The Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) initiative is a major international programme motivated by the rapid changes in Arctic climate observed over the last few decades. This is driven by an accelerated rise in the mean temperature of the Arctic; which is warming at 2-3 times the mean global rate. The most visible change is the dramatic reduction in sea ice extent, particularly of the summer minimum, which is decreasing at a rate of 13% per decade.
These rapid changes are the result of a combination of feedback processes – the best known is the ice albedo feedback, whereby the loss of ice exposes the land or sea surface beneath, lowering the area mean albedo and allowing more solar radiation to be absorbed, which warms the surface and enhances ice melt. Other feedbacks relate to the vertical profiles of atmospheric temperature and humidity, cloud properties, and large-scale atmospheric circulation. While climate models also show enhanced warming in the Arctic, they do not reproduce many of the observed details of the change; for example they do not reproduce the very rapid decline in the summer sea ice minimum observed over the last 10 years, and there are big differences between models. This has a significant impact on our ability to predict the future state of climate system. Poor model performance results from multiple leading-order deficiencies in their representation of physical processes in the Arctic system. MOSAiC aims to address these through a large-scale coordinated approach, making simultaneous measurements of the many interdependent processes relevant to climate over a full calendar year. This approach is necessary because of the strong linkages and feedbacks between different parts of the Arctic climate system and the strong seasonality in many processes. The observational campaign will take place on, and around, the icebreaker Polarstern, which will be frozen in at the edge of the pack ice at the end of the summer melt. This provides ready access to both multi-year ice within the pack and to freshly forming ice just outside it. Measurements will be made of all components of the surface energy budget on both the upper and lower sides of the ice, along with ice thickness, temperature, physical properties, topography, and deformation over time. The processes controlling the energy budget, including synoptic-scale forcing, cloud properties, turbulent mixing, and the interactions between them, will be studied in detail. The measurements will be complemented by an extensive modelling programme, spanning a full range from small scale process studies up to global climate modelling. The last study to attempt anything remotely comparable to MOSAiC was the Surface Heat Budget of the Arctic (SHEBA) project, which undertook a much more limited set of measurements over 11 months in 1997-1998. The area where SHEBA took place is now ice-free every summer, emphasising the changes that have taken place since then. This proposal is a core contribution to the atmospheric science component of MOSAiC. We will make detailed measurements of lower atmosphere mean and turbulent dynamics for the duration of the measurement campaign. Several remote sensing systems (Doppler sodar and lidar) will be deployed to make continuous measurements mean wind profiles throughout the boundary layer. Retrievals of the vertical turbulence structure will allow the complex interactions between clouds, the boundary layer, and the surface to be understood and thus the primary controls on the surface energy budget and ice melt and formation to be better represented in models. Such detailed measurements of the vertical dynamic structure of the boundary layer will be unique in the Arctic, no comparable data set exists. In addition to our science they underpin many aspects of MOSAiC science being undertaken by other groups, and are considered essential by the MOSAiC atmospheric science coordinators.