Richard R. Forster

A spatially calibrated model of annual accumulation rate on the Greenland Ice Sheet (1958-2007)

[1] Past estimates of Greenland Ice Sheet accumulation rates have been multiyear climatologies based on ice/firn cores and coastal precipitation records. Existing annually resolved estimates have incompletely quantified uncertainty, owing primarily to incomplete spatial coverage. This study improves upon these shortcomings by calibrating annual (1958–2007) solid precipitation output from the Fifth Generation Mesoscale Model modified for polar climates (Polar MM5) using firn core and meteorological station data. The calibration employs spatial interpolation of regionally derived linear correction functions. Residual uncertainties exhibit coherent spatial patterns, which are modeled via spatial interpolation of root mean squared errors. Mean 1958–2007 Greenland Ice Sheet annual accumulation rate is 337 ± 48 mm/yr water equivalent (w.e.) or 591 ± 83 Gt/yr. Annual estimates contain one standard deviation uncertainties of 74 mm/yr w.e., 22%, or 129 Gt/yr. Accumulation rates in southeast Greenland are found to exceed 2000 mm/yr w.e. and to dominate interannual variability in Greenland Ice Sheet total accumulated mass, representing 31% of the whole. Accumulation rates in the southeast are of sufficient magnitude to affect the sign of Greenland mass balance during some years. The only statistically significant temporal change in total ice sheet accumulation in the 1958–2007 period occurred between 1960 and 1972, when a simultaneous accumulation increase and decrease occurred in west and east Greenland, respectively. No statistically significant uniform change in ice sheet‐wide accumulation is evident after 1972. However, regional changesdooccur,includingan accumulation increaseonthewestcoast post‐1992.Thehigh accumulation rates of 2002–2003 appear to be confined to the southeast.

Efficient meltwater drainage through supraglacial streams and rivers on the southwest Greenland ice sheet

Significance Meltwater runoff from the Greenland ice sheet is a key contributor to global sea level rise and is expected to increase in the future, but it has received little observational study. We used satellite and in situ technologies to assess surface drainage conditions on the southwestern ablation surface after an extreme 2012 melting event. We conclude that the ice sheet surface is efficiently drained under optimal conditions, that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater release from the ice sheet. Thermally incised meltwater channels that flow each summer across melt-prone surfaces of the Greenland ice sheet have received little direct study. We use high-resolution WorldView-1/2 satellite mapping and in situ measurements to characterize supraglacial water storage, drainage pattern, and discharge across 6,812 km2 of southwest Greenland in July 2012, after a record melt event. Efficient surface drainage was routed through 523 high-order stream/river channel networks, all of which terminated in moulins before reaching the ice edge. Low surface water storage (3.6 ± 0.9 cm), negligible impoundment by supraglacial lakes or topographic depressions, and high discharge to moulins (2.54–2.81 cm⋅d−1) indicate that the surface drainage system conveyed its own storage volume every <2 d to the bed. Moulin discharges mapped inside ∼52% of the source ice watershed for Isortoq, a major proglacial river, totaled ∼41–98% of observed proglacial discharge, highlighting the importance of supraglacial river drainage to true outflow from the ice edge. However, Isortoq discharges tended lower than runoff simulations from the Modèle Atmosphérique Régional (MAR) regional climate model (0.056–0.112 km3⋅d−1 vs. ∼0.103 km3⋅d−1), and when integrated over the melt season, totaled just 37–75% of MAR, suggesting nontrivial subglacial water storage even in this melt-prone region of the ice sheet. We conclude that (i) the interior surface of the ice sheet can be efficiently drained under optimal conditions, (ii) that digital elevation models alone cannot fully describe supraglacial drainage and its connection to subglacial systems, and (iii) that predicting outflow from climate models alone, without recognition of subglacial processes, may overestimate true meltwater export from the ice sheet to the ocean.

Fusion of hyperspectral and radar data using the IHS transformation to enhance urban surface features

Abstract The Intensity–Hue–Saturation (IHS) transformation is used to integrate the high spectral resolution, provided by hyperspectral data (Airborne Visible Infrared Imaging Spectrometer, AVIRIS), and the surface texture information, derived from radar data (Topographic Synthetic Aperture Radar, TOPSAR), into a single image of an urban area. This transformed image is superimposed on the Digital Elevation Model (DEM) data derived from TOPSAR data to create a 3D perspective view. The ambiguity of several urban land cover types is resolved to a larger degree using the higher spectral and spatial resolutions and the synergistic visual content provided by the fused image in a 3D perspective. For urban areas that are at risk from geological hazards (e.g., avalanches, mudflows, and debris flows), the fused image provides a cost-effective product, rich in the information necessary for assessment and mitigation of these hazards.

Estimation of discharge from braided glacial rivers using ERS 1 synthetic aperture radar: first results

Using multitemporal ERS 1 synthetic aperture radar (SAR) satellite imagery and simultaneous ground measurements of streamflow, a strong correlation (R2 = 0.89) was found between water surface area and discharge for a braided glacial river in British Columbia, Canada. Satellite-derived effective width (We) was found to vary with discharge (Q) as We = 27.5Q0.42, where We is defined as the total water surface area within a 10 km × 3 km control section, divided by the section length. This “area/discharge rating curve” yields instantaneous discharge estimates with a mean error of ±275 m3/s for ground-measured flows that ranged from 242 to 6350 m3/s.

Flow velocities of Alaskan glaciers

Our poor understanding of tidewater glacier dynamics remains the primary source of uncertainty in sea level rise projections. On the ice sheets, mass lost from tidewater calving exceeds the amount lost from surface melting. In Alaska, the magnitude of calving mass loss remains unconstrained, yet immense calving losses have been observed. With 20% of the global new-water sea level rise coming from Alaska, partitioning of mass loss sources in Alaska is needed to improve sea level rise projections. Here we present the first regionally comprehensive map of glacier flow velocities in Central Alaska. These data reveal that the majority of the regional downstream flux is constrained to only a few coastal glaciers. We find regional calving losses are 17.1 Gt a(-1), which is equivalent to 36% of the total annual mass change throughout Central Alaska.

Interferometric Radar Observations of Glaciar San Rafael, Chile

Interferometric radar observations of Glaciar San Rafael, Chile, were collected in October 1994 by NASAs Spaceborne Imaging Radar C (SIR-C) at both L- (24 cm) and C-band frequency (5.6 cm), with vertical transmit and receive polarization. The C-band data did not yield good geophysical products, because the temporal coherence of the signal was significantly reduced after 24 h. The L-band data were, however, successfully employed to map the surface topography of the icefield with a 10 m uncertainty in height, and measure ice velocity with a precision of 4 mm d -1 or 1.4 ma -1 . The corresponding error in strain rates is 0.05 a -1 at a 30 m horizontal spacing. The one-dimensional interferometric velocities were subsequently converted to horizontal displacements by assuming a flow direction and complemented by feature-tracking results near the calving front. The results provide a comprehensive view of the ice-flow dynamics of Glaciar San Rafael. The glacier has a core of rapid flow, 4.5 km in width and 3.5° in average slope, surrounded by slowermoving ice, not by rock. Ice velocity is 2.6 m d -1 or 0.95 km a -1 near the equilibrium-line altitude (1200 m), increasing rapidly before the glacier enters the narrower terminal valley, to reach 17.5 m d -1 or 6.4 km a -1 at the calving front. Strain rates are dominated by lateral shearing at the glacier margins (0.4-0.7 a -1 ), except for the terminal-valley section, where longitudinal strain rates average close to 1 a -1 . This spectacular longitudinal increase in ice velocity in the last few kilometers may be a fundamental feature of tidewater glaciers.

Sediment plume response to surface melting and supraglacial lake drainages on the Greenland ice sheet

Increased mass losses from the Greenland ice sheet and inferred contributions to sea-level rise have heightened the need for hydrologic observations of meltwater exiting the ice sheet. We explore whether temporal variations in ice-sheet surface hydrology can be linked to the development of a downstream sediment plume in Kangerlussuaq Fjord by comparing: (1) plume area and suspended sediment concentration from Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and field data; (2) ice-sheet melt extent from Special Sensor Microwave/Imager (SSM/I) passive microwave data; and (3) supraglacial lake drainage events from MODIS. Results confirm that the origin of the sediment plume is meltwater release from the ice sheet. Interannual variations in plume area reflect interannual variations in surface melting. Plumes appear almost immediately with seasonal surface-melt onset, provided the estuary is free of landfast sea ice. A seasonal hysteresis between melt extent and plume area suggests late-season exhaustion in sediment supply. Analysis of plume sensitivity to supraglacial events is less conclusive, with 69% of melt pulses and 38% of lake drainage events triggering an increase in plume area. We conclude that remote sensing of sediment plume behavior offers a novel tool for detecting the presence, timing and interannual variability of meltwater release from the ice sheet.

Shuttle imaging radar (SIR‐C/X‐SAR) reveals near‐surface properties of the South Patagonian Icefield

Shuttle imaging radar C/X band synthetic aperture radar (SIR-C/X-SAR) views of the South Patagonian Icefield in southern Chile and Argentina demonstrate the ability of spaceborne multiparameter radar to detect climatically driven, intra-annual changes in the snow and ice conditions on glaciers. The SIR-C/X-SAR system aboard space shuttle Endeavor acquired images during two 11-day missions in April and October 1994. The radar signatures of differing snow and ice conditions are distinctive and homogeneous over large areas of the ice fields. The signatures are characterized mainly by (1) backscatter amplitude levels, (2) relative amplitudes of the C and L bands (wavelengths of 5.7 and 24 cm, respectively), and (3) polarization properties indicative of volumetric or surface scattering. The radar signatures are interpreted by correlating the radar characteristics with elevation of the snow and ice surfaces and with changes in the meteorological conditions. We are able to define four “radar glacier zones”: (zone A) a relatively dry snow zone with dominant C band returns; (zone B) a moderately wet snow zone with dominant L band returns; (zone C) a wet snow zone with weak returns in all bands; and (zone D) bare ice and/or heavily crevassed surfaces with strong returns in all bands. The spatial changes in the radar glacier zones between April and October are consistent with colder temperatures recorded in October, producing drier snow conditions at lower elevations than in April.

Understanding Greenland ice sheet hydrology using an integrated multi-scale approach

Improved understanding of Greenland ice sheet hydrology is critically important for assessing its impact on current and future ice sheet dynamics and global sea level rise. This has motivated the collection and integration of in situ observations, model development, and remote sensing efforts to quantify meltwater production, as well as its phase changes, transport, and export. Particularly urgent is a better understanding of albedo feedbacks leading to enhanced surface melt, potential positive feedbacks between ice sheet hydrology and dynamics, and meltwater retention in firn. These processes are not isolated, but must be understood as part of a continuum of processes within an integrated system. This letter describes a systems approach to the study of Greenland ice sheet hydrology, emphasizing component interconnections and feedbacks, and highlighting research and observational needs.

Greenland Ice Sheet Mass Balance Reconstruction. Part I: Net Snow Accumulation (1600–2009)

Ice core data are combined with Regional Atmospheric Climate Model version 2 (RACMO2) output (1958‐2010) to develop a reconstruction of Greenland ice sheet net snow accumulation rate, ^ At(G), spanning the years 1600‐2009. Regression parameters from regional climate model (RCM) output regressed on 86 ice cores are used with available cores in a given year resulting in the reconstructed values. Each core site’s residual variance is used to inversely weight the cores’ respective contributions. The interannual amplitude of the reconstructed accumulation rate is damped by the regressions and is thus calibrated to match that of the RCM data. Uncertainty and significance of changes is measured using statistical models. A 12% or 86 Gt yr 21 increase in ice sheet accumulation rate is found from the end of the Little Ice Age in ;1840 to the last decade of the reconstruction. This 1840‐1996 trend is 30% higher than that of 1600‐2009, suggesting an accelerating accumulation rate. The correlation of ^ At(G) with the average surface air temperature in the Northern Hemisphere (SATNHt) remains positive through time, while the correlation of ^ At(G) with local near-surface air temperatures or North Atlantic sea surface temperatures is inconsistent, suggestinga hemispheric-scale climateconnection.An annualsensitivityof ^ At(G) to SATNHtof 6.8%K 21 or 51 Gt K 21 is found. The reconstuction, ^ At(G), correlates consistently highly with the North Atlantic Oscillation index. However, at the 11-yr time scale, the sign of this correlation flips four times in the 1870‐2005 period.