GRACE Science Team Meeting

Session: B.1 - Multidisciplinary Science
(Convener: )

TIME TITLE
11:00-11:15 Introduction of the web-based Mascon Visualization Tool
First Author: Michael Croteau
Co-Authors: B.D. Loomis, D.N. Wiese, R.S. Nerem
11:15-11:30 Geocenter motion time series derived from GRACE GPS or LAGEOS observations
First Author: Zhigui Kang
Co-Authors: B. Tapley, J. Chen, J. Ries, S. Bettadpur
11:30-11:45 Uncertainty in GRACE estimates of the mass redistributions at the Earth surface and implications for the global water and sea level budgets
First Author: Alejandro Blazquez
Co-Authors: B. Meyssignac, J.M. Lemoine, E. Berthier, A. Ribes, A. Cazenave
11:45-12:00 Caspian Sea level change and validation of GRACE and GRACE-FO
First Author: Jianli Chen
Co-Authors:
12:00-12:15 Detecting the Acceleration of Sea Level Rise in the Satellite Altimeter Record and its Validation with GRACE
First Author: R. Steven Nerem
Co-Authors: B. D. Beckley, J. Fasullo, B. D. Hamlington, D. Masters, and G. T. Mitchum
POSTER Investigation of Approaches for Long-Wavelength Gravity Recovery from GPS-Based Orbit Determination of Low-Earth Orbiters
First Author: Shailen Desai
Co-Authors: B.J. Haines, D. Kuang

Title: Introduction of the web-based Mascon Visualization Tool
Presenter: Croteau, Michael
Co-Authors: B.D. Loomis, D.N. Wiese, R.S. Nerem

Abstract: We have developed a web-based interactive portal for exploring and visualizing GRACE mascon data, available at http://ccar.colorado.edu/grace. This tool provides an intuitive and approachable method for working with these products, both for initial exploratory work and more detailed research. The tool allows users to interact with GRACE data at both regional and basin scales, as well as with the individual mascons themselves. The tool currently supports both the JPL RL05M v02 and GSFC v2.3b mascon products, while initial work has begun on additionally implementing CSR's mascon product. All usage best practices for each solution have been incorporated into the tool to ensure correct usage of the data by end users, allowing experts and non-experts alike to use GRACE mascon data with confidence. In this talk, we demonstrate use of the tool for exploring signals of interest, such as basin water storage and ice sheet mass loss.

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Title: Geocenter motion time series derived from GRACE GPS or LAGEOS observations
Presenter: Kang, Zhigui
Co-Authors: B. Tapley, J. Chen, J. Ries, S. Bettadpur

Abstract: In order to estimate the total mass variability from Gravity Recovery And Climate Experiment (GRACE), the geocenter motion time series are required. Currently, there are two sources of regular monthly estimates for the series: one derived from satellite laser ranging data; another derived from GRACE monthly gravity estimates combined with ocean model output. With the passage of time, long enough GPS tracking data from GRACE satellites are acquired for studying the geocenter motion. And at same time, the approaches, products, reference system, and models for GRACE precise orbit determination (POD) have also been significantly improved. Those aspects are very important for determining geocenter motion from GRACE GPS observations. Based on those motivations, the GRACE GPS data are reprocessed for geocenter motion study. In addition, geocenter motion is also derived from satellite laser ranging (SLR) observations to the LAGEOS satellites for comparisons. The main purpose of this study is to present the recent advances and results for determining geocenter variations using GRACE GPS and LAGEOS observations. In this study, daily geocenter time series from GRACE GPS for the time span of 2003 to 2016, and 28-day geocenter variations from LAGEOS covering the years 1992-2016, have been derived. Internal comparison between the GRACE-A and GRACE-B geocenter time series and external comparison between the GRACE and LAGEOS show very good agreement. To verify the results, the annual variations of geocenter motions from this study have been externally compared with other recent geocenter motion solutions as well as predictions from geophysical models. The comparisons show good agreements in both amplitude and phase. This means that the geocenter variations derived from the GRACE GPS observations could be used for some applications based on GRACE Level-2 products. Therefore, the geocenter time series from GRACE GPS can provide consistent products for GRACE users.

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Title: Uncertainty in GRACE estimates of the mass redistributions at the Earth surface and implications for the global water and sea level budgets
Presenter: Blazquez, Alejandro
Co-Authors: B. Meyssignac, J.M. Lemoine, E. Berthier, A. Ribes, A. Cazenave

Abstract: Observations from GRACE provide quantitative estimates of the global water budget components. However, these estimates are uncertain as they show discrepancies when different parameters are used in the processing of the GRACE data. We examine trends in ocean mass, ice loss from Antarctica, Greenland, Arctic islands and water storage in land and in glaciers from GRACE data (2005-2014) and quantify the associated uncertainty. We considered variations in 6 different GRACE processing parameters, namely the processing center of the raw GRACE solutions, the geocenter motion, C2,0, the filtering, the leakage correction and the GIA. Considering all possible combinations of the different processing parameters lead to an ensemble of 1620 post-processed GRACE solutions which is assumed to cover the uncertainty range of GRACE estimates. The ensemble-mean trend in all global water budget components agree within uncertainties with previous estimates based on different sources of observations. The uncertainty (at the 90% confidence level) in the global water budget is ±0.46mm/yr. It is systematically larger than previous estimates because it takes into account for the first time the uncertainty in the geocenter motion correction. We find that the uncertainty in the geocenter motion and GIA corrections dominate the uncertainty in GRACE estimate of the global water budget and hampers accurate closure of the global water budget and the global sea level budget. This uncertainty in GRACE estimate implies an uncertainty in the net warming of the ocean and the Earth energy budget of ±0.32W.m-2 when inferred using the sea level budget approach.

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Title: Caspian Sea level change and validation of GRACE and GRACE-FO
Presenter: Chen, Jianli
Co-Authors:

Abstract: The Caspian Sea is the largest enclosed inland body of water on Earth, with a surface area of ~ 371,000 km^2. Satellite altimeter measurements show a declining Caspian Sea level at a rate of ~ 9 cm/yr since 2005, on top of strong seasonal variations of almost 40 cm from peak to peak. The long-term and peak-to-peak seasonal Caspian Sea mass changes are ~ 33 km3/yr and 148 km3, respectively. With a clearly defined geographic region and dominant signal magnitudes, variations in the Caspian Sea level and associated mass changes provide an excellent way to validate GRACE data in the form of both spherical harmonic (SH) and mascon solutions and the applied data processing methods. After correcting for spatial leakage in GRACE SH estimates, and accounting for steric and terrestrial water processes, GRACE and altimeter observations are in complete agreement at seasonal and longer time scales, including linear trends. After proper leakage corrections, both JPL and CSR mascon solutions also agree well with altimeter observations. The excellent agreement between GRACE and altimeter estimates provides not only important validation of GRACE observations, but also a possible means to help bridge the expected gap between the GRACE and GRACE Follow-On missions.

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Title: Detecting the Acceleration of Sea Level Rise in the Satellite Altimeter Record and its Validation with GRACE
Presenter: Nerem, R. Steven
Co-Authors: B. D. Beckley, J. Fasullo, B. D. Hamlington, D. Masters, and G. T. Mitchum

Abstract: We now have a 25-year record of global average sea level change from satellite altimeter missions such as TOPEX/Poseidon, Jason-1, Jason-2, and Jason-3. This record shows considerable interannual and decadal variability superimposed on a long-term trend of about 3 mm/year. We show that after the interannual and decadal variability is accounted for and after performing a small adjustment to the TOPEX data, the long-term trend is accelerating. If the rate of sea level rise were to continue at 3 mm/year, this would result in 30 cm of sea level rise over a century. However, when this rate is combined with the acceleration we have detected, this would more than double the sea level rise over a century (~65 cm). We have also compared the rate and acceleration of sea level derived from satellite altimetry to the rate and acceleration of ice mass loss as determined from GRACE data (2002-present). When the GRACE results are added to the rate and acceleration of thermosteric sea level change, we found quite good agreement with the altimeter estimates, even though the time periods differ. Determining the acceleration of sea level change will become more robust as the satellite record lengthens. GRACE and GRACE Follow-On will be valuable tools for validating these estimates and pinpointing the causes of the acceleration.

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Poster Title: Investigation of Approaches for Long-Wavelength Gravity Recovery from GPS-Based Orbit Determination of Low-Earth Orbiters
Presenter: Desai, Shailen
Co-Authors: B.J. Haines, D. Kuang

Abstract: In this poster, we show results from our ongoing investigation of approaches for long-wavelength time-variable gravity (TVG) recovery using precise orbit determination (POD) of Low-Earth Orbiters (LEOs). The optimal recovery of TVG while appropriately accounting for non-gravitational perturbing forces and systematic errors that influence the POD of LEOs serves as the motivation for our investigation. Our goal is to leverage the continuously increasing number of low-Earth orbiting satellites that are tracked using onboard Global Positioning System (GPS) receivers. The GPS-based POD approach benefits from high-accuracy, and continuous tracking of the satellite’s orbital motions.

We adopt a mixed-dynamic GPS-based POD approach for this investigation, without the benefit of inter-satellite ranging data, and directly using the GPS phase and range tracking data. Our POD solutions simultaneously estimate deterministic parameters (e.g., 20 by 20 TVG field) together with stochastic perturbing accelerations to account for non-gravitational force model errors, while allowing for errors in the GPS-based reference frame. We also investigate and compare determination of TVG independently for each considered LEO, and simultaneous determination of TVG from a combination of LEOs. Our focus thus far has been to consider GPS-based POD from the GRACE (A and B), CHAMP, and SWARM (A, B, and C) satellites.

Our results demonstrate the trade-off between higher TVG sensitivity (but larger drag model errors) from lower altitude satellites and lower TVG sensitivity (but lower drag model errors) from higher altitude satellites. For example, with our current approach, the 530 km altitude SWARM-B satellite appears to provide better TVG estimates than the lower (450 km) altitude SWARM-A and C satellites. Similarly, the error in TVG estimates from the 340 km altitude CHAMP satellite is an order of magnitude larger than from the higher altitude GRACE satellites, primarily due to our use of GRACE accelerometer data to account for non-gravitational forces. Our long-wavelength TVG estimates from SWARM-B have similar accuracy to those from GRACE with its accelerometer data except for the degree 2 spherical harmonic components. Future work will continue to tailor our mixed dynamic GPS-based POD approach, investigate improvements to drag models, while also incorporating other LEOs with onboard GPS receivers (e.g., Jasons, Sentinel-3, etc).

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