Contributors: Lin Gilbert (University of Leeds), Sebastian B. Simonsen (Technical University of Denmark), Jan Wuite (ENVEO IT GmbH)
Issued by: University of Leeds / Lin Gilbert
Date: 04/05/2021
Ref: C3S_312b_Lot4.D2.IS.5_v3.0_SEC_Product_Quality_Assurance_Document_i1.0.docx
Official reference number service contract: 2018/C3S_312b_Lot4_EODC/SC2
History of modifications
List of datasets covered by this document
Related documents
Acronyms
Scope of the document
This document is the Product Quality Assurance Document for Surface Elevation Change (SEC) as part of the Copernicus Ice Sheets and Ice Shelves service. It describes the validated datasets and the methods used for validation.
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
We document here the validation methods and results from the CDR v3 for the two Polar region SEC products.
1. Validated products
1.1. Surface elevation change, Antarctica – D3.IS.6.1
The product is a netCDF file containing monthly gridded maps of the rate of surface elevation change over the Antarctic region, including ice sheet, ice shelves, ice rises and islands. Rates are given in m/year. The grid is based on a polar stereographic projection with central meridian 0E, standard parallel 71S, no false northing or false easting, using ellipsoid WGS84 (see National Geospatial Intelligence Agency, 2004). Its resolution is 25km by 25km.
The rates of change in each map are derived from the 5-year period centred on that map's timestamp. The timestamps are one month apart. The initial climate data record (CDR) contains surface elevation change rate maps centred on November 1994 to May 2018 Due to the 5-year aggregation window, the data used to derive each map extends 2.5 years to each side of the central timestamps, so the initial v3 CDR uses data from May 1992 to November 2020, and each monthly interim CDR (ICDR) adds one map. The ICDRS are accumulative, containing all previous data as well as the latest monthly map. The v3 CDR and ICDRs incorporate reprocessed v2 data and extend the timeseries beyond the end of v2. As well as maps, timestamps and grid definitions, the product includes uncertainties on each rate, based on the combination of three uncertainty sources – input data, cross-calibration between satellites, and derivation of the rate of change from modelling. Flags are given for where rates have been produced, for geographical surface type, and for geographical regions of high slope. An example map is shown below in Figure 1 .
Figure 1: Example surface elevation change map from product D3.IS.6.1
1.2. Surface elevation change, Greenland – D3.IS.6.2
The surface elevation change product is available as a single NetCDF-file containing monthly gridded maps of the rate of surface elevation change over the Greenland ice sheet. Following glaciological conventions, the rates of elevation change are given in m/year. The data are posted in a polar stereographic projection (EPSG:3413), with central meridian 45W, standard parallel 70N, no false northing or false easting, using ellipsoid WGS84 (see National Geospatial Intelligence Agency, 2004) and a resolution is 25 km by 25km.
The rates of change in each map are derived from the 3-year or 5-year period centred on that map's timestamp. The timestamps are one month apart. The difference in time period is due to the enhanced capabilities of CryoSat-2 which enables the surface elevation change to be derived for shorter time periods. The initial climate data record (CDR) contains maps centred on November 1994 to April 2018, and each monthly interim CDR (ICDR) adds one map. The ICDRs are accumulative, containing all previous data as well as the latest monthly map. With the introduction of the present version 3 of the CDR, we take advantage of an updated surface elevation algorithm yielding an estimate of the elevation change at all months within the temporal window (3-year or 5-year period) and provide this solution in the CDR/iCDRs. This update results in a reduction of the time delay of the solution from 1.5 years (the latest solution for April 2018 was provided at the end of 2019) to 6 months. Hence, at the end of 2020 we provide elevations changes until July 2020.
As well as maps, timestamps and grid definitions, the product includes uncertainties on each rate, based on the combination of three uncertainty sources – input data, cross-calibration between satellites, and derivation of the rate of change from modelling. Flags are given for where rates have been produced, for geographical surface type, and for geographical regions of high slope. An example map is shown in Figure 2.
Figure 2: Example of accumulated surface elevation change map produced from product D3.IS.6.2
2. Description of validating datasets
2.1. Surface elevation change – D3.IS.6.1 and D3.IS.6.2
Independent estimates of the rate of surface elevation change at discrete locations and over specific time periods are provided by the Airborne Topographic Mapper, a scanning laser altimeter flown on board aircraft by Operation IceBridge (OIB) (Studinger 2014). Observation campaigns are conducted in the hemispheric spring in most years, and dataset production usually lags by a year. The validating dataset used by both hemispheric SEC products is the level 4 product, ATM L4 Surface Elevation Rate of Change V001. This can be obtained free of charge on registration, from https://icebridge.gsfc.nasa.gov/
. The surface elevation rate of change in this dataset is calculated from the change in elevations recorded at the same location between two overflights. Operation IceBridge ceased at the end of the Antarctic campaign in 2020.
3. Description of product validation methodology
3.1. Surface elevation change, Antarctica – D3.IS.6.1
The validation method is a comparison of SEC results with all OIB ATM measurements from the validating dataset that coincide in time and space. This is not a direct comparison to the product dataset, as the ATM overflights are highly irregular. Instead we use the underlying elevation change timeseries and analysis methods from which the product dataset was derived to produce results comparable to the ATM.
To make the surface elevation change product, first we divide the Antarctic ice region into a 25km by 25km grid. In each grid cell we use crossover analysis to derive a series of elevation change values (dh) over time from each of six radar altimetry missions – ERS1, ERS2, Envisat, CryoSat-2 and Sentinel-3A and Sentinel-3B. The six timeseries are cross-calibrated and combined to produce a single timeseries of elevation change (i.e., dh vs t).
We can use the dh timeseries in each cell to derive the rate of change of surface elevation, ie dh/dt, at any point in time. In the product the rate is calculated over a 5 year window that advances in monthly steps, however, for validation we apply the same time periods as the ATM to the grid cell timeseries in which the ATM took a measurement.
We averaged the rates of elevation change computed from pairs of ATM overflights in 25km cells to match the spatial resolution of the satellite data. Similarly, we averaged the rates of elevation change, derived by linear least-squares fitting from the surface elevation change data, comparable to the ATM pairs in length of time and grid cell location.
The comparison was then restricted to where the root mean square of the ATM results and the standard deviation of the satellite results was less than or equal to 0.4 m/year, and the time period between overflights was at least 2 years.
The airborne data is sampled at 250m spatial resolution along flight-lines that preferentially sample fast-thinning ice. Because of this, its measurements tend to be biased to a higher value when compared to coarsely gridded data (Flament and Remy 2012, McMillan 2014). To take account of this, a bias factor is estimated using a high-resolution (1km by 1km) map of ice velocity (Rignot et al 2011). This bias factor is the velocity of the 1km by 1km grid cell containing the ATM measurement with respect to the average velocity of the 25km by 25km AIS cell containing it, averaged over all measurements used.
The validation results come from the comparison of the averaged satellite and biased airborne measurements. A map of the airborne measurement locations used, a scatterplot of the two components of the comparison and a histogram of their differences are shown, and the mean and standard deviation of the differences derived.
3.2. Surface elevation change, Greenland – D3.IS.6.2
The validation method is a comparison of surface elevation change results with all OIB ATM measurements, for the Greenland ice sheet, with at least 3-years between the pairs of overflights. As the ATM L4 product provides elevation change estimates of repeated flightpaths, the locations and observational period are highly irregular. To ensure consistency in the inter-comparison we construct a record based on the C3S surface elevation product, which coincides with the ATM-L4 data both in space and time. The gridded 25 km solution from satellite altimetry (the C3S surface elevation product) is interpolated onto the individual ATM-observations in space by linear least-squares fitting.
This reconstructed dataset is only available for internal use in the validation effort but is derived solely from the NetCDF-files available at CDS.
The comparison is then restricted to where the root mean square of the ATM results and the standard deviation of the satellite results is less than or equal to 5 m. As the ATM data is sampled at 250m spatial resolution along flight-lines that preferentially sample fast-thinning ice, its measurements tend to be biased to a higher value when compared to coarsely gridded data (Flament and Remy 2012, McMillan 2014). Different measures have been taken to limit the bias, however, for the Greenland ice sheet validation we note this possible bias but neglect any corrections for it.
4. Summary of validation results
4.1. Surface elevation change, Antarctica – D3.IS.6.1
Here, we provide a short summary of the validation results, for further details please see related document, the Product Quality Assessment Report [D1].
Figure 3 shows the comparison results. All OIB ATM Antarctic data available from the portal, as of November 2020, were initially considered. However, flights were concentrated on the West Antarctic Ice Sheet, Filchner-Ronne Ice Shelf and surrounding regions. In total 222732 points of comparison, covering 443 grid cells, were used. The cell-averaged resistant mean difference (ie excluding outliers of more than 3 sigma) was 0.027 ± 0.65 m/yr, with a correlation coefficient of 0.58. This is within the Global Climate Observing System (GCOS) target accuracy of 0.1 m/yr, as specified by the C3S project.
Figure 3: Validation results – D3.IS.6.1 Left: map of locations where comparisons could be made. Middle: Scattergram of grid-cell-averaged dh/dt (m/yr) from ATM and C3S, one point per cell. If the values agreed perfectly, they would lie along the red line indicated. Right: Histogram of differences of grid-cell-averaged dh/dt (m/yr), ATM minus C3S, mean of distribution marked by the red line.
5. Surface elevation change, Greenland – D3.IS.6.2
Here, we provide a short summary of the validation results, for further details please see related document, the Product Quality Assessment Report.
Figure 4 shows the result of the inter-comparison between the OIB ATM and the C3S surface elevation changes. The monthly time-series of surface elevation change grids makes it possible to tailor the time-series to resolve the timespan of ATM repeat locations on the Greenland ice sheet. Based on more than 25k observations, distributed both in time and space, a median bias of -0.013±0.47 m/yr in relation to the ATM data is found. This shows the product compliance to the GCOS requirement of 0.1 m/yr.
Figure 4: Comparison of the rate of elevation change observed in the ATM data and the C3S Greenland surface elevation product; iCDRv2 and CDRv3. As the ATM level 4 data consist of data from all repeats of older flight paths, this inter-comparison is based on observations from 1993 flightpath until 2017. The upper-left panel shows the point-point agreement, alongside the one-to-one line. The lower-left panel shows the complete distribution for all years, which is averaged in the right panel to show the spatial distribution.
References
Flament, T. and F. Remy (2012). Antarctica volume change from 10 years of Envisat altimetry.
Conference paper, International Geoscience and Remote Sensing Symposium (IGARSS), 2012 IEEE International. DOI: 10.1109/IGARSS.2012.6351149
McMillan, M., A. Shepherd, A. Sundal, K. Briggs, A. Muir, A. Ridout, A. Hogg and D. Wingham (2014). "Increased ice losses from Antarctica detected by CryoSat-2." Geophysical Research Letters 41 (11): 3899 -3905
National Geospatial Intelligence Agency, 2004. https:earth-info.nga.mil/GandG/publications/tr8350.2/tr8350_2.html
Rastner, P., Bolch, T., Mölg, N., Machguth, H., Le Bris, R., Paul, F. 2012, updated 2018, The first complete inventory of the local glaciers and ice caps on Greenland. The Cryosphere, 6, 1483-1495. (doi:10.5194/tc-6-1483-2012)
Rignot, E., J. Mouginot and B. Scheuchl (2011). "Ice Flow of the AIS." Science 333 (6048): 1427-1430
Studinger, M. (2014). IceBridge ATM L4 Surface Elevation Rate of Change, Version 299 1, Antarctica subset. N. S. a. I. D. C. D. A. A. Center. Boulder, Colorado, USA. DOI: 10.5067/BCW6CI3TXOCY
