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

A quantitative relationship between observed sea-ice roughness and simulate d large-scale deformation work is established in order to provide new means for model validation and a better representation of the sea-ice component in climate modelling. Sea-ice roughness is introduced as an additional prog nostic variable in a dynamic-thermodynamic sea-ice model with a viscous-pla stic rheology. It is defined as the accumulated work of internal forces act ing upon an ice volume, given in energy per area. A fraction of this total deformation work is transferred to the potential energy stored in pressure ridges. Using ridge geometries and distribution functions from observations , observable quantities like mean pressure ridge height, ridge frequency as well as volumetric and areal fractions of deformed ice are derived from th e simulated ice roughness. Comparisons of these simulated quantities with m easurements (submarine-borne sonars, laser altimeters on helicopters) show good agreement. Satellite-borne observations of sea-ice roughness now under development will provide an even larger data set which will be used for mo del verification. Additionally roughness-dependent drag coefficients are in troduced to account for the effect on the momentum exchange between ocean a nd atmosphere due to the form drag of roughness elements. The simulations i ndicate that the inclusion of sea-ice roughness provides for a more realist ic representation of the boundary layer processes in climate models.