Turbulence beneath sea ice and leads: A coupled sea ice/large‐eddy simulation study

The importance of leads, sea ice motion, and frazil ice on the wintertime ocean boundary layer was examined by using a large‐eddy simulation turbulence model coupled to a thermodynamic slab ice model. Coupling was achieved through exchange coefficients that accounted for the differing diffusion rates of heat and salinity. Frazil ice concentrations were modeled by using an ice crystal parameterization with constant crystal size and shape. Stationary ice without leads produced cellular structures similar to atmospheric convection without winds. Ice motion caused this pattern to break down into a series of streaks aligned with the flow. Eddy fluxes were strongly affected by ice motion with relatively larger entrainment fluxes at the mixed layer base under moving ice, whereas stationary ice produced larger fluxes near the top of the boundary layer. Opening of leads caused significant changes in the turbulent structure of the boundary layer. Leads in stationary ice produced concentrated plumes of higher‐salinity water beneath the lead. Ice motion caused the lead convection to follow preexisting convective rolls, enhancing the roll circulation salinity and vertical velocity under the lead. Comparison of model time series data with observations from the Arctic Leads Experiment showed general agreement for both pack ice and lead conditions. Simulated heat flux carried by frazil ice had a prominent role in the upper boundary layer, suggesting that frazil ice is important in the heat budget of ice‐covered oceans.