1. GSICS VIS/NIR subgroup report
David Doelling (sub-lead-facilitator), and all other VIS/NIR members
GSICS Annual Meeting, Darmstadt, Germany, March 24-28, 2014

2. Outline An example of how calibration impacts retrievals
Calibration accuracy through time
Reference sensor and traceability to future instruments and methods
Comparison of visible calibration techniques
Coordination of GPRC for common short and long term goals
Proposed forward progress
This years goals

3. CERES fluxes MODIS C5 DCC stability
CERES measures the Earth’s broadband shortwave and longwave fluxes
MODIS cloud properties aid in converting measured CERES radiances to fluxes, since the SW BRDF is scene based,
absolute calibration is tied to CERES instrument
The current CERES Edition 3 products uses a combination of MODIS C4 and C5 radiances
CERES users try to correlate cloud/aerosol/land retrievals with CERES fluxes to evaluate climate change
MODIS C5 DCC stability
Wu et al. 2013

4. The effect of MODIS band 1 calibration on retrieved optical depth
Compute global monthly mean optical depth from MODIS pixel level retrievals
Deseasonalize to determine trend over time
A 2.5% MODIS calibration drift has resulted in a ~8% decrease in cloud optical depth
Which may be falsely interpreted as climate change
Collection 6 stable to 1%/decade
GSICS calibration goal is to improve and harmonize cloud/aerosol/land retrievals
+0.4
+0.4
Terra-MODIS C5
Aqua-MODIS C5
+0.0
+0.0
-0.4
2000
2005
2012
-0.4
2002
2007
2012

5. VIS/NIR Calibration Milestones
1980 ISCCP 5% calibration accuracy
“The Earth is more stable than your sensor” Bill Rossow
1995 to 2000 First solar diffusers for VIS bands
ERS2-ATSR2, TRMM-VIRS, Terra-MODIS
2006 MODIS band 1 C5 calibration 2%/decade trend
2010 MODIS band 1 C6 calibration 1%/decade trend
2014 Predicted VIIRS stability
Desert Calibration stability 1%/decade
DCC calibration stability 0.5%/decade (Bhatt et al. 2014)
Predicted anthropogenic SW forcing, 0.3%/decade
2025 CLARREO/TRUTHS absolute calibration accuracy 0.3% 2K
Solar Cycle amplitude 0.1%/11years

6. Reference Calibration
Reference Instrument or reference calibration tied to radiative transfer (RT) predicted radiances?
How traceable are the radiative transfer (RT) radiances
Dependent on RT method and inputs, how traceable in time are they?
The inputs were based on MODIS cloud properties, which are tied then to the MODIS calibration
Either way we agree on reference instrument for traceability
Okuyama. GSICS 2009 web meeting
RT based
Reference Instrument based

7. VIS/NIR reference instrument
Ideal traceable on orbit calibration hyper-spectral instrument such as the future CLARREO/TRUTHS
If the calibration is traceable on orbit you need no overlap between successive instruments
You can monitor change with an equivalent instrument years in the future
Now given todays instruments
The data must be freely available, user friendly, and widely used, very familiar with the community (retrievals well documented)
Must be a well characterized sensor and stability referenced through a solar diffuser and/or moon
Navigation, instrument radiometric noise, polarization knowledge
Must be traceable (inter-calibrated) with future instruments

8. Traceable inter-calibration
if the VIS reference instruments can be faithfully inter-calibrated, then the absolute calibration of a future instrument can be reference back in time
Eventually future instruments will have excellent absolute and SI traceable calibration
Factors that increase the confidence of inter-calibration
Overlapping successive or follow on operational instruments, such as NPP/JPSS-VIIRS, Metop-SG-MetImage
Length of overlap period
Similar spectral channels, known SRFs
Similar (equator crossing time) well maintained (non drifting LEO) orbits, to observe entire dynamic range over all earth targets

9. Reference Instrument
The reference instrument absolute calibration is secondary as long as the calibration is very stable and well characterized
However it is important that the absolute calibration can be traced to the reference instrument for all contemporary operational visible calibration methods
This allows the comparison of calibration methods
This allows uniform cloud/aerosol/land retrievals over multiple platforms both spatially and temporally
This allows future calibration enhancements to improve the entire record
GSICS goal is to verify the operational instrument stability so that the reference instrument calibration can be transferred effectively

10. Visible Calibration Methods
No traceable reference VIS hyper-spectral instrument is available, therefor a multiple calibration approach is taken
Agreement among methods validates the resulting calibration
Each method has its advantages and disadvantages
Hewison, GSICS annual meeting 2009

11. VIS calibration technique comparison (not comprehensive)
GEO Method
Stability at TOA
SBAF
Traceable to reference
Sampling
Historical application
Lunar
•Geological variability
• Earth view optics?
ROLO model
at MODIS/VIIRS phase angles
•Monthly
•Long phase and libration period
Dependent on sampling
DCC
•0.5%/decade (VIIRS)
• tropopause
Nearly flat spectrally
•DCC raymatch
•Domain mean
Large sample collective method
Reference calibration difficult
Rayliegh
RT depends
Wind speed
Chlorophyll
RT at surface, good for interband
Reference/RT compare
Use coincident MODIS .8µm
Need 0.8µm reflectivity
Desert
1.0%/decade (VIIRS)
• surface
Need observed hyper-spectral
SADE with atmosphere
Not available over all GEO domains
Long term invariance not known
Liquid Water Clouds
Direct compare
RT model
MODIS cloud properties
Dependent on reference orbit
Need coincident reference obs
Ray-match
Needed for clear-sky and low clouds
MODIS radiances

12. GSICS GEO VIS ATBDs
ATBD is the first step to implement VIS method within GSICS
VIS ATBDs in GSICS library, effort started in 2011
Liquid Water Clouds (JMA) – Arata Okuyama
Liquid Water Clouds (SNU) B.J. Sohn
Lunar (NOAA) – Fred Wu
Stellar (NOAA) – Fred Wu
DCC (NASA) – David Doelling
DCC (SNU) – B.J. Sohn
(MODIS/GEO) Ray-matching (NASA) – David Doelling
Combining Methods (EUMETSAT) – Tim Hewison
VIS ATBDs TBD
Rayleigh (CNES) draft status? - Bertrand Fougnie
Desert (CNES) – Patrice Henry
Desert (EUMETSAT) – Sebastien Wagner
Sunglint (NOAA) Andy Heidinger

13. Implementation of VIS methods Fred Wu, GSICS web meeting 2009
Common VIS method goals
Common effort
Common product (requires more GPRC commitment)
Common algorithm (creates more uniformity)
Common VIS calibration product goals
Quantify the bias
Correct the bias (benefits the retrieval community)
Diagnose the bias
Common VIS record goals
Common algorithm during reference sensor record
Common algorithm for reference sensor and historical record
Common GPRC goals
Best to coordinate various GPRC goals and commitments

14. VIS calibration method product timeline
Calibration method timeline
Year 1: Agree on calibration approach from existing methods or a combined approach
Year 2: GPRC’s implement common approach – have verification dataset to ensure uniform implementation
Year 3: GPRC’s deliver submission/demonstration phase products to GSICS – formatted, documented, peer reviewed, user tested
Year 4: GPRC’s deliver pre-operational/operation phase products, version control, made public, accepted
Can method implementations be staggered to accelerate products?
Fangfang Yu, GPPA workflow 2013
2012
2013
2014
2015
2016
2017
2018
DCC
Lunar
Rayleigh?

15. VIS calibration product timeline
Do we spend time to modify method for historical instruments?
Do we spend time to modify method for LEO instruments?
How will these efforts be redirected when the next generation GEOs are launched in a few years?
These will have their calibration stability tied to solar views
The methodology will be straight forward once either CLARREO or TRUTHS hyper-spectral radiances are available for calibration
Probably at least 10 years from now

16. Goals/Discussion for this year
DCC
Agree on methodology (Version 1)
Product, bias monitoring and path to demonstration product
Determine DCC reference calibration tied to MODIS and SBAF
Lunar
Agree on methodology that applies uniformly to all
Reference ROLO with MODIS at MODIS phase angles
Improve ROLO with Plaiedes?
Rayleigh
Requires 0.86µm channel, use coincident MODIS 0.86µm?
Prepare for next generation GEO
Explore next technique
Desert (IVOS), liquid water, combining methods