NOAA MSL12 Ocean Color - Near Real Time - VIIRS single-sensor SNPP and NOAA-20

Ocean Color satellite sensors measure visible light at specific wavelengths which leaves the surface of the ocean and arrives at the top of the atmosphere where the sensor is located. From these visible spectral measurements, along with simultaneous measurements in the near infrared (NIR) and the short wave infrared (SWIR) wavelengths, the color of the ocean, or normalized water leaving radiances (nL_w), can be calculated. Then, the nL_ws are used to derive other ocean properties such as the concentration of chlorophyll-a (chlor-a or sometimes chl, which is the green pigment responsible for photosynthesis and therefore an indicator of the amount of phytoplankton biomass in the ocean water) and the coefficient of extinction for downwelling irradiance (K_d(PAR) and K_d(490) which are related to water clarity).

The ocean color datasets described here are from the Visible and Infrared Imaging Radiometer Suite (VIIRS) sensor aboard the Suomi-NPP satellite (SNPP) which was launched in November 2011 and NOAA-20 launched in November 2017.  Global life-of-mission science quality data are available beginning on 2 January 2012 for SNPP.  NOAA ocean color data are processed using NOAA Multi-Sensor Level 1 to Level 2 processing system (MSL12) developed by the NOAA/STAR Ocean Color Team. See documentation tab for relevant publications describing the system and algorithms.

The VIIRS near real time products and the science quality collection differ in several ways (Table 1). Generally, the near real time data are best suited for applications where timeliness of reasonable quality data is most important whereas science quality data are best suited for applications where consistent, best available quality are required and a ~2 week delay is acceptable.

The 2-sensor merged daily 4 km global products are derived from MSL12 v1.3 for both SNPP plus NOAA-20.  Note that sole sensor SNPP products are processed from MSL12 v1.2 and sole sensor NOAA-20 products are processed from MSL12 v 1.3.  The upgrade to MSL12 v1.3 for SNPP is planned (along with a mission-long reprocessing for science quality).  

Standard VIIRS ocean color data Level 2 products (both near real time and science quality) include:

• Normalized water-leaving (nL_w) radiance at five visible bands (nominal center wavelengths)
  • M1 (410nm)
  • M2 (443nm)
  • M3 (486nm)
  • M4 (551nm)
  • I1 (638 nm)
  • M5 (671nm)
• Chlorophyll-a concentration
• Diffuse attenuation coefficient at 490 nm (K_d490), and Diffuse attenuation coefficient of photosynthetically active radiation (K_dPAR)
• QA score (an empirical product quality assessment )

See Table 2 for quick reference matrix of additional near real time VIIRS ocean color products. The Ocean Color Granule Selector and the new CoastWatch Data Portal are online tools to interactively preview, choose and download L2 granule files. Data files are in NetCDF4/CF but differ from past NOAA CoastWatch products in their use of hierarchical groups to store attributes, variables, and dimensions. The CoastWatch Utilities Software has been updated to work with these files. Additional metadata can be obtained using THREDDS services such as ISO or OPeNDAP.

*See Figure 1 for locations of CoastWatch regions

**See Figure 2 for description and identification of sectors.

***Under development for April 2019 release.

Figure 1. Seven heritage CoastWatch "CONUS" regions (clockwise from top right: Great Lakes, NorthEast, SouthEast, Caribbean, Gulf of America, Hawaii, West Coast).

Figure 2. Twenty-four sectors identified for file naming convention. Sectors enable downloads of select subset regions from global high resolution VIIRS ocean color science quality data.