GRACE Mascons - Current Products

 

Name: GSFC.glb.200301_201607_v02.4

Summary: Standard solution; not corrected for GIA; comparable to GRACE Project Level-2 spherical harmonics (GSM) with post-processing corrections applied

Applications: Terrestrial water storage and cryosphere after removal of GIA

Available formats: HDF5, ASCII

Region: Global

Name: GSFC.glb.200301_201607_v02.4-ICE6G

Summary: Standard solution with ICE6G GIA model removed [Peltier et al., 2015]

Applications: Terrestrial water storage and cryosphere

Available formats: HDF5, ASCII

Region: Global

Name: GSFC.glb.200301_201607_v02.4-GeruoA

Summary: Standard solution with GeruoA GIA model removed [A et al., 2013]

Applications: Terrestrial water storage and cryosphere

Available formats: HDF5, ASCII

Region: Global

Name: GSFC.glb.200301_201607_v02.4+ECMWF+MOG2D

Summary: ECMWF+MOG2D atmosphere and ocean de-aliasing model restored

Applications: Total time-variable gravity signal including AOD model

Available formats: HDF5,  ASCII

Region: Global

Name: GSFC.ocn.200301_201607_v02.4_OBP-GeruoA

Summary: GAD product restored and GeruoA GIA model removed

Applications: Ocean bottom pressure

Available formats: HDF5,  ASCII

Region: Ocean

Name: GSFC.ocn.200301_201607_v02.4_SLA-GeruoA

Summary: GAD restored; mean atmospheric pressure removed; GeruoA GIA model removed 

Applications: Ocean mass, comparable to steric-corrected sea level anomalies

Available formats: HDF5,  ASCII

Region: Ocean


HDF5 format documentation: GSFC_mascons_HDF5_format_v02.4.pdf 

 

Notes about v02.4:

  • v02.4 solution is identical to v02.3b solution
  • v02.4 contains new mascon uncertainties including leakage (see HDF format documentation)
  • A manuscript documenting all current procedures is currently in review

 

Solution, background models, and post-processing details:

  • Solution span: Jan 2003 – July 2016
  • Mean removed: 2004.0 – 2016.0
  • Iterated to convergence [Luthcke et al., 2013]
  • Background static field: GOCO-05S with epoch of 2008.0
  • Tide model: GOT4.7 to degree/order 90 [Ray 1999]
  • Atmosphere and ocean de-aliasing (AOD): ECMWF+MOG2D [Carrere and Lyard, 2003]
  • Geocenter correction is applied [Swenson et al., 2008]
  • C20 is replaced with GRACE TN-07 [Cheng et al., 2013]
  • C21/S21 trends are corrected following [Wahr et al., 2015]
  • Antarctic Ice Sheet mascons for July 2015 have been linearly interpolated due to GRACE data gaps 

 

Forward models and the GSFC solution strategy:

Since the beginning of the GRACE mission it has been standard practice to apply atmosphere and ocean de-aliasing products when processing the Level 1B data in order to directly remove these high-frequency signals from the inter-satellite measurements and the distributed gravity solutions. [Luthcke et al., 2006] and [Sabaka et al., 2010] demonstrated the benefit of also modeling global hydrology towards a further reduction in the measurement residuals and the mitigation of signal leakage. The GSFC mascon estimation procedure applies this general approach by modeling the best-fit trend and annual time-variable gravity signals as observed by a prior GSFC mascon solution (v1.1) and glacial isostatic adjustment (GIA). We also employ solution iteration [Luthcke et al., 2013], where the estimated updates to the mascons are applied as corrections to the forward model for a subsequent processing of the Level 1B data. The final solution is the sum of the initial forward model, and the mascon adjustments for the first and second iterations. The motivation of this iterative approach is to minimize the potential effect of the mascon regularization design, where the regularization for the final iteration is based on a binned analysis of the post-fit range-acceleration residuals. Over land and ice the mascons primarily describe the total variability in water content, and over the ocean we provide separate solutions for ocean bottom pressure and sea level anomalies. The global solutions are available with two different GIA model corrections.

 

De-aliasing products and ocean-only mascons:  

The GSFC ocean bottom pressure (OBP) and sea level anomaly (SLA) products make use of the GAC and GAD, which are products distributed by the GRACE Project. Over the ocean, both the GAC and GAD contain the sum of the OMCT non-tidal ocean model and the ECMWF atmospheric variability. They differ in that the GAC contains the vertically integrated atmospheric mass variability required for precise orbit determination, while the GAD applies the atmospheric surface pressure as required to relate GRACE measurements to ocean bottom pressure. The definitions of GSM, GAC and GAD products are described by Bettadpur [2012] and Flechtner et al. [2014]. Ocean bottom pressure from GRACE Project spherical harmonics is computed by: GSM+GAD-GIA.  The GSFC procedures currently apply a different ocean model (MOG2D) in the Level-1B data processing than the GRACE Project (OMCT).  In order to provide ocean products that are consistent with other solutions, we must restore our own de-aliasing model (ECMWF+MOG2D) and then remove the GRACE Project de-aliasing model (GAC) prior to restoring the GAD.  To produce a product (SLA) that is comparable to steric-corrected altimetry measurements of sea level anomalies, we must remove the spatial mean of atmospheric surface pressure over the ocean, which is equal to the mean of the GAD as the non-tidal ocean component conserves mass.  

 

When using this data please cite:

Luthcke, S.B., T.J. Sabaka, B.D. Loomis, et al. (2013), Antarctica, Greenland and Gulf of Alaska land ice evolution from an iterated GRACE global mascon solution,J. Glac.; 59(216), 613-631, doi:10.3189/2013JoG12J147


Contact:

Bryant Loomis: Bryant.D.Loomis@nasa.gov
Scott Luthcke: Scott.B.Luthcke@nasa.gov

 

References:

  • A. Geruo, J. Wahr, and S. Zhong (2013), Computations of the viscoelastic response of a 3-D compressible Earth to surface loading: an application to Glacial Isostatic Adjustment in Antarctica and Canada, Geophys. J. Int., 192, 557-572,doi:10.1093/gji/ggs030
  • Bettadpur, S(2012), GRACE Level-2 Gravity Field Product User Handbook, GRACE 327-734 (CSR-GR-03-01), Center for Space Research, The University of Texas at Austin.
  • Carrere, L., and F. Lyard (2003), Modeling the barotropic re- sponse of the global ocean to atmospheric wind and pressure forcing- Comparisons with observations, Geophys. Res. Lett., 30(6), 1275.
  • Cheng, M. K., B. D. Tapley, and J. C. Ries (2013), Deceleration in the Earth's oblateness, J. Geophys. Res., 118, 1-8,doi:10.1002/jgrb.50058.
  • Flechtner, F., H. Dobslaw and E. Fagiolini (2014), AOD1B Product Description Document for Product Release 05, GRACE 327-750 (GR-GFZ-AOD-0001), GFZ German Research Centre for Geosciences, Department 1: Geodesy and Remote Sensing.
  • Han, S.-C., R. Riva, J. Sauber, and E. Okal (2013), Source parameter inversion for recent great earthquakes from a decade-long observation of global gravity fields, J. Geophys. Res. Solid Earth, 118, 1240-1267, doi:10.1002/jgrb.50116
  • Luthcke, S. B., T. J. Sabaka, B. D. Loomis, et al. (2013), Antarctica, Greenland and Gulf of Alaska land ice evolution from an iterated GRACE global mascon solution, J. Glac.,59 (216), 613-631, doi:10.3189/2013JoG12J147.
  • Luthcke, S. B., H. J. Zwally, W. Abdalati, D. D. Rowlands, R. D. Ray, R. S. Nerem, F. G. Lemoine, J. J. McCarthy and D.S. Chinn (2006), Recent Greenland Ice Mass Loss by Drainage System from Satellite Gravity Observations, Science 314, 1286, doi:10.1126/science.1130776
  • Peltier, W. R., D. F. Argus, and R. Drummond (2015), Space geodesy constrains ice-age terminal deglaciation: The global ICE-6G C (VM5a) model, J. Geophys. Res. Solid Earth, 120, 450-487, doi:10.1002/2014JB011176.
  • Ray, R. (1999), A global ocean tide model from Topex/Poseidon altimetry: GOT99.2. NASA Tech. Memo 209478.
  • Ray, R.D. and S.B. Luthcke (2006), Tide model errors and GRACE gravimetry: towards a more realistic assessment, Geophys. J. Int., 167, 1055-1059, doi: 10.1111/j.1365-246X.2006.03229.x
  • Sabaka, T. J., D. D. Rowlands, S. B. Luthcke, and J. P. Boy (2010), Improving global mass flux solutions from GRACE through forward modeling and continuous time-correlation, J. Geophys. Res., doi:10.1029/2010JB007533.
  • Swenson S., D. Chambers and J. Wahr (2008), Estimating geocenter variations from a combination of GRACE and ocean model output, J. Geophys. Res.-Solid Earth,113(B8), B08410, doi:10.1029/2007JB005338.
  • Wahr, J., R. S. Nerem, and S. V. Bettadpur (2015), The pole tide and its effect on GRACE time-variable gravity measurements: Implications for estimates of surface mass variations, J. Geophys. Res. Solid Earth, 120, 4597-4615,doi: 10.1002/2015JB011986.

This work was supported by the NASA MEaSUREs Program.