VIIRS Diffuse Attenuation Coefficient at 490 nm (Kd490)

Diffuse attenuation coefficient at 490 nm (Kd490) is a measure of water turbidity or clarity, i.e. the amount of light reduction (attenuation) in the water due to scattering by particles or due to light absorption by the water itself. Higher Kd490 values mean more attenuation due to more turbid water, and lower Kd490 values mean less attenuation due to more clear water.

NOAA CoastWatch produces ocean color products, including Kd490, from the Visible Infrared Imaging Radiometer Suite (VIIRS) on-board the polar-orbiting Suomi-NPP and NOAA-20 satellites (Wang et al., 2017).

Atmospheric correction uses the near-infrared (NIR) approach of Jiang and Wang (2014), which is an iterative NIR approach combining the NIR approachs of Bailey et al. (2010), MUMM (or Ruddick et al. (2000)), and Wang et al. (2012), referred to as the BMW atmospheric correction (see Jiang and Wang, 2014, for detailed references). While short-wave infrared (SWIR) approaches, like that used for NOAA's MODIS ocean color products, generate more accurate results for coastal waters than NIR approaches, the improved BMW approach of Jiang and Wang (2014) was found to offer the most accurate results compared to other NIR approaches.

Diffuse attenuation coefficient is generated using the Wang et al. (2009) algorithm, in which separate algorithms for clear open ocean and turbid coastal waters are weighted into a combined product. This approach improves accuracy across ocean conditions. This algorithm was developed for the MODIS-Aqua instrument using in-situ data from the NASA SeaBASS database.

Validation for Kd490 has only occurred for MODIS-Aqua to date. MODIS-Aqua Kd490 data were compared with in-situ Kd490 from the NASA SeaBASS database, representing open ocean and some turbid coastal measurements. MODIS Kd490 data reasonably agreed with the in-situ measurements, with a mean ratio of 1.037 (Wang et al., 2009). To evaluate VIIRS Kd490 performance, images of seasonal averages from 2012-2013 were visually compared between MODIS and VIIRS and found to be similar (Wang et al., 2017). There is no quantitative validation of the VIIRS Kd490 data product.

CoastWatch East Coast Node currently distributes mapped gridded VIIRS Kd490 from the Suomi-NPP satellite for the U.S. east coast. Sensor nominal spatial resolution is 750 m. Data is science-quality (i.e., post-processed for increased accuracy) from the beginning of the Suomi-NPP/VIIRS mission (Jan 1, 2012) up to the current year. Current year data is of near-realtime quality (i.e. reduced accuracy for quick turn-around data within 24 hours).

• Filename pre-fix:
• VRSRCW: Science-Quality, 2012 - year prior to current year
• VRSUCW: Near-Realtime, current year