Interactions between the atmosphere and the surface of the planet are mediated by turbulent fluxes – chaotic mixing that transports momentum, heat, moisture, and trace gases between the two. Turbulent mixing spans scales from millimetres to hundreds of meters – much smaller than the grid scale of numerical models; such sub-grid-scale processes must be parameterized (or represented) in terms of simpler model variables such as mean wind speed, air temperature, humidity, etc. These parameterizations are developed by making direct measurements of the fluxes themselves (a very difficult and expensive undertaking) and then determining their empirical relationships with the simpler (and more easily measured) variables such as wind speed, temperatures etc.
This proposal aims to address long-standing issues with the parameterization of turbulent fluxes over sea ice. The remote location and harsh conditions of both Arctic and Antarctic sea ice means that very few direct measurements of the fluxes have ever been made, and models must rely on parameterizations developed at lower latitudes, e.g. over ice-free areas of the oceans. The very different conditions that occur over sea ice – a high degree of spatial variability, strong temperature contrasts between the ice and open water in leads in the ice, and strongly stable atmospheric conditions in winter – mean that the parameterizations developed at lower latitudes are often not appropriate, and models tend to do a poor job of representing the surface fluxes. The current generation of models fails to represent the mean changes in sea ice extent compared to satellite observations over the last 35 years or so, and produce often wildly inaccurate seasonal forecasts of ice extent even just a few months in advance. The growth and melt of sea ice is controlled by the surface energy budget, and the turbulent fluxes between the ice and the atmosphere are a critical component of that budget. The solar and terrestrial radiation fluxes dominate the budget, but the turbulent fluxes control the atmospheric boundary-layer structure, and influence the development of boundary-layer clouds which are the dominant control on the radiation fluxes. So all of these fluxes are inter-linked and consequently a failure to properly represent the air-ice turbulent fluxes has a knock-on influence on the surface radiation balance through their impact on clouds. An accurate representation of turbulent fluxes is thus essential for accurate predictions of weather, sea ice and the climate system. On short timescales the recent reduction of Arctic sea ice, and the accompanying increase in commercial activity in the Arctic (shipping, tourism, petrochemical extraction, etc) means that there is an urgent need for accurate operational forecasts of weather, sea ice and other environmental factors on timescales from days to seasons. Delivering these will require a much improved representation of the surface exchange processes that control the atmospheric boundary layer and properties of clouds within it, and contribute to the surface energy budget, and hence ice melt/freeze, and ice drift. Significant progress has been made over the last 5 years in developing theoretical models of the physical processes that control the surface fluxes, such as form drag at ice edges, ridges, melt ponds, and ice/water temperature contrasts. However there is a need for in situ measurements to test these parameterizations and to evaluate their performance. This project will utilise a very extensive set of surface flux and sea-ice measurements made during two recent (2014 and 2016) cruises in the Arctic Ocean, totalling 18 weeks, to develop state-of-the-art parameterizations for momentum, heat, and water vapour that are tuned to real-world conditions. We will implement these parameterizations within the Met Office Unified Model, and evaluate their impact on the atmosphere, and on the climate system, over a range of timescales.