Browsing by Author "Shuman, Christopher A."
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Item The 32-year record-high surface melt in 2019/2020 on the northern George VI Ice Shelf, Antarctic Peninsula(Copernicus Publications, 2020-10-22) Banwell, Alison F.; Datta, Rajashree Tri; Dell, Rebecca L.; Moussavi, Mahsa; Brucker, Ludovic; Picard, Ghislain; Shuman, Christopher A.; Stevens, Laura A.In the 2019/2020 austral summer, the surface melt duration and extent on the northern George VI Ice Shelf (GVIIS) was exceptional compared to the 31 previous summers of dramatically lower melt. This finding is based on analysis of near-continuous 41-year satellite microwave radiometer (and scatterometer) data, which are sensitive to meltwater on the ice-shelf surface and in the near-surface snow. Using optical satellite imagery from Landsat 8 (since 2013) and Sentinel-2 (since 2017), record volumes of surface meltwater ponding are also observed on north GVIIS in 2019/2020, with 23 % of the surface area covered by 0.62 km3 of meltwater on January 19. These exceptional melt and surface ponding conditions in 2019/2020 were driven by sustained air temperatures ≥ 0 °C for anomalously long periods (55–90 hours) from late November onwards, likely driven by warmer northwesterly and northeasterly low-speed winds. Increased surface ponding on ice shelves may threaten their stability through increased potential for hydrofracture initiation; a risk that may increase due to firn air content depletion in response to near-surface melting.Item Decadal-Length Composite Inland West Antarctic Temperature Records(AMS, 2001-05-01) Shuman, Christopher A.; Stearns, Charles R.Decadal-length, daily average, temperature records have been generated for four inland West Antarctic sites by combining automatic weather station (AWS) and satellite passive microwave brightness temperature records. These records are composites due to the difficulty in maintaining continuously operating AWS in Antarctica for multiyear to multidecade periods. Calibration of 37-GHz, vertical polarization, brightness temperature data during periods of known air temperature by emissivity modeling allows the resulting calibrated brightness temperatures (TC) to be inserted into data gaps with constrained errors. By the same technique, but with reduced constraints, TC data were also developed through periods before AWS unit installation or after removal. The resulting composite records indicate that temperature change is not consistent in sign or magnitude from location to location across the West Antarctic region. Linear regression analysis shows an approximate 0.9°C increase over 19 yr at AWS Byrd (0.045 yr⁻¹ ±0.135°C), a 0.9°C cooling over 12 yr at AWS Lettau (−0.078 yr⁻¹ ±0.178°C), a 3°C cooling over 10 yr at AWS Lynn (−0.305 yr⁻¹ ±0.314°C), and a 2°C warming over 19 yr at AWS Siple (0.111 yr⁻¹ ±0.079°C). Only the Siple trend is statistically significant at the 95% confidence level however. The temperature increases at Siple and possibly Byrd are suggestive of a broader regional warming documented at sites on the Antarctic Peninsula. The cooling suggested by the shorter records in the vicinity of the Ross Ice Shelf is consistent with results recently reported by Comiso and suggests that significant regional differences exist. Continued data acquisition should enable detection of the magnitude and direction of potential longer-term changes.Item An empirical algorithm to map perennial firn aquifers and ice slabs within the Greenland Ice Sheet using satellite L-band microwave radiometry(EGU, 2022-01-13) Miller, Julie Z.; Culberg, Riley; Long, David G; Shuman, Christopher A.; Schroeder, Dustin M.; Brodzik, Mary J.Perennial firn aquifers are subsurface meltwater reservoirs consisting of a meters-thick water-saturated firn layer that can form on spatial scales as large as tens of kilometers. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting and high snow accumulation. Widespread perennial firn aquifers have been identified within the Greenland Ice Sheet (GrIS) via field expeditions, airborne ice-penetrating radar surveys, and satellite microwave sensors. In contrast, ice slabs are nearly continuous ice layers that can also form on spatial scales as large as tens of kilometers as a result of surface and subsurface water-saturated snow and firn layers sequentially refreezing following multiple melting seasons. They have been observed within the percolation facies of glaciated regions experiencing intense seasonal surface melting but in areas where snow accumulation is at least 25 % lower as compared to perennial firn aquifer areas. Widespread ice slabs have recently been identified within the GrIS via field expeditions and airborne ice-penetrating radar surveys, specifically in areas where perennial firn aquifers typically do not form. However, ice slabs have yet to be identified from space. Together, these two ice sheet features represent distinct, but related, sub-facies within the broader percolation facies of the GrIS that can be defined primarily by differences in snow accumulation, which influences the englacial hydrology and thermal characteristics of firn layers at depth. Here, for the first time, we use enhanced-resolution vertically polarized L-band brightness temperature (Tᵥᴮ) imagery (2015–2019) generated using observations collected over the GrIS by NASA's Soil Moisture Active Passive (SMAP) satellite to map perennial firn aquifer and ice slab areas together as a continuous englacial hydrological system. We use an empirical algorithm previously developed to map the extent of Greenland's perennial firn aquifers via fitting exponentially decreasing temporal L-band signatures to a set of sigmoidal curves. This algorithm is recalibrated to also map the extent of ice slab areas using airborne ice-penetrating radar surveys collected by NASA's Operation IceBridge (OIB) campaigns (2010–2017). Our SMAP-derived maps show that between 2015 and 2019, perennial firn aquifer areas extended over 64 000 km², and ice slab areas extended over 76 000 km². Combined together, these sub-facies are the equivalent of 24 % of the percolation facies of the GrIS. As Greenland's climate continues to warm, seasonal surface melting will increase in extent, intensity, and duration. Quantifying the possible rapid expansion of these sub-facies using satellite L-band microwave radiometry has significant implications for understanding ice-sheet-wide variability in englacial hydrology that may drive meltwater-induced hydrofracturing and accelerated ice flow as well as high-elevation meltwater runoff that can impact the mass balance and stability of the GrIS.Item Mapping Greenland accumulation rates using observations of thermal emission at 4.5-cm wavelength(AGU, 2001-12-01) Winebrenner, Dale P.; Arthern, Robert J.; Shuman, Christopher A.Accurate predictions of sea level rise over the coming century will require improved knowledge of the processes controlling accumulation on the great ice sheets. The sparsity of accumulation rate observations, both temporally and spatially, hinder development of this understanding. We introduce a new method to observe accumulation rates (averaged over roughly a decade) using satellite observations of microwave emission at 4.5-cm wavelength, focusing in this paper on Greenland. At this wavelength, scattering by the grain fabric in firn is unimportant relative to quasi-reflection from density (and thus dielectric permittivity) stratification. We show observationally a strong link between random firn density stratification, on scales of millimeters to centimeters, and accumulation rate. We then show theoretically how the observed density stratification can produce and is consistent with observations of polarization of 4.5-cm-wavelength emission. We employ observations from the Scanning Multichannel Microwave Radiometer (SMMR) and previously published ground observations of accumulation rates in Greenland to complete specification of the relationship between accumulation rate and polarization. The relationship is sufficiently accurate to serve as a basis for mapping accumulation rate fields. We compare our satellite-derived maps with previously published maps based on syntheses of ground data. We find broad agreement between the two types of maps, though the satellite-derived map indicates more strongly the importance of topography and prevailing weather patterns in determining detailed accumulation rate patterns. Finally, we discuss possible refinements and the prospects for improved satellite-derived maps based on a new generation of sensors about to be launched.Item Melt Detection Over Greenland Using Smap Radiometer Observations(IEEE, 2021-02-17) Mousavi, Seyedmohammad; Colliander, Andreas; Miller, Julie; Entekhabi, Dara; Johnson, Joel T.; Shuman, Christopher A.; Kimball, John S.; Courville, Zoe R.Microwave measurements have been previously used to detect melt events due to their sensitivity to the presence of liquid water in snow. Since NASA's SMAP mission offers a valuable set of low frequency radiometer measurements, SMAP measurements have been used as a tool to detect melt events. SMAP's L-band radiometer also covers virtually the entire Greenland ice sheet twice daily. The overpasses center on morning and evening hours as the satellite is on a 6AM/6PM equator-crossing orbit, and the spatial resolution of the instrument is about 40 km. In this paper, the response of L-band measurements to surface melting of the ice sheet from 2015 through 2019 melt seasons is investigated. It is shown that the Greenland ice sheet experienced an unusually strong melt event at the end of July 2019, which extended the melt area across much of dry snow zone of the ice sheet over a period of two days.Item Seasonal δ³⁴S variations in two high elevation snow pits measured by ³³S–³⁶S double spike thermal ionization mass spectrometry(Elsevier, 2008-08-01) Mann, Jacqueline L.; Shuman, Christopher A.; Kelly, W. Robert; Kreutz, Karl J.δ³⁴S and sulfate concentrations were determined in snow pit samples using a thermal ionization mass spectrometric technique capable of 0.2‰ accuracy and requires ≈5 μg (0.16 μmol) natural S. The technique utilizes a ³³S–³⁶S double spike for instrumental mass fractionation correction, and has been applied to snow pit samples collected from the Inilchek Glacier, Kyrgyzstan and from Summit, Greenland. These δ³⁴S determinations provide the first high-resolution seasonal data for these sites, and are used to estimate seasonal sulfate sources. Deuterium (δD) and oxygen (δ¹⁸O) isotope data show that the Inilchek and Summit snow pit samples represent precipitation over ≈20 months. The δ³⁴S values for the Inilchek ranged from +2.6 ± 0.4‰ to +7.6 ± 0.4‰ on sample sizes ranging from 0.3 to 1.8 μmol S. δ³⁴S values for Greenland ranged from +3.6 ± 0.7‰ to +13.3 ± 5‰ for sample sizes ranging from 0.05 to 0.29 μmol S. The SO₄²⁻ concentration ranged from 92.6 ± 0.4 to 1049 ± 4 ng/g for the Inilchek and 18 ± 9 to 93 ± 6 ng/g for the Greenland snow pit. Anthropogenic sulfate dominates throughout the sampled time interval for both sites based on mass balance considerations. Additionally, both sites exhibit a seasonal signature in both δ³⁴S and SO₄²⁻ concentration. The thermal ionization mass spectrometric technique has three advantages compared to gas source isotopic methods: (1) sample size requirements of this technique are 10-fold less permitting access to the higher resolution S isotope record of low concentration snow and ice, (2) the double spike technique permits δ³⁴S and S concentration to be determined simultaneously, and (3) the double spike is an internal standard.