GRACE Science Team Meeting

B.1.1 to B.1.5 - Multidisciplinary
(Convener: )

Geocenter Motion and Degree-1 Surface Mass Variations - Reconciling Results from Direct SLR Determination and Inverse Estimation
(X. Wu, C.Abbondanza, Z. Altamimi, T.M. Chin, X. Collilieux, R.S. Gross, M.B. Heflin,Y. Jiang, J.W. Parker)

Ice sheets and land water mass contributions to the sea level fingerprint from GRACE, InSAR, and a regional climate model output
(C-H. Hsu, I. Velicogna)

Closing the global mean sea level rise budget
(J.L. Chen)

Australia's Unique Influence on Global Sea Level in 2010-2011
(J. Fasullo, C. Boening, F. Landerer, S. Nerem)

GRACE Contributions to Hydroclimatic Analysis
(M. Rodell)

Title: Geocenter Motion and Degree-1 Surface Mass Variations - Reconciling Results from Direct SLR Determination and Inverse Estimation
Presenter: Wu, Xiaoping
Co-Authors: X. Wu; C.Abbondanza; Z. Altamimi; T.M. Chin;X. Collilieux; R.S. Gross; M.B. Heflin; Y. Jiang; J.W. Parker

Abstract: The longest-wavelength surface mass variations are the three degree-one spherical harmonic components involving hemispherical mass exchanges. The mass load causes geocenter motion between the center-of-mass of the total Earth system (CM) and the center-of-figure of the solid Earth surface (CF), and deforms the solid Earth. Estimation of the degree-1 surface mass changes through CM-CF motion or degree-1 mass redistribution and deformation signatures from space geodetic techniques can thus complement GRACE's n>1 time-variable gravity data to form a complete change spectrum up to a high resolution. Currently, SLR is considered the most accurate technique for direct geocenter motion determination. By tracking satellite motion from ground stations, SLR determines the motion between CM and the geometric center of its ground network (CN). This motion is then used to approximate CM-CF and subsequently for deriving degree-1 mass changes. The inverse estimation uses GRACE, relative GPS ground deformation data, and ocean bottom pressure models to solve for time variable surface mass coefficients up to high degrees including the n=1 components. Annual geocenter motion results from the two methods agree quite well along the Y- and Z-axes but differ by more than a millimeter along the X-axis. Recently, we realized an experimental Terrestrial Reference Frame (TRF) through station time series using the Kalman filter and the RTS smoother. The TRF has its origin defined at nearly instantaneous CM using weekly SLR measurement time series. VLBI, GNSS and DORIS time series are combined weekly with those of SLR and tied to the geocentric (CM) reference frame through local tie measurements and co-motion constraints on co-located geodetic stations. The unified geocentric time series of the four geodetic techniques provide a much better network geometry for direct geodetic determination of geocenter motion. Results from this direct approach using a multi-technique network compare favorably with those obtained from joint inversions of GPS/GRACE data and ocean bottom pressure models. The discrepancy in X-component between direct SLR orbit-tracking and inverse determined geocenter motions is largely reconciled with the new unified network.

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Title: Ice sheets and land water mass contributions to the sea level fingerprint from GRACE, InSAR, and a regional climate model output
Presenter: Hsu, Chia-Wei
Co-Authors: I. Velicogna

Abstract: We generate seasonal, inter-annual, and long-term regional sea level variations based on observations of rapid water mass changes on land, which is also called the sea level fingerprint. We use 10 years of monthly gravity field data from the GRACE satellite mission to determine the rapid water mass changes on land during the 2002-2012 period. For the ice sheets, we use both GRACE time series and ice mass changes estimated using the mass budget approach. We evaluate the fingerprint for each ice sheet and for the land hydrological contribution, and we determine the relative contribution of each component and how the fingerprint changes with time. Before calculating the sea level fingerprint, the GRACE data are scaled to recover the signal amplitude. Over land, we use the GLDAS hydrological model to calculate the scaling factor. Over the ice sheets, we use the spatial pattern of ice mass from the mass budget method. We estimate a scaling factor for the seasonal and the long-term component of the signal, and evaluate the sensitivity of the fingerprints to different spatial patterns of ice mass change. Finally, we use 40 years of ice mass changes from the mass budget method to evaluate ice sheet impacts on the sea level fingerprint over a longer time period. This work was conducted at the University of California Irvine and at Caltech's Jet Propulsion Laboratory under a contract with the National Aeronautics and Space Administration.

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Title: Closing the global mean sea level rise budget
Presenter: Chen, Jianli
Co-Authors: J.L. Chen

Abstract: Changes in global mean sea level reflect mainly the sum of three contributions: water mass changes in the ocean basins, steric changes (i.e., ocean volume changes due to density changes), and variable ocean basin volumes due to post-glacial rebound and other influences. Closing the global sea level budget implies reconciling these various observations and estimates, and has been a challenging topic. We show that the closure of sea level rise budget can be achieved by reducing leakage into the oceans of terrestrial signals in GRACE data through global forward modeling. The study covers the period 2005-2011 when Argo float coverage is reasonably global. Argo data indicate a steric sea level rise of 0.60 ± 0.27 mm/yr in this period. A new mass estimate from GRACE is 1.80 ± 0.47 mm/yr, with the majority (~ 1.73 ± 0.28 mm/yr) from polar ice sheets and mountain glaciers melting. The sum of steric and mass effects in this period is 2.40 ± 0.54 mm/yr, which agrees very well with the altimeter rate of 2.39 ± 0.48 mm/yr. The significantly larger influence of melting ice sheets and mountain glaciers on observed sea level rise (~75%) is contrary to previous IPCC assessments, but consistent with widely reported accelerated ice meltings in recent years.

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Title: Australia's Unique Influence on Global Sea Level in 2010-2011
Presenter: Boening, Carmen
Co-Authors: J. Fasullo; C. Boening; F. Landerer; S. Nerem

Abstract: In 2011, a significant drop in global sea level occurred that was unprecedented in the altimeter era and concurrent with an exceptionally strong La Niña. This analysis examines multiple datasets in exploring the physical basis for the drop's exceptional intensity and persistence. Australia's hydrologic surface mass anomaly is shown to have been a dominant contributor to the 2011 global total and associated precipitation anomalies were among the highest on record. The persistence of Australia's mass anomaly is attributed to the continent's unique surface hydrology, which includes expansive arheic and endorheic basins that impede runoff to ocean. Based on Australia's key role, attribution of sea level variability is addressed. The modulating influences of the Indian Ocean Dipole and Southern Annular Mode on La Niña teleconnections are found to be key drivers of anomalous precipitation in the continent's interior and the associated surface mass, and sea level responses.

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Title: GRACE Contributions to Hydroclimatic Analysis
Presenter: Rodell, Matthew
Co-Authors: M. Rodell

Abstract: With more than ten years of data available, GRACE is now beginning to contribute to studies of hydroclimatic variability, including assessments of natural seasonal cycles, interannual variability, and emerging trends. This presentation focuses on three such applications. The first is GRACE's role in NASA's Energy and Water Cycle Study (NEWS) Water and Energy Cycle Climatology project, which has developed "state of the global water cycle" and "state of the global energy cycle" assessments for roughly the first decade of the millennium based on data from modern ground and space based observing systems and data integrating models. The second application is a groundwater and terrestrial water storage section for the Bulletin of the American Meteorological Society's annual "State of the Climate" special issue. The third is an evolving global map of trends in terrestrial water storage.

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