MODIS-Terra Ocean Color Reprocessing 2014.0
Introduction
The Ocean Biology Processing Group (OBPG) completed a full-mission ocean color reprocessing of the MODIS-Terra (MODIST) ocean color dataset in April 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 MODIST sensor-specific details of the reprocessing, and provide an assessment of data quality and impact relative to the previous MODIST reprocessing version R2013.0 .
Sensor-specific Processing Details
Source Data
As in previous reprocessings, R2014.0 starts from the MODIST Level-1A files. The Level-1A source files are unchanged for this reprocessing.
Instrument Calibration
The MODIST instrument is known to be difficult to calibrate in the shorter wavelengths, especially below 500nm (Franz et al. 2008) . The main contributors to this challenge are the degradation of the reflective properties of the primary mirror (most likely due to an incident that occurred during prelaunch characterization), and the malfunction of parts of the solar diffuser calibration mechanism (the door that usually protects the solar diffuser from degradation due to solar irradiance held open since 2003). In addition to lunar and solar diffuser measurements, the MODIS Calibration Support Team (MCST) has been using desert sites to improve the calibration trending of several bands. The OBPG uses the MCST calibration as a starting point. The NIR bands are assumed to be well calibrated, with only a striping correction applied. For the remaining bands, a cross-calibration is performed (Kwiatkowska et al. 2008). The cross-calibration procedure uses the water-leaving radiances from other sensors (SeaWiFS from 2000 until 2004, MODIS Aqua from 2004 until 2016) to derive an expected TOA radiance field, using the MODIS Terra viewing and illumination geometries. Instrument gain and polarization sensitivity coefficients are than derived as a function of time and scan angle to minimize the differences between MODIST observed TOA radiances and the expected TOA radiances. The plots below show the cross-calibration coefficients M11 (gain) and m12 and m13 (polarization sensitivity) as a function of time for the end of scan (pixel 1250) and band 8 (412nm). Solid (dashed) lines are for mirror sides 1 (2), different colors show detectors 1-10. Starting with this reprocessing, the OBPG is now trending the m13 coefficient as well. In previous reprocessings, the change in m13 was so small that it was ignored.
| M11 of 412nm band | m12 of 412nm band | m13 of 412nm band |
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Vicarious_Calibration
The OBPG applies an additional vicarious calibration during Level-2 processing (Franz et al. 2007). The 869nm band is assumed to be correctly calibrated from on-orbit solar diffuser measurements. The 748nm band is adjusted using match-ups from the South Pacific Gyre to force the aerosol type retrievals to agree, on average, with the aerosol type observed at the Tahiti AERONET site. The calibration of the visible bands (412nm to 678nm) is then adjusted to produce retrievals that match, on average, measurements from the Marine Optical Buoy (MOBy) near Lanai Hawaii. The vicarious calibration gains derived in this manner are:
| Wavelength (nm) | 412 | 443 | 469 | 488 | 531 | 547 | 555 | 645 | 667 | 678 | 748 | 859 | 869 | 1240 | 1640 | 2130 |
| Gain | 0.9917 | 0.9923 | 1.0034 | 0.9958 | 0.9959 | 0.9951 | 0.9956 | 1.0321 | 0.9963 | 1.0015 | 0.9995 | 0.9986 | 1.0 | 1.0 | 1.0 | 1.0 |
Level-2 Processing
The algorithms employed and products produced from MODIS are as described in the R2014.0 Ocean Color Reprocessing General Description. 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 R2013.0 reprocessing, a comparative timeseries analysis was performed (see Franz 2009 for details on the evaluation approach). The impact of all calibration, algorithm, and ancillary data changes on remote sensing reflectance is shown in this comparison of monthly mean ocean color product trends for various globally-distributed geographic subsets. Sample trends for Rrs and Chlorophyll over global mean deep-water (> 1000m) are shown below.
| R2013.0 & R2014.0 | R2014.0/R2013.0 |
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| R2014.0 MODIS-Terra & SeaWIFS | R2014.0 MODIS-Terra & MODIS-Aqua |
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Comparison with In Situ Measurements
Validation of the remote sensing reflectance retrievals was performed relative to all available match-ups from SeaBASS holdings and additional match-up analysis with Level 2 data from the Aerosol Robotic Network - Ocean Color (AERONET-OC). Statistical analysis of the satellite to in situ match-ups as of this writing is provided below. Also shown are scatter plots and frequency distribution comparisons.
Validation results: SeaBASS Rrs holdings
| Product Name | MODIS Terra Range | In situ Range | # | Best Fit Slope | Best Fit Intercept | R2 | Median Ratio | Abs % Difference | RMSE |
|---|---|---|---|---|---|---|---|---|---|
| Rrs412 | -0.00297, 0.01683 | 0.00074, 0.01680 | 467 | 1.05677 | -0.00104 | 0.80727 | 0.84791 | 26.29227 | 0.00177 |
| Rrs443 | -0.00163, 0.01353 | 0.00079, 0.01514 | 726 | 1.00347 | -0.00053 | 0.78836 | 0.90167 | 17.66078 | 0.00130 |
| Rrs488 | 0.00007, 0.01767 | 0.00109, 0.01936 | 739 | 0.96035 | -0.00034 | 0.79406 | 0.89214 | 14.60500 | 0.00113 |
| Rrs531 | 0.00068, 0.01683 | 0.00106, 0.01871 | 170 | 0.99660 | -0.00022 | 0.84760 | 0.96306 | 13.14775 | 0.00152 |
| Rrs547 | 0.00100, 0.01613 | 0.00130, 0.01965 | 103 | 0.96950 | -0.00008 | 0.82961 | 0.97770 | 15.67407 | 0.00193 |
| Rrs667 | -0.00048, 0.01230 | 0.00002, 0.01100 | 469 | 0.92541 | -0.00005 | 0.86283 | 0.83273 | 39.15653 | 0.00054 |
| Rrs678 | 0.00025, 0.00413 | 0.00038, 0.00575 | 11 | 0.82973 | 0.00027 | 0.70449 | 0.83348 | 33.67266 | 0.00081 |
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Validation results: AERONET-OC Level 2 Match-ups
| Product Name | MODIS Terra Range | In situ Range | # | Best Fit Slope | Best Fit Intercept | R2 | Median Ratio | Abs % Difference | RMSE |
|---|---|---|---|---|---|---|---|---|---|
| Rrs412 | -0.00378, 0.01516 | -0.00000, 0.01580 | 2902 | 1.05362 | -0.00079 | 0.66323 | 0.72434 | 49.13177 | 0.00155 |
| Rrs443 | -0.00204, 0.01914 | 0.00007, 0.01783 | 2916 | 1.03186 | -0.00024 | 0.80521 | 0.93433 | 25.81365 | 0.00117 |
| Rrs488 | -0.00011, 0.02515 | 0.00039, 0.02467 | 2576 | 0.91336 | -0.00029 | 0.89044 | 0.85128 | 20.30292 | 0.00133 |
| Rrs531 | 0.00085, 0.03040 | 0.00113, 0.02689 | 1455 | 0.91255 | -0.00022 | 0.90605 | 0.87582 | 15.94000 | 0.00142 |
| Rrs547 | 0.00034, 0.03199 | 0.00031, 0.02866 | 2916 | 0.91482 | -0.00022 | 0.90384 | 0.86886 | 16.02610 | 0.00130 |
| Rrs667 | -0.00109, 0.02091 | 0.00002, 0.01623 | 2509 | 1.07463 | -0.00031 | 0.80839 | 0.79292 | 28.57211 | 0.00055 |
| Rrs678 | -0.00032, 0.01053 | 0.00002, 0.00965 | 411 | 1.06194 | -0.00041 | 0.85263 | 0.63145 | 47.94635 | 0.00052 |
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Validation results: SeaBASS Chlorophyll holdings
Validation of the chlorophyll retrievals was performed relative to all available match-ups in SeaBASS.
| Product Name | MODIS Terra Range | In situ Range | # | Best Fit Slope* | Best Fit Intercept* | R2* | Median Ratio | Abs % Difference | RMSE* |
|---|---|---|---|---|---|---|---|---|---|
| chlor_a | 0.03276, 135.68965 | 0.00100, 50.84000 | 1167 | 1.00148 | 0.04206 | 0.75432 | 1.03935 | 38.08246 | 0.33462 |
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