Dorothee Coppens

Outline of the presentation IASI processing stability IASI-A reprocessing Purpose Validation of the improvements Schedule 3) IASI-B Non-linearity update Why changing the non-linearity correction? Preliminary results Next steps

1) IASI-A used as GSICS IR reference IASI characteristics: Continuous spectral coverage: 645-2760 cm-1 Spectral resolution: 0.5 cm-1 (sampling of 0.25 cm-1) Spectral accuracy better than 2 ppm IASI L1 products quality: Radiometric and spectral calibrations are done every 8 seconds Level 1c products are harmonized (IASI SRF removed)  Easy use of the data for the users IASI L1 processing stability: Accurate IASI on-board/on-ground monitoring to perform updates before any degradation can be observed  Only one small adjustment in 10 years !!! (February 2011)

IASI-A reprocessing

2) IASI-A reprocessing IASI calibration is very stable since 10 years Small remaining instrument effect before 2011 in pixel 2 Format evolution in 2010 based on operations needs:  New information has been added on the L1 products: Cloud information Detailed on-board flagging for the monitoring To take the opportunity to fill the gaps in the data records due to any anomaly on the processing chain

2) Improvement of Spectral Harmonisation Since July 2007, inter-pixel radiance differences in the IASI level 1C products exceeding 0.1 K were observed in some regions of the spectra The on-ground processing algorithm has been update in February 2011. Reference file Reprocessed file

2) New Information in the Products: Cloud information Cloud and land mask information from AVHRR have been included in the IASI products which did not exist in the processed data before 2010 The following figure shows the cloud and land fraction in a reprocessed product from October 2007: Cloud Fraction Land Fraction

2) IASI-A reprocessing: Filling Gaps Missing products due to some anomaly are recovered in the reprocessed products. Reference file Reprocessed file IASI-A reprocessing

IASI-B non-linearity correction

3) Why Changing the non-linearity correction ? Comparison IASI-B with: IASI-A  Radiometric residual bias, higher in band 1 ~ 0.1 K

3) Why Changing the non-linearity correction ? Comparison IASI-B with: Observed bias of -0.1K AIRS  Observed bias of -0.05K CrIS

3) EUMETSAT: Final ISSWG Statement in June 2017  The justifications were: The new corrections will improve the quality of IASI data The correction will be implemented for IASI-C  Must be applied on IASI-B in order to get a IASI-B / IASI-C consistent system The sooner the better, as IASI-B will replace IASI-A as the GSICS reference The request to apply the correction on board IASI-A in order to establish a consistent system IASI-A / IASI-B / IASI-C during a period before Metop-A drift becomes too large Delay of 6 months between the IASI-B and IASI-A corrections should be sufficient to characterize the effect of the change on IASI-B alone Implementation schedule following a clear statement from ISSWG: The non-linearity correction for IASI-B shall be updated as soon as possible  New non-linearity implemented on-board as for 2 August 2017 The non-linearity correction for IASI-A shall be implemented 6 months after IASI-B

3) Preliminary results in December 2017 – CNES Comparison IASI-B with: IASI-A AIRS CrIS  Better agreement with IASI-A Residual bias of +0.05K with AIRS  Residual bias of +0.1K with CrIS

3) Status of the new Non-Linearity – EUMETSAT Based on OBS-CALC methodology CALC are done with: RTM: RTTOV 10.2 Temperature, humidity and O3 from ECMWF forecasts Selection of scenes: close to nadir (satza < 20 deg.) night time (sunza ≥ 120 deg.) No polar regions (|lat| ≤ 60 deg.) Over sea clear sky

3) IASI cold scenes Following Manning (SPIE, 2017), we average IASI scenes with Tb=200 ± 1 K @ 939 cm-1 i.e. thick tropical clouds These scenes are expected to be identical from a geophysical point of view; and indeed, the s of the mean spectrum is below 0.2%  possibility to compare different instruments

3) Cold Scenes – results IASI/B-IASI/A 2014 2015 2016 2018

3) IASI warm scenes A similar approach can be followed for warm scenes: we average IASI scenes with Tb=288 ± 1 K @ 939 cm-1 The variability is much higher; however, using grand averages, it is still possible to compare instruments from year to year

3) Warm scenes – results IASI/B-IASI/A 2014 2015 Current status of the new Non-Linearity – EUMETSAT Current status of the new Non-Linearity – EUMETSAT 2016 2018

3) Status in March 2018 On EUMETSAT side: the first results are promising and show that IASI-A should be updated as well First estimation from ISSWG members were inconsistent  Need more time to have a full picture of the impact Update of IASI-A in February 2018 has been put on hold A Task Force (ISSWG members) has been organised – collocation meeting is April 6th, 2018. Outcome of the task force will be discussed at the ISSWG and presented at the IASI RevEx, in June 2018, where the decision will be taken regarding the update of IASI-A (and IASI-C)

Summary IASI stability: Very stable since 10 years, with only one small adjustment in 2011. IASI-A reprocessing: IASI-A has been reprocessed for the period 2007-2016 to: Update the spectral harmonization before 2011 Include the cloud/land information, missing before 2010 Fill small gaps in the data records Validation will be completed by the end of March 2018 3) IASI-B non-linearity correction IASI-B first results are encouraging More time is needed on the users side to assess the impact A Task Force (ISSWG members) has been organised – collocation meeting is April 6th, 2018  Trigger the IASI-A update or not