GRACE Science Team Meeting

Session: B.3 - Cryosphere
(Convener: )

13:30-13:45 Update on the state of the Cryosphere with GRACE
First Author: Isabella Velicogna
Co-Authors: s
13:45-14:00 Two decades of ice mass loss from SLR versus GRACE
First Author: Jennifer Bonin
Co-Authors: D. Chambers. M. Cheng
14:00-14:15 Integrating GRACE and GNET GPS in modeling the deglaciation of Greenland
First Author: Per Knudsen
14:15-14:30 Exploration of Combination of GRACE, ICESat, and GPS Data over Antarctica
First Author: Ryan Hardy
Co-Authors: R. S. Nerem; D.N. Wiese
14:30-14:45 Regionally Optimized GRACE Processing on Totten and Moscow University Glaciers
First Author: Yara Mohajerani
Co-Authors: I. Velicogna, E. Rignot, T. Sutterley

Title: Update on the state of the Cryosphere with GRACE
Presenter: Velicogna, Isabella

Abstract: In this presentation, we will discuss the state of mass balance of mountain glaciers and ice caps (GIC), and the Greenland and Antarctica ice sheets up to present (July 2017). The data reveal the acceleration in mass loss of the GIC at the regional level, the cold summer spell of 2017 in Greenland, and the general increase in data noise in 2017. We also perform an inter-comparison of different GRACE time-series on the GIC and ice sheets with results from Regional Climate Models (RCM) such as RACMO2.4 and MAR3.6. The results highlight differences between GRACE time series and their significance and deviations of these time series from RCM and possible causes for these differences. We conclude on how this analysis helps constrain uncertainties in ice sheet and glacier mass balance.

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Title: Two decades of ice mass loss from SLR versus GRACE
Presenter: Bonin, Jennifer
Co-Authors: D. Chambers. M. Cheng

Abstract: Surface mass balance (SMB) represents a significant portion of the overall mass balance of the Greenland and Antarctic ice sheets. Accurate modeling of the SMB processes is key to our understanding of the past, present and future evolution of the ice sheets in response to climate forcings. Characterizing errors and improving the models is necessary to enhance our understanding of the SMB processes driving change, but also to isolate other processes such as dynamic ice loss through the combined reduction of satellite, airborne, in-situ and model data. In this study we quantify the performance of the MERRA-2 and RACMO SMB models for the Greenland and Antarctic ice sheets using satellite gravity observations from the GRACE mission.

We use Satellite Laser Ranging (SLR) to extend the time series of ice mass change back in time to 1994. Because the SLR series is of far lesser spatial resolution than GRACE (5x5 spherical harmonics rather than 60x60), we apply a constrained inversion technique to better localize the signal. Simulations with idealized 5x5 harmonics have demonstrated that mass changes in Greenland and Antarctica cannot be separated from each other, but that the sum total of mass change in the two can be accurately depicted using the inversion method. Using real SLR and GRACE data, we find that when using monthly-averaged observations, SLR does not have the accuracy required to match GRACE and correctly solve for the mass change over Greenland and Antarctica. However, when we use a six-monthly or yearly collection of observations to create the input SLR fields, the inversion technique produces a long-term pattern of mass change which matches what is seen by GRACE quite well, and extends back in time before GRACE as well.

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Title: Integrating GRACE and GNET GPS in modeling the deglaciation of Greenland
Presenter: Knudsen, Per

Abstract: Through the recent years it has been demonstrated that changes in the Greenland ice sheet can be observed by GPS receivers mounted on the adjacent bedrock. Especially, the Greenland GPS Network (GNET) has proven that GPS is a valuable technique for detecting mass changes through the Earths elastic response.

An integration of GNET with other observations of the Greenland ice sheet, e.g. satellite altimetry and GRACE, has made studies of GIA progressing significantly. In this study, we aim at improving the monitoring of the ice sheet mass by utilizing the redundancy for reducing the influence of errors and to fill in at data voids and, not at least to bridge the gap between GRACE and GRACE FO. Initial analyses are carried out to link GRACE and GNET time series empirically. EOF analyses are carried out to extract the main part of the variability and to isolate errors. Subsequently, linear regression analyses are carried out and used in the integration. Preliminary results are derived and inter-compared.

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Title: Exploration of Combination of GRACE, ICESat, and GPS Data over Antarctica
Presenter: Hardy, Ryan
Co-Authors: R. S. Nerem; D.N. Wiese

Abstract: The major geodetic observation techniques available over Antarctica offer distinct perspectives on the effects of ice-sheet mass change and glacial isostatic adjustment (GIA). GRACE gravity data offer direct measurement of total mass change due to both processes; altimetry measures ice sheet and bedrock elevation change; and the motion of bedrock-mounted GPS receivers reflects both elastic loading and GIA. The combination of these techniques, which are distinguished in both spatial and temporal resolution, presents an opportunity for combination that can increase both resolutions. Our goal is to combine GRACE, ICESat, and GPS into a single model of disaggregated monthly Antarctic ice mass change and GIA, improving the spatial resolution offered by GRACE. We present attempts to combine these data over Antarctica and discuss the challenges of reaching this goal.

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Title: Regionally Optimized GRACE Processing on Totten and Moscow University Glaciers
Presenter: Mohajerani, Yara
Co-Authors: I. Velicogna, E. Rignot, T. Sutterley

Abstract: The Antarctic ice sheet is losing mass at an accelerating rate, with a sea level contribution that changed from 0.08mm/yr from 1992 to 2001 to 0.4mm/yr from 2002 to 2011. While most of this contribution comes from West Antarctica, Totten Glacier has the largest discharge of ice in East Antarctica, with a sea level rise potential of 3.9 m. Furthermore, the drainage basin of Totten Glacier, along the neighboring Moscow University Glacier are below sea level, extending hundreds of kilometers inland. Therefore, obtaining regional estimates of these drainage basins is of critical importance. The GRACE (Gravity Recovery and Climate Experiment) satellite has been providing mass balance time-series from geoid changes since 2002. Numerous mascon and harmonic GRACE solutions are available from different processing centers. Here we use a set of regionally-optimized mascons to study the mass balance of Totten and Moscow University glaciers. We obtain a trend of -16.53±4.09Gt/yr with an acceleration of -1.97±1.75Gt/yr2 for the two glaciers for the period April 2002 to December 2016 using the Ivins et al (2013) GIA model (errors include leakage, GIA, and regression errors). Our basin-scale results agree well with independent mass budget method (MBM) estimates. In addition, we use the results to evaluate surface mass balance models in this area. We calculate the MBM time-series within the area covered by our mascons using RACMO2.3 and MAR3.6.4. RACMO2.3 shows much better agreement with the GRACE estimates. Within the common period from April 2002 to December 2015, the MBM trends derived from RACMO2.3 and MAR3.6.4 were -15.58±1.83Gt/yr and -6.75±1.54Gt/yr respectively, while the GRACE time-series had a trend of -14.89±2.70 Gt/yr, which is in agreement with the RACMO-derived mass budget trend (only including regression errors here).

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