (clouds & aerosols)

In 2010 SCAR sponsored a workshop on Antarctic clouds hosted by Byrd Polar Centre which proposed an International programme to coordinate the investigation of Antarctic clouds. It was suggested that this programme would have a series of special observing periods in Antarctic when both intensive ground based measurements would be made at the same time as in-situ measurements would be made with instrumented aircraft. It was hoped that such a programme would also include both regional and global modelling to help understand the observations and act as a test bed for new parameterisations of clouds.

As a first step to formation of an international programme the SCAR Action Group on Antarctic Clouds and Aerosols (ACA) was set up. This group’s long term goal is to organise an international large scale campaign to investigate clouds and aerosols in Antarctica.

The IPCC has identified the indirect aerosol effect as the biggest uncertainty in the Earth’s climate system. For this reason in many regions worldwide great efforts are being made to measure aerosols and cloud microphysical properties and the associated effect on the climate.

However, in the Antarctic very few measurements have been made of cloud microphysical properties. Climate model representations of the Antarctic climate rely on parameterisations of clouds and cloud properties that based on mid latitude observations and these are likely to be wrong -for example the number nuclei available for the formation of cloud particles will be very different in the pristine air over the Antarctic.

Also the source of many of the cloud nuclei found at high southern latitudes may be either the exposed rock or biological activity in the seas surrounding Antarctica. Climate change will affect both these sources and so they may act as strong climate feedback. Therefore we must understand the process that may be important for the formation of aerosols and Antarctic clouds if we are to successfully model climate change both regionally in Antarctica and globally.

Terms of Reference

The Antarctic Clouds and Aerosols (ACA) Action Group was set up to have the following objectives:

1. Encourage the development of a climatology of Cloud Microphysical properties over both the Antarctic Coastal area and over the Antarctic plateau. It hoped that this climatology will be the product of international cooperation, using ground based observation, in-situ measurements and satellite remote sensing.
2. Identification of the sources of cloud nuclei that are important for the formation of Antarctic clouds.
3. The development of new improved parameterisations of Antarctic clouds for inclusion within regional and climate models.