Study Of Length-of-day Geophysical Excitations Based On Satellite Time-varying Gravity And Surficial Fluids Models

The excitations length-of-day(LOD)variations include both the tidal and non-tidal parts.At time scales shorter than about two years,non-tidal LOD variations are mainly excited by angular momentum exchanges between the atmospheric,oceanic,and continental hydrological fluid envelopes and the underlying solid Earth.But,neither agreement among different geophysical models for the fluid dynamics nor consistency with geodetic observations of LOD has reached satisfactory levels.This is mainly ascribed to significant discrepancies and uncertainties in the theories and assumptions adopted by different modeling groups,in their numerical methods,in the accuracy and coverage of global input data fields,in the data length and quality,and also in the observation accuracy,etc.With the improvement of various observation methods,the accuracy of LOD observations have reached a quite high level(about ±0.012 ?s),which makes it possible to improve those geophysical models by taking LOD observations as a reference.On the other hand,satellite laser ranging(SLR)is currently the most reliable technique for observing the geodynamic shape factor(J2).Since the J2 term has a proportional relationship with the C20 term of the Earth's gravitational field,the observations for J2 have been regarded as a reliable data source by relevant geodetic studies.Also,C20 results from GRACE are replaced by those results from SLR.Based on careful comparisons with more accurate geodetic measurements and satellite gravimetry products(from SLR),observed length-of day(LOD)can provide strong constraints to evaluate or form combined geophysical models(the total of geophysical excitations,matter + motion),and C20 geopotential time series from SLR can provide strong constraints to those matter term of geophysical excitations.Then we construct new geophysical model combinations.In addition,we use two different LOD observation series as references,they are 14C04 series from IERS and SPACE2015 series from JPL.Obtaining improved geophysical excitations while discussing the influences of these two different LOD observation series to our LDC/WMC improved results.Those low-frequency parts of LOD variations(such as 5?6 years signals,decadal years signals,etc.)do not mainly come from the mass transportation and relative motion of the Earth surface liquid layer.Therefore,considering the total signal of the LOD variations will make an adverse effect on our improved result for this study.Based on wavelet decomposition results,we extract several narrow-band components to compare in addition to considering the total signals,and then we make refinements to the least difference combination(LDC)method proposed by Chen et al.,(2013 b)to compare and improve those geodetic and geophysical excitations for every seasonal and high frequencies component.Two combination variants,called the weighted mean combination(WMC2 and WMC4),are also evaluated.All the multi-model methods attempt to extract the best-modeled frequency components from each geophysical model by relying on geodetic excitation and the C20 series as references.The comparative performances of the three combinations LDC,WMC2 and WMC4 and the original single models are determined.We find that(1)the Estimating the Circulation and Climate of the Ocean(ECCO)and Max-Planck-Institute for Meteorology Ocean Model(MPIOM)give a more reliable view of the ocean redistributions than the Ocean Model for Circulation and Tides(OMCT)used by European Centre for Medium-Range Weather Forecasts(ECMWF),especially for the annual component;(2)C20 series from SLR can provide a rigorous constraint for the total matter excitation of the geophysical fluids,especially for broadband parts;(3)the Sea-Level Angular Momentum functions(SLAM)term,correcting for sea level effects(global mass balance)put forward by the Earth System Modelling group at GFZ German Research Centre for Geosciences(ESMGFZ),can significantly improve the Hydrospheric Effective Angular Momentum functions(HAM)matter terms;(4)the LDC/WMC combinations are much better than the original individual geophysical model excitations,reducing the magnitude of unexplained LOD excitations to roughly the 10 ?s level;(5)for the seasonal LOD excitations,the level of residual LOD variations after removing models or model combinations is remarkably invariant with respect to LOD periods between?2 months and-3 years,being 12 to 14 ?s for the best original models and 7 to 12 ?s for our combinations;(6)while differences between the IERS 14C04 and the JPL SPACE2015 geodetic LOD time series are not negligible,errors in both series are still not large compared to the geophysical models(for periods>2 months)so the impact on excitation studies is minimal except at semi-annual periods and usually 14C04 compares better with excitation models.But for high-frequencies component,the observations from SPACE2015 is better than 14C04.The improved geophysical models are recommended to replace the original ones as they present overwhelming advantages.