B.2
Cryosphere

B.2

Cryosphere
Orals
| Wed, 19 Oct, 14:39–15:03 (CEST), 18:03–18:15 (CEST)|Lecture Hall, Building H

Orals: Wed, 19 Oct, 14:39–18:15 | Lecture Hall, Building H

14:39–14:51
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GSTM2022-14
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On-site presentation
Combination of Geometric and Gravimetric Data across Greenland for the Estimation of Changing Ice Masses 
Matthias Graf and Roland Pail

In this study, a method is developed which allows to combine gravimetric and geometric data in order to improve the spatial resolution of the resulting mass balance estimate. The so called equivalent ice density of the changing ice volume is estimated within a mathematical inversion model which includes as observations geometric information about the volumetric change of the ice sheet and the resulting gravity change. In the presentation, detailed insights into the mathematical and stochastic procedures will be delivered.

As gravimetric information, monthly GRACE gravity fields are applied. They have a limited spatial resolution of a few 100 km, but allow direct conclusions about the true mass changes across Greenland. The ice volumes are described by a product of ESA’s Climate Change Initiative which is derived from altimetry data. They have a very fine spatial resolution (down to a few km), but are not directly sensitive to the density of the changing ice volumes.

By combining both datasets in a common mathematical model, the advantages of both data types (direct sensitivity to mass vs. high spatial resolution) are used to improve the spatial resolution of mass balance estimates across Greenland. The result will be a map of mass trends which has the same spatial resolution like the input map of geometric changes. At the same time, the high-resolution distribution of surface-mass changes is reasoned in the GRACE gravity fields. The resulting distribution is compared to the results of other studiers regarding the spatial structure of mass changes and the total mass loss.

However, due to limits concerning the computational effort regarding the applied computers, it is only possible to recognize structures which correspond to the applied sampling rate of the grid on which the density distribution is estimated. Furthermore, regularization methods due to near-singularities of the normal equation matrix have to be applied. Along the coasts due to the limited resolution of the input GRACE gravity fields, leakage effects occur in the sense of a physically unreasonable assignment of mass changes to nearby oceanic surfaces. However, those effects can be partly reduced through the here presented combination approach.

How to cite: Graf, M. and Pail, R.: Combination of Geometric and Gravimetric Data across Greenland for the Estimation of Changing Ice Masses, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-14, https://doi.org/10.5194/gstm2022-14, 2022.

14:51–15:03
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GSTM2022-69
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On-site presentation
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Estimation of mass variations within Greenland by an inversion of GRACE/GFO spherical harmonic coefficients into a global set of mascon-type mass anomalies 
Pavel Ditmar

Satellite gravimetry is one of the primary tools to monitor mass variations of ice sheets. In this study, we apply data from GRACE and GRACE Follow-on (GFO) missions to quantify ice mass variations per Drainage Basin (DB) of the Greenland Ice Sheet (GrIS). To that end, we invert monthly sets of GRACE/GFO-based sets of spherical harmonic coefficients into mascon-type mass anomalies defined on a global equiangular grid. To suppress noise in the obtained estimates, a first-order Tikhonov regularization is applied. Regularization parameters are defined separately for ocean and land areas to ensure a much higher smoothness of mass anomalies over the ocean, as compared to the land. Such an approach offers a number of advantages, as compared to an estimation of mascon-type mass anomalies within a limited region only: (i) there is no need in low-degree coefficients that cannot be provided by GRACE/GFO missions with a sufficient accuracy (such as degree-1, C2,0, and C3,0 coefficients); (ii) it is not necessary to incorporate a correction for a leakage of hydrological and ice-related signals into Greenland from surrounding areas, since those signals are co-estimated; (iii) error propagation is straightforward.

In our study, we use as input the monthly ITSG-Grace2018 solutions produced at Graz Technical University (Austria). The resulting mass anomalies are estimated on a global 1o × 0.4o grid. Ultimately, they are integrated to yield the total mass anomaly per DB (the GrIS being split into six DBs).To enable the data inversion in spite of a large number of unknown parameters, we exploit the pre-conditioned conjugate gradient method. Two different regularization parameters are used within the study area: (i) a relatively small parameter for the Greenland’s coastal area and the periphery of the GrIS and (ii) a much larger regularization parameter for the inner part of the GrIS. To find the optimal values of the regularization parameters, we make use of a numerical study. We simulate a realistic signal and noise for one particular month (namely, Jan. 2019). Then, we select the regularization parameters that result in the smallest RMS differences between the estimated and the “true” mass anomalies per DB. Those parameters ensure a sufficient suppression of random noise in the mass anomaly estimates, while avoiding a substantial signal leakage between the DBs. The optimal values of the regularization parameters are applied to process all the available GRACE/GFO monthly solutions in the time interval from Apr. 2002 to Dec. 2021. This allows us to produce a time-series of mass anomalies for each of the six DBs under consideration, as well as for the territory of Greenland as a whole. The obtained results were submitted to the IMBIE-3 inter-comparison project.

How to cite: Ditmar, P.: Estimation of mass variations within Greenland by an inversion of GRACE/GFO spherical harmonic coefficients into a global set of mascon-type mass anomalies, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-69, https://doi.org/10.5194/gstm2022-69, 2022.

18:03–18:15
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GSTM2022-99
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On-site presentation
Greenland, Antarctica and Glaciers and Ice Caps mass balance from GRACE/GRACE-FO and other data 
Isabella Velicogna, Chia-Chun Liang, Tyler Sutterley, Geruo A, Enrico Ciraci, and Felix Landerer

We discuss the state of mass balance of glaciers and ice sheets from 2002 to present using data from GRACE/GRACE-FO missions, after filling the gap between missions. We compare data processed by different centers (JPL, CSR, GFZ) and evaluate various Glacial Isotatic Adjustment models.  In Greenland, the data indicate a persistent mass loss at 251 Gt/yr, with an acceleration of 3 Gt/yr/yr, and large summer losses (400-600 Gt) in 2012, 2017, 2019. The mass balance regime has been evolving significantly in recent years, especially in the North, which holds the largest potential for rapid sea level rise. In Antarctica, ongoing mass losses in the Amundsen Sea Embayment of West Antarctica (122 Gt/yr), Antarctic Peninsula (26 Gt/yr), and Wilkes Land in East Antarctica (33 Gt/yr) dominate a small but significant increase in snowfall in the Queen Maud Land sector of East Antarctica since 2009 (47 Gt/yr). For the GIC, the mass loss averages 274 Gt/yr, with an acceleration of 4 Gt/yr/yr. The largest contributors are in the Arctic: Canadian Archipelago (70 Gt/yr), Alaska (72 Gt/yr), Russian Arctic (21 Gt/yr), Svalbard and Iceland (29 Gt/yr)) versus the southern hemisphere which is dominated by Patagonia (35 Gt/yr). High Mountain Asia averages 22 Gt/yr mass loss, with a large inter-annual variability. In regions not dominated by ice dynamics, the GRACE results compare better every year with output products from regional climate models (MAR, RACMO) forced by ERA5 and with global models such as NASA’s MERRA-2, which offers interesting perspectives for model development.

 

How to cite: Velicogna, I., Liang, C.-C., Sutterley, T., A, G., Ciraci, E., and Landerer, F.: Greenland, Antarctica and Glaciers and Ice Caps mass balance from GRACE/GRACE-FO and other data, GRACE/GRACE-FO Science Team Meeting 2022, Potsdam, Germany, 18–20 Oct 2022, GSTM2022-99, https://doi.org/10.5194/gstm2022-99, 2022.