NASA Langley Research Center / Science Directorate CERES SW channel spectral darkening: Rev1...

NASA Langley Research Center / Science Directorate

CERES SW channel spectral darkening: Rev1 adjustments and

beyond

Contributors: Norman Loeb, Jennifer Hubble & Brett Matthews

Grant Matthews, CERES IWGGIST 24, 15 Dec 05, Imperial College


• 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


CERES Scanning Radiometer Instrument


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


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


CERES Spectral Response of 3 Channels


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


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


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

viewing onboard lamps)?


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)


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


Plot of normilized Earth spectra compared to fractional transmission of polymerized contaminant transmission (1um thick)

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Clear ocean spectrum

Allsky spectrum


How can we quantify and remove this change in response for each instrument without 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.


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


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


• 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

In order to fully compensate for darkening effects in the Edition 3 spectral response the following activities are underway:


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


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

Plot of normilized Earth spectra compared to fractional transmission of polymerized contaminant transmission (1um thick)

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Clear ocean spectrum

Allsky spectrum


Fix/Darkening model comparison with TERRA Allsky filtered direct compare

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Fix/Dark model


Fix/Darkening model comparison with TERRA Clear Ocean filtered direct compare

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


Fix/Darkening model comparison with AQUA Allsky filtered direct compare

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


• 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

Summary and Conclusions

NASA Langley Research Center / Science Directorate CERES SW channel spectral darkening: Rev1...

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