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The diffuse attenuation coefficient in water indicates how strongly light intensity at a specified wavelength is attenuated within the water column. This parameter has wide applicability in ocean optics, as it is directly related to the presence of scattering particles in the water column, either organic or inorganic, and thus is an indication of water clarity. The diffuse attenuation coefficient at 490 nm (K490) indicates the turbidity of the water column - how visible light in the blue to green region of the spectrum penetrates within the water column. The value of K490 represents the rate which light at 490 nm is attenuated with depth. For example a K490 of 0.1/meter means that light intensity will be reduced one natural log within 10 meters of water. Thus, for a K490 of 0.1, one attenuation length is 10 meters. Higher K490 value means smaller attenuation depth, and lower clarity of ocean water. CoastWatch provides MODIS K490 products in near-real time from global data collected by the NASA Goddard Space Flight Center (GSFC). NOAA obtains Level-0 data from NASA GSFC in 5-minute granules. These Level- 0 datasets are processed to geolocated, calibrated radiances (Level 1b) and derived MODIS data products (Level 2) using NASA SeaDAS 'msl12' software. These products are then mapped (Level 3) to the CoastWatch geographic regions. Further details on the algorithm for K490 can be found from the NASA Ocean Color website at: http://oceancolor.gsfc.nasa.gov/PRODUCTS/k490.html.
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