CZCS Ocean Color Reprocessing 2014.0

Introduction

The Ocean Biology Processing Group (OBPG) completed a full-mission ocean color reprocessing of the Coastal Zone Color Scanner (CZCS) dataset on 10 December 2015. This reprocessing is part of a multi-mission effort to update common algorithms, product suites, and data formats across all supported missions. Sensor-independent changes are detailed in the R2014.0 Ocean Color Reprocessing General Description. Here we describe the CZCS sensor-specific details of the reprocessing, and provide an assessment of data quality and impact relative to the previous reprocessing.

Sensor-specific Processing Details

Source Data

Level-1A source data are unchanged. The CZCS data was originally obtained in a raw format associated with the original 9-track tape storage (CRTT format). Each file contained approximately 2 minutes of data. There were many files with duplicate or overlapping data segments, and many files with corrupted data. A more detailed discussion of the CZCS data collection is here. The CRTT files were first converted to Hierarchical Data Format (HDF). The HDF Level-1A format incorporates the raw counts from the CRTT files, as well as all relevant telemetry and quality information. Extensive quality control was performed to filter-out the corrupted data. The small files were then merged to form one file per orbit, with best quality selected where duplicate scans were found. In addition, the OBPG incorporated the navigation biases described in Gregg and Conkright (2002) and orbit vectors obtained from archives of the SMMR instrument on NIMBUS-7 to improve geolocation and knowledge of the view path geometries, respectively.

Calibration

The CZCS instrument calibration is unchanged from that employed in as detailed for the R2006.0 reprocessing and carried forward to the R2010.0 multi-mission reprocessing. The vicarious calibration (Franz et al. 2007, Werdell et al. 2007) was updated to account for changes to the atmospheric correction algorithm and ancillary sources.

Wavelength (nm) 443 520 550 670
Gain 1.0394 0.9903 1.0061 1.000

Level-2 Processing

Again, the algorithms employed and products produced from CZCS are as described in the R2014.0 Ocean Color Reprocessing General Description, however, the inherent optical properties (IOP) were not produced for this sensor, due to the spectral limitation of retrievable spectral water-leaving reflectances available for the algorithm inversions. Sensor-specific center wavelength values used in processing and product naming are as shown in the table above.

Impact and Quality Assessment

Impact of Reprocessing on Timeseries

To assess the impact of this R2014.0 reprocessing relative to the previous R2010.0 reprocessing, a comparative timeseries analysis was performed. See the whitepaper Franz, B.A. (2009), "Methods for Assessing the Quality and Consistency of Ocean Color Products" for details on the approach. The impact of all calibration, algorithm, and ancillary data changes on remote sensing reflectance is a substantial increase in the blue and decrease in the green to red spectral bands, as shown in this comparison of the monthly mean Rrs in various globally-distributed geographic subsets. Impact on the chlorophyll time-series is to lower the chlorophyll concentration by approximately 30% over most of the global oceans. This brings the chlorophyll concentrations into much better agreement with more recent observations from SeaWiFS, as shown by this analysis of CZCS (1978-1986) temporal trends relative to SeaWiFS (1998-2006), and the same analysis for the R2010.0 CZCS and SeaWiFS.

Comparison with In Situ Measurements

Validation of the remote sensing reflectance retrievals was performed relative to all available match-ups from SeaBASS holdings. However, the number of match-ups are quite small, as there were a limited number in situ measurements from the CZCS time period. Only chlorophyll match-ups are shown because 1 or fewer data points were available for other types of products.

Validation results: SeaBASS Rrs holdings

Product
Name
CZCS RangeIn situ Range#Best Fit
Slope*
Best Fit
Intercept*
R2*Median RatioAbs %
Difference
RMSE*
chlor_a2.51521, 18.131883.79500, 22.3500080.99342-0.166520.189040.7008552.650870.32844