Assessment of Degree-2 Zonal Gravitational Changes from GRACE, Earth Rotation, Climate Models, and Satellite Laser Rangi
Four independent time series of degree-2 zonal gravitational variations ΔC20 are compared for the period April 2002 to February 2008. We examine estimates from the Gravity Recovery and Climate Experiment (GRACE), Earth rotation variations, climate models
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Assessment of Degree-2 Zonal Gravitational Changes from GRACE, Earth Rotation, Climate Models, and Satellite Laser Ranging J.L. Chen and C.R. Wilson
Abstract Four independent time series of degree-2 zonal gravitational variations C20 are compared for the period April 2002 to February 2008. We examine estimates from the Gravity Recovery and Climate Experiment (GRACE), Earth rotation variations, climate models (AOW), and satellite laser ranging (SLR). At the annual period, all C20 estimates agree remarkably well, and good correlation is found among these time series at nonseasonal time scales as well. SLR and AOW C20 time series show the best agreement in a broad band of frequencies with the maximum cross-correlation coefficient of 0.86 at nonseasonal time scales. GRACE monthly C20 estimates are subject to significant aliasing effects due to errors in high-frequency tide models, especially the S2 and K2 tides. Correctly removing winds and ocean currents and other motion related excitations from length-ofday (LOD) observations plays a key role in estimating C20 from LOD, especially at interannual or longer time scales.
88.1 Introduction The Earth gravitational change is caused by mass redistribution within the Earth system, including the atmosphere, ocean, hydrosphere, and cryosphere. The degree-2 zonal spherical harmonics C20 represent one of the longest wavelengths’ gravitational variations, commonly referring to the “oblateness” of the
J.L. Chen () Center for Space Research, University of Texas, Austin, TX 78712, USA e-mail: [email protected]
Earth. Air and water contributions dominate at periods less than a few years (Chen et al., 2000), while mass redistribution within the solid Earth due to tectonics and postglacial rebound (PGR) is the primary contributor at decadal and longer time scales (Mitrovica and Peltier, 1993). Satellite laser ranging (SLR) has been an effective technique for measuring low degree gravitational changes, with time series extending over more than 2 decades. SLR is especially useful in measuring the lowest degree even zonal harmonics (especially C20 , also called -J2 in the literature) (Yoder et al., 1983). SLR estimates of C20 show variations over a broad frequency band, with seasonal variability most prominent (Chen et al., 2000). A linear trend (∼0.116 × 10–10 per year) in C20 is well determined from SLR (Yoder et al., 1983), and widely accepted as PGR effect following the last glacial maximum (Mitrovica and Peltier, 1993). Advancements in processing methods, and tracking of multiple satellites have improved SLR estimates, especially of individual low-degree zonal terms (Cheng and Tapley, 2004). Advanced climate models of the atmosphere, ocean, and land hydrology provide independent estimates of Earth’s gravity changes. Models give time variations in global gridded fields of atmospheric surface pressure, ocean bottom pressure, and terrestrial water storage, which can be used to estimate gravity change. Model estimates are generally consistent with geodetic observations, e
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