ostst hobart 2007 – sla consistency between jason-1 and topex data - 1 - sla consistency between...

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


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


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


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


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



(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


New SSB, New orbitsLSE TP, MLE4 J1

-2 cm +2 cm


(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.


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


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)


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

ostst hobart 2007 – sla consistency between jason-1 and topex data - 1 - sla consistency between...

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