MODIS B35 and B36 Radiometric Biases

1. MODIS B35 and B36 Radiometric Biases Difficult Question What is the physical nature of MODIS biases wrt AIRS observations? Chris Moeller, University of Wisconsin Jan 25, 2010 MODIS Calibration Workshop (This work is an extension of work begun by Dave Tobin at U. Wisc.)

2. Tobin’s work suggests spectral influence MODIS Band 35 (13.9 mm) AIRS-MODIS,no shift AIRS-MODIS,MODIS SRF shifted by +0.8 cm-1 (15.5 nm) Tb diff (K) AIRS-MODIS: unshifted, shifted 140 120 100 80 60 40 20 0 Radiance (mW/m2 sr cm-1) 13.5 13.6 13,7 13,8 13.9 14.0 14.1 14.2 14.3 Wavelength (um)

3. AIRS minus MODIS, Band 35 vs B31 Scene Temp No Spectral Shift No OOB

4. AIRS minus MODIS, Band 35, with 0.8 cm-1 SRF shift w/Spectral Shift A spectral shift of 15.5 nm to shorter wavelengths of the MODIS B35 RSR is very effective at removing scene temperature dependence from the bias between AIRS and MODIS observations. Intriguing!

5. But….. • Could MODIS prelaunch RSR character-ization of B35 have been off by ~16nm? • Could there have been a postlaunch shift in the MODIS B35 RSR of ~16nm? • Might there be undetected OOB influence in MODIS that also could explain the AIRS-MODIS behavior? • Some combination of all of the above?

7. AIRS minus MODIS, Band 35 vs B31 Scene Temp No Spectral Shift No OOB

8. AIRS minus MODIS, Band 35 vs B31 Scene Temp OOB region: 13.16 – 13.33um OOB5_750cc CP 2005

9. AIRS minus MODIS, Band 35 vs B31 Scene Temp w/Spectral Shift

10. AIRS minus MODIS, Band 35 vs B31 Scene Temp No Spectral Shift No OOB

11. AIRS minus MODIS, Band 35 vs B31 Scene Temp OOB region: 13.16 – 13.33um OOB5_750cc CP 2005

12. AIRS minus MODIS, Band 35 vs B31 Scene Temp w/Spectral Shift

13. AIRS minus MODIS, Band 35 vs B31 Scene Temp No Spectral Shift No OOB

14. AIRS minus MODIS, Band 35 vs B31 Scene Temp OOB region: 13.16 – 13.33um OOB5_750cc CP 2005

15. AIRS minus MODIS, Band 35 vs B31 Scene Temp w/Spectral Shift

16. AIRS minus MODIS, Band 35 vs B31 Scene Temp No Spectral Shift No OOB

17. AIRS minus MODIS, Band 35 vs B31 Scene Temp OOB region: 13.16 – 13.33um OOB5_750cc CP 2005

18. AIRS minus MODIS, Band 35 vs B31 Scene Temp w/Spectral Shift

19. AIRS minus MODIS, Band 36 vs B31 Scene Temp No Spectral Shift No OOB

20. AIRS minus MODIS, Band 36 vs B31 Scene Temp OOB region: 13.16 – 13.33um (same spectral OOB leak as used for B35) OOB5_750cc CP 2005

21. AIRS minus MODIS, Band 36 vs B31 Scene Temp w/Spectral Shift

22. MOD06 CTH shows less bias using spectral shift RSR (Red) MODIS MOD06 (Cloud Top Heights) Retrievals

23. Summary • It appears that AIRS-MODIS biases for MODIS Band 35 (and 36) can be largely explained by either a spectral shift or an OOB filter leak. • Which of these two mechanisms is the true cause of AIRS-MODIS biases is not known. • MOD06 (Cloud Top Property) statistics indicate improved agreement with Calipso observations after applying the spectral shift to MODIS bands 34-36.

24. A quick look at Aqua B35 and 36 RVS Stability as portrayed by AIRS-MODIS comparisons • In general RVS appears to be stable over 5 year period of monthly comparisons • Some suggestion that small changes (< 0.2 K) may be occurring near the End-of-Scan on MODIS • No way of being certain that this is a MODIS effect

25. MODIS End-of-Scan MODIS Begin-of-Scan

26. MODIS End-of-Scan MODIS Begin-of-Scan

27. MODIS End-of-Scan MODIS Begin-of-Scan

28. MODIS End-of-Scan MODIS Begin-of-Scan

29. MODIS End-of-Scan MODIS Begin-of-Scan

30. MODIS End-of-Scan MODIS Begin-of-Scan