LES Modeling of Wind over Antarctic Snow-Ice Formations using a Dynamic Surface Roughness Approach

Surface drag over sea-ice is a primary control in driving sea-ice drift and deformation – both important processes for climate and weather prediction models. A series of high-resolution Large Eddy Simulations (LES) of fully-developed boundary-layer flow over snow-ice surfaces of Antarctic sea ice floes are here performed to study surface drag and roughness parameters at process scales from 1 cm to 100 m. Snow/ice surface morphology was obtained using a Terrestrial Laser Scanner during the SIPEX II (Sea Ice Physics and Ecosystem experiment II) research voyage to East Antarctica (September-November 2012). The effects of large-scale surface features on the wind flow (those features that can be resolved in LES) are accounted for through an immersed boundary method (IBM). Conversely, the drag forces caused by subgrid-scale features of the surface are accounted for through a parameterization. However, the effective hydrodynamic roughness length (z0) for snow surfaces on sea ice is not known. Hence, a dynamic approach is adopted, in which z0 is determined using the first-principles based constraint that the total momentum flux (drag) must be independent from grid-filter scale. This dynamic surface roughness model is inspired by the Germano identity, traditionally used to determine model parameters for closing subgrid-scale stresses in the bulk of a turbulent flow. The model is found to be robust and results in accurate flow predictions (resolution invariant). A preliminary estimate of the dimensionless subgrid-scale roughness parameter (α) is here provided. This could be of use in climate, weather prediction and scalar transport models to compute the effective z0.