Sea-ice roughness and drag coefficients in a dynamic–thermodynamic sea-ice model for the Arctic

Abstract

A quantitative relationship between observed sea-ice roughness and simulated large-scaledeformation work is established in order to provide new means for model validation and abetter representation of the sea-ice component in climate modelling. Sea-ice roughness is introducedas an additional prognostic variable in a dynamic–thermodynamic sea-ice model with aviscous-plastic rheology. It is defined as the accumulated work of internal forces acting uponan ice volume, given in energy per area. A fraction of this total deformation work is transferredto the potential energy stored in pressure ridges. Using ridge geometries and distribution functionsfrom observations, observable quantities like mean pressure ridge height, ridge frequencyas well as volumetric and areal fractions of deformed ice are derived from the simulated iceroughness. Comparisons of these simulated quantities with measurements (submarine-bornesonars, laser altimeters on helicopters) show good agreement. Satellite-borne observations ofsea-ice roughness now under development will provide an even larger data set which will beused for model verification. Additionally roughness-dependent drag coefficients are introducedto account for the effect on the momentum exchange between ocean and atmosphere due tothe form drag of roughness elements. The simulations indicate that the inclusion of sea-iceroughness provides for a more realistic representation of the boundary layer processes in climatemodels.