Temporal and Spatial Variations of the Aerodynamic Roughness Length in the Ablation Zone of the Greenland Ice Sheet |
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| Authors: | C J P P Smeets M R van den Broeke |
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| Institution: | (1) Institute for Marine and Atmospheric Research Utrecht, Princetonplein 5, 3584 CC Utrecht, The Netherlands |
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| 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 (z
0) is a key parameter. We present, for the first time, spatial and temporal variations of z
0 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 z
0 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 z
0≈ 10−4 m throughout the ablation area. At the beginning of summer, snow melt uncovers ice hummocks and z
0 quickly increases well above 10−2 m in the lower ablation area. At the end of summer melt, hummocky ice dominates the surface with z
0 > 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 z
0 = 10−5 m. At the beginning of winter, we observed that single snow events have the potential to lower z
0 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. |
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| Keywords: | Ablation area Aerodynamic roughness length Drag partition model Greenland Ice sheet Ice surface roughness |
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