The concept of drag partitioning to parameterise the surface roughness of sea ice is validated using topography data of regions with high sea ice concentrations. The parameterised drag is compared to measurements obtained by aircraft and ship. The form drag can well be expressed as a function of mean ridge heights and spacings averaged over flight legs of 12 km, if an improved approximation for the coefficient of resistance of a single ridge is used. We find a good agreement between the parameterised and observed drag coefficients. The highest sea ice roughness was encountered close to coastal regions and the lowest in the central Arctic.

Abstract. The influence of a single pressure ridge of 4.5 m height on the structure of the atmospheric surface layer is studied. The field of the mean wind velocity is governed by typical features of a Bernoulli effect with a speedup over the crest and a shadowing effect downwind of the ridge. It is found that the turbulence generated by the ridge compensates for the deformation of the flow field by mixing momentum downward. Both mean and turbulent fields are restored to their upwind values at a distance of -• 300 m downwind of the ridge, which is equivalent to an aspect ratio of -• 0.015. The level of maximum turbulence generated by the ridge is characterized by a linear relationship. A formulation for the determination of the form drag of a single ridge is proposed and generalized toward an ensemble of ridges. We estimate that the form drag contributes < 50 % to the total drag exerted by a typically ice covered sea surface on the atmospheric flow.

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This chapter summarises mesoscale modelling studies, which were carried out during the ACSYS decade until 2005. They were aiming at the parameterisation and improved understanding of processes in the Arctic boundary layer over the open ocean and marginal sea ice zones and over the Greenland ice sheet. It is shown that progress has been achieved with the parameterization of fluxes in strong convective situations such as cold-air outbreaks and convection over leads. A first step was made towards the parameterization of the lead-induced turbulence for high-resolution, but non-eddy resolving models. Progress has also been made with the parameterization of the near-surface atmospheric fluxes of energy and momentum modified by sea ice pressure ridges and by ice floe edges. Other studies brought new insight into the complex processes influencing sea ice transport and atmospheric stability over sea ice. Improved understanding was obtained on the cloud effects on the snow/ice surface temperature and further on the near-surface turbulent fluxes. Finally, open questions are addressed, which remained after the ACSYS decade for future programmes having been started in the years after 2005