Mass-related excitation of polar motion: an assessment of the new RL06 GRACE gravity field models

The new Release-06 (RL06) Gravity Recovery and Climate Experiment (GRACE) gravity field solutions are evaluated by converting them into equatorial effective angular momentum functions (so-called excitation functions) for polar motion and comparing these to respective time series based on space-geodetic observations (geodetic excitation). The same is performed for the older RL05 solutions using identical processing. Maps of equivalent water heights derived from both releases show that the signal-to-noise ratio is significantly improved in RL06. The derived polar motion excitation functions from RL05 and RL06 differ by about 15%. An analysis of the contributions of different Earth subsystems revealed that the release update mainly influenced the hydrological (12%) and oceanic excitations (17%), but it has a relatively small impact on the cryospheric excitations related to Antarctica (4%) and Greenland (1%). The RL06 data from different GRACE processing centers are more consistent among each other than the previous RL05 data. Comparisons of the GRACE-based excitation functions with the geodetic and model-based oceanic excitations show that the latest release update improved the agreement by about 2 to 15 percentage points.