Atmospheric ice nuclei in the Arctic

The Arctic climate is changing twice as fast as the global average and these dramatic changes are evident in the decreases in sea ice extent over the last few decades. The lowest sea ice cover to date was recorded in 2007 and recent data suggests sea ice cover this year may be even lower. Clouds play a major role in the Arctic climate and therefore influence the extent of sea ice, but our understanding of these clouds is very poor. Low level, visually thick, clouds in much of the world tend to have a cooling effect, because they reflect sunlight back into space that would otherwise be absorbed at the surface. However, in the Arctic this albedo effect is not as important because the surface, often being covered in snow and ice, is already highly reflective and Arctic clouds therefore tend to warm instead of cooling. Warming in the Arctic can, in turn, lead to sea ice break-up which exposes dark underlying sea water. The sea water absorbs more of the sun's energy, thus amplifying the original warming. Hence, small changes in cloud properties or coverage can lead to dramatic changes in the Arctic climate; this is where the proposed research project comes in.

A large portion of clouds, including those found in the Arctic region, are categorized as mixed phase clouds. This means they contain both supercooled water droplets and ice crystals (for a demonstration of supercooled water see: http://www.youtube.com/watch?v=0JtBZGXd5zo). Liquid cloud droplets can exist in a supercooled state well below zero degrees centigrade without freezing. Freezing will, however, be observed if the droplets contain a particle known as an ice nucleus that can catalyze ice formation and growth. Ice formation dramatically alters a cloud's properties and therefore its influence on climate. At lower latitudes, ice nuclei are typically made up of desert dusts, soot or even bacteria. But the composition and source of ice nuclei in the Arctic environment remains a mystery.

A likely source of ice nuclei in the Arctic is the ocean. Particles emitted at the sea surface, through the action of waves breaking and bubble bursting, may serve as ice nuclei when they are lofted into the atmosphere and are incorporated in cloud droplets. This source of ice nuclei has not yet been quantified. We will be the first to make measurements of ice nuclei in the central Arctic region. We will make measurements of ice nuclei in the surface layers of the sea from a research ship as well as measuring airborne ice nuclei from the BAe-146 research aircraft.

The sea's surface contains a wide range of bacteria, viruses, plankton and other materials which are ejected into the atmosphere and may cause ice to form. We will use state-of-the-art equipment developed at Leeds to measure how well sea-derived particles and particles sampled in the atmosphere nucleate ice. We will piggy back on a NERC funded project called ACACCIA, which not only represents excellent value for money (since the ship and aircraft are already paid for under ACCACIA), but is a unique opportunity to access this remote region.

Results from the proposed study will build upon previous work performed in the Murray laboratory and generate quantitative results that can be directly used to improve computer-based cloud, aerosol and climate models. Our results will further our understanding of these mysterious and important mixed phase clouds and, in turn, the global climate.

Impact summary

The following groups will benefit from our proposed work:

1) Asymptote Ltd (www.asymptote.co.uk). We have strong links with Dr John Morris (project partner) of Asymptote Ltd, a Cambridge based company. This company specialises in technology for crystal engineering and has diverse interests. These include cryopreservation, freeze drying of valuable proteins, ice formation in jet fuel and hydrate formation in gas pipelines.

Asymptote's immediate role in the current project is that they are loaning us a Stirling cooler and associated software which is worth £15 K. In addition, embedding Asymptote in the project means any knowledge with commercial value will be identified and capitalised on.

Murray recently published a paper in collaboration with Morris on supercooling and freezing of water in Jet fuel (Murray et al., 2011). In this project we utilised the LECC instrument which was constructed for Murray's NERC fellowship on ice cloud formation. This collaboration was established as part of an investigation surrounding the crash-landing of a passenger jet at Heathrow in 2008. In addition to a paper in the journal Fuel, we hosted a meeting in Leeds with Airbus, who have initiated a follow-up project on this subject. This work has been included as a case study on NERC's Science Impact Database (SID: http://sid.nerc.ac.uk/details.aspx?id=337&cookieConsent=A).

We have also recently jointly published an article on the cryopreservation of sperm cells (Morris et al., 2012). Murray employed atmospheric aerosol particle freezing models to the case of sperm cells in order to improve the strategy for preservation of valuable biological samples.

These examples show how our work not only contribute to further understanding in atmospheric science, but can be used to help resolve technical and engineering problems in other fields as well. Our collaboration with Asymptote Ltd allows us to realise this potential.

2) The Intergovernmental Panel on Climate Change (IPCC) and policy makers. Regular assessments from the IPCC have a direct impact on policy makers and therefore our society. It is of the utmost importance that the data used in models, on which the IPCC assessments are based, is of the highest quality. Our study will contribute to improving the data which underpins these models. This will be achieved, in part, through links to ACCACIA. Results from the proposed study will underpin global cloud and aerosol modelling work. This will lead to improved assessments of climate change and better-informed policy makers.

3) The weather forecasting community will benefit. We have direct links to the Met Office through our Academic Partnership and specifically with Dr Ben Shipway who is working on Arctic mixed phase clouds (see letter of support from Shipway). In addition, the Met Office plans to incorporate the knowledge gained though ACCACIA in its Unified Model. We will make our data available to all ACCACIA stakeholders including the Met Office. The Arctic may seem remote, but it directly influences UK weather through advection of Arctic air masses, and indirectly influences local weather through changes in the tracks of North Atlantic storms associated with changes to surface pressure fields in a warmer Arctic. Hence, greater understanding of the microphysical processes and feedbacks in the Arctic will lead to improved performance of numerical weather prediction for mid-to-high latitude regions (including the UK).

See Pathways to impact for more details how these impacts will be realised.

References

Morris, G. J., Acton, E., Murray, B. J., and Fonseca, F.: Freezing injury: The special case of the sperm cell, Cryobiology, 64, 71-80, 10.1016/j.cryobiol.2011.12.002, 2012.

Murray, B. J., Broadley, S. L., and Morris, G. J.: Supercooling of water droplets in jet aviation fuel Fuel, 90, 433-435, doi:410.1016/j.fuel.2010.1008.1018 2011.