Contributors: S. B. Simonsen (Technical University of Denmark)
Issued by: Technical University of Denmark / SB Simonsen
Date: 18/10/2022
Ref: C3S2_312a_Lot4.WP1-PDDP-IS-v2_202306_SEC_PQAD-v5_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 the 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). An increase in surface elevation over time at a given location indicates a gain of ice or snow at that location, and conversely, a 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.
Scope of the document
This document is the Product Quality Assurance Document (PQAD) for Surface Elevation Change (SEC) of the Greenland ice sheet as part of the Copernicus Ice Sheets and Ice Shelves service. It describes the validated datasets and the methods used for validation. The SEC of the Antarctic ice sheets and ice shelves is provided only up to version 4.0. Please refer to the PQAD version 4 [D1] for the most up-to-date documentation regarding the Antarctic ice sheets and ice shelves SEC.
Executive summary
This document is the Product Quality Assurance Document (PQAD) for Greenland Surface Elevation Change (SEC) of the Greenland ice sheet as part of the Copernicus Ice Sheets and Ice Shelves service. It describes the validated datasets and the methods used for validation of the Climate Data Record (CDR) v4.0 for the Greenland SEC products. The same methods will be used to validate the v5.0 CDR. The Antarctic ice sheet SEC is provided only up to version 4.0, please refer to the PQAD version 4 [D1] for the most up-to-date Antarctic ice sheet SEC documentation.
In Section 1 we describe the Greenland (version 5.0) product that will be validated. It contains gridded, monthly estimates of surface elevation change rate over glaciated regions in Greenland.
In Section 2 we describe the datasets used for validation, and how to obtain them. This data is gathered by an aircraft-mounted laser altimeter, as part of National Aeronautics and Space Administration (NASA)’s Operation IceBridge (OIB), which has been surveying Earth’s polar ice regions since 1993. The last campaign season of OIB took place in 2019, and the subsequent data collected by Ice, Cloud and Elevation Satellite (ICESat)-2 is the most appropriate for validation purposes. The suitability of ICESat-2 data for validation is also described and discussed.
In Section 2.2 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/ICESat-2 and compare them.
In Section 4 we describe the validation results for the previous release of the Greenland product – (version 4.0). 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 product
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 and a resolution is 25 km by 25 km.
The rates of change in each map are derived from 3-year (CryoSat-2 and Sentinel-3) or 5-year (European Remote-sensing Satellite (ERS)-1, ERS-2, and Envisat) periods centred on that map's timestamp. The timestamps are one month apart. The difference in time periods (3 years and 5 years, respectively) is due to the enhanced capabilities of CryoSat-2 and Sentinel-3, enabling robust surface elevation changes to be derived for shorter periods than for the earlier missions. The initial CDR contains maps centered 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 CDR version 3.0, 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 CDR version 3.0, this update reduced 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 CDR version 5.0 follows upon this approach and iCDRs will provide elevation change with a half-year delay, starting January 2024.
Apart from maps, timestamps, and grid definitions, the product includes uncertainties on each rate of elevation change map, 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 of accumulated elevation changes is shown in Figure 1.
Figure 1: Example of accumulated surface elevation change map produced from product WP2-FDDP-SEC-CDR-GrIS
2. Description of validation datasets
2.1. Operation IceBridge Airborne Topographic Mapper
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 and the dataset production usually lags by a year.
Operation IceBridge ceased at the end of the Arctic campaign in 2020, and at time of writing (June 2023), the level 4 (surface elevation rate of change) product for Greenland is only available up to 2018, i.e. it is unchanged since the version 3.0 validation.
The validating dataset used is the Operation IceBridge ATM level 4 version 001. This can be obtained free of charge on registration1. This dataset's surface elevation rate of change 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 are shown in Figure 2.
Figure 2: Operation IceBridge flight paths for all campaigns over Greenland. Credit: Operation IceBridge Data Portal at https://nsidc.org/icebridge/portal/map2
Summary specifications for the validating dataset are given in Table 1 below3.
Table 1: Summary specifications for Operation IceBridge L4 Surface Elevation Rate of Change product, used for validation.
Temporal Coverage | 23 June 1993 to 1 May 2018 |
|---|---|
Temporal Resolution | 1 year |
Spatial Resolution | 250 m by 250 m |
Spatial Coverage | Greenland; N: 90 S: 60 E: 180 W: -180 |
2.2. ICESat-2 ATL15
The ICESat-2 ATL15 data product is a specific dataset derived from the Advanced Topographic Laser Altimeter System (ATLAS) aboard the ICESat-2 satellite. It provides valuable information about the surface elevation changes of Earth's ice sheets, glaciers, and ice shelves. The ATL15 data product focuses on the ice sheet elevation change rates, offering monthly and annual estimates. It utilizes the precise measurements obtained by the ICESat-2 satellite, which uses laser beams to measure the time it takes for the laser pulse to travel to the Earth's surface and back. By analysing the time-of-flight data, the satellite can accurately determine the elevation of the ice surface. The data product includes information about surface elevation changes, surface slope, and surface roughness. It also incorporates data quality measures, such as signal-to-noise ratio and various error estimates, providing an assessment of the data reliability. The ATL15 is derived from the higher-order data product of collocated height measurements provided by ATL11 (Smith, 2021). In the following validations, the data subset "dhdt_lag8" is used, which provides biennial height-change-rate estimates in the period late-2019 to mid-2020. The ATL15 data are available online4 after the creation of a free NASA Earthdata account.
3. Description of product validation methodology
The validation procedure is intended to give a measure of the validity 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 time series and analysis methods from which the product dataset was derived to produce results comparable to the ATM data.
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 the related Algorithm Theoretical Basis Document (ATBD) [D2]) at a 25 km by 25 km grid posting. This 25 km grid posting requires the OIB ATM measurements to be re-sampled to a similar resolution. For 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 flight paths, the locations and observational period are highly irregular, we ensure consistency in the inter-comparison by constructing a new record based on the Copernicus Climate Change Service (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 Climate Data Store (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 3.
Figure 3: Surface elevation change product validation flow chart.
Validation efforts utilizing ICESat-2 data closely follow the established procedure employed with OIB data, despite the transition from airborne to satellite laser altimetry. This consistent approach ensures a robust and reliable comparison of results based on the two datasets. The Key Performance Indicators (KPIs) defined by the C3S project is the target uncertainty of the dataset, which is set by the GCOS5 at 0.1m/yr. The target is achieved if the mean difference between the C3S product and the validation dataset is less than this value.
4. Summary of the most recent validation results
Here, we provide a short summary of the validation results for CDR version 4.0. The full version 5.0 validation and discussion will be published in the Product Quality Assessment Report [D3]. 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 25,000 observations, distributed both in time and space, a median bias of -0.02 m/yr in relation to the ATM data is found. This shows the product compliance with the GCOS requirement of 0.1 m/yr.
Additionally, Table 2 provides a summary of the statistics derived from the intercomparison between the ATL15 data product and CDR version 4.0. These statistics offer insights into the performance and agreement between the different data products. Furthermore, Figure 5 visually displays the map of the data products, specifically covering the period from July 2019 to July 2021. It is worth noting that both the ATL15 data product and CDR version 4.0 adhere to the GCOS target requirements.
Figure 4: (Panel a) The point-to-point correlations between the C3S-surface elevation change and the OIB surface elevation estimate. (Panel b) The histogram of the point-to-point differences between C3S and OIB surface elevation change. Panels a and b, also includes the older version of the CDR (v3.0) for refence. (Panel c) The spatial distribution of the point-to-point inter-comparison with the OIB ATM surface elevation estimates.
Table 2: Results of ICESat-2 ATL15 validation
Time period | Median | Standard deviation | Mean absolute deviation |
|---|---|---|---|
Oct. 2018 – Oct. 2020 | -3 cm/yr | 44 cm/yr | 8 cm/yr |
Jan. 2019 – Dec. 2020 | -3 cm/yr | 39 cm/yr | 7 cm/yr |
Apr. 2019 – Apr. 2021 | -2 cm/yr | 43 cm/yr | 7 cm/yr |
Jul. 2019 – Jul. 2021 | -6 cm/yr | 28 cm/yr | 6 cm/yr |
Figure 5: Validation against ICESat-2 ALT15 average elevation change from July 2019 to July 2021. (left) C3S SEC, (middle) ICESat-2 SEC and (right) the difference between the estimates, showing a mean difference of 2 cm/year and a standard deviation of 28 cm/year.
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
Smith, B. (2021). ICESat-2 Algorithm Theoretical Basis Document for Land Ice DEM and Land Ice Height Change Release 001 Algorithm Theoretical Basis Document (ATBD) for Land-ice DEM (ATL14) and Land-ice height change (ATL15). https://icesatiimis.gsfc.nasa.gov
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
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 (Contribution agreement signed on 22/07/2021). 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.
