Aerosol-Cloud Coupling And Climate Interactions in the Arctic

The climate of the Arctic is changing faster than that almost anywhere else on Earth, warming at a rate of twice the global average. This warming is accompanied by a rapid melting of the sea ice - 2007 saw a record minimum in summer ice extent, and the years since have seen the 2nd and 3rd lowest summer ice extents on record - and a thinning of the ice that remains from year to year. The strong warming in the Arctic is due to several positive feedback processes, including a sea-ice albedo feedback (warmer conditions melt ice, lowering the average reflectivity of the mixed ice/ocean surface and thus absorbing more solar radiation, leading to increased ice melt and further lowering of the albedo) and several cloud feedbacks. Over most of the globe low clouds act to cool the surface since they reflect sunlight; over the arctic the highly reflective ice surface reduces the significance of cloud reflectivity, and the absorption of infrared radiation by cloud water droplets becomes the dominant effect - this acts to trap heat below cloud, warming the surface.
Although climate models generally show a strong greenhouse warming effect in the Arctic, they also disagree with each other more in the Arctic than anywhere else, producing a wider range of possible future climate conditions. The models also tend not to be able to reproduce current Arctic climate conditions very accurately. This large uncertainty in models of the Arctic climate results primarily from poor representation of physical processes within the models, and some unique and particularly challenging conditions. The largest single source of uncertainty is the representation of clouds. The models use simple representations of cloud properties that were developed from observations in mid latitude or tropical cloud systems - very different conditions from those that exist in the Arctic.

This project will make airborne in situ measurements of cloud microphysical properties, the vertical structure of the boundary layer and aerosol properties, and the fluxes of solar and infra red radiation above, below, and within cloud. It will also measure the production rates and properties of aerosol at the surface and their variability with season and extent of sea ice cover. These measurements will be used, along with a range of numerical models of aerosol and cloud processes, and atmospheric dynamics to evaluate the interactions between sea ice extent, aerosol production and cloud properties. New and improved descriptions of these processes suitable for use within climate models will be developed, tested, and implemented within the MetOffice climate model HadGEM. The ability of the current MetOffice models to reproduce the observed Arctic cloud and boundary layer properties will be tested, and the impact of the new parameterization schemes evaluated.
Finally we will undertake a series of climate simulations to examine how future climate will evolve, and the feedbacks between warming of the Arctic, melting of sea ice, production of aerosol, and the properties of clouds evaluated.

This study has potentially wide impacts throughout the climate modelling community, both within academia and government agencies. Our collaboration with the Met Office guarantees both that all our results will be available directly to the Met Office and Hadley Centre, and that the necessary expertise with the UM is available to this study. A primary goal of the project is the development and implementation of new/improved parameterizations of aerosol, cloud, and boundary layer processes within the UM.

Improvements to parameterization schemes for boundary layer turbulent processes, low level cloud representation, and cloud radiative properties for large scale models will:

- Improve fidelity of climate predictions - this is essential if an accurate assessment of future climate change is to be achieved. This need is particularly pressing for the Arctic regions due to the rapid rate of observed change, and the expected continuation (perhaps acceleration) of this change, but also impacts on predicted climate for the rest of the world.

- Improved performance of numerical weather prediction for mid-to-high latitude regions (including the UK). The Arctic can seem remote, but there are direct influences on UK weather through advection of Arctic air masses, and indirect influence through changes in the tracks of North Atlantic storms associated with changes to surface pressure field in a warmer Arctic.

Improvements to predictive capabilities in the Arctic have impacts for climate prediction around the world. Reducing uncertainty in climate prediction is essential if policy makers are to be able to implement effective plans for limiting the human impacts of climate change.