Time-varying gravity observed by the Gravity Recovery and Climate Experiment (GRACE) satellites measures surface water and ice mass redistribution driven by weather and climate forcing and has emerged as one of the most important data... more
Time-varying gravity observed by the Gravity Recovery and Climate Experiment (GRACE) satellites measures surface water and ice mass redistribution driven by weather and climate forcing and has emerged as one of the most important data types in measuring changes in Earth’s climate. However, spatial leakage of GRACE signals, especially in coastal areas, has been a recognized limitation in quantitatively assessing mass change. It is evident that larger terrestrial signals in coastal regions spread into the oceans and vice versa and various remedies have been developed to address this problem. An especially successful one has been Forward Modeling but it requires knowledge of geographical locations of mass change to be fully effective. In this study, we develop a new method to suppress leakage effects using a linear least squares operator applied to GRACE spherical harmonic data. The method is effectively a constrained deconvolution of smoothing inherent in GRACE data. It assumes that o...
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We carry out a comprehensive error assessment of Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GFO) Release‐6 (RL06) solutions from the Center for Space Research (CSR) at the University of Texas at Austin, NASA Jet... more
We carry out a comprehensive error assessment of Gravity Recovery and Climate Experiment (GRACE) and GRACE Follow‐On (GFO) Release‐6 (RL06) solutions from the Center for Space Research (CSR) at the University of Texas at Austin, NASA Jet Propulsion Laboratory (JPL), and Geoforschungszentrum (GFZ). The study covers the period April 2002 to August 2020 and uses two different methods, one based upon open ocean residuals (OOR) and the other a Three‐Cornered Hat (TCH) calculation. General results from the two methods are similar. With 300 km Gaussian smoothing OOR RMS errors for CSR, JPL, and GFZ solutions are ∼2.01, 3.19, and 3.67 cm, respectively. With additional decorrelation filtering OOR RMS values are reduced to ∼1.24, 1.53, and 1.69 cm, respectively. TCH analysis also shows that CSR has the lowest noise levels with similar RMS values, and additional decorrelation filtering reduces error levels. TCH may underestimate errors if there are common errors among geophysical background models. Errors in GFO's first two years (25 solutions for 2018.06 to 2020.08) are comparable to those of GRACE when zonal degree 2 and 3 coefficients are replaced by Satellite Laser Ranging estimates. The OOR method reveals mismodeled intra‐seasonal dynamic ocean signals associated with the Argentine Gyre during de‐aliasing, while the TCH method shows differences between ocean tide models near Australia and Antarctica. Both OOR and TCH RMS analysis offer a means to assess the noise level of GRACE/GFO estimated mass change. The actual uncertainty of GRACE/GFO estimate averaged (or totaled) over a given region is also affected by other error sources.
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Research Interests: Geology, Multidisciplinary, Sea Ice, Sea level rise, Microwave Remote Sensing, and 15 morePerformance Model, Geographic distribution, Geophysical, Climate Warming, Dynamic Model of WSN, Ice Sheets, Mass Loss, Simulation Model, Ice Sheet, Interannual Variability, dynamic model, Greenland ice sheet, Model simulation, Climatic condition, and Model performance
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ABSTRACT Measurements of gravity changes by the GRACE satellite mission may be interpreted as terrestrial water storage variations (TWS). One of the primary interests in GRACE lies in the opportunity to disagregate GRACE data to extract... more
ABSTRACT Measurements of gravity changes by the GRACE satellite mission may be interpreted as terrestrial water storage variations (TWS). One of the primary interests in GRACE lies in the opportunity to disagregate GRACE data to extract changes in groundwater storage (GWS) and monitor, for example, depletion due to irrigation. The main hydrogeological information is associated with the amplitude of water storage variations. The objective of this study was to assess the reliability of water storage change estimates from GRACE and quantify uncertainties depending on processing strategies. Computing basin-scale TWS requires compensation for limited spatial resolution and to account for bias/leakage, which generally affects signal amplitude. Bias and leakage corrections are generally conducted by using a-proiri information from Land Surface Models (LSMs), which are affected by uncertainties and do not necessarily model all storage components. We address GRACE TWS and GWS reliability considering (1) a complete error budget to measure the impact of a-priori information in GRACE processing and (2)The impact of sub-resolution mass distribution on basin-scale estimates of water storage. THe High Plains aquifer, California Central Valley aquifer and Euphrates-Tigris River basin are used as examples and we show optimal integration of models and ground-based data with GRACE data to improve water storage changes for water resource planning and management.
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While GRACE (Gravity Recovery and Climate Experiment) satellites are increasingly being used to monitor water storage changes globally, the impact of spatial distribution of water storage within a basin is generally ignored but may be... more
While GRACE (Gravity Recovery and Climate Experiment) satellites are increasingly being used to monitor water storage changes globally, the impact of spatial distribution of water storage within a basin is generally ignored but may be substantial. In many basins, water may be stored in reservoirs, lakes, flooded areas, small aquifer systems, 5 and other localized regions with sizes typically below GRACE resolution. The objective of this study was to assess the impact of non-uniform water storage distribution on GRACE ...
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Monitoring global freshwater discharge is critical to understanding a range of climatologic, geomorphologic, hydrologic and ecologic Earth system processes. However, no comprehensive global streamflow observing network for the world's... more
Monitoring global freshwater discharge is critical to understanding a range of climatologic, geomorphologic, hydrologic and ecologic Earth system processes. However, no comprehensive global streamflow observing network for the world's major continental watersheds currently exists. Here we propose a method for estimating monthly basin discharge for large river basins based on the use of new GRACE estimates of terrestrial water storage
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ABSTRACT The objective of this paper is to examine the seasonal variations in the oceans and atmosphere that force the Earth's annual wobble, and to determine whether motions of air and water are a significant source of... more
ABSTRACT The objective of this paper is to examine the seasonal variations in the oceans and atmosphere that force the Earth's annual wobble, and to determine whether motions of air and water are a significant source of Chandler-wobble excitation. Although our investigation is similar to the one undertaken by Munk and Hassan over 15 years ago, we come to entirely different conclusions, largely because of differences in the details of our analysis. We find that the oceans and atmosphere are not observed well enough to fully explain the annual wobble, although much of it can be accounted for by annual changes in atmospheric mass distribution and continental water storage. Near the Chandler frequency there is evidence of significant coherence between polar motion and atmospheric pressure observations for the years 1901 and 1970, suggesting that the atmosphere is important in maintaining the Chandler wobble. The magnitude of meteorological variation appears to be large enough to account for more than half, and perhaps most of the Chandler wobble variance.
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We estimate global terrestrial water storage (TWS) changes using the time-variable gravity fields from the Gravity Recovery and Climate Experiment (GRACE) twin satellites gravity mission during the first three years. The high degree and... more
We estimate global terrestrial water storage (TWS) changes using the time-variable gravity fields from the Gravity Recovery and Climate Experiment (GRACE) twin satellites gravity mission during the first three years. The high degree and order spherical harmonics of GRACE-observed time-variable gravity fields are dominated by noise. We construct two optimized variance-dependent smoothing methods that can more effectively reduce the high
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Mass redistributions within the Earth system, especially in its geophysical fluid envelope, the atmosphere, ocean, and continental water produce observable changes in geocenter motion, which is currently detectable from space geodetic... more
Mass redistributions within the Earth system, especially in its geophysical fluid envelope, the atmosphere, ocean, and continental water produce observable changes in geocenter motion, which is currently detectable from space geodetic techniques. We investigate mass variations in the atmosphere and continental water cycle and their potential contributions to geocenter motion using surface pressure, soil moisture, and snow accumulation from the
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We have estimated the Chandler frequency from a variety of polar motion time series derived from optical and space geodetic data which span various time periods from 1846 through the early 1990s. Estimates of F vary, depending upon which... more
We have estimated the Chandler frequency from a variety of polar motion time series derived from optical and space geodetic data which span various time periods from 1846 through the early 1990s. Estimates of F vary, depending upon which time series is employed, but the variation is not significant when associated intervals of confidence are considered; thus there is no
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Five separate polar motion series are examined in order to understand what portion of their variations at periods exceeding several years represents true polar motion. The data since the development of space-geodetic techniques (by... more
Five separate polar motion series are examined in order to understand what portion of their variations at periods exceeding several years represents true polar motion. The data since the development of space-geodetic techniques (by themselves insufficient for study of long-period motion), and a variety of historical astrometric data sets, allow the following tentative conclusions: retrograde long-period polar motion below about m0.2 cpy (cycles per year) in pre-space-geodetic data (pre-1976) is dominantly noise. For 1976-1992, there is poor agreement between space-geodetic and astrometric series over the range m0.2 to +0.2 cpy, demonstrating that classical astrometry lacked the precision to monitor polar motion in this frequency range. It is concluded that all the pre-1976 astrometric polar motion data are likely to be dominated by noise at periods exceeding about 10 years. The exception to this is possibly a linear trend found in some astrometric and space geodetic series. At frequencies above prograde +0.2 cpy (periods shorter than about 5 years), historical astrometric data may be of sufficient quality for comparisons with geophysical excitation time series. Even in the era of space geodesy, significant differences are found in long-period variations in published polar motion time series.
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Four different basin functions are developed to estimate water storage variations within individual river basins from time variations in the Stokes coefficients now available from the GRACE mission. The four basin functions are evaluated... more
Four different basin functions are developed to estimate water storage variations within individual river basins from time variations in the Stokes coefficients now available from the GRACE mission. The four basin functions are evaluated using simulated data. Basin functions differ in how they minimize effects of three major error sources: measurement error; leakage of signal from one region to another;
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We computed zonal geopotential coefficients from average seasonal variations in global air and water mass distribution. These coefficients are used to predict the seasonal variations of LAGEOS' and... more
We computed zonal geopotential coefficients from average seasonal variations in global air and water mass distribution. These coefficients are used to predict the seasonal variations of LAGEOS' and Starlette's orbital node, δΩ, and the seasonal δJ 3 for Starlette. A ...
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Summary A new reduction of the International Latitude Service Observations for the years 1899–1977 has recently been completed under the direction of Dr S. Yumi and the International Astronomical Union Commission 19. This paper examines... more
Summary A new reduction of the International Latitude Service Observations for the years 1899–1977 has recently been completed under the direction of Dr S. Yumi and the International Astronomical Union Commission 19. This paper examines the annual, ...
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Recent studies based on various ocean general circulation models (OGCMs) demonstrate that the oceans are a major contributor to polar motion excitations. In this paper, we analyse and compare observed non-atmospheric polar motion... more
Recent studies based on various ocean general circulation models (OGCMs) demonstrate that the oceans are a major contributor to polar motion excitations. In this paper, we analyse and compare observed non-atmospheric polar motion excitations with oceanic angular momentum (OAM) variations determined from four OGCMs, which include the parallel ocean climate model (POCM), a barotropic ocean model (BOM), the Estimating the Circulation and Climate of the Ocean (ECCO) non-data-assimilating model (ECCO-NDA) and the ECCO data-assimilating model (ECCO-DA). The data to be analysed span a 5-yr overlapped period from 1993 to 1997. At annual timescale, considerable discrepancies exist between POCM and the other three models, which result mainly from differences in annual components of the forcing wind fields. At semi-annual timescale, however, POCM shows better phase agreement with observed non-atmospheric polar motion excitation than the other three ocean models. At intraseasonal timescales, ECCO-DA yields better agreement with observations, and reduces the variance of non-atmospheric excitations by ~60 per cent, 10-20 per cent more than those explained by the other three models. However, at the very short periods of 4-20 days, the BOM estimates could explain about half of the observed variance, twice as much as that by ECCO-NDA, and also shows considerably better correlation with observations. Due to different modelling schemes and methods, significant discrepancies could arise with respect to the quality of modelling large-scale oceanic mass redistribution and current variation. A complete understanding of global oceanic contributions to polar motion excitation still remains a challenge.
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We estimate terrestrial water storage variations using time variable gravity changes observed by the Gravity Recovery and Climate Experiment (GRACE) satellites during the first 2 years of the mission. We examine how treatment of... more
We estimate terrestrial water storage variations using time variable gravity changes observed by the Gravity Recovery and Climate Experiment (GRACE) satellites during the first 2 years of the mission. We examine how treatment of low-degree gravitational changes and geocenter variations affect GRACE based estimates of basin-scale water storage changes, using independently derived low-degree harmonics from Earth rotation (EOP) and satellite
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The Gravity Recovery and Climate Experiment (GRACE) satellites provide observations of water storage variation at regional scales. However, when focusing on a region of interest, limited spatial resolution and noise contamination can... more
The Gravity Recovery and Climate Experiment (GRACE) satellites provide observations of water storage variation at regional scales. However, when focusing on a region of interest, limited spatial resolution and noise contamination can cause estimation bias and spatial leakage, problems that are exacerbated as the region of interest approaches the GRACE resolution limit of a few hundred km. Reliable estimates of
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Thirteen monthly GRACE gravity solutions from August 2002 to December 2004 are examined to recover terrestrial water storage changes, and to investigate likely errors in the estimates. To understand limitations in GRACE results, simulated... more
Thirteen monthly GRACE gravity solutions from August 2002 to December 2004 are examined to recover terrestrial water storage changes, and to investigate likely errors in the estimates. To understand limitations in GRACE results, simulated monthly GRACE solutions are generated using numerical climate models, GRACE error levels, and the GRACE Atmospheric and Ocean Dealiasing (AOD) model. The simulated data are compared
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The Chandler wobble frequency F and quality factor Q are estimated by least squares using polar motion data in combination with proxy excitations derived from atmospheric data. Monte Carlo tests show that this approach can provide useful... more
The Chandler wobble frequency F and quality factor Q are estimated by least squares using polar motion data in combination with proxy excitations derived from atmospheric data. Monte Carlo tests show that this approach can provide useful improvements over traditional maximum likelihood methods, which use only observed polar motion. With less than a decade of good simultaneous polar motion and