Time Variable Gravity

Implementation issues for Jason

L. Cerri, S. Houry, J.P. Berthias

Ocean Topography Science Team Meeting - Hobart, Australia – March 2007

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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

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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

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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

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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

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

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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?

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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