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

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,  and off- nadir angle (  2). New instrumental corrections tables and rain flag has been updated using a Ku/C band  relation 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  and off-nadir angle significantly modifies the behavior of Ku and C band . Modification of the rain flagging process

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

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

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

Change of the Ku/C band relation For MLE3  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

Comparison of Ku/C band relations

Distribution of  /rms  AGC/rms Same Gaussian distribution

Impact of MLE4 on  distribution Version A : weak dependency of  and  agc on  ² Version B: strong dependency of  on  ² MLE4 estimation of  ² impact the  Ku

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

Latitudinal distributions of flagged samples Small difference of latitudinal distribution

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

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

Rain products are also available on Cersat web site

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