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Modification of JASON rain flag for MLE4 processing

Modification of JASON rain flag for MLE4 processing. J.Tournadre 1 IFREMER. Since October 21, 2005, new Jason-1 operational processing chain for IGDR and GDR products. Change from MLE 3 to MLE4 retracking algorithm.

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Modification of JASON rain flag for MLE4 processing

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  1. Modification of JASON rain flag for MLE4 processing J.Tournadre 1IFREMER

  2. Since October 21, 2005, new Jason-1 operational processing chain for IGDR and GDR products. • Change from MLE 3 to MLE4 retracking algorithm. • Based on a second-order altimeter echo model, MLE4 simultaneously retrieves epoch, SWH, s0 and off-nadir angle (z2). • New instrumental corrections tables and rain flag has been updated using a Ku/C band s0relation computed from the first 30 version A GDR cycles. • MLE4 algorithm is certainly more robust for large o-nadir angles, • However, the simultaneous retrieval of s0 and off-nadir angle significantly modifies the behavior of Ku and C band s0. • Modification of the rain flagging process

  3. Analysis of 3 JASON cycles : Comparison of AGC and s0 Cycle 20 v. A and B Cycle 50 v. A Cycle 160 v. B AGC: Automatic gain control use to modulate the waveform amplitude MLE4 modification of the AGC /s0 relation. Rms of difference grows from 0.15 dB to 0.45dB for Ku band

  4. Sample to Sample noise MLE 4 introduces noise in s0 Ku signal 0.45 dB compared to 0.15dB for AGC and s0 version A

  5. Operational Rain flag • Detection of significant attenuation of Ku band s0 compared to C band s0 • Use of Ku/C band s0 « rain free » relation • Criterion on JMR liquid water content Lz to insure the presence  of cloud.

  6. Change of the Ku/C band relation • For MLE3 s0 and AGC Ku/C band relations are identical • Rms at the same level. • For MLE4 large difference for s0>15dB • Rms 20 to 50% larger

  7. Comparison of Ku/C band relations

  8. DistributionofDs0/rmsDAGC/rms Same Gaussian distribution

  9. 50 20 Impact of MLE4 on Ds0 distribution Version A : weak dependency of Ds0 and Dagc on z² Version B: strong dependency of Ds0 on z² MLE4 estimation of z² impact the s0 Ku

  10. Difference in rain flagging Red dots samples flagged using the s0 relation Green dots: flagged using the AGC relation For Version A AGC and s0 flagged ensemble are only marginally different For version B the two ensembles are statistically different

  11. Latitudinal distributions of flagged samples • Small difference of latitudinal distribution

  12. Distribution of off-nadir angle for flagged samples • Gaussian distribution slightly skew toward + values for V.A • Larger rms and strong bias toward negative values for V.B

  13. Solution : change from s0 to AGC for rain flagging • Compute the Ku/C band AGC for the whole Jason archive. Mean relation defined • Program available for rain flagging

  14. Rain products are also available on Cersat web site

  15. Conclusion: • Any change modify more things than was originally planed.

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