Seasonal evolution of Ku- and Ka-band backscattering horizon over snow on first-year and multiyear sea ice

Arctic sea ice area has been mapped for nearly four decades using the long-term data record provided by successive passive microwave satellite missions; showing an accelerated pace of ice loss since 1979. Less is known about how much the ice has also thinned, in part because of the lack of a similarly long-term and consistent data record on sea ice thickness. Radar altimeters, such as the one flown on the European Space Agency (ESA)'s CryoSat-2 (CS2) since April 2010, and the SARAL/AltiKa satellite, launched in February 2013 as part of a joint mission by the Centre National d'Etudes Spatiales (CNES) and the Indian Space Research Organization (ISRO), are now providing pan-Arctic (or up to 81.5N for AltiKa) thickness observations. However, one key uncertainty in using these data is how far the radar actually penetrates into the overlying snow cover. The general assumption has been that the radar return is from the snow-ice interface at Ku-band (CS2) frequencies, and from the snow-air interface at Ka-band (AltiKa) frequencies. Using this information together with assumptions on the depth of the overlying snow pack and its density, scientists can then convert the radar returns into total ice thickness assuming hydrostatic equilibrium. However, field evidence has put this general assumption into question, even for a homogeneous snowpack. A further complication is the lack of knowledge on how deep the snow pack is and its density. Typically, snow depth and density information based on a climatology constructed over thick multiyear ice in the 1980s have been used. However, as the total area in the sea ice cover has declined, there is now a larger proportion of first-year sea ice in the Arctic Basin. Snow over first-year ice tends to be more saline than over multiyear ice, and as such it has the potential for a significant impact on the radar returns. In addition, autumn and winter freeze-up has been delayed by several weeks to months in certain regions of the Arctic, shortening the duration for accumulation of snow. Given these current uncertainties, it is difficult to accurately assess how sea ice thickness is changing from year to year and over the long-term.

Because sea ice is an important indicator of climate change, plays a fundamental role in the Arctic energy and freshwater balance, and is a key component of the marine ecosystem, it is essential that we improve the accuracy of thickness retrievals from radar altimetry. This project aims to do just that by making ground-based observations of the radar penetration depth over a full annual cycle at both Ku- and Ka-band frequencies, from autumn freeze-up, through winter snow metamorphism and summer melt. This information, together with detailed snow pack characteristics, will allow us to assess how changes in snow accumulation, snow morphology and snow salinity impact Ku- and Ka-band penetration factors. The MOSAiC drifting station provides a unique opportunity, possibly the only opportunity, to obtain a benchmark dataset that involves coherent field, airborne and satellite data. Analysis of this information will enable scientists to better characterize how the physical properties of the snow pack (above different ice types) influence the penetration of Ka and Ku band radar. Importantly, we will be able to evaluate the seasonal evolution of the snow pack over first-year (sea ice greater than a few cm) and multiyear sea ice. MOSAiC additionally provides the opportunity for year-round observations of snow depth and density that will allow for assessment of the validity of climatological assumptions typically employed in thickness retrievals from radar altimetry and provide data for validation of snow depth products. These activities are essential in order to improve sea ice thickness retrievals from radar altimetry over the many ice and snow conditions found in the Arctic.

Arctic change presents opportunities, but it also presents regional and global challenges and risks. Although the UK is not an Arctic nation, it plays a proactive role on international Arctic bodies, such as our observer status in the Arctic Council. Over the past 12 months policy makers, businesses leaders and philanthropists have all been involved in meetings dedicated to the Arctic. A good example is the White House Science Ministerial which brought together leaders from around the world, with the aim to expand joint collaborations focused on Arctic science.

To ensure our programme's impact goes beyond catchy headlines we will work in close contact with our Project Partners, MOSAiC and with the UK Arctic Office, to ensure the widest dissemination, and the involvement of a broad range of stakeholders. All data sets will be made freely available on a project website, with targeted adverts to key listservs (e.g. CRYOLIST). We will implement a range of activities with the aim of achieving the greatest potential for knowledge dissemination and socio-economic impact. We list some examples below.

Press office network: The press offices at BAS and UCL have a wide expertise in communicating with the media and public, as well as assisting and training staff in doing so. In order to attain a broader communication strategy, we will build on our established working relationship between our institutions. By doing so we are able to keep our press offices fully abreast of our project, and to facilitate communication to a wider audience, as well to co-ordinate 'on message' media activities. When appropriate, we will actively pursue a reputable scientific journalist (BBC or similar) to participate and record our cruise and scientific discoveries.

Outreach: Wider interest groups include school children, students and the general public. We will engage with them via outreach channels available to UCL and BAS, UCL, including the press offices, websites and publications such as NERC's Planet Earth magazine, and presentations to the local community. Our cruise blogs will be of particular interest, as will our real time imagery from the ship. We will use the knowledge gained within our project to develop new and innovative outreach activities to educate and enthuse a broad range of audiences about the importance of the Arctic in the climate system.

Website: The UCL CPOM website is the central point for gathering, redistributing, disseminating and enhancing project information and provides information on different aspects of the project, its topics, education and data accessibility. Additional impact will be achieved through, ship blogs, real-time data feeds from our time on MOSAiC, a Press Room for journalists, and more.

Training and Education: Training and education within our programme will mostly be delivered through a post-graduate post, who will spend time at both UCL and BAS. The purpose of the exchange is to encourage the post-doc to interact with different UK institutions and establish lifelong collaborations.

Expert workshop: In the later stages of our project we will run a 2-day international impact workshop on radar altimetry and sea ice thickness retrievals. Beside our international invitees we will provide dedicated space for key UK and MOSAiC groups. This workshop will consolidate our current understanding and include the new insight from this project, as well as to identify the pathways to incorporating the knowledge gained into more accurate sea ice thickness estimates.