Residual and Unmodeled Ocean Tide Signal From 20+ Years of GRACE and GRACE-FO Global Gravity Field Models

We analyze remaining ocean tide signal in K/Ka-band range-rate (RR) postfit residuals, obtained after estimation of monthly gravity field solutions from 21.5 years of Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow-On sensor data. Low-pass filtered and numerically differentiated residuals are assigned to $5°\times 5°$ grids and a spectral analysis is performed using Lomb-Scargle periodograms. We identified enhanced amplitudes at over 30 ocean tide periods. Spectral replicas revealed several tides from sub-semidiurnal bands. Increased ocean tide amplitudes are located in expected regions, that is, in high-latitude, coastal and shallow water regions, although some tides also show distinct patterns over the open ocean. While most identified tides are considered during processing, and therefore the amplitudes represent residual signal w.r.t. the ocean tide model, several unmodeled tides were found, including astronomical degree-3 tides ${{}^{3}M}_1$, ${{}^{3}N}_2$, ${{}^{3}L}_2$, ${{}^{3}M}_3$, and radiational and/or compound tides $S_3$, $R_3$${/SK}_3$, $T_3$${/SP}_3$, ${2SM}_2$ and ${2MK}_3$${/MO}_3$. The astronomical degree-3 tides were observed on a global level for the first time a few years ago in altimeter data. We are unaware of any global data-constrained solutions for the other tides. The amplitude patterns of these tides exhibit similarities to purely hydrodynamic solutions, and altimeter observations (astronomical degree-3 only). The sensitivity of the satellites to these rather small tidal effects demands their inclusion into the gravity field recovery processing to reduce orbit modeling errors and a possible aliasing. The conducted study shows enormous potential of RR postfit residuals analysis for validating ocean tide models and improving gravity field recovery processing strategies.