MODIS/Aqua Ocean Reprocessing #1
The Ocean Biology Processing Group (OBPG) has recently completed a series of calibration and algorithm modifications and evaluations in preparation for the first complete reprocessing of ocean products derived from the MODIS/Aqua mission. The evaluations have included a variety of alternate instrument calibrations developed in collaboration with the MODIS Characterization Support Team (MCST), and the most promising of these alternate instrument calibrations is being proposed for this reprocessing. Other recommended changes include updates to the atmospheric tables and minor enhancements to the atmospheric algorithms, as well as modifications to the definition of normalized water leaving radiance (nLw). Any algorithm changes adopted for this reprocessing of MODIS/Aqua will likley be proposed for the next reprocessing of SeaWiFS, OCTS, and CZCS (where applicable).
The OBPG makes use of a number of standard methods to evaluate the quality of ocean color products and assess the impact of calibration and algorithm changes. A description of the analysis techniques and procedures is available here.
Status of Current Operational Products
While the initial OBPG processing of MODIS/Aqua addressed a number of significant errors, some remaining issues and unexplained inconsistencies between MODIS and SeaWiFS products were still evident. These included:
- Inconsistency in the annual cycle of global water-leaving radiances
- Long-term change and seasonal deviations relative to SeaWIFS global trends
- Differences relative to SeaWiFS which increase with latitude or solar zenith angle
- Very large differences relative to SeaWiFS in global mean aerosol optical thickness retrievals
In addition, and of more immediate concern, is the significant deviation from sensor-to-sensor agreement in recent months, due to the use of an out-dated MODIS/Aqua solar diffuser calibration. The latest comparisons between MODIS/Aqua and SeaWiFS operational products are posted here.
- Updated instrument calibration
- New Level-2 straylight flag set and straylight masked in Level-3 products
- Updated atmospheric tables and Rayleigh pressure corrections
- Normalization of Lw for reflection/refraction effects on solar irradiance
- Normalization of Lw for nonisotropic upwelling radiance (f/Q)
- New products
Evaluation of Proposed Reprocessing Configuration
We believe that most of the proposed changes should be adopted for the next reprocessing of MODIS/Aqua, and that the major issue will be whether to choose the configuration with f/Q correction or without f/Q correction.
For a quick overview of results:
For more details:
- Validation against in situ match-ups
- Trend analysis for proposed configuration #1, all changes except f/Q
- Trend analysis for proposd configuration #2, all changes including f/Q
- Effect of f/Q correction on MODIS/Aqua Trends
- Effect of f/Q correction on SeaWiFS Trends
The evaluation time-series datasets, in the form of Level-3 SMI products for SeaWiFS and MODIS, with and without f/Q applied, can be made available by request. However, if you just want images, they are available from the "Trend analysis" pages listed above.