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 (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.

The seasonal cycle and interannual variability of surface energy balance and melt in the ablation zone of the west Greenland ice sheet

“…Table 1 lists the sensor specifications and Table 2 the period of operation, location information and basic climate and surface energy balance statistics. Radiation, temperature and humidity observations are corrected along the lines described in Van den Broeke et al (2004, 2008a. Owing to a datalogger failure, S6 misses data from September 2007 to August 2008, and for several weeks in June, July and August 2010.…”

“…Instrument levels (heights above the surface) are reconstructed based on the derived snow depths and reference measurements during annual station visits. In combination with the two-level wind speed, temperature and humidity data this information is used to calculate 20-day running means of surface momentum roughness z 0 (Smeets and Van den Broeke, 2008a;Van den Broeke et al, 2009b). Kipp en Zonen CM3 305 to 2800 nm EADT ± 10 % Pyrradiometer Kipp en Zonen CG3 5000 to 50 000 nm EADT ± 10 % Snow height Campbell SR50 0.5 to 10 m 0.01 m or 0.4 %…”

“…All terms are in W m −2 and defined positive when directed towards the surface. Here we briefly repeat the main model characteristics; for a more detailed model description, the reader is referred to Van den Broeke (2004, 2008a, b, 2009b.…”

“…If T s exceeds the melting point, it is reset to 273.15 K and all remaining energy is used for melt. This way of working assumes a closed energy balance and therewith differs from Van den Broeke (2008a, 2009b, who did not require melt rate to be quantified and therefore could use "observed" T s (from LW out ) to calculate the radiation balance and turbulent fluxes. Equation (1) assumes all energy to be absorbed at the surface, neglecting penetration of shortwave radiation to levels below the surface.…”

“…The AWS are equipped with radiation sensors and two measurement levels for temperature, humidity and wind speed, which makes them especially suitable for SEB studies. Previously, the first four years of AWS data (2003)(2004)(2005)(2006)(2007) were used to assess the radiation and turbulent driven heat exchange (Van den Broeke, 2008a, 2009b) and the surface mass balance . This study presents an update, using the longer time series to present the average seasonal cycle of the full energy balance with special reference to interannual variability, including a thorough evaluation of the model under melting and non-melting conditions.…”