NASA 2012.1 Reprocessing of MERIS for Ocean Color
Through a recent data exchange agreement between NASA and ESA, the OBPG acquired the full MERIS global reduced resolution (RR) dataset spanning the operational period of the MERIS/Envisat mission from March 2002 to April 2012, and acquisition of all available full-resolution (FR) data is in progress. The OBPG is now developing the capability to process and distribute the MERIS data using the same algorithms and distribution mechanisms currently applied to NASA ocean color missions such as MODIS and SeaWiFS. The goal of this activity is to produce global Level-2 and Level-3 ocean color products from MERIS that are fully compatible with products from NASA heritage sensors, to facilitate data intercomparison studies and support multi-sensor applications of the international ocean color time-series. The initial processing of MERIS RR data by NASA was denoted as 2012.0 and effectively ran the MERIS data through the NASA ocean color processing codes "as-is", using a previously published vicarius calibration (Melin et al 2011). In the 2012.1 reprocessing described here, a changes weas made to the atmospheric correction processing, and an updated vicarious calibration was derived, resulting in significant improvement in relative agreement with the SeaWiFS and MODIS-Aqua ocean color record.
2. Instrument Calibration
The MERIS global RR data (nominal 1-km resolution) was acquired from ESA as Level-1B calibrated radiances following the MERIS 3rd reprocessing by ESA. The Level-1B source data was not modified for this reprocessing.
3. Atmospheric Correction Algorithm
For the ocean color reprocessing of MERIS by NASA, the OBPG is using the standard NASA atmospheric correction, i.e., the same algorithm and software that are currently used in standard processing of MODIS, SeaWiFS, and other ocean color sensors. To account for the sensor bandpass differences, the detector-averaged MERIS relative spectral response functions were used to derive the nominal center wavelengths and band-averaged atmospheric parameters following the same bandpass integration procedures and sources used for all other NASA'supported ocean color sensors. MERIS-specific Rayleigh and aerosol tables were then generated using the same vector radiative transfer code and methods (Ahmad et al., 2010).
The standard NASA atmospheric correction uses two bands in the near infrared (NIR) to derive the contribution of aerosols to the observed reflectance. In the initial processing of MERIS RR data by NASA, the 754nm and 865nm bands were used for this purpose, as they are closest to the standard MODIS and SeaWiFS atmospheric correction bands. The 754nm band, however, is a narrow, low gain band designed for clouds, and it was found to be highly sensitive to a change in the instrument operational configuration in late 2004 to mid-2006 where the radiometric offset control loop was disabled (OCL-off). In the NASA 2012.1 reprocessing of MERIS, the aerosol selection was switched to use the 779nm and 865nm band pair.
4. Vicarious Calibration
For all sensors, the OBPG applies an additional vicarious calibration during Level-1B to Level-2 processing (Franz et al., 2007). For the 2012.1 reprocessing, the vicarious calibration was recomputed to be consistent with the atmospheric correction algorithm. The near infrared channel at 865nm was assumed to have a gain of 1.0, and the 779nm channel was adjusted to force average agreement of the aerosol type (Angstrom exponent) at the South Pacific Gyre with AERONET measurements at Tahiti. Using this NIR calibration for the atmospheric correction, all visible bands were than calibrated to force average agreement of water-leaving radiance measurements with the Marine Optical Buoy (MOBY). The 762nm oxygen absorption band and the NIR bands longward of 865nm were not calibrated. The table below shows the resulting vicarious gains.
5. Derived Products and Algorithms
The OBPG is currently producing a subset of the standard NASA Level-2 ocean color product suite. This includes the water-leaving "remote sensing" reflectance (Rrs) in each visible spectral band, aerosol optical thickness at 865nm (aot_865), aerosol Angstrom exponent at 443nm relative to 865nm (angstrom), chlorophyll concentration (chlor_a) based on the OC4 algorithm, and marine diffuse attenuation (Kd_490) based on the KD2 algorithm. Note that the chlor_a and Kd_490 algorithms were specifically tuned for the nominal center wavelengths listed above. In this reprocessing, the particulate organic and inorganic carbon (poc and pic) products were also included, but with minimal verification as to algorithm performance for MERIS band centers. Expansion of the standard product suite to include photosynthetically available radiation (PAR) and chlorophyll fluorescence is anticipated. Generation of additional evaluation products, e.g., inherent optical properties, is also likely as confidence is gained in the quality of the Rrs retrievals.
6. File Formats and Naming Conventions
The standard MERIS Level-1B (as obtained from ESA) and the NASA Level-2 ocean color products are being distributed through the Level-1/2 Browser and via the online Ocean Color Archive for direct access and bulk download. The Level-1B products are identical to those distributed by ESA, and they are still in the Envisat N1 format.
The Level-2 ocean color files produced by NASA follow the naming convention Myyyydddhhmmss.L2_RR_OC, where yyyy is year, ddd is day, hh is hour, mm is minute, and ss is second, and the date/time indicates the observation time of the first line of the granule. The file format is the same Level-2 format (HDF4) used for all other NASA ocean color sensor products. The OBPG is also distributing MERIS Level-3 products derived from the NASA Level-2 processing of the RR data. The Level-3 products are binned to 4.6-km spatial resolution (identical to MODIS), composited to daily, 8-day, monthly, and seasonal products, and distributed in the same binned and mapped HDF4 formats as other NASA'sensors. The Level-3 binned and mapped products are available from the evaluation tab of the Level-3 Browser and directly from the Ocean Color Archive.
For a comparative analysis of MERIS 2012.1 products and trends relative to the NASA ocean color time-series, see:
In situ match-ups coming soon.