Contributors: Lin Gilbert (University of Leeds), Sebastian B. Simonsen (Technical University of Denmark), Rene Forsberg (Technical University of Denmark), Jan Wuite (ENVEO IT GmbH)
Issued by: Technical University of Denmark / Sebastian B. Simonsen
Date: 30/06/2021
Ref:C3S_312b_Lot4_D2.IS.4-v3.0_Product_Quality_Assessment_Report_GMB_i1.0
Official reference number service contract: 2018/C3S_312b_Lot4_EODC/SC2
History of modifications
List of datasets covered by this document
Related documents
Acronyms
1. Scope of the document
This document is the Product Quality Assessment Report for the Copernicus Ice Sheets and Ice Shelves service, gravimetric mass balance ECV. It presents results of the quality assessment for the provided datasets and a discussion of how well Global Climate Observing System (GCOS) and user requirements have been met.
2. Executive summary
The service addresses three essential climate variables (ECVs) by providing four separate products.
- Ice velocity is given for Greenland in product D3.IS.4
- Gravimetric mass balance is given for Greenland and Antarctica in product D3.IS.5
- Surface elevation change is given for
- Antarctica in product D3.IS.6.1
- Greenland in product D3.IS.6.2
In this document we provide results for the gravimetric mass balance CDR v3.0. This gravimetric mass balance dataset was brokered as a whole from the beginning of the GRACE mission in 2002 at the beginning of the project. Version updates are as following:
- Version 2: Brokering of Greenland GRACE-FO data.
- Version 3: Brokering of Antarctic GRACE-FO data.
The basic methodology and validation for the L1 processing of the gravimetric mass balance dataset has not changed since v1.0 of this document 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.
The addition of data from the GRACE-FO mission have been confirmed in several papers and ESA CCI deliverables (Velicogna et al, 2020), so a near-continuous GMB time series from 2002-2020 is now available, except for the one-year gap between GRACE and GRACE-FO in 2017/18.
The overall estimates of the accuracy of the GMB data for Greenland is around 15 GT/yr, and for Antarctica 30 GT/yr, the latter larger error mainly due to large errors in GIA models for Antarctica.
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 AIS and GIS CCI data portal.
3. Product validation methodology
The GMB products are brokered from the two ice sheet CCI products and relevant documentation is available at:
- GIS: Product Validation and Intercomparison Report (PVIR), version 1.0, available at: https://climate.esa.int/media/documents/ST-DTU-ESA-GISCCI-PVIR-001_v1.0.pdf
- AIS: Product Validation and Intercomparison Report (PVIR), version 1.3, available at: https://climate.esa.int/media/documents/ST-UL-ESA-AISCCI-PVIR-001_v1.3a.pdf
End-to-end uncertainty estimates of the ECV's from the recent phase of the CCI project are available at
- GIS: https://climate.esa.int/media/documents/ST-DTU-ESA-GISCCI-E3UB-001-v1.1.pdf
- AIS: https://climate.esa.int/media/documents/ST-UL-ESA-AISCCI-E3UB-001-v1.0.pdf
These documents are continuously being updated on a yearly basis (less frequently for Antarctica)
4. Validation results
Please refer to the CCI ice sheets Product Validation and Intercomparison Reports (PVIRs) and End-to-End error budget documents as given in Section 3 above. The summary of the Greenland basins accuracy point to 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).
5. 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 snow fall in the interior. The IMBIE assessments (Shepherd et al., 2018; Shepherd et al., 2019) provide the only icesheet-wide current assessments. The GMB data brokered to C3S are the same data which went into the IMBIE studies, within the IMBIE time frame.
6. 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, 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, 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 resolution (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 IV and 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 are in 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'.
Table 1: GCOS target requirements for Ice Mass Change (source: GCOS Implementation Plan, 2016) in comparison to C3S GMB product (Green shows current status, yellow shows planned).
Requirement | C3S and GCOS target requirements | C3S 312b Lot 4 Products |
Product Specification | ||
Parameter of interest | Ice Mass Change | Gravimetric mass balance (GMB) |
Unit | km3/yr | Basins: GT/yr, Grids: mm water-eq./yr. |
Product aggregation | Not specified | GRACE mass change grids |
Spatial resolution | 50 km | 50 km, interpolated, real resolution 250-300 km |
Record length | 2002-2020 | 2002-2020 |
Revisit time | Monthly | Monthly (basin estimates only) |
Product accuracy | 10 km3/yr | 30 GT/yr for Antarctica and 15 GT/yr for Greenland (total mass change; GIA uncertainty) |
Product stability | 10 km3/yr | 10 GT/yr |
Quality flags | Not specified | None |
Uncertainty | Grid | GMB grids with associated error estimate |
References
Barletta, V.R., Sørensen, L.S. and Forsberg, R. (2013). Scatter of mass changes estimates at basin scale for Greenland and Antarctica. The Cryosphere, 7, 1411-1432
Bolch, T., L. Sandberg Sørensen, S. B. Simonsen, N. Mölg, H. Machguth, P. Rastner, F. Paul: Mass loss of Greenland's glaciers and ice caps 2003–2008 revealed from ICESat laser altimetry data. Geoph. Res. Letters, 2013, doi.org/10.1002/grl.50270.
Forsberg, R., Sørensen, L. & Simonsen, S: Greenland and Antarctica Ice Sheet Mass Changes and Effects on Global Sea Level. Surveys in Geophysics (2017) 38: 89. doi:10.1007/s10712-016-9398-7.
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: http://esa-icesheets-greenland-cci.org/index.php?q=webfm_send/169
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: http://esa-icesheets-greenland-cci.org/index.php?q=webfm_send/180
Groh, A., and Horwath, M. (2016). The method of tailored sensitivity kernels for GRACE mass change estimates. Geophysical Research Abstracts, 18, EGU2016-12065.
Hvidberg, C.S., et al., User Requirements Document for the Ice_Sheets_cci project of ESA's Climate Change Initiative, version 1.5, 03 Aug 2012. Available from: http://www.esa-icesheets-cci.org/
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This document has been produced in the context of the Copernicus Climate Change Service (C3S).
The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement signed on 11/11/2014). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.
The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.
