SeaWiFS Ocean Reprocessing 5.2

The primary purposes of SeaWiFS reprocessing 5.2 are to:

  • Update the sensor degradation model to account for a slight change in the radiometric response of the SeaWiFS instrument due to variations in the instrument electronics temperatures and.
  • Correct a trend identified in the ratio of two detector gain settings used in the transfer of the lunar calibration to the Earth view data.

Calibration and Characterization Updates

Temperature Dependence

The performance of the SeaWiFS normalized water-leaving radiances have demonstrated remarkable consistency for most of the mission to date. Recently, analyses of the interannual repeatability have shown measurable deviations from the mission trends. Starting in late 2005, the global nLw averages have deviated from the previous results by typically 3% to 5%. Based on an analysis of the lunar time-series, these deviations were determined to be associated with a change in the instrument response to temperature, most notably for the 765 and 865-nm bands (7 and 8, respectively). The effect is evident as a periodic signal in the lunar calibration time series starting in mid-2005. A revised temperature correction was derived from the lunar measurements and applied to both the calibration data and the Earth-view data. The lunar time series with the revised temperature corrections shows that the periodic calibration drift in bands 7 and 8 has been reduced significantly.

Commanded Gain Drift

The SeaWiFS lunar calibrations are performed using a different set of gains for each band than the normal Earth-viewing data collection, in order to optimize the sensor signal for the calibration. The ratio of the lunar to Earth view gains is monitored via daily intergain calibration sequences. A recent mission-long analysis of the gain calibrations has shown a slight drift in the gain ratios for the NIR bands (7 and 8) that are used for atmospheric correction. This drift has now been incorporated into the lunar calibration time series, significantly reducing an observed time trend in epsilon, which affects the atmospheric corrections for all of the visible bands. The net effect has been to essentially remove mission-long trends in the mid-to-long wavelength (490 nm through 670 nm) nLw.

Vicarious Calibration

Changes in the instrument temporal calibration also effect the vicarious calibration to ground truth. The vicarious calibration was rederived using all available MOBY measurements.

Reprocessing 5.1 Lunar Calibration Trend Reprocessing 5.2 Lunar Calibration Trend
Lunar Calibration, r5.1 Trend Diagram Lunar Calibration, r5.2 Trend Diagram

For further details, see:
SeaWiFS Calibration Drift

Effect of Correction on Derived Products

The plots below show trends in some derived products for the original Reprocessing 5.1 and for the proposed Reprocessing 5.2 (after the instrument calibration update). The trends (symbols) were derived by subtraction of the annual cycle for global oligotrophic waters from a mission time-series of 4-day global oligoritophic averages. The blue line is a 7-point running average, while the grey-shaded area shows the range of linear fits that can be drawn when considering a 2-sigma uncertainty on the fitting coefficients.

Normalized Water-leaving Radiance at 510nm (Oligotrophic Waters)
Mission Trend: Reprocessing 5.1 Mission Trend: Reprocessing 5.2
Oligotrophic Waters, r5.1 Mission Trend Diagram Oligotrophic Waters, nLw, r5.2 Mission Trend Diagram

Chlorophyll a (Oligotrophic Waters)
Mission Trend: Reprocessing 5.1 Mission Trend: Reprocessing 5.2
Oligotrophic Waters, Chlor-a, r5.1 Mission Trend Diagram Oligotrophic Waters, Chlor-a, r5.1 Mission Trend Diagram

The plots show that reprocessing 5.2 flattens the trend at 510-nm, such that the linear fits are consistent with the zero trend line. This is consistent with geophysical expectation. The trends in oligotrophic chlorophyll are effectively unchanged (of order 0.001 mg/m^3/year).

For further details, see:
SeaWiFS Reprocessing 5.2 versus SeaWiFS Reprocessing 5.1
SeaWiFS Reprocessing 5.1 Anomaly Trends
SeaWiFS Reprocessing 5.2 Anomaly Trends

Validation Results

The in situ validation shows an overall improvement with reprocessing 5.2 over reprocessing 5.1 results.

Reprocessing 5.1 - Deep Water Subset
Band N Median Ratio % Difference
nLw 412nm 154 1.030 10.783
nLw 555nm 242 0.976 16.637
chlorophyll 271 0.922 25.996
Reprocessing 5.2 - Deep Water Subset
Band N Median Ratio % Difference
nLw 412nm 153 1.005 11.762
nLw 555nm 236 0.961 15.955
chlorophyll 383 1.001 29.801

For complete validation analysis results, see:
Reprocessing 5.1 Validation
Reprocessing 5.2 Validation

Generation of SMIs for All Current Operational Products

With this mini-reprocessing, we will produce Standard Mapped Images (SMI) for all current operational products. Previously SMIs were created for chlorophyll a, Kd490, tau865, angstrom510 and nLw_555. Additionally, the SMI files will be 16-bit, rather than the previous 8-bit scaling. While this change will increase the size of the SMI files, it also greatly reduces digitization artifacts in the data. The MODIS Level-3 products will eventually be regenerated for consistency with the 16-bit format and expanded map product suite.

Elimination of Subordinate Files for Level 3 Bin Data

With this mini-reprocessing, we will produce self-contained Level 3 bin files, with all bin data contained in a single file. Previously, the Level 3 bin files consisted of a main file with one subordinate file for each data product.