1. Earth System Data Record of mass transport from time-variable gravity data Victor Zlotnicki 1, Matthieu Talpe 2, F. Lemoine 3, R. Steven Nerem 2, Felix Landerer 1, Michael Watkins 1. 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 2 Univ. of Colorado, Boulder, CO. 3 NASA Goddard Space Flight Ctr, Greenbelt, MD 1 JPL Document Clearance CL#14-3086

2. Gravity changes track water storage changes 2

3. Four ways to measure gravity from space satell tracking from earth (laser, doppler) (since 1975) high low sat sat tracking: GPS (since 1992) low-low sat-sat tracking GRACE (since 2002) gradiometry GOCE (mostly time-mean) (2009-2013) 3 oldest newest

4. GRACE is best, why bother with others? GRACE has provided the highest resolution and accuracy estimates of time changes in water/ice, but need: to extend the time series in time before GRACE launch to patch a possible gap between GRACE and GRACE Follow-on 4

5. GPS + laser satellite tracking 1 M.Weigelt, T. Van Dam, et al, 2014, GRACE STM Mtg. 5

6. GPS + laser satellite tracking 2 6 M.Weigelt, T. Van Dam, et al, 2014, GRACE STM Mtg. wavelength ~ 40,000km/l 4,000 2,000 1,333 1,000 800 667 ~wavelength (km)

7. GPS + laser satellite tracking 3 7 K. Sosnica, A. Jaggi, M.Weigelt et al, 2014, GRACE STM Mtg.

8. DORIS + Laser satellite tracking 1 8 F. Lemoine, GSFC, 2014

9. DORIS + Laser satellite tracking EOF reconstruction 9

10. 10 DORIS + Laser satellite tracking EOF reconstruction

11. 11 DORIS + Laser satellite tracking EOF reconstruction Greenland

12. Differences in GRACE processing no instrument bias differences between different satellites, main diff is spatial resolution But, differences due to processing choices can have spatial patterns. Example: RMS difference between two GRACE processing centers, using the same underlying GRACE data 12

13. How to validate mass fluxes 1 Validation OCEAN – comp to bottom pressure recorders ( Boening, GRL 2008 ) – comp to altimetry-argo ( Chambers, Bonin 2012 ) – comp to ocean+data model ( Chambers, Bonin 2012 ) ICE – comp Greenland to SMB ( Velicogna 2014 ) – comp Amundsen SMB, Icesat, Envisat ( Velicogna 2015 ) LAND – comp to GPS deformation in Calif. ( Argus & Landerer, 2015 ) – comp to well data ( Swenson, 2008 ) 13

14. 14 How to validate mass fluxes 2 Amundsen Sea Embayment, WAIS. 1)GRACE 2)mass budget method 3)Envisat radar altimetry 4)Icesat and airborne OIB laser altimetry Sutterley, Velicogna, et al GRL 2015 Bottom Pressure Recorder s, 8S, 125W ( Zlotnicki, Williams, Hughes, Boening, 2013 )

15. Summary, Discussion GRACE is the most accurate method to determine time-variable mass flux globally 2003 to 2015+, >300-500km. GFO in 2017. The time series can be extended with GPS+SLR or Doris+SLR (or all 3), at lower spatial resolution > ~2,000-8,000km There are no biases introduced by the different satellites, but there can be systematic differences due to processing choices There are several methods to validate gravity-derived mass fluxes, but limited in space and time 15

16. BACKUP 16

17. Gravity data measures mass flux The longest wavelength, J2, measured since 1976. Glacial isostatic adjustment + Greenland and Antarctica ice sheet melt, measured by SLR. Greenland, Antarctica, glaciers ice mass loss measured by GRACE since 2003 Land total water content: GRACE Ocean mass and bottom pressure: GRACE 17 J2: Cheng, Tapley JGR 2013 Velicogna & Wahr, GRL 2013 Greenland ice mass

18. 18 DORIS + Laser satellite tracking EOF reconstruction

19. Magnitude of orbit perturbations (hence sensitivity) diminishes with altitude. Sensitivity is not uniform across all coefficients of a given degree, especially given only SLR or DORIS tracking. Envisat has sensitivity to many terms at order 1 – but not all terms are separable from SLR+DORIS data alone on that satellite. (mm) Orbit Perturbations (mm) for SLR+DORIS satellites from effects of Time-Variable Gravity (Cryosat2, Envisat, Jason-2) Jason-2Envisat Cryosat-2 F. Lemoine, GSFC, 2014

20. Correlations of SLR+DORIS 5x5 solution with GRGS GRACE+Lageos solution (2003-2012) C coefficientsS coefficients C20, C22, sectorals, match GRACE solutions well; For C31, C32 and coefficients at L=5, the agreement becomes less good. F. Lemoine, GSFC, 2014