Abstract:
The Moderate Resolution Imaging Spectroradiometer (MODIS) is the key instrument aboard the Terra (EOS AM) and Aqua (EOS PM) satellites. It is an optical scanner that views the Earth in 36 channels with spatial resolution ranging from 250 meters to 1 kilometer.
The MODIS-Aqua Level 1A files that are produced, utilized, and distributed by the Ocean Biology Processing Group (OBPG) are subsetted to remove bands and data that are not utilized for Ocean (SST and Ocean Color) product generation. The Level 1B data set contains calibrated and geolocated at-aperture radiances generated from Level 1A sensor counts and GEO files.
Each Level-2 &granule& represents 5 minutes of Terra or Aqua viewing. The ocean color products are only collected in daylight, resulting in 144 granules per day. The sea surface temperature products are collected both day and night, resulting in 288 granules per day. Pixels have 1 km spatial resolution. There are 1354 pixels across the granule, and 2030 rows down the granule. Each Level-2 product is generated from the corresponding Level-1B product. Eleven ocean color parameters and three sea surface temperature parameters are produced. Each pixel also has up to 32 flags and masks that indicate the presence of non-ocean data, algorithm failures and warnings.
At Level-3, the products can be generalized into three functional groups: binned 4.6 km, mapped 4.6 km, and mapped 9 km.
Level-3 binned products are global products. Spatial bins have 4.63 km spatial resolution in a sinusoidal equal-area grid. Only bins with data values are present; land bins and bins with no data are not stored in the files. All 11 ocean color parameters and three sea surface temperature parameters are available in multiple temporal composites (Daily, Weekly, Monthly, Seasonal, Yearly, Cumulative). Each Level-3 binned Ocean Color granule contains only data that have met data quality standards as indicated by the Level-2 flags and masks. SST products can contain data of varying data quality levels, but each bin will only contain the best quality data available for the time period.
The Level-3 standard mapped products are global products as well. Grid points for the entire globe are present in the data granules, including fill values for land and unfilled data points. Spatial resolutions of 4.6 and 9.2 km are available. Five ocean color parameters and three sea surface temperature parameters are available in multiple temporal composites (Daily, Weekly, Monthly, Seasonal, Yearly, Cumulative). Maps are in an equal-angle latitude-longitude grid (cylindrical equidistant map projection).
Parameters 1-6 are normalized water leaving radiances at 412, 443, 488, 531, 551, and 667 nm. These are radiances received at spacecraft altitudes modified principally by solar irradiance scattering and absorption from the air (molecular (Rayleigh) scattering), from particles suspended in the air (aerosol (Mie) scattering), ozone (selective absorption), sea surface reflections (Sun and sky glitter), and finally, from within the ocean, water molecules and dissolved and suspended particles. Derived water-leaving radiances normalized to a constant sun angle, are essential information for the production of most other MODIS ocean products. The MODIS water-leaving radiance algorithm builds upon both CZCS and SeaWiFS algorithms, taking advantage of improvements in the instrument. For more details please see: http://oceancolor.gsfc.nasa.gov/DOCS/MSL12/
Parameter 7 is the chlorophyll-a concentration. The algorithm used is an empirical 3-band formulation (OC3M) developed by O'Reilly et al, based on the maximum ratio of either 443 nm or 488 nm to 551 nm surface reflectance. The coefficients in the formula are based on analysis of the NASA bio-Optical Marine Algorithm Data set (NOMAD) ( http://seabass.gsfc.nasa.gov/ ). For more details of the empirical chlorophyll algorithm please see: http://oceancolor.gsfc.nasa.gov/cgi/postlaunch_tech_memo.pl?11
Parameter 8 is the ocean water attenuation coefficient, or diffuse attenuation coefficient at 490 nm (K490). The summary for parameter 7 is relevant here, as well.
Parameters 9-11 are ocean aerosol properties: aerosol optical thickness (AOT) at 869 nm (Tau 869), epsilon 748nm/869nm, and angstrom coefficient 531nm/869nm. AOT is a measure of atmospheric aerosol concentration and the effect that they have on the transmission of light. Epsilon is an index describing the two aerosol models selected by the atmospheric correction algorithm. The angstrom coefficient describes the spectral behavior and size distribution of the aerosol particles in the AOT.
Sea-surface temperatures are produced as follows: Day using MODIS channels 31 and 32 (11 micron), Night using MODIS channels 31 and 32, and Night using MODIS channels 22 and 23 (4 micron). SST product determination is based on infrared retrievals of ocean temperature which are corrected for atmospheric absorption. Processing details can be found at http://oceancolor.gsfc.nasa.gov/DOCS/modis_sst/
