Last modified on Dec 01, 2023 17:39

Contributors: L. Gilbert (University of Leeds), S. B. Simonsen (Technical University of Denmark), J. Wuite (ENVEO IT GmbH)

Issued by: University of Leeds / Lin Gilbert

Date: 02/12/2022

Ref: C3S2_312a_Lot4.WP1-PDDP-IS-v1_202206_SEC_PQAD-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

Bias: The tendency of an instrument to preferentially make measurements over a certain type of surface.

Bias factor: A value calculated from external data sources to mitigate the effects of instrument bias on measurements.

Climate Data Record (CDR): A time series of measurements of sufficient length, consistency and continuity to determine climate variability and change.

Crossover analysis: A method for deriving elevation change at locations where the orbits of a single or multiple satellites cross.

Cross-calibration: A method that merges datasets from multiple satellites into one consistent dataset. 

Laser altimeter: An instrument mounted on an aircraft or spacecraft that measures altitude from the ground surface below by timing how long it takes a pulse of laser light to travel to ground, reflect, and return to the craft.

Radar altimeter: An instrument mounted on an aircraft or spacecraft that measures altitude from the ground surface below by timing how long it takes a pulse of radio waves to travel to ground, reflect, and return to the craft.

Surface Elevation Change (SEC): The surface elevation of a point on an ice sheet is the height of the ice sheet surface above a reference geoid (a hypothetical solid figure whose surface corresponds to mean sea level and its imagined extension under land areas). Increase in surface elevation over time at a given location indicates a gain of ice or snow at that location, and conversely decrease indicates a loss. The surface elevation change product provides the rate of change given at monthly intervals at each location on a grid covering the ice sheet. The definition of the grid projection includes the geoid used. Given the rates of change, absolute change can be calculated for any time period.

Uncertainty: An estimate of the error in a measurement, due to limitations in the measuring instrument or statistical fluctuations in the quantity being measured.

Validation: Comparison between two independent datasets to test their agreement.

List of Tables

Table 1. Summary contents of Antarctic SEC product
Table 2. Summary specifications for Operation IceBridge L4 Surface Elevation Rate of Change product, used for validation

List of Figures

Figure 1. Example surface elevation change map from SEC product over Antarctic Ice Sheet.
Figure 2. Example of accumulated surface elevation change map produced from SEC product over Greenland Ice Sheet.
Figure 3. Operation IceBridge flight paths for all campaigns over Greenland (left) and Antarctica (right). Credit: Operation IceBridge Data Portal at https://nsidc.org/icebridge/portal/map
Figure 4. Flowchart for producing a pair of datapoints, one from the ATM and one from the SEC product, which can be directly compared. The full validation dataset uses all comparable pairs from each grid cell and ATM period.
Figure 5. Greenland validation flow chart.
Figure 6. Validation results from the v3 CDR SEC over Antarctic Ice Sheet. Left: map of locations where comparisons were 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.
Figure 7. 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.

Scope of the document

This document is the Product Quality Assurance Document for Surface Elevation Change (SEC) of the Antarctic and Greenland ice sheets and ice shelves as part of the Copernicus Ice Sheets and Ice Shelves service. It describes the validated datasets and the methods used for validation.

The service addresses three essential climate variables (ECVs) by providing four separate products.

• Ice velocity is given for Greenland in product WP2-FDDP-IV-CDR
• Gravimetric mass balance is given for Greenland and Antarctica in product WP2-FDDP-GMB-CDR
• Surface elevation change is given for:
  • Antarctica in product WP2-FDDP-SEC-CDR-AntIS
  • Greenland in product WP2-FDDP-SEC-CDR-GrIS

Executive summary

We document here the validation methods and results from the Climate Data Record (CDR) v3 for the two Polar region SEC products. The same methods will be used to validate the v4 CDR.

In section 1 we describe the two products that will be validated

• Antarctica - WP2-FDDP-SEC-CDR-AntIS
• Greenland - WP2-FDDP-SEC-CDR-GrIS

Both contain gridded, monthly estimates of surface elevation change rate over glaciated regions. Although the two products take the same input datasets and have the same output format, they necessarily use different map projections and processing methods, and so have been split to avoid possible confusion.

In section 2 we describe the dataset used for validation, and how to obtain it. This data is gathered by an aircraft-mounted laser altimeter, as part of NASA's Operation IceBridge (OIB), which has been surveying Earth's polar ice regions since 1993.

In section 3 we describe the methodology for validation. Essentially, our data products are derived from an underlying dataset which charts surface elevation change over time. We use this underlying dataset to create estimates of surface elevation change rate matching those produced by OIB, and compare them. Due to regional differences in both ice dynamics and OIB overflights, Antarctica and Greenland use slightly varying validation methods.

In section 4 we describe the validation results for the previous release of the products, v3. These demonstrate that the difference between each product and its equivalent OIB dataset is within the Global Climate Observing System (GCOS) target requirement for measurement uncertainty.

1. Validated products

1.1. Surface elevation change, Antarctica

The product is a netCDF4 file containing monthly gridded maps of the rate of surface elevation change over the Antarctic region, including ice sheet, ice shelves, ice rises and glaciated islands. It uses input data from radar altimeters on board ERS1, ERS2, Envisat, CryoSat-2, Sentinel-3A and Sentinel-3B satellites. Input data, and the output contents are described in detail in related document [D2], the Algorithm Theoretical Basis Document (ATBD). A summary of the contents is given in Table 1.

Table 1: Summary contents of Antarctic SEC product

The initial CDR contains surface elevation change rate maps centred on November 1994 to May 2020², and each monthly interim CDR (ICDR) adds one map. The ICDRS are cumulative, containing all previous data as well as the latest monthly map. The v4 CDR and ICDRs incorporate reprocessed v3 data and extend the timeseries beyond the end of v3. An example map is shown below in Figure 1.

Figure 1. Example surface elevation change map from SEC product over Antarctic Ice Sheet.

1.2. Surface elevation change, Greenland

The surface elevation change product is available as a single netCDF4 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, 2012) and a resolution is 25 km by 25 km.

The rates of change in each map are derived from the 3-year (CryoSat-2 and Sentinel-3) or 5-year (ERS-1, ERS-2 and Envisat) 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 CDR contains maps centred on November 1994 to April 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. With the introduction of CDRv3, the updated surface elevation algorithm now provides individual monthly estimates of the elevation within the temporal window (3-year or 5-year period). For the CDRv3 this update resulted 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. The CDRv4 follows upon this approach and iCDRs will provide elevation change with a half-year delay, starting July 2022.

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 SEC product over Greenland Ice Sheet.

2. Description of validating datasets

2.1. Surface elevation change – Antarctica and Greenland

Independent estimates of the rate of surface elevation change at discrete locations and over specific time periods are provided by the Airborne Topographic Mapper (ATM), a scanning laser altimeter flown on board aircraft by Operation IceBridge (Studinger 2014). Observation campaigns are conducted in the hemispheric spring, from 1993 to 2020 in Greenland and from 2002 to 2019 in Antarctica. Dataset production usually lags by a year.

Operation IceBridge ceased at the end of the Arctic campaign in 2020, and at time of writing (September 2022), the level 4 product for Greenland is only available up to 2018, and for the Antarctic up to 2016, i.e. it is unchanged since the v3 validation.

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/ ³. The surface elevation rate of change in this dataset is calculated from the change in elevations recorded at the same location between two overflights. Due to differences in flight patterns between campaigns, not all campaigns are represented in this product as some locations were only overflown once. The flight paths for all campaigns over Greenland and Antarctica are shown in Figure 3.

Figure 3: Operation IceBridge flight paths for all campaigns over Greenland (left) and Antarctica (right). Credit: Operation IceBridge Data Portal at https://nsidc.org/icebridge/portal/map

Summary specifications for the validating dataset are given in Table 2 below⁴.

Table 2: Summary specifications for Operation IceBridge L4 Surface Elevation Rate of Change product, used for validation

3. Description of product validation methodology

3.1. Surface elevation change, Antarctica

The validation procedure is intended to give a measure of the uncertainty of the SEC product by comparing it to another SEC dataset made by different instruments and methods.

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 25 km by 25 km 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). This process is described in more detail in related document [D2], the ATBD – crossover analysis in section 1.3.2, cross-calibration in section 1.3.3 and outputs in section 1.4.

We can use the dh timeseries in each cell to derive the rate of change of surface elevation, i.e. 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 25 km 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 restricted to where the root mean square (rms) of the ATM results and the standard deviation of the satellite results was less than or equal to 0.4 m/year, to only use results of good quality (taken from the 3-sigma clipped mean rms of the ATM dataset). It was also restricted to where the time period between ATM overflights was at least 2 years, to allow for the inclusion of at least two seasonal elevation change cycles. In general, more cycles would be preferable, but in Antarctica 25% of the ATM dataset had a time period of 2 to 3 years, so we chose to include that data rather than further restrict the comparison datapoints.

The airborne data is sampled at 250 m 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) such as the SEC product. To take account of this, a bias factor is estimated using a high-resolution (1 km by 1 km) map of ice velocity (Rignot et al. 2011). This bias factor is the velocity of the 1 km by 1 km grid cell containing the ATM measurement with respect to the average velocity of the 25 km by 25 km Antarctic Ice Sheet (AIS) cell containing it, averaged over all measurements used.

A flowchart of the method for producing comparable datapoints is given in Figure 4.

Figure 4. Flowchart for producing a pair of datapoints, one from the ATM and one from the SEC product, which can be directly compared. The full validation dataset uses all comparable pairs from each grid cell and ATM period.

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 produced and the mean and standard deviation of the differences derived. As an example, the v3 results are described in section 4.1, with the output plots shown in .

One of the Key Performance Indicators (KPIs) defined by the C3S project is the target uncertainty of the dataset, which is set by the GCOS at 0.1m/yr⁵. If the mean difference between the product and the validation dataset is less than this value, then the target is achieved.

3.2. Surface elevation change, Greenland

The validation procedure is intended to give a measure of the uncertainty of the SEC product by comparing it to another SEC dataset made by different instruments and methods. Here, we compare the 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.

The surface elevation change product for Greenland is based on the optimal combination of the crossover, repeat-track and plan-fitting method (described in more detail in related document [D2], the ATBD, section 2.3.1.6) at a 25 km by 25 km grid posting. This 25 km grid posting require the OIB ATM measurements to pre-processed to similar resolution. Specific for Greenland validation, we first collect all OIB ATM measurements, 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, we ensure consistency in the inter-comparison by constructing a new 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 publicly available netCDF4-files available through the 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 similar requirements are used for internal production of the gridded C3S surface elevation. Further, the ATM data is sampled at 250 m 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). Hence, the 5-meter threshold is a measure to limit the biases between the two datasets, however, we do note that other possible bias is present between the two datasets (e.g. resolution of surface roughness etc.), but no further attempts are made to correct for it.  A flowchart of the method for producing comparable datapoints is given in Figure 5.

Figure 5. Greenland validation flow chart.

The Key Performance Indicators (KPIs) defined by the C3S project is the target uncertainty of the dataset, which is set by the GCOS at 0.1m/yr⁶. If the mean difference between the product and the validation dataset is less than this value, then the target is achieved. 

4. Summary of the most recent validation results

4.1. Surface elevation change, Antarctica

In this section, we provide a short summary of the v3 validation results, which used the same analysis method as described above for v4. The full v4 validation and discussion will be published in the Product Quality Assessment Report [D1].

Figure 6 shows the comparison results. All OIB ATM Antarctic data available from the portal⁷ described in section 2.1, as of November 2021, were 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 (i.e. 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 GCOS target accuracy of 0.1 m/yr.

Figure 6: Validation results from the v3 CDR SEC over Antarctic Ice Sheet. Left: map of locations where comparisons were 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.

4.2. Surface elevation change, Greenland

Here, we provide a short summary of the validation results, which used the same analysis method as described above for v4.  The full v4 validation and discussion will be published in the Product Quality Assessment Report [D1].

Figure 7 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 25,000 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 7. 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, (2012). World Geodetic System 1984.  https://www.unoosa.org/pdf/icg/2012/template/WGS_84.pdf  (URL resource last viewed 2^nd December 2022)

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

5. Related articles

• Page:
  ECV - Glaciers, Ice Sheets, Lakes, Soil Moisture
• Page:
  IV Version 1.2; SEC Version 2.0; GMB Version 2.0: Product User Guide Specification (PUGS)
• Page:
  IV Version 1.2; SEC Version 2.0; GMB Version 2.0: System Quality Assurance Document (SQAD)
• Page:
  IV Version 1.2; SEC Version 2.0; GMB Version 2.0: Product Quality Assurance Document (PQAD)
• Page:
  IV Version 1.2; SEC Version 2.0; GMB Version 2.0: Product Quality Assessment Report (PQAR)