Time-Variable Gravity Analysis: Implementation Challenges and Insights from Hobart Meeting

1. Time Variable Gravity Implementation issues for Jason L. Cerri, S. Houry, J.P. Berthias Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

2. Introduction • Conclusions from Venice 2006 OSTS meeting • Time variable gravity variations (atmosphere, hydrology, etc) are important at the current accuracy level • No standard model but several candidates available • Goal of analysis was to identify suitable candidates • many sources for atmospheric gravity • impact evaluated and compared • all sources have flaws and advantages • nothing readily available for seasonal gravity • will be covered by other presentations during splinter Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

3. Available inputs for atmospheric gravity (1/3) • GRACE mission products • AOD1B product • input ECMWF MET fields (P/T/H) 0.5° resolution every 6 hours • oceanic response from baroclinic ocean model (since release RL03) • forced with ECMWF MET data • 2 year mean removed (2001-2002) • RL03 exhibits significant drifts => use of RL04 recommended • S1 and S2 tides not removed • except internally when generating forcing • 100 x 100 gravity field available • ftp://podaac.jpl.nasa.gov/pub/grace/data/AOD1B/RL04 • 1 file per month starting January 2001 (37 Mb) for RL04 (Jan. 2002 (27 Mb) for RL03) • no regular update of directory (latency around 1-3 months) • atmosphere only, ocean response only and sum available in files (exact > RL03) Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

4. Available inputs for atmospheric gravity (2/3) • Service of the atmospheric contribution to geopotential (Petrov & Boy, GSFC) http://gemini.gsfc.nasa.gov/agra/agra.html • AGRA product • input NCEP/NCAR reanalysis pressure field 2.5° resolution every 6 hours • oceanic response modeled according to inverted barometer hypothesis • variant with conservation of the total ocean water mass • land-sea mask (0.25° x 0.25°) available • long term mean removed • origin of mean not explained but gridded mean field available • S1 and S2 tides removed • model used not clear – gridded values available • 72 x 72 and 20 x 20 gravity fields available • http://lacerta.gsfc.nasa.gov/agra72 or http://lacerta.gsfc.nasa.gov/agra20 • 1 file per month starting January 1976 (2.2 Mb for 20x20, 26 Mb for 72x72) • latency around 3-4 days Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

5. Available inputs for atmospheric gravity (3/3) • GRGS • SHDP product • input ECMWF ground level pressure field 0.5° resolution every 6 hours • oceanic response modeled according to inverted barometer hypothesis • land-sea mask (0.25° x 0.25°) • no correction total ocean water mass • 2-year (1998-1999) mean removed • S1 and S2 tides not removed • 50 x 50 gravity field available • 1 file per week starting January 2001 (0.6 Mb) • latency around 5-10 days (weekly delivery) • GRGS GRACE processing uses a different product based on 3D ECMWF MET data and MOG2D ocean response (similar to AOD1B) Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

6. Comparison between products (1/2) • AOD1B and AGRA/SHDP differ in content • 2D vs 3D • ocean response • ground level equivalent pressure fields differ mostly at high latitudes • impact of the difference on S/C orbit at Jason altitude is small Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

7. Comparison between product (2/2) • Jason radial acceleration shows impact of local high/low pressure and confirm strong similarities between AGRA et SHDP Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

8. Selection of degree and order cut-off • The most significant contributions to radial acceleration are below order 20 (analysis on AOD1B product) Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

9. Evaluation of AOD1B and SHDP products • Test over cycles 22  99 • expansion limited to degree and order 20 • do not include degree 0 and degree 1 coefficients • S1/S2 atmospheric tide (Haurwitz & Cowley) removed before linear interpolation and added afterwards • Orbit evaluation • comparison to JPL06b • it is assumed that reduced dynamics absorbs variability in the gravity field so that JPL06b can be considered as a reference • SLR residuals Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

10. Improvements with Time-Varying Atmospheric Gravity • Radial difference of GDR-b with added atmospheric gravity and JPL06b reduced dynamic orbit • reduced by ~1 mm • no clear advantage to either SHDP or AOD1B Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

11. Improvements with Time-Varying Atmospheric Gravity • SLR residuals RMS globally reduced by ~1 mm • no clear advantage to either SHDP or AOD1B Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

12. AOD1B SHDP GDR-B Impact on GCEs Mean Radial Difference relative to JPL06b in mm(averaged over cycles 22 to 99 = Aug 02 to Sep 04) • Atmospheric gravity reduces geographically correlated differences with JPL06b reduced dynamics orbits • Improvement more significant with GRACE AOD1B than SHDP • probably result of better tuning of AOD1B mean Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

13. Need for standards for atmospheric TVG • Each product has its own mean atmospheric pressure field • differences in mean pressure field result in static gravity offsets • Mean pressure fields are often not available • as a consequence mean pressure fields used to produce gravity models are not available • Request: provide the atmospheric mean pressure field with the gravity models Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

14. Options for Jason POD • Usage of 3D product with sophisticated ocean response not justified • 2D surface pressure data and inverse barometer precise enough • Three possibilities • use of internal SHDP file with improved latency • not designed to be operational • use of GSFC AGRA product • operational ? future ? • convert surface pressure files available at SSALTO into gravity coefficient files • offers autonomy and long term availibility • but do we need an additional independent product? Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

15. Conclusions • Taking into account atmospheric gravity brings a small but well characterized improvement to the Jason orbit • Simplified approach sufficient for Jason • ground level pressure (2D) • inverse barometer • limited to degree and order 20 • Operational POD software ready • Origin of atmospheric gravity data still open • Models for other contributors to TVG are needed for evaluation • expected in the form of trend + annual + semi-annual terms Ocean Topography Science Team Meeting - Hobart, Australia – March 2007