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. 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

3. 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)

4. 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

5. 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)

6. 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

7. 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

8. 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)

9. 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

10. 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

11. 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

12. Ocean Topography Science Team Meeting - Hobart, Australia – March 2007 12 Impact on GCEs Mean Radial Difference relative to JPL06b in mm (averaged over cycles 22 to 99 = Aug 02 to Sep 04) GDR-B SHDP AOD1B ■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

13. 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

14. 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?

15. 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