Session: B.3 Cryosphere
Title: Extending the GRACE time series into the past and future using an SLR inversion
Presenter: Bonin, Jennifer
Abstract: We aim to fill the gap between GRACE and GRACE Follow-On, as well as extend the GRACE time series before 2002, specifically over Greenland and Antarctica. We use a least squares inversion technique to separate 5x5 spherical harmonic series into regional kernels, constraining the solution with apriori values from the RACMO2 regional climate model. We optimize the estimation technique's parameters via a simulation, then apply it to the 5x5 SLR and GRACE data. We compare the results of the SLR and GRACE inversions over different parts of Greenland and Antarctica, to determine if SLR can be used to fill in for GRACE when needed.
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Ice sheet surface mass balance from models and GRACE
Presenter: Luthcke, Scott
Co-Authors: B. Loomis
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 compare the spatial and temporal variation of the intrinsic modes determined from the SMB models and the NASA GSFC release 2 iterated high-resolution GRACE mascon solution. We take advantage of the mascon solution's improved signal to noise and minimization of signal leakage. The Ensemble Empirical Mode Decomposition (EEMD) adaptive filter is used to isolate seasonal, annual, and inter-annual modes in both the GRACE mascons and SMB model data. Spatial and temporal variations are compared and analyzed. The SMB model output of spatial and temporal surface mass change is incorporated into our forward modeling used in the formal reduction of the GRACE inter-satellite range-rate data. We analyze the new estimated mascon solution which has the SMB model output rigorously removed through forward modeling in the GRACE range-rate data reduction. The EEMD analysis is applied to the “delta mascon” solution and analyzed. The results of the EEMD analysis as well as the “delta mascon” solution are presented and interpreted. The techniques developed can be used to quantify the performance and improve other SMB models and to isolate other processes such as dynamic ice loss.
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Title: Impact of spatial variability of ice sheet mass loss on regional sea level
Presenter: Hsu, Chia-Wei
Co-Authors: I. Velicogna
Abstract: We generate static regional sea level variations (SLF) from the mass budget method (MBM) and evaluate the sensitivity of the inferred SLF on the spatial distribution in ice mass loss. We find that approximating the ice mass loss with localized point sources introduces a 25% underestimate of the regional sea level in the near field in Greenland and not considering a realistic ice mass distribution in the interior of the ice sheet yields errors of 30% in the near field of Greenland and Antarctica, and 15% in the far field of Greenland and 10 % in the far field of Antarctica. We conclude that in order to reconstruct realistic SLF solutions, we need to know the ice mass loss distribution at a scale of 1 degree over the entire ice sheet, and it is essential to employ a realistic distribution of changes in surface mass balance of the ice sheet interior, especially in Antarctica. As a result, we derive 1 deg x 1 deg ice sheet mass loss products using GRACE data that yield SLF products which will be made available to the scientific community at large via the NASA Sea Level Change Team (N-SLCT).
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Title: Assessments of long-term mass balance of the Canadian Arctic Archipelago glaciers and Greenland ice sheet from GRACE
Presenter: Chen, Jianli
Abstract: We provide improved quantifications of long-term mass balance the Canadian Arctic Archipelago (CAA) glaciers and Greenland ice sheet using GRACE satellite gravity measurements. As the CAA glaciers and Greenland ice sheet are closely located and mostly surrounded by oceans, leakage effects between CAA and Greenland, and between CAA/Greenland and surrounding ocean areas are expected to be a major challenge for accurate quantification of CAA and Greenland ice mass balance. In addition, how to treat the dominant GRACE north-south oriented striped noise will also affect estimated CAA and Greenland ice mass rates, due to the similar north-south spatial patterns of ice mass changes over CAA and east and west Greenland. Based on different treatments of GRACE leakage effect and striped noise, GRACE estimated CAA ice mass rates can range from - 77 Gt/yr to - 98 Gt/yr over the period 2003 to 2012, and over the same period the Greenland ice mass rates are estimated to be -258 Gt/yr to - 238 Gt/yr. In this presentation, we will discuss in details how different data processing methods and treatments of leakage effect and spatial noise will affect GRACE estimates at different spatial scales.
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Title: Using GRACE and Climate Model Simulations to Predict Mass Loss of Alaskan Glaciers Through 2100
Presenter: Swenson, Sean
Co-Authors: J. Wahr; S. Swenson; E. Burgess
Abstract: Glaciers in Alaska are presently losing mass at a rate of about 50 gt/yr, the largest ice loss rate of any mountain glacier system on Earth. How will this rate be affected by future changes in climate? We address this question using a two-step approach. (1) We use monthly GRACE gravity fields from August, 2002 through December, 2014, to compute a time series of Alaskan glacier mass variability. We construct a simple three-parameter model of the mass variability, that is based on monthly ERA-Interim snowfall and temperature fields. By adjusting those three parameters to best match the GRACE time series, we find we can explain 94% of the variance of the GRACE data. (2) We use simulated fields of monthly temperature and snowfall through 2100 from the Community Earth System Model, and apply this same simple model with these same parameter values to those simulated fields, to obtain predictions of mass variations through 2100. We conclude that mass loss rates will likely increase to roughly 80 to 90 gt/yr by 2100, with a total sea level rise contribution on the order of 20 mm during the twenty-first century.
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Title: Regional evaluation of an ice flow model for Greenland using GRACE mascons
Presenter: Wiese, David
Co-Authors: N. Schlegel, M. M. Watkins, E. Y. Larour, J. E. Box, X. Fettweis, M. R. van den Broeke
Abstract: Quantifying Greenland's future contribution to sea level rise is a challenging task and requires accurate estimates of ice flow sensitivity to changing climates. Transient ice flow models are promising tools for estimating future ice sheet behavior; however, confidence in these types of projections is low due to a scarcity of data for validation of model historical runs. For more than a decade, the Gravity Recovery and Climate Experiment (GRACE) has continuously acquired time-variable measurements of the Earth's gravity field and has provided unprecedented surveillance of mass balance of the ice sheets, offering an opportunity for ice sheet model evaluation. Here, we take advantage of a new high-resolution (~300 km) monthly mascon solution for the purpose of mass balance comparison with an independent, historical ice flow model simulation using the Ice Sheet System Model (ISSM). The comparison highlights which regions of the ice sheet model differ most from GRACE observations. Investigation of regional differences in trends and seasonal amplitudes between simulations forced with different Regional Climate Model (RCM)-based estimates of surface mass balance (SMB) allows us to make conclusions about the relative contributions of errors in both models of SMB and ice dynamics. We highlight the importance of utilizing an ice flow model for future projections rather than relying solely on projecting future changes in SMB. This study constitutes the first regional comparison of GRACE data and an ice sheet model. Conclusions will aid in the improvement of RCM SMB estimates as well as ice sheet simulation estimates of present and future rates of sea level rise.
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Title: Comparing GRACE and GNET GPS analyses of the 2013 'pause' in the deglaciation of Greenland
Presenter: Knudsen, Per
Co-Authors: M. Bevis; S.A. Khan
Abstract: Between the start of 2003 and the middle 2013, the total mass of ice in Greenland declined at an accelerating rate, and this rate of acceleration was a nearly constant -26.9 ± 2.8 Gt/yr2. Then, a dramatic reversal occurred, and almost no additional ice mass was lost in the subsequent year. We use observations from the Gravity Recovery and Climate Experiment (GRACE) and a network of Global Positioning System (GPS) receivers to study both the decade of accelerating ice loss, and the subsequent 'pause', focusing on the space-time structure of changes in ice mass. We use a spatial basis set of spherical Slepian functions, and assume that at any given point the temporal variation in mass can be expressed using a 4-term Fourier series (i.e. an annual cycle) superimposed on a polynomial in time (i.e. a trend). We show that the spatial pattern of the sustained, decade-long acceleration and of the mass anomaly associated with the pause are very similar, and so manifest the footprint of the ice sheet's sensitivity to climate change at the wavelengths resolved by GRACE.
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Title: Reconciling Apparent Winter Mass Loss over Greenland between October and November 2011
Presenter: Hardy, Ryan
Co-Authors: D. Wiese, N. Schlegel
Abstract: Greenland's cumulative mass loss curve shows a few unexpected and abrupt episodes of mass loss during Greenland's winters, when conditions are normally too cold for melt and runoff to drive mass loss. These events present an opportunity to explore systematic errors in GRACE, and may shed light on unmodeled ice sheet processes. We probe one of these events in October and November 2011, during which Greenland appears to lose ~70 Gt. Much of this signal originates from Greenland's northeast coast. We look for the signatures of this mass loss in other geodetic and geophysical data and investigate the role of leakage and atmospheric modeling errors in producing this signal.
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Title: Uncertainties in sheet mass balance in Greenland and Antarctica from GRACE and comparison with other methods
Presenter: Sutterley, Tyler
Co-Authors: I. Velicogna
Abstract: We present updated time series of ice mass loss derived from GRACE data for the regions that control the ice sheet mass loss in Greenland and Antarctica. We compare the results with surface mass balance estimates to highlight whether the mass loss is dominated by surface mass balance processes or by ice discharge. We investigate the effect of geocenter motion and of orbit repeat track of the satellite pair in spring 2015 on the long-term ice sheet mass balance estimates. Finally, we show agreement in specific regions of the ice sheets between four independent techniques: 1) GRACE, 2) laser altimetry, 3) radar altimetry and 4) the mass budget method.
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