Degree-1 Surface Mass Transport and Geocenter Motion

Abstract:

The longest-wavelength and hemisphere asymmetric surface mass transport is characterized by three degree-one spherical harmonic components. Such mass transport modes cause geocenter motion between the center-of-mass of the total Earth system (CM) and the center-of-figure of the solid Earth surface (CF), and deforms the solid Earth. GRACE’s K-band ranging data system is not sensitive to these three variation modes. For a complete spherical harmonic spectral coverage of mass transport, degree-1 surface mass changes estimated through geocenter motion or degree-1 mass/deformation signatures from other space geodetic techniques should be combined with GRACE’s time-variable gravity data. The degree-1 coefficients are critically important for mass variation assessments over large regions. For example, 1 mm error in geocenter motion can result in an error of 190 gigatons of global oceanic water mass change or, equivalently, an error of 0.5 mm of global mean sea level change when the geocenter motion is converted to degree-1 mass and combined with GRACE data. Yet, several different methods of geocenter motion estimation differ in results by more than 1 mm in annual amplitude. These differences have to be resolved after 13 years of successful GRACE operation. Recently, the difference between results from direct satellite laser ranging (SLR) determination and from a global inversion of Global Navigation Satellite System (GNSS) deformation measurements, GRACE, and an ocean bottom pressure (OBP) model has been largely reconciled as due to SLR’s sparse station distribution. This result and our current efforts to examine possible systematic errors in GNSS data and the OBP model will be discussed along with a future perspective.