OLCI Diffuse Attenuation Coefficient at 490 nm (Kd490)

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, courtesy of the European Union's Copernicus satellite program. 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. The Sentinel-3A satellite was launched on 16 February 2016. Sentinel-3B was launched on 25 April 2018 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 Medium Resolution Imaging Spectrometer (MERIS) instrument on the Envisat satellite, designed for measuring 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. With both Sentinel-3 satellites in orbit, the combined swath width of the two OLCI instruments allows near global coverage to be obtained on a daily basis.

Water reflectances at green and blue wavelengths compare well between Sentinel-3A/OLCI and Sentinel-3B/OLCI. However, users are cautioned that Kd490 values from OLCI may be inaccurate, particularly in coastal zones.

For more information on the ocean color algorithms, atmospheric correction, and product uncertainty, please consult the EUMETSAT Ocean Colour Service website. In particular, the Sentinel-3 OLCI Ocean Colour Processing Baseline contains periodically updated Product Notices with important quality information and the latest uncertainty assessments.

CoastWatch converts ESA's Kd490 swath data (Level-2) into mapped gridded files (Level-3), including mapped swaths and daily composites. CoastWatch obtains the EUMETSAT operational data on a near real-time basis and has not yet obtained reprocessed collections. As such, the quality of the data as stored at CoastWatch will vary over the time series.

CoastWatch East Code Node provides this data for the time period: March 2018 - present.

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