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. )5.0),(8.1max()( 0000 dBrmsf CCKu

mLz 200

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 distribution5020

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 flaggingRed 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.