GRWG IR Sub-Group Web Meeting 2017-12-14

GSICS Web Meeting on Reference Migration from IASI-A to IASI-B

  1. Dorothée Coppens (EUMETSAT) - Delaying the roll-out of non-linearity changes to IASI-A
  2. Double differencing of IASI-A/B against geostationary imagers' IR Channels
  3. Igor Tomazic (EUMETSAT) – Double differencing of IASI-A/B against SLSTR
  4. Elsa Jacquette (CNES) - IASI-A/B direct and indirect comparisons
  5. All - Discussion on strategy for migrating GSICS products from IASI-A to IASI-B
  6. All – Implications for the IR Reference Uncertainty Traceability Report (“IRRefUTable”)
  7. Dave Smith (RAL) - SLSTR traceability chain

    Other topics - may be defered until annual meeting
  8. Tim Trent (U Leicester) - Alternative Regression algorithms for inter-calibration (postponed to 2018 GRWG meeting)
  9. Chengli Qi (CMA) - Radiance Uncertainly estimate for HIRAS (postponed to 2018 GRWG meeting)
  10. Hui Xu (NOAA) - Using PCA regression to fill up spectral gap (postponed to 2018 GRWG meeting)
  11. Likun Wang (NOAA) - Fast and accurate collocation package of CrIS with GOES16 or other geostationary imager (postponed to 2018 GRWG meeting)
This web meeting of IR Sub-Group focused on the migration of the inter-calibration reference from the IASI on Metop-A to Metop-B, following the action raised at our last web meeting in June 2017.

It reviewed the results of several analyses of the relative performance of these reference instruments – for example, from GEO-LEO Double Differencing - particularly since the change to IASI-B onboard processing in August 2017.

A strategy to merge multiple references for NRT product and the implications for the IR Reference Uncertainty Traceability Report (“IRRefUTable”) were also discussed.


Guest Chair: Tim Hewison

CMA: Hanlie Xu

CNES: Elsa Jacquette

EUMETSAT: Tim Hewison, Alessandro Burini, Sebastien Wagner, Igor Tomazic, Viju John, Dorothée Coppens (PT)

JMA: Masaya Takahashi and Yusuke Yogo

KMA: Dohyeong Kim, Minju Gu, Eunkyu Kim and Hyeji Yang

NOAA: Fangfang Yu, Likun Wang, Manik Bali

RAL: Dave Smith

UoLeicester: Tim Trent


Dorothée Coppens reported that unexpected results had been found since the roll-out of the non-linear correction to Metop-B/IASI. These are not fully understood, and are currently under review. A decision will be made by an ISSWG Task Force in mid-March 2018 to whether to roll-out the correctoin to Metop-A/IASI. Previously this was planned to take place in Feb 2018, but has now been delayed until at least Sept 2018, after which the Metop-A orbit will begin to drift more rapidly - and may be cancelled altogether. The Task Force would be happy to consider a recommendation from GSICS, which Tim will formulate.

  • Mean DD is larger in long WL channels and for colder scenes.
  • IASI-B change caused DD to jump from ~0.04K colder than IASI-A to ~0.04K warmer (for standard scenes)
  • Longer WL channels showed the jumping pattern from minus to plus around 0.1K, larger jump at cold scenes after IASI non-line correction
  • After update of ABI scan mirror emis LUTs, WV channles are affected (ABI-CrIS)
  • For the anlaysis of DD, collocation, scan angle, and so on should be taken care.
  • Before scan mrrr emm, there is diurnal variation…. ---> thus update the values
  • -- AHI will be considered the update of the scann mrr emissivity
  • FY-4A channel 8 to 14
  • Show FY-2G DD : after Aug 2017 jump around 0.05 to 0.1K
  • Check the Hyperspectral collocation… for the WV channel large bias
  • Recommend not mean Temp but std temp
  • SLSTR : with IASI less than 0.1 K difference bewteen 220 and 280K
  • DD is ~0.05K function of SNO event, scene Temp similarly MSG/SEVIRI
  • S9 : det0 and det1 -- SRF slightly different, but small
IASI-A/B direct
  • The effects of IASI-B non-lin correction are depend on temp and WN (larger at cold temp, and longer WL)
  • A/B direct comparison: good except CO2, O3 band
  • Befere : negavtive --> after … flat pattern but small residuals due to A non-linearty
  • IASI-A non linearity table will be update : Feb 2019
  • 1st TF team meeting of ISSWG will be held in mid March 2018 --> Outputs will be reported at GSICS annual meeting
  • the change IASI-A, the inter-calibration with IASI-B and CrIS will be worse, and the update is postponed
  • IASI-A/B DD is no time dependent
IR Reference Sensor Traceability & Uncertainty Report
  • JMA (Masaya), CMA(Hanlie?), NOAA(??), KMA(Dohy) will contribute the IASI-A/B DD analysis…
Possible strategy for migrating GSICS products from IASI-A to IASI-B
  • Previously, Tim had proposed the concept of the Prime GSICS Corrections, whereby comparisons with different reference instruments are adjusted by delta corrections, based on double-differences. This is still felt to be a good approach for generating FCDRs.

  • But adoption of the idea in Near Real-Time Corrections has been slow, so a simpler alternative is now proposed:

  • One idea is to first perform a comparison of the results with all available references.

  • This could be used to establish a consensus reference, as a weighted average of the results from all the references that are within the consensus uncertainty.

  • Of course, this would result in jumps in the time series

– but they would be within the consensus uncertainty, which estimates the absolute uncertainty on the merged GSICS corrections.
  • Of course, this may be large than the uncertainty derived using a single reference, and this increase may be critical for some applications.

  • And we need to establish some values before deciding whether to adopt this approach.

Dave: SLSTR calibration
  • Non-blackness and temp gradiaents in flight made different between measured and actual BT


Tim will formulate the questionnaire for non-linearity of IASI-A to provide TF team of ISSWG in March 2018.
Topic revision: r12 - 14 Dec 2017, TimHewison
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