Temporal and spatial variations of the aerodynamic roughness length in the ablation zone of the greenland ice sheet

Abstract

To understand the response of the Greenland ice sheet to climate change the so-called ablation zone is of particular importance, since it accommodates the yearly net surface ice loss. In numerical models and for data analysis, the bulk aerodynamic method is often used to calculate the turbulent surface fluxes, for which the aerodynamic roughness length (z0) is a key parameter. We present, for the first time, spatial and temporal variations of z0 in the ablation area of the Greenland ice sheet using year-round data from three automatic weather stations and one eddy-correlation mast. The temporal variation of z0 is found to be very high in the lower ablation area (factor 500) with, at the end of the summer melt, a maximum in spatial variation for the whole ablation area of a factor 1000. The variation in time matches the onset of the accumulation and ablation season as recovered by sonic height rangers. During winter, snow accumulation and redistribution by snow drift lead to a uniform value of z0 ≈ 10-4 m throughout the ablation area. At the beginning of summer, snow melt uncovers ice hummocks and z0 quickly increases well above 10-2 m in the lower ablation area. At the end of summer melt, hummocky ice dominates the surface with z0 > 5 × 10-3 m up to 60 km from the ice edge. At the same time, the area close to the equilibrium line (about 90 km from the ice edge) remains very smooth with z0 = 10-5 m. At the beginning of winter, we observed that single snow events have the potential to lower z0 for a very rough ice surface by a factor of 20 to 50. The total surface drag of the abundant small-scale ice hummocks apparently dominates over the less frequent large domes and deep gullies. The latter results are verified by studying the individual drag contributions of hummocks and domes with a drag partition model. © The Author(s) 2008.