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Analysis of the high frequency content of Jason-1, Topex and Envisat data

Analysis of the high frequency content of Jason-1, Topex and Envisat data. Y. Faugere (CLS) A. Ollivier (CLS) P. Thibaut (CLS). Introduction. Purpose of the study : To compare the high frequency content of Jason-1, Topex and Envisat GDR products

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Analysis of the high frequency content of Jason-1, Topex and Envisat data

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  1. Analysis of the high frequency content of Jason-1, Topex and Envisat data Y. Faugere (CLS) A. Ollivier (CLS) P. Thibaut (CLS)

  2. Introduction Purpose of the study: To compare the high frequency content of Jason-1, Topex and Envisat GDR products • The analysed signal is SLA=[Orbit-range-MSS]. This signal includes instrumental noise, processing noise, correction noise, residual geophysical signals… • The 1Hz and the 20Hz signals have been analysed • Two types of method have been used to analyse such signals: a spectral analysis, and a filtering technique Plan: • Method and data used • Cross comparison of Jason-1 and Envisat HF content • Improvement of the Envisat/Jason-1 consistency with MLE4 • Impact of mispointing and SWH selection on the high frequency content • First results on the cross comparison of Jason-1 and Topex RGDR HF content • Conclusion

  3. Spectrum of oceanic signal s Noise hidden by oceanic signal Noise  Signal hidden by noise Method and data used • First method: spectral analysis of the SLA signal SLA(t) = s(t) + (t) , where s(t) is the geophysical signal and (t) is the noise A plateau on a power spectrum can be the signature of a white noise. Plateau  • 1Hz spectra are computed from 10 days of data • 20Hz spectraare computed from 2 days of data

  4. Method and data used • Second method: Filtering technique SLA=Orbit-Range-MSS High-pass filter (20km cut-off) HF(SLA) Standard deviation σ[HF(SLA)] in 2°x2° boxes

  5. Method and data used • 3 Missions with several retrackings: • Jason-1 • MLE3 (Cycles 128-135) and MLE4 (Cycle 172, 20) • Parameters are averaged from 20Hz estimations • Envisat • MLE3 (Cycle 39, 51) • Parameters are averaged from 18Hz estimations • Selection: • Calval editing • Ocean data where • Bathymetry < -1000m • Topex (Cycle 363) • LSE • range is averaged from 10Hz estimations • Other parameters are estimated at 1Hz • MAP • Idem as LSE

  6. Method and data used Calval editing criteria for Jason-1:

  7. Cross comparison of Jason-1 and Envisat • Improvement of the Envisat/Jason-1 consistency with MLE4: spectral analysis • Legend: • EN • J1 GdrA • J1 GdrB 1Hz Data • The strange shape of 1Hz Jason-1 spectra in the 0.1-0.4 Hz bandwidth disappears with the use of MLE4 retracking. 1Hz Envisat and Jason-1 spectra are superimposed.

  8. Cross comparison of Jason-1 and Envisat • Improvement of the Envisat/Jason-1 consistency with MLE4: spectral analysis • Legend: • EN • J1 GdrA • J1 GdrB 1Hz Data 20Hz Data • The strange shape of 1Hz Jason-1 spectra in the 0.1-0.4 Hz bandwidth disappears with the use of MLE4 retracking. 1Hz Envisat and Jason-1 spectra are superimposed • At 20Hz the higher energy in the 0.1-0.4 Hz bandwidth is reduced with the use of MLE4 retracking. Jason-1 HF energy is now lower than Envisat at all frequencies

  9. Cross comparison of Jason-1 and Envisat • Improvement of the Envisat/Jason-1 consistency with MLE4: Filtering technique σ²[HF(EN)] – σ²[HF(J1 GDRa)] -2cm² 2cm² • the higher Jason-1 energy in the wet areas (blue color) is the signature of the higher energy in the 0.1-0.4 Hz bandwidth seen on the 1 Hz spectra with J1 MLE3 retracking.

  10. Cross comparison of Jason-1 and Envisat • Improvement of the Envisat/Jason-1 consistency with MLE4: Filtering technique σ²[HF(EN)] – σ²[HF(J1 GDRa)] σ²[HF(EN)] – σ²[HF(J1 GDRb)] -2cm² 2cm² -2cm² 2cm² • the higher Jason-1 energy in the wet areas (blue color) is the signature of the higher energy in the 0.1-0.4 Hz bandwidth seen on the 1 Hz spectra with J1 MLE3 retracking. • The wet areas are no more visible when using MLE4 j1 retracking

  11. Cross comparison of Jason-1 and Envisat • Impact of mispointing selection on the high frequency content Envisat Jason-1 Small Mispointing (|X² - bias|<0.02deg2) -0.15 0.15 -0.15 0.15 deg2 deg2 0.3 0.15 -0.15 Mispointing over 1 Jason-1 pass (deg2) -20 -10 0 10 20 Latitude

  12. In red: small mispointing only In black: no selection 0.1 0 -0.1 Mispointing (deg2) 2 0 Water liquid content (Kg/m3) -1 -2 -3 Uncorrected SLA (m)

  13. Cross comparison of Jason-1 and Envisat • Impact of mispointing selection on the high frequency content • Legend: • EN (all data) • J1 (all data) • Low Mispointing EN • Low Mispointing J1 • 1Hz Data • 1Hz Envisat and Jason-1 are consistent in both cases. The HF energy is very sensitive to the mispointing selection

  14. 20Hz Data Cross comparison of Jason-1 and Envisat • Impact of mispointing selection on the high frequency content • Legend: • EN (all data) • J1 (alla data) • Low Mispointing EN • Low Mispointing J1 • 1Hz Data • 1Hz Envisat and Jason-1 are consistent in both cases. The HF energy is very sensitive to the mispointing selection • At 20Hz the plateau is not impacted by the selection on both satellites. However the energy between 0.1-0.4Hz is strongly reduced when selecting low mispointing

  15. Cross comparison of Jason-1 and Envisat • Impact of SWH selection on the high frequency content Envisat SWH histogram Jason-1 SWH histogram 0 11 0 11 m m Small SWH (0.5<SWH<1.5m) High SWH (3m<SWH<11m) Small SWH (0.5<SWH<1.5m) High SWH (3m<SWH<11m)

  16. Cross comparison of Jason-1 and Envisat Impact of SWH selection on HF content • Legend: • High SWH EN • High SWH J1 • Small SWH EN • Small SWH J1 • 1Hz Data • 1Hz Envisat and Jason-1 are superimposed in both cases. The noise level increases with the wave height. Moreover a sort of pseudo-plateau is visible on 1Hz spectra at 1Hz only for high waves

  17. 20Hz Data Cross comparison of Jason-1 and Envisat Impact of SWH selection on HF content • Legend: • High SWH EN • High SWH J1 • Small SWH EN • Small SWH J1 • 1Hz Data • 1Hz Envisat and Jason-1 are superimposed in both cases. The noise level increases with the wave height. Moreover a sort of pseudo-plateau is visible on 1Hz spectra at 1Hz only for high waves • At 20hz Envisat and Jason-1 spectra are closer for small waves (plateau almost superimposed). • The pseudo-plateau visible at 1Hz on high waves is not the signature of a instrumental white noise. It is the signature of the energy between 0.1-0.4Hz on the 20Hz spectra

  18. First results on the cross comparison of Jason-1 and Topex RGDR HF content Variance difference of HF content: J1 MLE4 (cycle 20) - TP LSE (Cycle 360) TP MAP TP LSE J1 MLE4 -1cm² 1cm² • TP LSE and J1 MLE4 1Hz spectra are very consistent • The Geographical distribution of the difference of HF content is not as homogeneous as for Jason-1/Envisat

  19. Conclusion • Cross comparison of Jason-1 and Envisat HF content • The energy at 0.1-0.4Hz is reduced with the use of MLE4 on Jason- at 1Hz and 20Hz. GdrB enables then to Improve the Envisat/Jason-1 consistency at 1Hz and 20Hz. • BUT: • the perfect superimposition of 1Hz Envisat and Jason-1 spectra is with the lower energy on Jason-1 20Hz spectrum. Why? • There is a remaining suspicious high energy in the 0.1-0.4Hzbandwidth on 1Hz spectra • At 20Hz, in the case of small waves selection, the energy is reduced at all frequencies whereas only the remaining energy in the bandwidth 0.1-0.4Hz is reduced in the case of the low mispointing selection => the remaining suspicious high energy in the0.1-0.4Hzbandwidth is due to perturbed data with high mispointing values => The pseudo-plateau visible at 1Hz on high waves is not the signature of an instrumental white noise. It is linked to the energy between 0.1-0.4Hz on the 20Hz spectra • First results on the cross comparison of Jason-1and Topex RGDR HF content • Good consistency between Topex LSE and Jason-1 MLE4

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