Modelling of Summertime Arctic Stratus

The Arctic is a region of exceptional sensitivity to climate change: the observed rate of temperature increase is twice that of the rest of the world, and there is strong evidence for thinning and retreat of Arctic ice pack ice, which reached a record minimum during 2007. Although models agree that the Arctic will continue to have a strong response to increasing greenhouse gas concentrations, they also show much greater variability between models here than elsewhere, and fail even to reproduce current conditions adequately. The response of the Arctic to continued climate change is thus extremely uncertain. Model performance in the Arctic is poor in large part because the parameterizations used are derived from observations in mid-latitudes or the tropics; there are very few measurements within the Arctic, with the result that our understanding of the processes controlling local conditions: cloud dynamics and microphysics, radiative forcing, surface exchange, etc, is poor. This PhD project will study the dynamical and microphysical processes in summertime Arctic stratus clouds using the Met Office Large Eddy Model and a single column version of the Unified Model. The aims are to understand the interactions between the cloud dynamics and microphysics, radiative forcing, and surface coupling, with the ultimate objective of improving the representation of Arctic boundary layer clouds in climate models. The project will assess 1) how the dynamics of Arctic stratus differ from typical marine stratocumulus, 2) what the role of entrainment is in maintaining the cloud in the unique humidity structure of the Arctic lower atmosphere, 3) how the cloud responds to different sources of aerosol: from the surface or entrained from above, since it is currently unknown where the CCN required to maintain Arctic stratus derive from. Such studies are of paramount importance since Arctic stratus is the dominant controlling factor for the surface energy budget, and differ substantially from their mid-latitude counterparts; failure to adequately represent these clouds in climate models makes it impossible to properly simulate Arctic climate. The project is directly linked to a major International Polar Year (IPY) field study taking place in the central Arctic in summer 2008: the Arctic Summer Cloud-Ocean Study (ASCOS). The ASCOS programme has been organised to address the issues above from an observational stance, and will provide an extensive data set for this project to draw upon. Measurements of direct relevance to this study include: aerosol physics and chemistry, boundary layer mean and turbulence structure from the surface through cloud top, surface energy budget, radiative fluxes, and cloud properties - both remotely sensed by cloud radar, lidar, microwave radiometers, etc, and in-situ measurements from a NASA P3 research aircraft. The ASCOS observations provide both the basis for establishing initial conditions for the modelling studies, and a means of assessing the fidelity of the simulations. The modelling study will in turn enhance the observational study by allowing controlled tests of the relative importance of the various factors controlling the cloud properties. The link to ASCOS and the IPY make the project particularly timely, provides access to a unique data set, and adds significant value to current NERC funded research. This link also provides exceptional training opportunities through interaction with the many international collaborators, involvement in ASCOS and IPY workshops and international meetings on polar science following the IPY. The primary supervisor (Brooks) is PI on the UK contribution to ASCOS, and a member of the programme steering group. The supervision team has extensive experience of analysing observational data, integrating it into models, and undertaking detailed process studies; Met Office collaboration results from their strong interest in resolving the issue of Arctic stratus in the UM.