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The Gravity Recovery and Climate Experiment (GRACE)

The Gravity Recovery and Climate Experiment (GRACE). Matt King School of Civil Engineering and Geosciences Newcastle University. Outline. Overview of GRACE mission Analysis approaches & error sources Static gravity field Temporal gravity field Case studies Ice mass balance Hydrology

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The Gravity Recovery and Climate Experiment (GRACE)

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  1. The Gravity Recovery and Climate Experiment • (GRACE) Matt King School of Civil Engineering and Geosciences Newcastle University

  2. Outline • Overview of GRACE mission • Analysis approaches & error sources • Static gravity field • Temporal gravity field • Case studies • Ice mass balance • Hydrology • Ocean processes • The Great Sumatra Earthquake

  3. Overview of GRACE mission • Observing since 2002, planned 7+ yr mission life • Twin-satellites, ~500->400km above Earth’s surface, ~200km apart • Key components: • Inter-satellite microwave ranging (KBR) • GPS • Accelerometer (non-gravitational forces) • Star Tracker

  4. Overview of GRACE mission • Global gravity field determined every 10-30 days • Mass change determined from gravity change with accuracy ~2cm water equivalent at ~400km scales • Addresses fundamental need in science and society • Many large scale processes in Earth System poorly/inaccurately understood (or no redundancy to check)

  5. Overview of GRACE mission

  6. Overview of GRACE mission • Secondary focus: Occultation

  7. GRACE Error Budget Wahr et al, GRL, 2004

  8. Data Analysis Approaches • Two main (and fundamentally different) approaches • Global Spherical Harmonics • Most common approach • Uses (and adjusts) GPS data, range-rate data • In global solutions mismodeling in one area is propagated globally by aliasing through once per revolution effects in the orbit solution Forward model (trend) Forward model (annual amp) Wouters et al., GRL, 2008 Forward model SH GRACE observed SH

  9. Data Analysis Approaches • Mass concentrations - “Mascons” • Yields a direct estimate of the mass flux over a given region over a specific time interval • Step 1: Biases and trends in the accelerometer data removed through calibration • Step 2: The mascons and orbit parameters are exclusively estimated from the KBRR data themselves (no GPS) • Generally 400km at 10day intervals (no spatial smoothing)

  10. Data Analysis Approaches • Same data, different centres… Sasgen et al., EPSL, 2007

  11. Error Sources • GRACE measures total mass • All mass change signal other than one desired needs to be removed (modelled) • E.g., if after ice mass change, need to model all ocean (tidal and non-tidal), hydrology, atmospheric, mantle processes • In general, atmosphere, oceans taken as known and removed at data analysis stage • In reality all these models have errors, with varying levels in different locations (hence the need for GRACE!) • Major errors • Glacial Isostatic Adjustment in Antarctica • Tides everywhere, especially over oceans

  12. Error Sources • S2 (12h) aliasing TPXO6.2-CSR4 mm; 161-day TPXO6.2-FES2004 Moore & King, JGR, 2008

  13. Error Sources • Mitigating them – one common approach • Gaussian averaging kernel with a radius (i.e., half-width) of X km Wahr et al., GRL, 2006 Wahr et al., JGR, 1998

  14. Static Gravity Field Pre-GRACE (satellite data) GRACE only http://www.csr.utexas.edu/grace/gallery/gravity/Old_Gravity_Field.html

  15. Temporal Gravity Field • Relative to mean static field • First results: • GRACE annual signal • Model ocean+hydrology model • Errors in both Wahr et al, GRL, 2004

  16. Case Study I: Ice Mass Balance

  17. Case Study I: Ice Mass Balance GIA Model (900km smoothed) GIA Model (unsmoothed) GRACE Chen et al, Science, 2006

  18. Case Study I: Ice Mass Balance • GIA - Same Earth Model, different ice histories (courtesy: G. Milne, Uni Ottawa)

  19. Case Study II: Ice Mass Balance Luthcke et al., Science, 2006

  20. Case Study II: Hydrology Mascon approach SH approach: Tapley et al, Science

  21. Case Study III: Ocean Processes • Ocean currents from Geoid(incl. GRACE)+altimetry http://grace.jpl.nasa.gov/data/dot/

  22. Case Study IV: The Great Sumatra Earthquake Han et al., Science, 2006

  23. Conclusions • GRACE has revolutionised our understanding of static and temporal gravity fields • Offers new insights into processes in the Earth System • Ice mass balance, hydrological signal, ocean processes, mantle processes… • Lots more work to do • No follow-on mission yet planned

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