Abstract
The waveform retracking algorithm is a key factor that affects the accuracy of elevation change from satellite altimetry over an ice sheet. The elevation change results from four waveform retracker algorithms (ICE1/ICE2/Sea Ice/OCEAN) provided by the Satellite with ARgos and ALtiKa (SARAL/ALtiKa) data were compared using repeated SARAL data between March 2013 and April 2016 to determine the optimal retracker in the crossovers of descending and ascending orbits over a Greenland ice sheet (GrIS). The ICE1 provided slightly better results than the three other algorithms with the lowest standard deviation (SD) of 0.30 m year−1. Further comparison was also conducted between the Satellite with ARgos and ALtiKa (SARAL) and Operation ICEBridge laser data, thereby indicating that ICE1 was the best retracker with an Root Mean Square Error (RMSE) of 0.43 m year−1. The distribution of elevation change rate and uncertainties over Greenland from SARAL were presented using the selected ICE1 retracker with a volume loss of 40 ± 12 km3 year−1. This volume loss did not include the fast-changing coastal areas of the GrIS. A large thinning was observed in Jakobshavn Isbræ, and a trend that extended far inland was also found from 2013–2016. Furthermore, a melting ice sheet was observed in the large areas northwest over the GrIS.
Highlights
The mass balance of the Greenland ice sheet (GrIS) is an important factor in estimating the rate of increase of global sea levels
The elevation change rate differences at crossovers can reflect the accuracy of Satellite with ARgos and ALtiKa (SARAL)
The calculation included five steps: (i) the elevation change rates were estimated by the RT method along-track for all orbits; (ii) the results of elevation change rate for ascending and descending orbits were extracted from results for all orbits; (iii) quadratic polynomials were fitted to derive the approximate location of crossover for the ascending and descending orbits; (iv) the precise location of the crossover was estimated by iteration of (ii) and (iii); and (v) the elevation change rate differences of the crossovers were calculated [30]
Summary
The mass balance of the Greenland ice sheet (GrIS) is an important factor in estimating the rate of increase of global sea levels. The mass balance of the GrIS is modelled by combining surface mass balance (SMB) from a climate model and ice flux across ice grounding lines from interferometry and other measurements. Such GrIS SMB models may undergo model uncertainties and the inter-annual mass variations of GrIS [3,4]. GRACE-based estimations of the mass balance of Greenland typically have spatial resolutions of approximately 300 km and can be further degraded by its systematic errors such as north-south stripes, data noises and errors introduced by the uncertainties of models that represent temporal gravity effects
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