Validation of GRACE/GRACE-FO Solutions Using Caspian Sea Level Change

Abstract

To validate gravity recovery and climate experiment (GRACE)/ GRACE follow-on (GRACE-FO) gravity solutions, we compare satellite altimetry observations of Caspian Sea level (CSL) change with CSL estimates from satellite gravity from April 2002 to December 2020. We use GRACE/GRACE-FO Release 6 GSM fields [spherical harmonics (SH)] from the three processing centers [Center for Space Research (CSR), Jet Propulsion Laboratory (JPL), and Geoscience Research Center (GFZ)] and three mascon solutions from CSR, JPL, and Goddard Space Flight Center (GSFC). CSL change is a regional scale signal that should be reasonably well resolved by satellite gravity measurements, but spatial leakage and other corrections are still required. We computed an average of smoothed SH solutions and those that are both smoothed and decorrelation filtered (to remove north–south stripe noise). Averaging mitigates attenuating effects of decorrelation filtering on the north–south oriented Caspian Sea Signal. After spatial leakage, terrestrial water storage, and steric corrections, most GRACE/GRACE-FO CSL estimates agree remarkably well with the altimetry series over a range of time scales as measured by trend and seasonal components and at other frequencies. The linear CSL trend from altimetry is −7.55 ± 0.17 cm/yr, while GRACE/GRACE-FO values range from −7.30 ± 0.17 to −8.66 ± 0.20 cm/yr. Annual amplitudes from altimetry are (17.75 ± 1.28 cm) with GRACE/GRACE-FO values in the range 17.05 ± 1.49 to 19.16 ± 1.55 cm, with good phase agreement. The GSFC mascon solution shows substantially smaller annual amplitude (11.62 ± 1.04 cm) than others. We found no bias between GRACE and GRACE-FO, but GRACE-FO shows larger root-mean-square differences from altimetry. Among the three standard SH solutions, those from CSR show the best agreement with altimetry.

Keywords

• Caspian Sea level (CSL)
• GRACE/GRACE-FO
• gravity
• satellite altimeter
• satellite gravimetry