Post on 08-Jan-2016
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OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 1 -
SLA consistency between Jason-1 and TOPEX/Poseidon data
M.Ablain, S.Philipps, J.Dorandeu, - CLS
N.Picot - CNES
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 2 -
Introduction
• Jason-1 and TOPEX data have been reprocessed over the tandem period (Jason-1 cycles 1 to 21, TOPEX cycles 344 to 364)
• Evolutions in J1 GDR “B” and T/P RGDR:– New orbits based on GRACE gravity model– New retracking algorithms (MLE4 for Jason-1, MLE5 & MAP for TOPEX)– Updated or new geophysical corrections (tidal models, DAC HF correction, …)
• Objective : check the impact of the reprocessing on the sea surface height consistency (SLA) between Jason-1 and TOPEX
• Objectives:
– Impact of new orbits on SLA consistency
– Impact of new range on SLA consistency
– Impact of new Sea State Bias on SLA consistency
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 3 -
1 - Impact of new orbits
• Raw SLA differences between TOPEX and J1 :
– SSH not corrected for geophysical corrections => T/P & Jason-1 on the same pass with a delay of 72s.
– SSB is not applied
• Use of old orbits :– Large structures appear : North Atlantic,
Indonesia => (+/- 5 cm)– Trackiness strongly visible
• Use of new orbits allows to :– Remove trackiness– Strongly reduce Patterns in North Atlantic
and Indonesia– Evidence new structures partly explained
by SSB discrepancies
(TP- J 1)=- 8.4 cm
(TP- J 1)=- 8.1 cm
Use of new orbits (GRACE)Use of new orbits (GRACE)Use of new orbits (GRACE)
-2 cm
-2 cm
+2 cm
+2 cm
No SSB, Old orbitsOld ranges
No SSB, New orbitsOld ranges
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 4 -
2 - Impact of new range : TOPEX
• Impact of new TOPEX LSE range : – Patches are strongly reduced
-2 cm+2 cm
(TP- J 1)=- 8.1 cm
-2 cm +2 cm
(TP- J 1)=- 7.7 cm
No SSB, New orbitsMGDR TP, MLE3 J1 No SSB, New orbits
LSE TP, MLE3 J1
Use of TP LSE range
Jason-1 and TOPEX SLA are more homogeneous with the TOPEX LSE range : SSB discrepancies are reduced
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 5 -
2 - Impact of new range : Jason-1
• Impact of new Jason-1 MLE4 range :
– weak impact on the mean differences,– the consistency is slightly better in the
Indian Ocean
-2 cm +2 cm
(TP- J 1)=- 7.7 cm
-2 cm +2 cm
(TP-J1)=-7.6 cm
Use of Jason-1 MLE4 range
No SSB, New orbitsLSE TP, MLE3 J1
No SSB, New orbitsLSE TP, MLE4 J1
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 6 -
(TP- J 1)=- 6.9 cm
3 - Impact of new SSB
• New SSB corrections have been computed
– Jason-1 : Labroue, Venice 2006– TOPEX : collinear method using RGDR
(LSE) => Labroue’s talk.
• These new TOPEX and J1 SSB models are now much closer than before.
-2 cm +2 cm
(TP-J1)=-7.6 cm
• Discrepancies between J1 and T/P are further reduced when applying these new SSB models in the SLA calculation, in addition to the new orbits.
• However, an East/West patch (< 1cm)
remains, but it is not correlated with SWH.
Use of new SSB
No SSB, New orbitsLSE TP, MLE4 J1
New SSB, New orbitsLSE TP, MLE4 J1
-2 cm +2 cm
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 7 -
(TP- J 1)=- 6.9 cm
(TP- J 1)=- 7.1 cm
4 - Impact of GSFC orbit
• New orbits are provided by CNES for Jason-1(GDR ‘B’) and GSFC for TOPEX (RGDR).
• Using GSFC orbits similar for Jason-1 and TOPEX, allows us to remove the East/West signal
Even if orbits are best and more homogenous between TOPEX and Jason-1, weak systematic discrepancies remain (< 1cm).
-2 cm
-2 cm
+2 cm
+2 cmUse of new orbits (GRACE)Use of new orbits (GRACE)Use of new orbits (GRACE)
New SSB, range, orbitsOrbit : J1-CNES/TP-GSFC
New SSB,New rangesOrbit : J1-GSFC/TP-GSFC
• Equatorial band is visible : due to hemispheric bias when
separating ascending and descending passes.
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 8 -
4 - Hemispheric bias
• A strong hemispheric bias (between -2 cm and +2 cm) is highlighted when separating ascending and descending passes.
• Bias mainly due to TOPEX data.
• Present on TOPEX M-GDR data but not easily detectable with former orbits
Waveform leakage problem explains this hemispheric effect.
• Use of new TOPEX LSE range increases this ascending/descending effect
-2 cm
-2 cm
+2 cm
+2 cm
Ascending passes
Descending passes
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 9 -
5 - Variance of SLA differences
• Variance of Jason-1/TOPEX SLA differences computed after filtering out SLA signals smaller than 50 km (in order to remove the SSH high frequency content).
• Using GDR ‘A’ for Jason-1 and MGDR for T/P:
– variance is about 7.4 cm²– larger differences in strong waves areas
due to SSB discrepancies
• Using GDR ‘B’ for Jason-1 and RGDR for T/P:
– variance is reduced by 2 cm² – Variance is mainly reduced in strong
wave areas showing the better SSB consistency between Jason-1 and T/P.
0 cm² 20cm²
GDR ‘A’ / MGDR
GDR ‘B’ / RGDR
0 cm² 20cm²
Variance=5.31 cm² (2.30 cm RMS)
Variance=7.37 cm² (2.71 cm RMS)
OSTST Hobart 2007 – SLA consistency between Jason-1 and TOPEX data- 10 -
Conclusion
• T/P / Jason-1 SLA consistency show large improvements :– from new orbits for both TOPEX and Jason-1
• Reduction of apparent trackiness• New signals can now be detected (SSB differences, North/South – ascending/descending
signals on TOPEX)• Weak East/West signal (< 1cm) have been evidenced between CNES and GSFC orbits
– from new TOPEX retracking : strongly reduces the differences between TOPEX and Jason SSB
– from new SSB : SSB estimations are now much more consistent
• SLA discrepancies remain :– Large ascending/descending hemispheric signals still remain in TOPEX data due to
leakage waveform problem
– LSE range seems more impacted by this leakage problem than the MGDR range