1. Radiometric inter-comparison of IASI : Miscellaneous results
Presented by Denis Jouglet (CNES, on behalf for:
LMD (N.A. Scott, L. Crépeau, R. Armante, J. Pernin, A. Chédin)
LATMOS / LPPMA (S. Payan, C. Camy-Peyret, J. Bureau et al.)
EUMETSAT (D. Coppens)
CNES IASI team (E. Péquignot, V. Lonjou et al.)

2. IASI,HIRS stand-alone + inter-calibration
From LMD (N. Scott et al.): IIR/MODIS/SEVIRI and
IASI,HIRS stand-alone + inter-calibration
From EUMETSAT (D. Coppens):
IASI radiance monitoring + IASI/HIRS
From LATMOS (S. Payan et al.):
IASI/GOSAT (TANSO-FTS)
From CNES (V. Lonjou et al.):
IASI/AVHRR

3. IASI,HIRS stand-alone + inter-calibration
From LMD (N. Scott et al.): IIR/MODIS/SEVIRI and
IASI,HIRS stand-alone + inter-calibration
From EUMETSAT (D. Coppens):
IASI radiance monitoring + IASI/HIRS
From LATMOS (S. Payan et al.):
IASI/GOSAT (TANSO-FTS)
From CNES (V. Lonjou et al.):
IASI/AVHRR

4. Quality control of satellite data @ LMD
 within the frame of the processing and validation of level1 and level2 satellite data combining forward and inverse radiative transfer modeling
- TOVS (NOAA/NASA Pathfinder Programme),
- ATOVS,
- AIRS/Aqua
- IASI/MetOpA IASI/MetOpB Closely related to CNES/TEC/Activities
- IIR/Calipso
 within the frame of the GSICS activities
 All along the instrument/satellite life time, we are looking for:
Unstabilities , Spurious Trends, Night/Day differences ,
Asymetric behavior along scan line, …
But also:
Natural variability, Natural trends
N.A. Scott, L. Crépeau, R. Armante, J. Pernin,
A. Chédin

5. Ascertain the quality of observed radiances A multi-step procedure @ LMD
« Inter-calibration »(*)
 synergy between instruments onboard sounders/imagers based on comparisons of
observed radiances with radiances observed from other sounders
« Stand alone »(*)
 Comparison of observed BT with simulated BT based on forward RT model + in-situ
(R/S) observations, for every instrument involved in the inter calibration approach
Aim : identify which instrument deviates from the other(s).
- compare wide ranges of brightness temperatures (clear or cloudy) (inter-calibration)
- study each channel of each instrument, independently of the others (stand alone).
Models and databases involved in the two approaches
Satellite data archive at LMD: TOVS, ATOVS, AIRS, IASI, AMSU, MHS, …
Forward Radiative Transfer Model: 4A/OP
Off-Line data bases LMD: GEISA, TIGR, ARSA
Off-Line data bases, other: Ecmwf Reanalyses, …
(*) cf previous GSICS CNES/LMD joint presentations, and IASI Conf’, Hyères, 2013

6. The ARSA (Analyzed RadioSounding Archive) Database
When collocated with satellite observations, RAOBs may be used to
reveal errors or trends
LMD has elaborated the ARSA database:
starting from observations by worldwide distributed radiosonde stations
combining them with surface and other auxiliary observations (ECMWF ERA_Interim reanalyses, ACE_Scisat level2 products, …)
in order to make it suitable for
forward and inverse radiative transfer models
validation of time series of level1 and level2 data
GSICS ACTIVITIES
ARSA : Long term validated and stable description of the atmospheric and surface state

7. The ARSA database
From the raw radiosonde measurements extracted from ECMWF up to the converged ARSA product
 several (fully automatized) severe quality control and extrapolation steps.
Missing information (ozone profiles, surface temperature, …) taken from
ERA_Interim and ACE_Scisat level 2 products.
A 43-level description of the atmosphere between surface and hPa
including P, T, H2O, Ozone profiles, Surface temperature, Geolocation +
date/time
ARSA starts in January 1979 and is extended onwards on a monthly LMD
So far: A total of
> 4.9 million profiles
From a total of ~22 millions considered
ARSA is available upon request at LMD.

8. Inter Calibration: IIR/Calipso, Modis/Aqua and Seviri/Meteosat
Ascertain the quality of observed radiances : Recent Results IIR/Calipso
Inter Calibration: IIR/Calipso, Modis/Aqua and Seviri/Meteosat
Study From April 2006  current month at Global scale
Statistics: day/night, sea, 5 latitude bands, 10 BT ranges (from ~ 200 to 330K)
Our statistics reveal night-day differences in the 30N-60N latitude band in summer for IIR and Modis companion channels (illustration for June 2008)
- CNES evidenced a calibration bias related to the elapsed time after the Night to Day transition, therefore at different latitudes according to the season, and expected in the range of latitudes where the IIR-MODIS hysteresis effect is observed.
Further investigations are on going.
30°N
(1) N.A. Scott et al, GSICS Quarterly Vol. 3, No. 3, 2009
(2) A. Garnier, J. Pelon et al.
Results presented at the Calipso Icare/IPSL/CNES Revex, Sept.2012

9. Ascertain the quality of observed radiances : Results MetOpA
The stand alone aproach applied to MetOpA (July 2007 to current month)
reveals a bias due to the data processing in the cm-1
Required : 4A/OP + ARSA + Sat. Observations + Clear/Cloud/Aerosols Flag
History:
Unexpected variation of BT residuals (red curve)  no plausible LMD
BT residuals, K
Meanwhile at CNES:
Numerical error identified during the data processing from level1a  level1b (Gibbs effects)
Corrected by the CNES/TEC (Toulouse) in 2010
Wavenumber, cm-1
Happy end:
When LMD after CNES/TEC correction  blue curve : as expected

10. For IASI and HIRS and Land / Sea ; Day / Night cases
IASI MetOpA stand alone: from July 2007 onwards (September 2012)
Every single month: Computation of BT residuals  4AOP+ARSA+clear sky
STDV of BT residuals (°K) vs wavenumber
BT residuals (°K) vs wavenumber 0- 2-
2 -
-2 -
And also : Generation of Time series of monthly means of residuals (°K)
For IASI and HIRS and Land / Sea ; Day / Night cases
For selected CO2, O3, CO, N2O, CH4, … , and window channels
To identify natural trends, seasonal variations, unwanted trends (from auxiliary datasets and/or companion instruments)
CO2 channel, 696 cm-1
I I July Aug 2012
0.6- 0-
0.4-
-0.4-
I I
July Aug 2012
CO channel, 2143 cm-1
I I
July Aug 2012
0.5-
O3 channel, cm-1
-1.0-
0.2- 10

11. Ascertain the quality of observed radiances : Results MetOpA
The stand alone approach applied to MetOpA
allows to detect scan angle effects through the computation
of mean (July 2007 to current month) values of BT residuals vs scan position
AMSU ch. 10
IASI
cm-1
Left Y-Axis BT residuals,K
Right Y-Axis Nb. items
AMSU ch 11
IASI
cm-1
X-axis:  secant (SatZenAngle) : <0  left side ; >0  right side of the orbit
Residuals from 4A/OP with ARSA as input, in 100km/3h space/time windows with IASI obs.

12. MetOpA vs MetOpB: very first results
Trial dissemination starting : January 23rd  THANKS ++++ to CNES, Eumetsat, Ether
Desarchiving Metop A and Metop B through the Ether/CNES/IPSL data centre
Level 1c, Level 1d archived at LMD
Since then:
Inter calibration : (IASI-HIRS)/MetOpA + (IASI-HIRS)/MetOpB
What about the status of HIRS on MetopB?
MetOpA
Sequence of observed BT differences of IASI and HIRS companion channels for a given secant, for clear and cloudy spots
MetOpB
Day : January 2013, 24th
HIRS ch4
HIRS ch2

13. MetOpA vs MetOpB: very first results (cont’ned)
Stand Alone: : Difference of BT residuals IASI/MetOpA and IASI/MetOpB .
4A/OP colocated with ECMWF analyses.
So far: -30°,+30°, night, sea, tropical.
Space window= 15km.
> 30 days of satellite data processed.
For Feb. 2013, 17th
~ 2000 colocations
+0.2K
-0.2K

14. MetOpB MetOpA Stand alone aproach : Satellite zenith angle dependence?
Hovmöller diagram of BT residuals versus position along the scan line
MeOpA and MetOpB, January 2013, 24th
NB: 4A/OP+Ecmwf analysis instead of usual 4A/OP+ARSA
MetOpB
MetOpA

15. CONCLUSIONS PERSPECTIVE
Stand-alone associated to inter-calibration:Powerful procedures to identify unexpected or undesired radiances behaviors
However: requires permanent validation of all the actors involved in these procedures
Applications
At LMD:
BAU for Level1 and Level2 data processing
Continue the validation and distribution of ARSA database
Within the frame of GSICS: discussion and exchange of results and data with CNES and international participants

16. IASI,HIRS stand-alone + inter-calibration
From LMD (N. Scott et al.): IIR/MODIS/SEVIRI and
IASI,HIRS stand-alone + inter-calibration
From EUMETSAT (D. Coppens):
IASI radiance monitoring + IASI/HIRS
From LATMOS (S. Payan et al.):
IASI/GOSAT (TANSO-FTS)
From CNES (V. Lonjou et al.):
IASI/AVHRR

17. Inter-comparison of IASI-A and IASI-B spectra
The methodology is based on the radiance monitoring performed at EUMETSAT:
For each IASI: Comparison of measured and modelled IASI L1C spectra generated by RTTOV v9.3,
Use of 3-hour ECMWF forecast files, taken 1 hour around the forecasts,
Clear situation: Less than 2% of clouds,
Polar regions have not been taken into account, only latitude within [-60°:60°],
Only large band around the Nadir (Scan Position 10 to 20),
Over sea, by night.
Credits: Dorothée Coppens

18. Inter-comparison of IASI-A and IASI-B spectra
December of January ~ comparisons
(IASI-B Obs-Calc) – (IASI-A Obs-Calc)
IASI-A Obs-Calc
IASI-B Obs-Calc
NedT differences at 280K (in K)
NedT differences at 280K (in K)
Wave numbers (in cm-1)
Wave numbers (in cm-1)
Conclusions: IASI-B and IASI-A radiometric calibrations are very close (~0.1K)
Credits: Dorothée Coppens

19. IASI-HIRS radiance comparison for Metop-A and Metop-B
Daily average IASI (HIRS like) – HIRS (ch8 for example)
(~6000 comparisons per day for clear scenes, colocation <3 km)
HIRS (on board Metop-A and B) ch8 spectral response
IASI-A – HIRS-A
IASI-B – HIRS-B
NedT differences at 280K (in K)
Wavenumber (cm-1)
 Spectral integration of IASI channels over the HIRS ISRF
Conclusions:
IASI-A / HIRS-A & IASI-B / HIRS-B radiometric calibrations are very close for the channel 8 (~0.15K), other channels also studied
IASI-A / IASI-B via HIRS also very close (~0,015K)
Days
Credits: Dorothée Coppens

20. IASI,HIRS stand-alone + inter-calibration
From LMD (N. Scott et al.): IIR/MODIS/SEVIRI and
IASI,HIRS stand-alone + inter-calibration
From EUMETSAT (D. Coppens):
IASI radiance monitoring + IASI/HIRS
From LATMOS (S. Payan et al.):
IASI/GOSAT (TANSO-FTS)
From CNES (V. Lonjou et al.):
IASI/AVHRR

21. Motivation and method
Motivation: Study of the annual carbon budget with GOSAT
Use both IASI and GOSAT to increase the amount of data
Potential of TIR has to be explored, B4 of TANSO‐FTS not yet used extensively
Method:
Compare IASI and GOSAT results in coincidence at the poles (orbit crossing)
Compare GOSAT B4 [700–1800cm‐1] to IASI B1 [645–1210 cm‐1] and B2 [1210–2000 cm‐1]
Periods april 2010 (CNES balloon campaign) and july/august 2010
Comparison of spectra at level 2: retrieved surface temperatures
Retrievals in 4 windows:
[940‐980 cm‐1 ] Tsurf and CO2
[980‐1100 cm‐1] Tsurf and O3
[1240‐1320 cm‐1] Tsurf and CH4
[1120‐1200 cm‐1] Tsurf and N2O
Cloud filtering, refine coincidences
Credits: Sébastien Payan

22. Results
Conclusion:
Retrieved Tsurf from coincident IASI and GOSAT spectra are well correlated
 good IASI / GOSAT B4 radiometric comparison
Weak dependence of correlation with the distance or time difference  reasonable statistics achievable, but need a more realistic average value
Credits: Sébastien Payan

23. IASI,HIRS stand-alone + inter-calibration
From LMD (N. Scott et al.): IIR/MODIS/SEVIRI and
IASI,HIRS stand-alone + inter-calibration
From EUMETSAT (D. Coppens):
IASI radiance monitoring + IASI/HIRS
From LATMOS (S. Payan et al.):
IASI/GOSAT (TANSO-FTS)
From CNES (V. Lonjou et al.):
IASI/AVHRR

24. IASI / AVHRR inter-calibration: method
Conditions:
IASI and AVHRR are on the same satellite, they see the same scene
IASI is at high spectral resolution but low spatial resolution
AVHRR is at high spatial resolution but low spectral resolution
Method:
No selection of data
IASI spectra integrated in the AVHRR4 and AVHRR5 spectral filters (IASI-LikeAVHRR4 and IASI-LikeAVHRR5)
AVHRR4 image spatially integrated in the IASI PSFs (AVHRR4-Like IASI)
AVHRR5 image spatially integrated in the IASI PSFs (AVHRR5-LikeIASI)
Example for IASI and AVHRR spatial resolutions
AVHRR spectral resolution (IASI 0.5 cm-1)

25. IASI-B / AVHRR inter-calibration
IASI-B / AVHRR ch4
IASI-B / AVHRR ch5
Input surface temperature distribution
Dependency wrt scene temperature is confirmed : -0.5K between 220K and 300K
Small dependency wrt scene temperature
Conclusions: The discrepancy between IASI-B/AVHRR-4 may be attributed to AVHRR-4, for 2 reasons:
IASI-B non-linearity potential impact on radiometry in B1 is almost constant over B1 band and lower than (max).
IASI-B/AVHRR-5 do not display the same feature as IASI-B/AVHRR-4 wrt scene temperature.

26. IASI-A / AVHRR inter-calibration
IASI-A / AVHRR ch4
IASI-A / AVHRR ch5
Input surface temperature distribution
Similar trends for IASI-AVHRR4 and IASI-AVHRR5, for all 4 sounder pixels.
Good consistency for cold scene, -0.5K bias for hottest scenes (300K).
Effect of non linearity on METOP-A are expected to be pixel dependent and not to exceed 0.05K over B1
We believe that IASI Sounder-AVHRR discrepancies are mainly due to AVHRR.

27. IASI-B / AVHRR inter-calibration
IASI-A / AVHRR Effect of scan angle IASI-B / AVHRR
A scan angle dependant bias of 0.2K is observed between IASI and AVHRR. The behaviour is the same on MetOp-A. As this bias is not observed in IASI / IIS intercalibration, it may be attributed to polarization effect of AVHRR scan mirror.

28. Motivation and method
Motivation: Study of the annual carbon budget with GOSAT
Use both IASI and GOSAT to increase the amount of data
Potential of TIR has to be explored, B4 of TANSO‐FTS not yet used extensively
Method:
Compare IASI and GOSAT results in coincidence at the poles (orbit crossing)
Compare GOSAT B4 [700–1800cm‐1] to IASI B1 [645–1210 cm‐1] and B2 [1210–2000 cm‐1]
Periods april 2010 (CNES balloon campaign) and july/august 2010
Comparison of spectra at level 2: retrieved surface temperatures
Retrievals in 4 windows:
[940‐980 cm‐1 ] Tsurf and CO2
[980‐1100 cm‐1] Tsurf and O3
[1240‐1320 cm‐1] Tsurf and CH4
[1120‐1200 cm‐1] Tsurf and N2O
Cloud filtering, refine coincidences
Credits: Sébastien Payan

29. IASI,HIRS : stand-alone + inter-calibration
From LMD (N. Scott et al.): IIR/MODIS/SEVIRI and
IASI,HIRS : stand-alone + inter-calibration
From EUMETSAT (D. Coppens):
IASI radiance monitoring + IASI/HIRS
From LATMOS (S. Payan et al.):
IASI/GOSAT (TANSO-FTS)
From CNES (V. Lonjou et al.):
IASI/AVHRR
General conclusion:
Many independents absolute and relative methods
 High confidence in the absolute calibration of IASI and continuity IASI-A/IASI-B