Initial NASA Processing of MERIS for Ocean Color

1. Introduction

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. This document describes the initial processing of MERIS RR data by NASA, which is denoted as 2012.0.

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.

3. Vicarious Calibration

For all sensors, the OBPG applies an additional vicarious calibration during Level-1B to Level-2 processing (Franz et al., 2007). A vicarious calibration of MERIS using NASA algorithms was previously developed by Melin et. al 2011 in support of the ESA Climate Change Initiative for Ocean Color (OC-CCI) Project. In that work, the near-infrared (NIR) bands were taken as-is from the instrument calibration. The visible band were then adjusted to produce retrievals that match, on average, the water-leaving radiances derived from coincident measurtements of the Marine Optical Buoy (MOBY) near Lanai, Hawaii (currently used for SeaWiFS and MODIS). This vicarious calibration was applied in the NASA MERIS 2012.0 processing.

4. Processing Algorithms and Derived Products

For the ocean color processing of MERIS, 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 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. Expansion of the standard product suite to include photosynthetically available radiation (PAR), particulate organic and inorganic carbon (poc and pic), 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.

5. 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.

6. Results

For a comparative analysis of MERIS 2012.0 products and trends relative to the NASA ocean color time-series, see: