Surface Melting Drives Fluctuations in Airborne Radar Penetration in West Central Greenland

Otosaka, Inés; Shepherd, Andrew; Casal, Tânia; Coccia, Alex; Davidson, Malcolm; Bella, Alessandro Di; Fettweis, Xavier; Forsberg, R.; Helm, Veit; Hogg, Anna E.; Hvidegaard, Sine Munk; Lemos, Adriano; Macedo, Karlus; Munneke, Peter Kuipers; Parrinello, Tommaso; Simonsen, Sebastian B.; Skourup, Henriette; Sørensen, Louise Sandberg

doi:10.1029/2020gl088293

Cited by 12 publications

(9 citation statements)

References 54 publications

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“…In the ablation zone, we assume that elevation change anomalies occur at the density of ice (917 kg/m 3 ), and across the inland runoff zone, and when the elevation changes are negative, we assume they occur at the density of firn. We obtained a firn density of 684 kg/m 3 from shallow cores collected recently in the region [54][55][56] rather than ice-sheet wide models as they are known to be biased high in the inland runoff zone 56 , though the choice has only a small (< 10 %) impact on our runoff solution because the region makes a small overall contribution (see Methods). Our approach is a simplification because it neglects mass accumulated during the summer period.…”

Section: Resultsmentioning

confidence: 99%

Increased variability in Greenland Ice Sheet runoff from satellite observations

et al. 2021

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Runoff from the Greenland Ice Sheet has increased over recent decades affecting global sea level, regional ocean circulation, and coastal marine ecosystems, and it now accounts for most of the contemporary mass imbalance. Estimates of runoff are typically derived from regional climate models because satellite records have been limited to assessments of melting extent. Here, we use CryoSat-2 satellite altimetry to produce direct measurements of Greenland’s runoff variability, based on seasonal changes in the ice sheet’s surface elevation. Between 2011 and 2020, Greenland’s ablation zone thinned on average by 1.4 ± 0.4 m each summer and thickened by 0.9 ± 0.4 m each winter. By adjusting for the steady-state divergence of ice, we estimate that runoff was 357 ± 58 Gt/yr on average – in close agreement with regional climate model simulations (root mean square difference of 47 to 60 Gt/yr). As well as being 21 % higher between 2011 and 2020 than over the preceding three decades, runoff is now also 60 % more variable from year-to-year as a consequence of large-scale fluctuations in atmospheric circulation. Because this variability is not captured in global climate model simulations, our satellite record of runoff should help to refine them and improve confidence in their projections.

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Section: Resultsmentioning

confidence: 99%

Increased variability in Greenland Ice Sheet runoff from satellite observations

et al. 2021

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“…The minimal snow cover was likely the reason for the excellent agreement observed in the external-mission analysis of the Helheim region. However, there was a decrease in radar penetration depth, which could have been caused by fluctuations in density due to summer melt events [42]. Furthermore, for Helheim, we analyzed the crossover differences to see if there was dependence on surface slope, and found that the standard deviation increased only slightly with increasing surface slope.…”

Section: Discussionmentioning

confidence: 99%

Regional Assessments of Surface Ice Elevations from Swath-Processed CryoSat-2 SARIn Data

et al. 2021

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The Arctic responds rapidly to climate change, and the melting of land ice is a major contributor to the observed present-day sea-level rise. The coastal regions of these ice-covered areas are showing the most dramatic changes in the form of widespread thinning. Therefore, it is vital to improve the monitoring of these areas to help us better understand their contribution to present-day sea levels. In this study, we derive ice-surface elevations from the swath processing of CryoSat-2 SARIn data, and evaluate the results in several Arctic regions. In contrast to the conventional retracking of radar data, swath processing greatly enhances spatial coverage as it uses the majority of information in the radar waveform to create a swath of elevation measurements. However, detailed validation procedures for swath-processed data are important to assess the performance of the method. Therefore, a range of validation activities were carried out to evaluate the performance of the swath processor in four different regions in the Arctic. We assessed accuracy by investigating both intramission crossover elevation differences, and comparisons to independent elevation data. The validation data consisted of both air- and spaceborne laser altimetry, and airborne X-band radar data. There were varying elevation biases between CryoSat-2 and the validation datasets. The best agreement was found for CryoSat-2 and ICESat-2 over the Helheim region in June 2019. To test the stability of the swath processor, we applied two different coherence thresholds. The number of data points was increased by approximately 25% when decreasing the coherence threshold in the processor from 0.8 to 0.6. However, depending on the region, this came with the cost of an increase of 33–65% in standard deviation of the intramission differences. Our study highlights the importance of selecting an appropriate coherence threshold for the swath processor. Coherence threshold should be chosen on a case-specific basis depending on the need for enhanced spatial coverage or accuracy.

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“…where dH/dt is the long-term elevation change rate, H 0 is the reference elevation at reference time t 0 , and s 1 and s 2 are coefficients of the trigonometric functions used to fit the seasonal elevation change. For CryoSat-2 SARin observations, the impact of changing scattering properties of the ice sheet surface should also be considered, because changing snow penetration depth has a significant effect on radar altimetry observations (Slater et al, 2019;Otosaka et al, 2020). To alleviate this impact, a backscatter correction factor is applied following Simonsen and Sørensen (2017).…”

Section: Methods For Elevation Change Estimation Elevation Difference Methods For Cryosat-2 Synthetic Aperture Interferometric Mode and Amentioning

confidence: 99%

“…Previous research studies have shown that the penetration depth in a dry snow zone can achieve several meters (Slater et al, 2019) and that abrupt changes were shown when an extreme melting event occurs (Nilsson et al, 2015;Slater et al, 2019;Otosaka et al, 2020). Although the actual impact of penetration on elevation observations is greatly mitigated by retracking algorithms, it might still lead to a deviation from radar altimeter-derived elevation changes (Gray et al, 2019;Otosaka et al, 2020). To assess whether the CryoSat-2 derived elevation change is affected by radar penetration, the GrIS elevation change between 2010 and 2019 was further estimated by CryoSat-2 observations from four summer months (June, July, August, and September) and compared with the result of CryoSat-2 observations from all 12 months.…”

Section: Impact Of Penetration Depth Of Cryosat-2 Radar Altimetrymentioning

confidence: 99%

Elevation and Volume Changes in Greenland Ice Sheet From 2010 to 2019 Derived From Altimetry Data

et al. 2021

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Long-term altimetry data are one of the major sources to analyze the change in global ice reserves. This study focuses on the elevation and volume changes in the Greenland ice sheet (GrIS) from 2010 to 2019 derived from altimetry observations. In this study, the methods for determining surface elevation change rates are discussed, and specific strategies are designed. A new elevation difference method is proposed for CryoSat-2 synthetic aperture interferometric (SARin) mode observations. Through validation with Airborne Topographic Mapper (ATM) data, this new method is proved to be effective for slope terrains at the margins of the ice sheet. Meanwhile, a surface fit method is applied for the flat interior of the ice sheet where low resolution mode (LRM) observations are provided. The results of elevation change rates in the GrIS from 2010 to 2019 are eventually calculated by combining CryoSat-2 and ATM observations. An elevation change rate of −11.83 ± 1.14 cm·a−1 is revealed, corresponding to a volume change rate of −200.22 ± 18.26 km3·a−1. The results are compared with the elevation changes determined by Ice, Cloud, and Land Elevation Satellite (ICESat) from 2003 to 2009. Our results show that the overall volume change rate in the GrIS slowed down by approximately 10% during the past decade, and that the main contributor of GrIS ice loss has shifted from the southeast coast to the west margin of the ice sheet.

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Surface Melting Drives Fluctuations in Airborne Radar Penetration in West Central Greenland

Cited by 12 publications

References 54 publications

Increased variability in Greenland Ice Sheet runoff from satellite observations

Increased variability in Greenland Ice Sheet runoff from satellite observations

Regional Assessments of Surface Ice Elevations from Swath-Processed CryoSat-2 SARIn Data

Elevation and Volume Changes in Greenland Ice Sheet From 2010 to 2019 Derived From Altimetry Data

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