In Situ Measurements of Surface Turbulent Exchange Over Arctic Sea Ice: Results from the ACSE Campaign

The turbulent exchange of momentum, heat, and water vapour between the atmosphere and ocean surface is a process which is poorly represented in weather and climate models. This is particularly the case in polar regions where the presence of sea ice alters surface roughness and thus modulates the strength of exchange. To capture these effects in models, surface exchange must be explicitly linked to surface conditions; this can be achieved through parameterisation of the transfer coefficients in terms of variables that characterise the surface. However, development of such parameterisations relies on the availability of high quality in situ measurements, which are both challenging and expensive to make.

Here, we present ship-based direct eddy covariance estimates of surface momentum and heat exchange over the Arctic Ocean obtained during the three-month Arctic Clouds in Summer Experiment (ACSE) in summer 2014. The extensive data set spans a range of surface conditions (both ice-free and ice-covered), with a large fraction of the measurements made within the Marginal Ice Zone (MIZ) during both melting and freezing periods. The 10 m neutral transfer coefficients are calculated from direct eddy covariance flux measurements and mean winds corrected for ship-induced flow distortion. We discuss the dependencies of the transfer coefficients on the surface conditions, which are characterised both qualitatively and quantitatively through analysis of shipborne digital imagery and use of infrared temperature sensors. In addition, satellite-based estimates of sea ice concentration from various platforms are matched with our in situ measurements, and the dependence of the transfer coefficients on these estimates is discussed. Our results are compared to recently published results and parameterisations. Finally, we comment on the challenge of making such measurements from an icebreaker, paying particular attention to flow distortion effects.