Andreas P. Ahlstrøm

A new present-day temperature parameterization for Greenland

Near-surface air temperature (2 m) over the Greenland ice sheet (GrIS) is parameterized using data from automatic weather stations located on land and on the ice sheet. The parameterization is expressed in terms of mean annual temperatures and mean July temperatures, both depending linearly on altitude, latitude and longitude. The temperature parameterization is compared to a previous study and is shown to be in better agreement with observations. The temperature parameterization is tested in a positive degree-day model to simulate the present (1996-2006) mean melt area extent of the GrIS. The model accounts for firn warming, rainfall and refreezing of meltwater, with different degree-day factors for ice and snow under warm and cold climate conditions. The simulated melt area extent is found to have reasonable agreement with satellite-derived observations.

Quantifying Energy and Mass Fluxes Controlling Godthåbsfjord Freshwater Input in a 5-km Simulation (1991–2012)*,+

AbstractFreshwater runoff to fjords with marine-terminating glaciers along the Greenland Ice Sheet margin has an impact on fjord circulation and potentially ice sheet mass balance through increasing heat transport to the glacier front. Here, the authors use the high-resolution (5.5 km) HIRHAM5 regional climate model, allowing high detail in topography and surface types, to estimate freshwater input to Godthabsfjord in southwest Greenland. Model output is compared to hydrometeorological observations and, while simulated daily variability in temperature and downwelling radiation shows high correlation with observations (typically >0.9), there are biases that impact the results. In particular, overestimated albedo leads to underestimation of melt and runoff at low elevations.In the model simulation (1991–2012), the ice sheet experiences increasing energy input from the surface turbulent heat flux (up to elevations of 2000 m) and shortwave radiation (at all elevations). Southerly wind anomalies and declining ...

Improving surface boundary conditions with focus on coupling snow densification and meltwater retention in large-scale ice-sheet models of Greenland

Snowpack changes during the melt season are often not incorporated in modelling studies of the surface mass balance of the Greenland ice sheet. Densification of snow accelerates when meltwater is present, due to percolation and subsequent refreezing, and needs to be incorporated in ice-sheet models for ablation calculations. In this study, simple parameterizations to calculate surface melt, snow densification and meltwater retention are included as surface boundary conditions in a large-scale ice-sheet model of Greenland. Coupling the snow densification and meltwater-retention processes achieves a separation of volume and mass changes of the surface layer, in order to determine the surface melt contribution to runoff. Experiments for present-day conditions show that snow depth at the onset of melt, mean annual near-surface air temperature and the mean density of the annual snow layer are key factors controlling the quantity and spatial distribution of meltwater runoff above the equilibrium line on the Greenland ice sheet.

Sudden increase in tidal response linked to calving and acceleration at a large Greenland outlet glacier

[1] Large calving events at Greenland’s largest outlet glaciers are associated with glacial earthquakes and near‐ instantaneous increases in glacier flow speed. At some glaciers and ice streams, flow is also modulated in a regular way by ocean tidal forcing at the terminus. At Helheim Glacier, analysis of geodetic data shows decimeter‐level periodic position variations in response to tidal forcing. However, we also observe transient increases of more than 100% in the glacier’s responsiveness to such tidal forcing following glacial‐earthquake calving events. The timing and amplitude of the changes correlate strongly with the step‐like increases in glacier speed and longitudinal strain rate associated with glacial earthquakes. The enhanced response to the ocean tides may be explained by a temporary disruption of the subglacial drainage system and a concomitant reduction of the friction at the ice‐bedrock interface, and suggests a new means by which geodetic data may be used to infer glacier properties. Citation: de Juan, J., et al. (2010), Sudden increase in tidal response linked to calving and acceleration at a large Greenland outlet glacier, Geophys. Res. Lett., 37, L12501,

Automatic glacier ablation measurements using pressure transducers

An instrument is described that automatically records ice ablation while eliminating the need for ablation stakes. A pressure transducer placed at the bottom of a hole drilled into the ice is connected by a hose to a bladder lying on the surface. Ice ablation is detected as a reduction in the hydrostatic pressure measured by the transducer.

Algae Drive Enhanced Darkening of Bare Ice on the Greenland Ice Sheet

Surface ablation of the Greenland ice sheet is amplified by surface darkening caused by light-absorbing impurities such as mineral dust, black carbon, and pigmented microbial cells. We present the first quantitative assessment of the microbial contribution to the ice sheet surface darkening, based on field measurements of surface reflectance and concentrations of light-absorbing impurities, including pigmented algae, during the 2014 melt season in the southwestern part of the ice sheet. The impact of algae on bare ice darkening in the study area was greater than that of non-algal impurities and yielded a net albedo reduction of 0.038 ± 0.0035 for each algal population doubling. We argue that algal growth is a crucial control of bare ice darkening, and incorporating the algal darkening effect will improve mass balance and sea level projections of the Greenland ice sheet and ice masses elsewhere.

Controls on the Basal Water Pressure in Subglacial Channels Near the Margin of the Greenland Ice Sheet

Assuming a channelized drainage system in steady state, we investigate the influence of enhanced surface melting on the water pressure in subglacial channels, compared to that of changes in conduit geometry, ice rheology and catchment variations. The analysis is carried out for a specific part of the western Greenland ice-sheet margin between 668 N and 66830 0 N using new high-resolution digital elevation models of the subglacial topography and the ice-sheet surface, based on an airborne ice-penetrating radar survey in 2003 and satellite repeat-track interferometric synthetic aperture radar analysis of European Remote-sensing Satellite 1 and 2 (ERS-1/-2) imagery, respectively. The water pressure is calculated up-glacier along a likely subglacial channel at distances of 1, 5 and 9 km from the outlet at the ice margin, using a modified version of Rothlisbergers equation. Our results show that for the margin of the western Greenland ice sheet, the water pressure in subglacial channels is not sensitive to realistic variations in catchment size and mean surface water input compared to small changes in conduit geometry and ice rheology.

Present-day temperature standard deviation parameterization for Greenland

Modelling the surface mass balance of the Greenland ice sheet (GrIS) in large-scale ice-sheet models using temperature parameterizations in relation with the positive degreeday (PDD) approach is highly sensitive to a parameter: the temperature standard deviation (Braithwaite, 1984; Reeh, 1991). The PDD method is a statistical approach that relates the totals of positive near-surface air temperatures to the amount of snow or ice that melts. The standard deviation of the near-surface air temperature, pdd, is important for PDD modelling because it indicates whether the temperature has been above freezing during a month even though the mean monthly near-surface air temperature value is below. Fausto and others (2009a) demonstrated that a uniform increase of pdd from 2.58C to 4.58C results in a 33% increase in the modelled melt area over Greenland where melt is >1mm. It is therefore important to constrain the pdd value with observations. In large-scale ice-sheet and surface massbalance models of Greenland, it is common that pdd is assigned a single value which typically spans the interval 4.5–5.58C (Greve, 2005; Goelzer and others, 2010; Greve and others, 2011; Sundal and others, 2011). The value of pdd is often used as a tuning parameter, instead of using the temperature standard deviations observed at the automatic weather stations (AWSs) on the ice sheet. To add to the temperature parameterization presented by Fausto and others (2009a), it is proposed to construct a similar distributed parameterization for the temperature standard deviation using the same dataset. Commonly, large-scale ice-sheet models over Greenland calculate the amount of melt using the PDD method by assuming an annual sinusoidal evolution of the near-surface air temperature (Reeh, 1991). The number of PDDs from the normal probability distribution around the monthly mean temperatures during the years, following Reeh (1991), is given as

A new regional high-resolution map of basal and surface topography for the Greenland ice-sheet margin at Paakitsoq, West Greenland

Abstract In 2005 an airborne survey was carried out from a Twin Otter aircraft at Pâkitsup Akuliarusersua (Paakitsoq) near Ilulissat in West Greenland. The survey aimed to measure ice thickness with a 60 MHz coherent radar and surface elevation with a scanning laser altimeter. Positioning information came from multiple on-board differential GPS units and an inertial navigation system. The region surveyed covers >80km along the ice margin and has a total area of ~2700km2 with varying density of measurements: the between-track distance was ~1 km near the margin, increasing to ~3km away from the margin. Regional high-resolution maps of basal topography under the Greenland ice sheet are useful for resolving important glaciological and hydrological questions and for enhancing related process studies, such as the influence of basal meltwater on ice dynamics. The ice-sheet margin in this region is also currently under consideration for hydropower development and has a long and continuing history of glaciological investigations, lately with emphasis on the connection between surface meltwater formation and surface velocity of the ice sheet. Here we present a new regional map of the surface and basal topography of the ice-sheet margin and discuss some of the implications for reported observations at Swiss Camp.

Mapping of a hydrological ice-sheet drainage basin on the West Greenland ice-sheet margin from ERS-1/-2 SAR interferometry, ice-radar measurement and modelling

Abstract The hydrological ice-sheet basin draining into the Tasersiaq lake, West Greenland (66°13’ N, 50°30’W), was delineated, first using standard digital elevation models (DEMs) for ice-sheet surface and bedrock, and subsequently using a new high-resolution dataset, with a surface DEM derived from repeat-track interferometric synthetic aperture radar (SAR) and a bedrock topography derived from an airborne 60 MHz ice-penetrating radar. The extent of the delineation was calculated from a water-pressure potential as a function of the ice-sheet surface and bedrock elevations and a hydraulic factor k describing the relative importance of the potential of the ice overburden pressure compared to the bedrock topography. Themeltwater run-off for the basin delineations was modelled with an energy-balance model calibrated with observed ice-sheet ablation and compared to a 25 year time series of measured basin run-off. The standard DEMs were found to be inadequate for delineation purposes, whereas delineations from high-resolution data were found to be very sensitive to changes in k in a non-linear way, causing a factor 5 change of basin area, corresponding to a doubling of the modelled runoff. The 50% standard deviation of the measured basin run-off could thus be explained by small year-to-year variations of the k-factor.