The evolution of the atmospheric boundary layer (ABL) during warm-air advection over sea ice in the Fram Strait is investigated. A hierarchy of numerical models is applied and the results are validated against aircraft observations. Operational HIRLAM (HIgh Resolution Limited Area Model) runs suffered from inaccurate information on the location of the sea ice edge. When this was improved, together with improving the horizontal resolution, the HIRLAM results became much better. Two-dimensional model runs with inflow boundary conditions prescribed according to the aircraft observations demonstrated that the contribution of thin ice (re-frozen leads) is important. The modelled turbulent fluxes of momentum, sensible heat and latent heat showed a large vertical divergence from the surface to the height of the aircraft observations. Above a 4-km-wide open lead, the modelled heat fluxes at the observation height of 24 m agreed reasonably well with the observations but were on average only 6% (sensible heat) and 13% (latent heat) of the modelled surface values. Model experiments showed that in this case relatively high values for the roughness length z0 over sea ice yielded better agreement with the observed wind speed than values commonly used in numerical models.