ASCOS Analysis - surface-cloud coupling in the arctic boundary layer

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The Arctic is a region of extreme sensitivity to climate change. Observations show its temperature to be increasing at twice the rate of the rest of the world. Models suggest this strong response will continue; however they also show a greater uncertainty here than for anywhere else in the world. The combination of rapid change and high uncertainty make improving predictive capability in the Arctic a matter of urgency. The strong climate response to increasing climate forcing by greenhouse gases is believed to be a result of several important feedback processes; for example the ice-albedo feedback, and several cloud-related feedbacks. Arctic stratus cloud is both extensive and long lived during the summer months. Their impact on the surface radiative budget, and hence on total energy budget, differs from that elsewhere in the world: sea ice and low clouds have very similar albedo, so that over ice the cloud has little impact on the solar radiation budget at the surface, and longwave (infra red) processes tend to dominate. Unlike anywhere else in the world, low cloud acts to warm the surface rather than cool it. The radiative properties of the clouds themselves also differ from lower latitudes because of different droplet size distributions. The Arctic has the lowest aerosol concentrations of anywhere on earth; a result of high deposition due to extensive cloud and fog, the great distance from strong continental aerosol sources, and the sea ice, which minimises marine sources. Aerosol provide the nuclei upon which cloud droplets form, so the low numbers mean the clouds have a smaller number of drops, which are consequently larger than typically found in mid-latitude stratus. The different drop-size distribution means Arctic stratus has different radiative properties than mid-latitude stratus: one of the reasons models represent their effects poorly. Small changes to cloud properties, whether from changes in aerosol availability, thermodynamic structure, or turbulent processes, can have a significant impact on their radiative properties. Small-scale processes must be parameterised in a simple form within climate models; these parameterisations must be based on measurement. Most measurements used have been from mid-latitudes or the tropics; thus the parameterizations derived from them are not necessarily appropriate for Arctic conditions. The difficulty and expense of making measurements in the Arctic has meant there are very few available against which to test the existing parameterisations or with which to develop new, more appropriate ones. The ASCOS field campaign achieved one of the most extensive and wide-ranging sets of measurements ever made in the central Arctic, and is intended to address the questions of what controls the properties of Arctic cloud. This proposal will use the unique ASCOS data set to study the interactions between the surface and the cloud that control the exchange of heat, water, and aerosol; and also the exchanges across cloud top with air from the free troposphere. It will study both the fundamental processes and examine how well they are represented within climate models, identify where parameterisations are failing, and propose alternative approaches more appropriate to the Arctic. The study will draw upon detailed in-situ and remote sensing measurements of lower-atmosphere structure, turbulent mixing, aerosol properties, cloud properties, and radiative fluxes to study the fundamental processes. These will be combined with large eddy simulation modelling to provide deeper insight into the 3-dimensional interactions, and allow perturbation experiments to be undertaken to study the relative importance of different parameters. Finally, the Met Office Unified Model will be used to study how parameterisations of these processes perform, and to discover weaknesses or failings within them. Ultimately this work will lead to improved parameterizations and more accurate predictions of future climate.