A parametrization, based on sea ice morphology, of the neutral atmospheric drag coefficients for weather prediction and climate models

摘要

A hierarchy of parametrizations of the neutral 10 m drag coefficientsudover polar sea ice with different morphology regimes is derived onudthe basis of a partitioning concept that splits the total surface drag into contributionsudof skin drag and form drag. The new derivation, which providesuddrag coefficients as a function of sea ice concentration and characteristic lengthudscales of roughness elements, needs fewer assumptions than previous similarudapproaches. It is shown that form drag variability can explain the variabilityudof surface drag in the marginal sea ice zone (MIZ) and in the summertimeudinner Arctic regions. In the MIZ, form drag is generated by floe edges;udin the inner Arctic, by edges at melt ponds and leads due to the elevationudof the ice surface relative to the open water surface. It is shown that an earlierudfit of observed neutral drag coefficients is obtained as a special case withinudthe new concept when specific simplifications are made which concern theudfloe and melt pond geometry. Due to the different surface morphologies inudthe MIZ and summertime Arctic, different functional dependencies of the dragudcoefficients on the sea ice concentration result. These differences cause onlyudminor differences between the MIZ and summertime drag coefficients in averageudconditions, but they might be locally important for atmospheric momentumudtransport to sea ice. The new parametrization formulae can be usedudfor present conditions but also for future climate scenarios with changing seaudice conditions.