MODIS/Terra Ocean Color Reprocessing 2018.0
The Ocean Biology Processing Group (OBPG) completed a full-mission ocean color reprocessing of the data from the Moderate Resolution Imaging Spectroreadiometer on Terra (MODIST). This reprocessing is part of a multi-mission effort to update the instrument calibrations and vicarious calibration. Sensor-independent changes are detailed in the R2018.0 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 R2014 MODIST reprocessing.
Sensor-specific Processing Details
As in previous reprocessings, R2018.0 starts from the MODIST Level-1A files. The Level-1A source files are unchanged for this reprocessing.
This reprocessing implements the Collection 6.1 Level-1B and Geolocation processing code. The L1B LUT format for MODIST changed slightly from the Collection 6.0 to incorporate new correction information to address cross-talk issues for several of the thermal IR bands. These changes do not impact ocean color or SST processing.
Instrument CalibrationThe 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 2003, MODIS Aqua from 2003 on) 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. Plots (a) – (c) show the band 8 (412nm) mirror side 1, detector averaged, cross-calibration coefficients M11 (gain) and m12 and m13 (polarization sensitivity) as a function of scan angle at selected years. Plots (d) – (f) show M11, m12, and m13 as a function of time for the end of scan (+50 degree scan angle), different colors show detectors 1-10.
The OBPG applies an additional vicarious calibration to VIIRS during Level-2 processing (Franz et al. 2007). The 869nm bands is assumed to be correctly calibrated based on prelaunch measurements and absolute calibration to the solar diffuser. The 748nm bands is then adjusted using match-ups from the South Pacific Gyre, to force the aerosol type retrievals to match, on average, the aerosol type observed at the Tahiti AERONET site. The calibration of 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.
A major change in this reprocessing is to make use of the updated MOBY time-series, which was reprocessed by the MOBY Operations Team (MOT) to incorporate advancements in MOBY instrument calibration and characterization as discussed in the R2018 General Description.
The updated vicarious gains for this reprocessing are reported in the table below.
The product algorithms, product suites, and data formats are identical to those employed in the R2014.0 MODISA reprocessing.
Impact and Quality Assessment
Comparison with In Situ Measurements
Validation of the remote sensing reflectance (Rrs) and chlorophyll retrievals was performed relative to all available in situ match-ups from SeaBASS and the Aerosol Robotic Network - Ocean Color (AERONET-OC), where only AERONET-OC Rrs data of quality level 2 were considered. Statistical analysis and scatter plots of the satellite to in situ match-ups are provided below, with results from R2014 also shown for reference. The R2018 Rrs retrievals show a reduction in mean absolute error (MAE) and mean bias (MB) relative to in situ measurements, especially in the 412 channel. This is primarily an impact of the revised MOBY measurements. Corresponding improvement in agreement was also found beween MODIST-retrieved and in situ chlorophyll, with a reduction in mean bias.
Rrs Validation Statistics for R2014 Reprocessing
Rrs Validation Statistics for R2018 Reprocessing
Franz, B.A., E.J. Kwiatkowska, G. Meister, & C.R. McClain (2008). Moderate Resolution Imaging Spectroradiometer on Terra: limitations for ocean color applications, Journal of Applied Remote Sensing, 2(1), 023525. http://dx.doi.org/10.1117/1.2957964
Franz, B. A. (2009). Methods for Assessing the Quality and Consistency of Ocean Color Products.
Kwiatkowska, E.J., B.A. Franz, G. Meister, C.R. McClain, X. Xiong (2008). Cross-Calibration of ocean color bands from Moderate Resolution Imaging Spectroradiometer on Terra platform. Applied optics, 47: (36) 6796-6810. http://dx.doi.org/10.1364/AO.47.006796