CERES SW channel spectral darkening: Rev1 adjustments and beyond

1. CERES SW channel spectral darkening: Rev1 adjustments and beyond Grant Matthews, CERES IWG GIST 24, 15 Dec 05, Imperial College Contributors: Norman Loeb, Jennifer Hubble & Brett Matthews

2. Introduction: CERES Instrument • Anomalous SW Drift caused by non-spectrally uniform SW darkening (as implied by LDEF, and GOME)? • CERES direct compare correlation to RAPS/Xtrack modes • “Rev1” adjustment using Xtrack instrument as calibration standard • Contamination model to derive Edition 3 spectral response • Summary

3. CERES Scanning Radiometer Instrument

4. What does SWICS data imply about change SW gains gains on the Terra instruments? Answer: The SW channel gains are stable to 0.1% on both Terra instruments FM1 and FM2

5. Anomalous -2% drift in Edition 2 Terra measured global SW Flux from the Earth?

6. CERES Spectral Response of 3 Channels

7. Could this be caused by a non-spectrally uniform darkening of the CERES SW optics as occurred on LDEF?

8. The GOME instrument narrowband channels show a similar spectral shape of darkening:

9. Could this be caused by a non-spectrally uniform darkening of the CERES SW optics as occurred on LDEF (undetectable by viewing onboard lamps)?

10. In order for contaminants to interact with space bound particulates and arrive at the filter the telescope would need to be looking in the RAM direction (occurring only in RAPs mode)

11. 1 RAPS, 1 Xtrack instrument at anytime. Direct Compare (DC) is the ratio of simultaneous nadir filtered radiance Instrument diagnostic that is INDEPENDENT OF CLIMATE

14. How can we quantify and remove this change in response for each instrumentwithout removing real climate trends? • Short term (Ed2_Rev1): Experience from LDEF suggests that the instrument in Xtrack mode will not change in SW response. Hence use Xtrack instrument as calibration standard in percent direct compare to derive change in RAPS instrument response to All sky and Clear Ocean. • Long term (Ed3): Develop comprehensive Contamination/UV exposure model and use percent direct compare to derive change to instrument spectral response.

16. Use Xtrack instrument as calibration standard and hence you can derive the separate instrument drift to be taken out in Rev1 adjustment

17. A table of Rev1 adjustment factors is issued via the quality summary, authors then use the description “Edition2_Rev1” http://eosweb.larc.nasa.gov/PRODOCS/ceres/ES8/Quality_Summaries/CER_ES8_Terra_Edition2.html http://eosweb.larc.nasa.gov/PRODOCS/ceres/ES8/Quality_Summaries/CER_ES8_Aqua_Edition2.html

21. In order to fully compensate for darkening effects in the Edition 3 spectral response the following activities are underway: • Stows are planned to directly measure any Xtrack darkening • New reduced noise full swath direct compare data using both instruments in Xtrack is under production • A physical model of the contaminant deposition and subsequent darkening is being developed and tuned to internal calibration and direct compare data. This will incorporate results from further stows and noise reduced direct compare

22. The contaminant model estimates the contaminant thickness and spectral properties given time spent in ram direction and SW radiance exposure:

23. Tune the model so its estimate of changes to filtered radiance matches the internal calibration lamps and direct compare:

26. The model can also be tuned to Aqua direct compare and internal calibration results:

28. Check model contaminant spectral properties match MISSE contaminant samples returned in Aug 05 by Discovery from ISS:

29. Summary and Conclusions • Rev 1 adjustments remove the majority of the spurious SW trends in Edition 2 data • Special operations and modeling currently underway to characterize and compensate for RAPs and any Xtrack darkening in Edition 3 SW spectral response • Possible implications for the design and operational modes of future broad band earth observation instruments