Reply on RC2

Yubin Fan

doi:10.5194/essd-2021-183-ac2

Abstract

Greenland digital elevation models (DEMs) are indispensable to fieldwork, ice velocity calculations and mass change estimations. Previous DEMs provided Greenland elevation information for different periods, but long temporal coverage introduced additional time uncertainty to scientific research. To provide a high-resolution DEM with a definite time, approximately 5.8 × 108 ICESat-2 observations from November 2018 to November 2019 were used to generate a new DEM for both the ice sheet and glaciers in peripheral Greenland. A spatiotemporal model fit process was first performed at 500 m resolution. To improve ICESat-2 data utilization, DEMs with 1 km and 2 km resolution across all of Greenland and an additional 5 km resolution in southernmost Greenland were used to fill the DEM gaps. Kriging interpolation was used to fill the remaining 2 % of void grids that were insufficiently observed by ICESat-2 measurements. IceBridge mission data acquired by the Airborne Topographic Mapper (ATM) Lidar system were used to evaluate the accuracy of the newly generated ICESat-2 DEM. Overall, the ICESat-2 DEM had a median difference of −0.48 m for all of Greenland, which agreed well with the IceBridge data, and the performance in the calculated and interpolated grids was similar. Better accuracy could be observed in the northern basins due to the larger proportion of calculated grids with 500 m resolution. The ICESat-2 DEM showed significant improvements in accuracy compared with other altimeter-derived DEMs. Compared to the DEM generated by image pairs, the accuracy was also significantly higher than those of the 1 km ArcticDEM and TanDEM. Similar performance between the ICESat-2 DEM and 500 m ArcticDEM indicated the high accuracy and reliability of the ICESat-2 DEM. Moreover, the ICESat-2 DEM performed better on northern aspects than the 500 m ArcticDEM. Overall, the ICESat-2 DEM showed great accuracy stability under various topographic conditions, hence providing a time-accurate DEM with high accuracy that will be helpful to study elevation and mass balance changes in Greenland. The Greenland DEM and its uncertainty are available at (https://data.tpdc.ac.cn/en/disallow/07497631-047548b5-ba53-c17f9076c72f/, Fan et al, 2021).

Highlights

30 The digital elevation model (DEM) of Greenland is important for fieldwork planning, numerical modelling verification, and ice movement tracking (Bamber et al, 2009; Bamber et al, 2013)
The ICESat-2 DEM shows the same pattern as the other published Greenland DEMs, with the highest elevation appearing in the interior ice sheets and showing a downward trend to the margins
We estimated the uncertainty with a median difference of -0.48 m for all of Greenland, which represents the upper bound of the ICESat-2 DEM bias

Summary

Introduction

30 The digital elevation model (DEM) of Greenland is important for fieldwork planning, numerical modelling verification, and ice movement tracking (Bamber et al, 2009; Bamber et al, 2013). ASTER and SPOT 5 measurements were used on the ice sheet periphery, and AVHRR photoclinometry was used in the inner and polar regions to derive the Greenland Ice Mapping Project (GIMP) DEM (version 1) (Howat et al, 2014). The ICESat DEM adopted a bi-quadratic surface to fit all ICESat 50 footprints within each 1 km grid, but the largest radius of 20 km in the low-latitude regions to some extent limited the ability to describe the small-scale elevation patterns at the Greenland margin (DiMarzio et al 2007). TanDEM-X and TerraSAR-X, high-resolution and allweather SAR data, were used to generate the Greenland DEM using differential interferometry (Zink et al, 2014), but the radar signal can penetrate into the snow, which causes the elevation to be underestimated. The mosaicked DEM is the 65 combination of images from many times, and it is difficult to quantify the exact time of the DEM, limiting its scientific applications

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