GEOMETRIC AND AERODYNAMIC ROUGHNESS OF SEA-ICE

The aerodynamic drag of Arctic sea ice is calculated using surface dat a, measured by an airborne laser altimeter and a digital camera in the marginal ice zone of Fram Strait. The influence of the surface morpho logy on the momentum transfer under neutral thermal stratification in the atmospheric boundary layer is derived with the aid of model concep ts, based on the partitioning of the surface drag into a form drag and a skin drag. The drag partitioning concept pays attention to the prob ability density functions of the geometric surface parameters. We foun d for the marginal ice zone that the form drag, caused by flee edges, can amount to 140% of the skin drag, while the effect of pressure ridg es never exceeded 40%. Due to the narrow spacing of obstacles, the ski n drag is significantly reduced by shadowing effects on the leeward si de of flee edges. For practical purposes, the fractional sea-ice cover age can be used to parameterize the drag coefficient C-dn, related to the 10 m-wind. C-dn increases from 1.2 . 10(-3) over open water to 2.8 . 10(-3) for 55% ice coverage and decreases to 1.5 . 10(-3) for 100% ice coverage. Aircraft turbulence measurements are used to compare the model values of C-dn With measurements. The correlation between measu red and modelled drag coefficients results in r(2) = 0.91, where r is the correlation coefficient.