Last modified on Nov 08, 2023 12:37

Contributors: S. B. Simonsen (Technical University of Denmark), V. R. Barletta (Technical University of Denmark), R. Forsberg (Technical University of Denmark)

Issued by: Technical University of Denmark / Sebastian B. Simonsen

Date: 17/08/2023

Ref: C3S2_312a_Lot4.WP2-FDDP-IS-v1_202212_GMB_PQAR-v4_i1.1

Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1

History of modifications

List of datasets covered by this document

Related documents

Acronyms

General definitions

Brokered Product: A brokered product is a pre-existing dataset to which the Copernicus Climate Change Service (C3S) acquires a license, for the purpose of including it in the Climate Data Store (CDS).

Firn: Old snow on glaciers and ice sheets which have survived a melt season.

Generated Product: A generated product is a dataset made specifically for C3S, for the purpose of including it in the CDS.

Gravimetric Mass Balance (GMB): The mass balance of an ice sheet is the net difference between mass gained from snow deposition and mass lost by melting or iceberg calving. This is essentially the same as the mass change of the ice sheet. When mass balance is derived from measured changes in the Earth's gravitational field, this is referred to as a gravimetric mass balance.

Scope of the document

This document is the Product Quality Assessment Report for the Copernicus Ice Sheets and Ice Shelves service, gravimetric mass balance Essential Climate Variable (ECV). It summarizes the quality assessment results for the brokered datasets and discusses how well the Global Climate Observing System (GCOS) and user requirements have been met.

The Ice Sheets and Ice Shelves service addresses three ECVs by providing four separate products:

• Ice velocity is given for Greenland in product WP2-FDDP-IV-CDR-v4.
• Gravimetric mass balance is given for Greenland and Antarctica in product WP2-FDDP-GMB-CDR-v4 and is a brokered dataset from the Greenland and Antarctic Climate Change Initiative (CCI) projects.
• Surface elevation change is given for:
  • Antarctica in product WP2-FDDP-SEC-CDR-AntIS-v4
  • Greenland in product WP2-FDDP-SEC-CDR-GrIS-v4

Executive summary

In this document, we summarize the results of the product quality assessment of the brokered gravimetric mass balance Climate Data Record (CDR) v4.0 of the Greenland and Antarctic ice sheets. This gravimetric mass balance dataset was brokered from the Greenland and Antarctic CCI projects, as a whole and version updates of the timeseries are as follows:

• Version 1/2: Brokering of Greenland Gravity Recovery And Climate Experiment (GRACE)-Follow-On (FO) data.
• Version 3: Brokering of Antarctic GRACE-FO data.
• Version 4: Brokering of the updated time series.

Summarizing the CCI ice sheet's Product Validation and Intercomparison Reports (PVIRs) and End-to-End error budget, the Greenland basins' accuracy indicates an average Gravimetric Mass Balance (GMB) trend error of 8 GT/yr, with the overall estimate of Greenland mass loss accuracy at 15 GT/yr; for Antarctica, these estimates are more than twice these numbers, due to the large uncertainty in Antarctic Glacial Isostatic Adjustment (GIA) models, as discussed in Shepherd at al. (2018).

1. Product validation methodology

The Gravimetric Mass Balance (GMB) products are brokered from the two ice sheet European Space Agency (ESA) Climate Change Initiative (CCI) products and contain data from the Gravity Recovery And Climate Experiment (GRACE) and GRACE-Follow-On (FO). Continuing the GRACE time series with the GRACE-FO mission has been confirmed in several papers (e.g., Velicogna et al, 2020). Hence, CCI delivers a near-continuous GMB time series from 2002-2020, with the exception of the one-year gap between GRACE and GRACE-FO in 2017/18 (Figure 1).

Figure 1: Total mass loss of Antarctica from GRACE / GRACE-FO missions 2003-2020, with monthly error estimates for Antarctica (upper) and Greenland (lower). The average mass for the period was -92 GT/yr for Antarctica, and -250 GT/yr for Greenland. From TU Dresden Antarctic Ice Sheet (AIS) and Greenland Ice Sheet (GrIS) CCI data portal.

For product validation the relevant CCI documents are the Product Validation and Intercomparison Report (PVIR), and the end-to-end uncertainty budget (E3UB) report both of which are available at:

• Greenland Ice Sheet (GrIS):
  • PVIR version 1.0, available at: https://climate.esa.int/media/documents/ST-DTU-ESA-GISCCI-PVIR-001_v1.0.pdf
  • E3UB: version 1.0, available at: https://climate.esa.int/media/documents/ST-DTU-ESA-GISCCI-E3UB-001-v1.1.pdf
• Antarctic Ice Sheet (AIS):
  • PVIR version 1.3, available at: https://climate.esa.int/media/documents/ST-UL-ESA-AISCCI-PVIR-001_v1.3a.pdf
  • E3UB: version 1.0, available at: https://climate.esa.int/media/documents/ST-UL-ESA-AISCCI-E3UB-001-v1.0.pdf ¹

These documents are continuously being updated on a yearly basis (less frequently for Antarctica).

The basic methodology and validation for the L1 processing of the gravimetric mass balance dataset has not changed since the first C3S-CDRv1.0 brokering and is still based on two different methods (mascon inversion for Greenland and spherical harmonic filtering for Antarctica, delivered by DTU and TU Dresden, respectively. Both methods are known from the CCI Greenland project to give essentially the same results, when applied in a consistent manner (the advantage of the mascon approach in Greenland is the ability to better separate "aliasing" from nearby Canadian ice caps), see Forsberg et al., (2017). In the following, we refer mainly to the E3UB documents for the full error propagation of the following error sources:

• GRACE solutions, by propagation of the scaled standard deviation.
• Glacial Isostatic Adjustment (GIA) model, by intercomparison of different models.
• Signal leakage, by analysing different solution areas.
• Degree-one, by intercomparison of different degree-one time-series.

2. Validation results

Please refer to the CCI ice sheets PVIRs and E3UB (see links above), which in summary of the Greenland basins' accuracy indicate an average GMB trend error of 8 GT/yr, with the overall estimate of Greenland mass loss accuracy at 15 GT/yr; for Antarctica, these estimates are more than twice these numbers, due to the large uncertainty in Antarctic GIA models, as discussed in Shepherd at al. (2018).

3. Application(s) specific assessments

GMB data from GRACE/GRACE-FO cannot be directly assessed by any other methods directly measuring gravity changes. Only indirect assessments are possible, e.g. by using elevation changes in conjunction with models of firn compaction and snow density, or by using the input-output method for mass loss of drainage basins, which again is dependent on assumptions on outlet glacier geometry, depths, and deep ice velocities, as well as model assumptions on snowfall in the interior. The Ice Sheet Mass Balance Inter-comparison Exercise (IMBIE) assessments (Shepherd et al., 2018; Shepherd et al., 2019) provide the only ice sheet-wide current assessments. The GMB data brokered to C3S are the same data that went into the IMBIE studies, within the IMBIE time frame.

4. Compliance with user requirements

The GRACE solution provided for the major drainage basins are brokered from the Greenland and the Antarctic ice sheet CCI projects. For both processing algorithms and uncertainty estimates we refer to Barletta, Sørensen and Forsberg (2013) and Groh and Horwath (2016).

The primary GCOS Implementation Plan, (GCOS-200, 2016), and ESA CCI user requirements (Forsberg et al., 2017) for Gravimetric mass balance are met in terms of horizontal resolution, record length and revisit times, see Table 1. If typical ice densities are assumed, the measurement uncertainties are at present about twice the requirement for Antarctica, and roughly met in Greenland, except for the spatial resolution. The 50 km C3S resolution is subsampled from data of only 250-300 km resolution; better resolution is not possible from current space gravity change missions due to a satellite orbit at 420-500 km above the Earth's surface.

A major issue for Greenland is whether to include or exclude the outlying ice caps. While C3S Ice Velocity (IV) and Surface Elevation Change (SEC) data only cover the main ice sheet, GMB data will include mass changes for all ice caps and glaciers in Greenland. This is due to its nature and inseparability of close ice mass bodies due to the resolution. The effect of the local ice caps is of the order of 30 GT/yr, depending on the definition of the "icesheet proper" (Bolch et al., 2013). If this is misunderstood, it could be a source of 'user error'.

Any update or new user requirement defined under the C3S service will be presented in the TRGAD (D3).

Table 1: GCOS target requirements for Ice Mass Change (source: GCOS Implementation Plan, 2016) in comparison to C3S GMB product (Green indicates compliances and yellow highlights the partial compliances).

References

Barletta, V. R., Sørensen, L. S., & Forsberg, R. (2013). Scatter of mass changes estimates at basin scale for Greenland and Antarctica. The Cryosphere, 7(5), 1411-1432.

Bolch, T., Sandberg Sørensen, L., Simonsen, S. B., Mölg, N., Machguth, H., Rastner, P., & Paul, F. (2013). Mass loss of Greenland's glaciers and ice caps 2003–2008 revealed from ICESat laser altimetry data. Geophysical Research Letters, 40(5), 875-881.

Forsberg, R., Sørensen, L., & Simonsen, S. (2017). Greenland and Antarctica ice sheet mass changes and effects on global sea level. In Integrative Study of the Mean Sea Level and Its Components (pp. 91-106). Springer, Cham.

Forsberg, R. et al. (2017) User Requirements Document (URD), ESA Climate Change Initiative (CCI), Greenland Ice Sheet (GIS) Essential Climate Variable (ECV), Version, 2.4, 22-11-2017, ST-DTU-ESA-GISCCI-URD-001, available at: https://climate.esa.int/media/documents/ST-DTU-ESA-GISCCI-URD-001_v1.0.pdf (URL resource last accessed 17^th August 2023)

Forsberg, R. et al. (2018) Product Validation and Intercomparison Report (PVIR), ESA Climate Change Initiative (CCI), Greenland Ice Sheet (GIS) Essential Climate Variable (ECV), Version, 3.0, 25-06-2018, ST-DTU-ESA-GISCCI-PVIR-001, available at: https://climate.esa.int/media/documents/ST-DTU-ESA-GISCCI-PVIR-001_v3.0.pdf (URL resource last accessed 17^th August 2023)

Groh, A., & Horwath, M. (2016, April). The method of tailored sensitivity kernels for GRACE mass change estimates. In EGU General Assembly Conference Abstracts (pp. EPSC2016-12065).
GCOS-200 (2016), The Global Observing System for Climate: Implementation Needs, World Meteorological Organization (WMO) (2016). https://library.wmo.int/doc_num.php?explnum_id=3417 (URL resource last accessed 17^th August 2023)

Hvidberg, C.S., et al. (2012), User Requirements Document for the Ice_Sheets_cci project of ESA's Climate Change Initiative, version 1.5, 03 Aug 2012.

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