The fluxes of energy and momentum over the Arctic sea ice covered ocean arestrongly influenced by the various sources of surface inhomogeneity. The firsttype of inhomogeneity concerns the existance of open water within theice covered regions. It occurs e.g., in the marginal sea ice zones and very often even in the inner Arctic regions in form of ice free leads and polynjas.During winter, the large differences between the near-surface air temperatureand the surface temperature of open water cause very strong heat fluxes fromthe ocean to the atmosphere. The second type of inhomogeneity concerns the surface topography. Roughness elements like ridges and floe edges are distributed irregularly, and they have a large influence on the near-surface transport of momentum from the atmospheric boundary layer (ABL) to sea ice.Investigations of the effect of the various sources of inhomogeneityon the Arctic ABL have been carried out in a subproject of the German contribution to the international ACSYS project. A nonhydrostatic mesoscale model (METRAS), mainly developed at the university of Hamburg in cooperation with AWI, was applied to several scenarios differingby the sea ice concentration, by the roughness of sea ice, and by themeteorological forcing. For a realistic representation of the effects ofinhomogeneity, new parameterizations of the surface roughness, suitablefor models of different scales, and of convection over leads for microscalemodels have been developed and applied to the model METRAS. It is shown bythe model runs that the Arctic ABL is significantly influenced by both typesof surface inhomogeneities and that they should be accountedfor in regional and large scale models.