Description: The firn density, temperature and liquid water content of the Greenland ice sheet have been modelled with the IMAU-FDM firn model. IMAU-FDM is forced at the surface with the latest output of the regional climate model RACMO2.3p2. The data is on a horizontal grid of 11x11 km and covers 1960-2016 with a 10-day temporal resolution. Here, time series of the firn air content (vertically integrated difference between firn and ice density (= 917 kg m-3)) and 10-m firn temperature are provided. All other IMAU-FDM output is available from the authors without conditions.

Description: We investigated time series of 17 Antarctic drainage basins from April 2002 until August 2016 using data from the satellite gravimetry mission GRACE, a multi-mission altimetry product, and products from regional climate and firn modeling. The model products are cumulated surface mass balance anomalies (cSMBA) derived from RACMO2 outputs and firn thickness change predicted by the firn densification model (FDM) IMAU-FDM. We simultaneously evaluated these data sets in a state-space model framework to separate time-variable contributions from ice-dynamics and climatological forcing to mass and volume changes of the drainage systems. We parametrize long-term changes by a trend with a time-variable rate. Further we separate residual cyclic, first-order auto regressive (AR(1)), and irregular short-term variations. For each drainage basin we provide a file that includes mass and volume time series of the input data sets and the estimated signals along with their uncertainty (single standard deviation). The basin numbers refer to drainage systems defined by Zwally et al. (2012).

Description: Greenland Ice Sheet surface melting has increased since the 1990s, affecting the rheology and scattering properties of the near‐surface firn. We combine firn cores and modeled firn densities with 7 years of CryoVEx airborne Ku‐band (13.5 GHz) radar profiles to quantify the impact of melting on microwave radar penetration in West Central Greenland. Although annual layers are present in the Ku‐band radar profiles to depths up to 15 m below the ice sheet surface, fluctuations in summer melting strongly affect the degree of radar penetration. The extreme melting in 2012, for example, caused an abrupt 6.2 ± 2.4 m decrease in Ku‐band radar penetration. Nevertheless, retracking the radar echoes mitigates this effect, producing surface heights that agree to within 13.9 cm of coincident airborne laser measurements. We also examine 2 years of Ka‐band (34.5 GHz) airborne radar data and show that the degree of penetration is half that of coincident Ku‐band.

Description: Glacial isostatic adjustment (GIA) is a major source of uncertainty for ice and ocean mass balance estimates derived from satellite gravimetry. In Antarctica the gravimetric effect of cryospheric mass change and GIA are of the same order of magnitude. Inverse estimates from geodetic observations hold some promise for mass signal separation. Here, we investigate the combination of satellite gravimetry and altimetry and demonstrate that the choice of input data sets and processing methods will influence the resultant GIA inverse estimate. This includes the combination that spans the full GRACE record (April 2002–August 2016). Additionally, we show the variations that arise from combining the actual time series of the differing data sets. Using the inferred trends, we assess the spread of GIA solutions owing to (1) the choice of different degree-1 and C20 products, (2) viable candidate surface-elevation-change products derived from different altimetry missions corresponding to different time intervals, and (3) the uncertainties associated with firn process models. Decomposing the total-mass signal into the ice mass and the GIA components is strongly dependent on properly correcting for an apparent bias in regions of small signal. Here our ab initio solutions force the mean GIA and GRACE trend over the low precipitation zone of East Antarctica to be zero. Without applying this bias correction, the overall spread of total-mass change and GIA-related mass change using differing degree-1 and C20 products is 68 and 72 Gt a−1, respectively, for the same time period (March 2003–October 2009). The bias correction method collapses this spread to 6 and 5 Gt a−1, respectively. We characterize the firn process model uncertainty empirically by analysing differences between two alternative surface mass balance products. The differences propagate to a 10 Gt a−1 spread in debiased GIA-related mass change estimates. The choice of the altimetry product poses the largest uncertainty on debiased mass change estimates. The spread of debiased GIA-related mass change amounts to 15 Gt a−1 for the period from March 2003 to October 2009. We found a spread of 49 Gt a−1 comparing results for the periods April 2002–August 2016 and July 2010–August 2016. Our findings point out limitations associated with data quality, data processing, and correction for apparent biases.

Description: Surface melt ponds form intermittently on several Antarctic ice shelves. Although implicated in ice-shelf break up, the consequences of such ponding for ice formation and ice-shelf structure have not been evaluated. Here we report the discovery of a massive subsurface ice layer, at least 16 km across, several kilometres long and tens of metres deep, located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and radar measurements with remote sensing and firn modelling to investigate the layer, found to be ~10 °C warmer and ~170 kg/m3 denser than anticipated in the absence of ponding and hitherto used in models of ice-shelf fracture and flow. Surface ponding and ice layers such as the one we report are likely to form on a wider range of Antarctic ice shelves in response to climatic warming in forthcoming decades.

Description: A common precursor to ice shelf disintegration, most notably that of Larsen B Ice Shelf, is unusually intense or prolonged surface melt and the presence of surface standing water. However, there has been little research into detailed patterns of melt on ice shelves or the nature of summer melt ponds. We investigated surface melt on Larsen C Ice Shelf at high resolution using Envisat advanced synthetic aperture radar (ASAR) data and explored melt ponds in a range of satellite images. The improved spatial resolution of SAR over alternative approaches revealed anomalously long melt duration in western inlets. Meteorological modelling explained this pattern by föhn winds which were common in this region.Melt ponds are difficult to detect using optical imagery because cloud-free conditions are rare in this region and ponds quickly freeze over, but can be monitored using SAR in all weather conditions. Melt ponds up to tens of kilometres in length were common in Cabinet Inlet, where melt duration was most prolonged. The pattern of melt explains the previously observed distribution of ice shelf densification, which in parts had reached levels that preceded the collapse of Larsen B Ice Shelf,suggesting a potential role for föhn winds in promoting unstable conditions on ice shelves.

Description: Surface melt and ponding has been observed on many Antarctic ice shelves and is implicated in ice-shelf collapse through firn compaction and hydrofracture (van den Broeke, 2005). Ice-shelf surface meltwater can percolate into the firn, transferring heat to deeper layers by refreezing (Vaughan, 2008). This can lead to warming and densification to the point where firn air content approaches zero (Holland and others, 2011), potentially impacting ice dynamics and fracture toughness. Surface processes also contribute to the recent thinning observed on Larsen C ice shelf (LCIS; Pritchard and others, 2012; Holland and others, 2015). In northern LCIS, föhn winds provide extra sensible heat to drive surface melt (Luckman and others, 2014). The NERC MIDAS Project (Impact of Melt on Ice Dynamics and Stability: 2014–2017) aims to investigate the mechanisms of melt and ponding, and test their impact on the stability of the LCIS. In November 2014 we visited Cabinet Inlet in northern LCIS to install an ARGOS-enabled automatic weather station, drill a 100 m borehole, and determine ice layering and temperature using an optical televiewer (OPTV) and logged thermistor string. The OPTV log shows a ~40 m layer of refrozen meltwater perched above glacier ice presumably advected from beyond the grounding line 40 km away. The thermistor data show the ice to be many degrees warmer than mean atmospheric temperatures would lead us to expect. We infer the source of this unusual ice configuration and temperature profile as föhn-driven surface melt. In April 2015, well after normal mean surface temperature had fallen below freezing, several föhn wind events lasting a few days raised the temperature well above 0°C and, in MODIS satellite data, melt ponds were seen to form. We continue to investigate using numerical modelling the potential impact on ice-shelf stability of melt, warming and massive ice layers.

Description: The surface mass balance (SMB) of the Larsen C ice shelf (LCIS), Antarctica, is poorly constrained due to a dearth of in situ observations. Combining several geophysical techniques, we reconstruct spatial and temporal patterns of SMB over the LCIS. Continuous time series of snow height (2.5–6 years) at five locations allow for multi-year estimates of seasonal and annual SMB over the LCIS. There is high interannual variability in SMB as well as spatial variability: in the north, SMB is 0.40+/-0.06 to 0.41+/-0.04mw.e. per year, while farther south, SMB is up to 0.50+/-0.05mw.e. per year. This difference between north and south is corroborated by winter snow accumulation derived from an airborne radar survey from 2009, which showed an average snow thickness of 0.34mw.e. north of 66° S, and 0.40mw.e. south of 68° S. Analysis of ground-penetrating radar from several field campaigns allows for a longer-term perspective of spatial variations in SMB: a particularly strong and coherent reflection horizon below 25–44m of waterequivalent ice and firn is observed in radargrams collected across the shelf. We propose that this horizon was formed synchronously across the ice shelf. Combining snow height observations, ground and airborne radar, and SMB output from a regional climate model yields a gridded estimate of SMB over the LCIS. It confirms that SMB increases from north to south, overprinted by a gradient of increasing SMB to the west, modulated in the west by föhn-induced sublimation. Previous observations show a strong decrease in firn air content toward the west, which we attribute to spatial patterns of melt, refreezing, and densification rather than SMB.

Description: Surface melt ponds now form frequently on ice shelves across the northern sector of the Antarctic Peninsula in response to regional warming and local föhn winds. A potentially important, but hitherto unknown, consequence of this surface melting and ponding is the formation of high-density near-surface ice from the refreezing of that water. We report the discovery and physical character of a massive subsurface ice layer located in an area of intense melting and intermittent ponding on Larsen C Ice Shelf, Antarctica. We combine borehole optical televiewer logging and ground-based radar measurements with remote sensing and firn modelling to investigate the formation and spatial extent of this layer, found to be tens of kilometres across and tens of metres deep. The presence of this ice layer has the effect of raising local ice shelf density by about 190 kgm-3 and temperature by 5 - 10 degrees C above values found in areas unaffected by ponding and hitherto used in models of ice-shelf fracture and flow.