- Title
- Analysis of time-variable gravity signal from GRACE data
- Creator
- Ghobadi Far, Khosro
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2020
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The Gravity Recovery and Climate Experiment (GRACE) satellite mission revolutionized our understanding of mass redistribution in the Earth system from 2002 to 2017 by measuring time-variable gravity field with unprecedented accuracy. The conventional data products of GRACE are global monthly-mean snapshots of Level-2 (L2) time-variable gravity, and Level-3 or mascon surface mass change. The global monthly fields are obtained from the fundamental measurements of inter-satellite ranging acquired by the K-band ranging (KBR) system. Relying exclusively on the monthly data confines the application of GRACE to geophysical processes that are mainly characterized by seasonal and inter-annual variations such as terrestrial water, ice and ocean mass change. The primary aim of this thesis is to show that direct analysis of inter-satellite ranging data opens the way for detecting new geophysical mass changes at time-scales of significantly less than one month, such as tsunamis. By pushing the limit of GRACE, this thesis brings new opportunities to study new areas of the Earth system mass change. To study the gravitational effect of regional mass changes using GRACE, we first develop a transfer function based on correlation-admittance spectral analysis for accurate estimation of line-of-sight gravity difference (LGD) from inter-satellite range-acceleration. The correlation spectrum between LGD and range-acceleration shows near-unity correlation for frequencies above 1 mHz or 5 cycles-per-revolution (CPR), and the admittance spectrum quantifies the LGD response to range-acceleration at the correlated frequency band. As the first application, we employ the GRACE LGD observations to quantify surface water storage change and calibrate the stream flow velocity of runoff routing models in large river basins. Our results show that the optimal stream flow velocity for the Amazon and Siberian basins is ~0.3 m/s, while surface water in the Congo and Parana basins is better simulated with a velocity larger than 2.0 m/s. Consequently, surface water change explains as much as half of total water storage anomaly in the Amazon, while its contribution in Congo and Parana basins is almost negligible at the monthly temporal resolution [Ghobadi-Far et al., 2018, JGR Solid Earth]. Secondly, we examine the gravitational effect of tsunami-induced transient ocean mass change at 500 km altitude and its observation using GRACE. By upward continuing the gravitational effect of tsunami wave field to satellite altitude and comparison with GRACE LGD, we show that GRACE satellites have detected the tsunamis triggered by the great 2004 Sumatra, 2010 Maule, and 2011 Tohoku earthquakes. GRACE provides an independent source of information useful to discriminate among various seismic source models. This study in particular points to the potential of GRACE Follow-On to deliver low-latency gravimetric data for monitoring transient mass change due to extreme events such as tsunamis and hurricanes [Ghobadi-Far et al., 2019, under review, J. Geodesy]. Regional-scale co- and post-seismic gravity changes caused by great earthquakes are now routinely observed by GRACE L2 time-variable gravity data. Earthquakes also excite global-scale transient gravity changes at certain frequencies associated with Earth’s free oscillations which could last up to several days. In this study, we examine the global transient gravity changes excited by Earth’s free oscillations using the GRACE inter-satellite ranging data. By extending the Kaula orbit perturbation theory, we show that excited frequencies in GRACE KBR data are described by a linear combination of eigenfrequencies of the normal modes, Earth’s rotation rate, and satellite angular velocity. Wavelet analysis of the actual KBR residuals in December 2004 reveals the existence of a significant transient signal after the 2004 Sumatra earthquake with a frequency of ~0.022 mHz, which could be potentially related to the largest excitation due to the “football” mode. However, GRACE accelerometer noise seems to affect the reliability of the obtained results [Ghobadi-Far et al., 2019, JGR Solid Earth]. As the final contribution in this thesis, we put forward a rigorous theory for determining improved surface mass change from GRACE L2 data. The L2 time-variable gravity data are conventionally converted into surface mass change on the spherical Earth. Considering the accuracy of the current L2 data, we show that such simplistic spherical geometry is no longer tenable. We derive a unique one-to-one spectral relationship between the ellipsoidal harmonic coefficients of geopotential and surface mass. In conjunction with our ellipsoidal formulation, the linear transformation between spherical and ellipsoidal geopotential coefficients enables us to determine mass change on the ellipsoid from GRACE L2 data. Using the L2 data to degree 60, we show that the ellipsoidal approach determines mass change rate better than the spherical method by 3 – 4 cm/yr, equivalent to 10 – 15 % increase of total signal, in Greenland and West Antarctica. Our study emphasizes the importance of the ellipsoidal approach for quantifying mass change at polar regions from GRACE and GRACE Follow-On L2 data [Ghobadi-Far et al., 2019, Geophy. J. Int.].
- Subject
- GRACE; time-variable gravity; surface water; tsunami; earth's free oscillations; ellipsoidal surface mass change; thesis by publication
- Identifier
- http://hdl.handle.net/1959.13/1413263
- Identifier
- uon:36603
- Rights
- Copyright 2020 Khosro Ghobadi Far
- Language
- eng
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