In polar regions, sea ice strongly influences the momentum and energy transfer between the atmosphere and the ocean. In order to study mesoscale interactions the nonhydrostatic atmosphere model METRAS is coupled to a dynamic-thermodynamic sea ice model. The coupled model can be used on spatial and temporal scales up to 500 km and up to 1 week respectively. It considers in detail interactions between the atmospheric boundary layer and partly ice covered regions like marginal ice zones and polynyas.
In the atmosphere model, the flux averaging method for determining the turbulent surface fluxes is modified by introducing a calculation of the form drag of sea ice. Only by using this extended flux averaging method calculated mean drag coefficients for heterogeneous ice-water-surfaces agree well with observations.
The sea ice model has 5 surface classes (open water and 4 ice thickness classes) which are especially designed for simulating the development of thin ice and its influence on the heat exchange between the ocean and the atmosphere. The sea ice is vertically devided in several layers. The diffusion equation considers brine pockets and penetrating solar radiation. The model includes a parameterization of the oceanic form drag of sea ice. The mixed-layer temperature is prescribed.
The coupled model is used to simulate the flow of cold air over medium-sized polynyas with diameters ranging from 20 km to 100 km. It is shown that large turbulent fluxes of heat and momentum over a polynya cause a spatially heterogeneous wind field. This is characterized by an increase of near surface wind velocity over the polynya and a considerable decrease on the downwind side of the polynya. For different drift velocities of the sea ice surrounding the polynya, its position changes and its horizontal extension decreases. Consequently, there is a gradual homogenization of the wind field. In case of freezing of a polynya, even a thin layer of new ice homogenizes the atmospheric flow fields considerably