Increased Low Degree Spherical Harmonic Influences on Polar Ice Sheet Mass Change Derived from GRACE Mission

Xiaoli Su,Yu Zhang,Junyi Guo,C K Shum,Zhicai Luo

doi:10.3390/rs12244178

Abstract

Replacing estimates of C20 from the Gravity Recovery and Climate Experiment (GRACE) monthly gravity field solutions by those from satellite laser ranging (SLR) data and including degree one terms has become a standard procedure for proper science applications in the satellite gravimetry community. Here, we assess the impact of degree one terms, SLR-based C20 and C30 estimates on GRACE-derived polar ice sheet mass variations. We report that degree one terms recommended for GRACE Release 06 (RL06) data have an impact of 2.5 times more than those for GRACE RL05 data on the mass trend estimates over the Greenland and the Antarctic ice sheets. The latest recommended C20 solutions in GRACE Technical Note 14 (TN14) affect the mass trend estimates of ice sheets in absolute value by more than 50%, as compared to those in TN11 and TN07. The SLR-based C30 replacement has some impact on the Antarctic ice sheet mass variations, mainly depending on the length of the study period. This study emphasizes that reliable solutions of low degree spherical harmonics are crucial for accurately deriving ice sheet mass balance from satellite gravimetry.

Highlights

With its launch in March 2002, the Gravity Recovery and Climate Experiment (GRACE) mission provided unique observations about the Earth’s temporal gravity field with unprecedented accuracy [1]
The annual amplitude of mass change caused by degree one terms corresponding to GRACE Release 06 (RL06) data is more than 30% larger than that over each ice sheet for GRACE Release 05 (RL05) data
We report that the trend of mass change caused by degree one terms corresponding to GRACE RL06 data in absolute value is 2.5 times larger than that for GRACE RL05 data

Summary

Introduction

With its launch in March 2002, the Gravity Recovery and Climate Experiment (GRACE) mission provided unique observations about the Earth’s temporal gravity field with unprecedented accuracy [1]. GRACE-based temporal gravity field solutions in the standard Level 2 (L2) data product consist of a series of spherical harmonic coefficients with degree and order up to at least 60. Regardless of the true cause, it has become a standard procedure to replace GRACE-determined C20 values with those provided by satellite laser ranging (SLR) data in order to accurately derive mass redistribution within the Earth’s system using GRACE monthly gravity field solutions. As the authors of [7] mentioned, there was a concern that SLR-provided C20 values could be inconsistent with the other spherical harmonic coefficients in GRACE monthly solutions They compared C20 estimates from SLR only and from the combination of SLR and GRACE, with the results concluding that estimates of C20 from SLR only were appropriate for science applications of GRACE data. It was recognized that these low degree zonal harmonics derived from SLR have non-negligible impacts on accurately deriving mass redistribution from GRACE data [15,18]

Results

Discussion

Conclusion