# Presentation: 352588

## Ocean Biology Processing Group Evaluations of CZCS & OCTS

**Chuck McClain, Gene Feldman, Bryan Franz, Sean Bailey, Jeremy Werdell, Fred Patt, Wayne Robinson**
**MODIS Science Team Meeting**
**October 31-November 2, 2006**

## OBPG Perspective & Objectives

- Apply vicarious calibration techniques & atmospheric correction & bio-optical algorithms used for SeaWiFS & MODIS, as possible.
- Develop new approaches & algorithms, if necessary
- New methods & algorithm must provide comparable results to operational versions
- Focus on Lwn’s as primary quantities of interest
- Evaluate data sets for climate research

## OBPG Accomplishments under the REASoN-CAN

**CZCS**
- Generated a merged local area coverage (MLAC) data set
- All duplicate scenes/subscenes eliminated using “best data” criteria
- Renavigated entire mission using ephemeris from Nimbus-7/SMMR
- MLAC data set placed on-line with browse and order capabilities similar to SeaWiFS & MODIS
- Evaluated sensor degradation and vicarious calibration using current processing algorithms and  models
- Evaluated derived product data quality (Lwn’s and chlorophyll-a)
**OCTS**
- Evaluated sensor degradation and vicarious calibration using current processing algorithms and models

## Previous CZCS & OCTS Processings

**CZCS**
- GSFC-U. Miami (1989)
- Feldman, G., & others, Ocean Color: Availability of the global data set, *EOS, Trans. Am. Geophys. Union*, 70(23), 634, 1989.
- Evans, R. H., & H. R. Gordon, Coastal zone color scanner “system calibration”: A retrospective examination, *J. Geophys. Res*., 99(C4), 7293-7307, 1994.  Referred to as EG94.
- Gregg et al. (2002)
- Gregg, W. W., M. E. Conkright, J. E. O’Reilly, F. S. Patt, M. H. Wang, J. A. Yoder, and N. W. Casey, NOAA-NASA Coastal Zone Color Scanner reanalysis effort, *Appl. Opt*., 41(9), 1651-1628, 2002.
- Laboratoire d’Oceanographie de Villefranche-U. Miami (2005)
- Antoine, D., A. Morel, H. R. Gordon, V. F. Banzon, & R. H. Evans, Bridging ocean color observations of the 1980s and 2000s in search of long-term trends, *J. Geophys. Res*., 110, C06009, 22 pp., 2005.

## Previous CZCS & OCTS Processings cont.

**OCTS**
- NASDA
- Shimada, M., H. Oaku, Y. Mitomi, & H. Murakami, Calibration of the Ocean Color & Temperature Scanner, *IEEE Trans. Geosci. & Remote Sens*., 37(3), 1484-1495, 1999.
- Gregg (1999)
- Gregg, W. W., Initial analysis of ocean color data from the ocean color and temperature scanner. I. Imagery analysis, Appl. Opt., 38(3), 476-485, 1999.
- Gregg W. W., F. S. Patt, & W. E. Esaias, Initial analysis of ocean color data from the ocean color and temperature scanner. II. Geometric and radiometric analysis, *Appl. Opt*., 38(27), 5692-5702, 1999.
- NASA-NASDA (2000; SIMBIOS Project)
- Wang, M., A. Isaacman, B. A. Franz, & C. R. McClain, Ocean-color property data derived from the Japanese Ocean Color and Temperature Scanner and the French Polarization and Directionality of Earth’s Reflectances: a comparison study, *Appl. Opt*., 41(6), 974-990, 2002.

## “Common” Processing Approach & Key Requirements

**Prelaunch characterization & calibration**
- Polarization sensitivity, response vs. scan, counts vs. radiance, relative spectral response, etc.
**On-orbit performance**
- Sensor loss of sensitivity vs. time
- Vicarious calibration
- CZCS: Clear-water radiances (very little concurrent radiometric data)
- OCTS: MOBY &/or clear-water radiances (very little concurrent radiometric data)
- SeaWiFS & MODIS: MOBY
**Atmospheric corrections**
- Standard multiple scattering Rayleigh, sun glint, & foam corrections
- Aerosols
- CZCS: 670 nm-based aerosol correction with turbid water reflectance correction
- OCTS, SeaWiFS, MODIS: Gordon & Wang aerosol correction (2 NIR band scheme) with turbid water NIR correction, Morel bidirectional reflectance, TOMS ozone
**Data quality masks and flags**
- Clouds, sun glint, etc.
**Chlorophyll-a**
- Empirical maximum ratio algorithms, e.g., OC4v4
**CZCS & OCTS product validation**
- Clear-water radiance comparisons
- Comparisons with SeaWiFS clear water Lwn’s, AOTs, etc.
- Time series analyses
- Comparisons with field data (generally sparse)

**None of these sensors have the same set of bands for ocean color.**

## CZCS (October,1978 - June,1986)

- Spectral coverage
- Bio-optical: 443, 520, and 550 nm (20 nm bandwidths)
- Small spectral difference between 520 & 550 limits chlorophyll-a algorithm accuracy at high concentrations
- Aerosol correction: 670 nm (20 nm bandwidth)
- Lack of additional NIR bands a major limitation for aerosol correction
- Cloud flag: 750 nm (100 nm bandwidth)
- Special features
- Polarization scrambler: _Significant polarization residuals remain_
- 4 science gains (Gains 2-4 ratios relative to Gain 1: 0.7, 0.55, 0.25)
- Most data collected with Gains 1 & 2
- Tilting for sun glint avoidance up to ±20o in 2o increments (mirror tilt)
- Internal calibration lamps
- Proved useless on-orbit
- Noon-time ascending orbit (~955 km altitude) with west to east scan
- Temporal & spatial coverage
- 825 m resolution @ nadir; ~±39o scan range, 1636 km swath
- Mission baseline of 10% global coverage
- Coverage extremely uneven over time and space (e.g., N.H. vs. S.H.)
- Additional information
- Limited prelaunch characterization
- Available:  radiance response, spectral response functions, SNR, polarization sensitivity (partial), modulation transfer function
- Not available:  response vs. scan, temperature dependence, point spread function, etc.
- Substantial electronic “over-shoot” off bright targets
- Bands saturate over clouds
- 8-bit digitization
- SNRs, Bands 1-4: 260, 260, 233, 143

## CZCS Polarization Uncertainty(Information from Ball Bros. final report)

- Piece-part depolarization scrambler test indicates 0.5% sensitivity to monochromatic light (wavelength not provided).
- System-level tests show greatest polarization sensitivity at 443 nm, 2-3% for 0 & ±10o mirror tilt (corresponds to ±20o viewing angle change).
- No information provided on polarization phase function.
- Validity of system-level test uncertain due to problems with test set-up.

## CZCS Mission History

**http://oceancolor.gsfc.nasa.gov/CZCS/czcs_processing/**

## CZCS Coverage: Total Mission

## SeaWiFS Coverage:  Total Mission

- Note:  Scale 3x CZCS coverage scale

## December

**December**

- 1978	       1979	     1980		  1981

- 1982	       1983	     1984		  1985

**June**

- 1979	       1980	     1981	                1982		  SeaWiFS

- 1983	       1984	     1985		1986

- Coastal Zone Color Scanner

-  SeaWiFS 2005

-  SeaWiFS 2006

- Monthly Coverage Comparisons for CZCS and SeaWiFS

## CZCS/SeaWiFS Coverage Time SeriesNormalized to SeaWiFS Monthly Coverage (9 km bins) Climatologies

## CZCS Degradation: EG94 & model-based estimates

- Model-based degradation derived at BATS (Sargasso Sea) using in situ chlorophyll observations

## Model-based CZCS Calibration*: BATSComparisons with SeaWiFS

- *Time dependence & vicarious gains

## CZCS NET Field Data Match-ups

**CZCS NET Field Data Match-ups**

**Current radiometric QC & match-up selection criteria applied**
**Roughly 10% of NET stations selected (% similar to that of recent data sets)**

## Multiple Lwn Distribution Peaks &N.H. - S.H. Disparity

- Fall 1979

- Fall 1981

- Spring 1981

**From EG94, June & Oct. 1981**

- Lwn(520)

## El Chichon Stratospheric Aerosols:Non-negligible Effect

**Molecular Atmosphere**

**El Chichon Stratospheric Aerosols:****Non-negligible Effect**

- Standard Atmospheric Correction
- Aerosol Layer:  Thickness ~ 2 km

## CZCS:  El Chichon Aerosols

- .

**M90**
**GW94**

- M90:  Marine aerosol with 90% humidity
- GW96: Gordon & Wang 1994
- GC88: Gordon & Costaño, 1988

## Change in the TOA Reflectance due to El Chichon Aerosols (CZCS, 443 nm)

**λ=443 *****nm *****τ=0.1**

**θ****o****=30****o**

**θ****o****=30****o**

**θ****o****=60****o**

**θ****o****=60****o**

**λ=443 *****nm *****τ=0.2**

**Change in the TOA Reflectance due to El Chichon Aerosols (CZCS, 443 *****nm*****)**

- Delta reflectance computed between two cases:
- M90 trophospheric layer only vs. El Chichon (King) stratospheric layer only.

## CZCS:  Comparison of CZCS & SeaWiFS Global Oligotrophic Lwn’s

- SeaWiFS- Solid  CZCS-Dashed

- 443 nm

- 510 nm
- 520 nm
- 550 nm
- 555 nm
- 670 nm

-     SeaWiFS- solid  CZCS-dashed

## CZCS:  Comparison of CZCS & SeaWiFS Global Oligotrophic Epsilons

- CZCS epsilon values primarily determined by the fixed M90 model used in the processing.

## CZCS:  Extreme Seasonality at 443 nmModel-based time dependence & vicarious calibration

- Mediterranean Sea

-       SeaWiFS-solid    CZCS-dashed

## CZCS Lwn’s:  Large BiasesModel time dependence with model-based vicarious calibration

- North Pacific

- CZCS Coverage Gap

## CZCS Electronic Overshoot (ringing):Revised Mueller (1988) algorithm

- Lwn(520): no mask

- Lwn(520): masked

## CZCS Chlorophyll-a Algorithm:520/550 band ratio problem

- The 520-550 band pair provides little spectral separation
- Ratio results in minimal  algorithm sensitivity: small errors in ratio produces large errors in chlorophyll

- Comparison of maximum band ratio algorithms

## CZCS:  OBPG Summary

- Global coverage inadequate for global climate data record status
- N. H. coverage may be suitable for certain hemispheric studies during early phase of mission
- Data quality varies with location based on comparisons with SeaWiFS
- Comparisons quite good at validation site (Bermuda)
- At other locations, large biases, either uniform or seasonal, observed
- Implication: sensor characterization inadequate
- Sensor degradation and behavior difficult (or impossible) to explain
- El Chichon aerosols do impact CZCS retrievals contrary to Gordon & Costaño (1988)
- Lack of validation data prohibits accurate assessment of radiometry and data quality

**OBPG assessment:  CZCS global data cannot be brought up to a level of accuracy comparable to SeaWiFS and MODIS & should not be used for global climate research.  A few regions for certain periods may be acceptable.**

## Spectral coverage
Bio-optical: 412, 443, 490, 520, 565, and 670 nm (20 nm bandwidths)
Aerosol correction: 765 & 865 nm (40 nm bandwidth)
10 detectors/band
Special features
Gains: 4
Tilting for sun glint avoidance up to ±20o, 0o
Tilts the scan mirror, not the instrument
Creates spatial separation of spectral data as scan angle increases
Introduces noise in the retrievals due to resampling required to achieve approximate co-registration
Internal calibration lamps (not useful)
Solar diffuser (not useful)
10:40 descending orbit
Temporal & spatial coverage
700 m resolution @ nadir; 1400 km swath
GAC data:  4th line, 5th pixel subsampling (only data available to OBPG)
Additional information
Limited prelaunch characterization
Available:  radiance vs. counts, spectral response functions
Not available:  response vs. scan, temperature dependence, polarization sensitivity, point spread function, etc.
10-bit digitization
SNRs, Bands 1-6: 779, 1373, 1453, 994, 988, 1603, 706, 637
Significant uncorrected straylight (ghosting)

- Bio-optical: 412, 443, 490, 520, 565, and 670 nm (20 nm bandwidths)
- Aerosol correction: 765 & 865 nm (40 nm bandwidth)
- 10 detectors/band
- Special features
- Gains: 4
- Tilting for sun glint avoidance up to ±20o, 0o
- Tilts the scan mirror, not the instrument
- Creates spatial separation of spectral data as scan angle increases
- Introduces noise in the retrievals due to resampling required to achieve approximate co-registration
- Internal calibration lamps (not useful)
- Solar diffuser (not useful)
- 10:40 descending orbit
- Temporal & spatial coverage
- 700 m resolution @ nadir; 1400 km swath
- GAC data:  4th line, 5th pixel subsampling _(only data available to OBPG)_
- Additional information
- Limited prelaunch characterization
- Available:  radiance vs. counts, spectral response functions
- Not available:  response vs. scan, temperature dependence, polarization sensitivity, point spread function, etc.
- 10-bit digitization
- SNRs, Bands 1-6: 779, 1373, 1453, 994, 988, 1603, 706, 637
- Significant uncorrected straylight (ghosting)

**OCTS****November, 1996 – June, 1997**

## OCTS Mission Timeline

**OCTS Mission Timeline**

- OBPG line:  Lwn(412) without NASDA nadir tilt calibration adjustment
- Lwn(412) analyses based on NIR atmospheric corrections w/o trends removed.

- NASDA

- OBPG

**http://oceancolor.gsfc.nasa.gov/OCTS/octs_processing.html**

## OCTS NIR Trends

- Analyses based on assuming zero ocean reflectance in open ocean.
- Single trend characterizations of 765 and 865 time dependence inadequate.
- Dual trend corrections (pre- & post-heating) required.

## OCTS(1996-1997) & SeaWiFS (1999-2000)Aerosol Optical Thicknesses (865 nm):  Deep-Water Averages

**Aerosol optical thicknesses similar to SeaWiFS:  No pronounced trend or discontinuity**

- Dual NIR trend analysis

- SeaWiFS-solid     OCTS-dashed

## OCTS Lwn Time Series:  Comparison with SeaWiFS (1999-2000) in Oligotrophic Waters

- Global average

- Hawaii

- Dual NIR trend analysis

- 443 nm
- 490 nm
- 520 nm
- 555 nm
- 565 nm
- 670 nm

- SeaWiFS-solid    OCTS-dashed

## OCTS Lwn Time Series:  Comparison with SeaWiFS (1999-2000) in High Latitude Waters

- North Atlantic: 55°N

- South Atlantic: 55°S

- Dual NIR trend analysis

- OCTS/SeaWiFS Ratio

## OCTS:  OBPG Summary

- Lack of radiometric validation data makes quantification of data accuracy difficult to impossible
- No overlap with SeaWiFS
- Simultaneous global POLDER data
- See Wang et al. (2002)
- Trends in water-leaving radiances coincide with El Niño onset: sensor & geophysical changes convolved.