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OLCI Diffuse Attenuation Coefficient at 490 nm (Kd490)

OLCI Kd490
Figure: OLCI Kd490, Florida East Coast at Cape Canaveral, October 16, 2017, 300 m

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 (Kd490) 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 Kd490 represents the rate which light at 490 nm is attenuated with depth. For example a Kd490 of 0.1/meter means that light intensity will be reduced one natural log within 10 meters of water. Thus, for a Kd490 of 0.1, one attenuation length is 10 meters. Higher Kd490 value means smaller attenuation depth, and lower clarity of ocean water.

CoastWatch provides Kd490 products in near-real time from the Ocean and Land Colour Instrument (OLCI) on-board the European Sentinel-3 satellites. These data offer a spatial resolution advantage over other satellite missions because of their 300 m spatial resolution, as opposed to a typical 1 km spatial resolution of ocean color products from other satellite sensors.

Sentinel-3 is a multi-instrument and multi-satellite mission to measure sea- and land-surface temperature, ocean and land color and sea-surface topography. Sentinel-3A was launched on 16 February 2016. Sentinel-3B is expected to launch in 2017 with the same instruments. See the European Space Agency (ESA) Sentinel-3 website for more information. The Sentinel-3 satellites have a mid-morning local overpass time (roughly 10 AM local time).

The Ocean and Land Colour Instrument (OLCI) is based on heritage from ESA's Envisat satellite Medium Resolution Imaging Spectrometer (MERIS) to measure visible and thermal infrared radiances. OLCI has 21 bands at wavelengths from 400 nm to 1020 nm, a spatial resolution of 300 m, and a swath width of 1270 km. When both Sentinel-3 satellites are on orbit, the combined swath width of both OLCI instruments will allow near global coverage to be obtained on a daily basis. Kd490 is generated using the MERIS 1999 Baseline Atmospheric Correction and the European OLCI M07 Kd490 algorithm. CoastWatch converts ESA's Kd490 swath data (Level-2) into mapped gridded files (Level-3), including mapped swaths and daily composites.

ESA states that OLCI Kd490 compares within 5% of MODIS-Aqua Kd490 in oligotrophic water when averaging spatially and temporally, but this percentage increases gradually in mesotrophic and eutrophic (turbid, nutrient-rich) water. Validation of OLCI Kd490 with in-situ data is still preliminary. Users are cautioned that coastal Kd490 values from OLCI may be inaccurate.

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