NOAA MSL12 Ocean Color, near real time, VIIRS multi-sensor SNPP + NOAA-20 daily merge

Ocean Color Level 3 VIIRS multi-sensor, daily, global, merge is produced through NOAA Multi-Sensor Level 1 to Level 2 processing system (MSL12).  Parameters from this product suite include chlorophyll-a concentration (pictured above for 15 January 2020) and diffuse attenuation coefficients (K_d(490) and K_d(PAR) (not shown). 

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 also aboard the NOAA-20 satellite launched in November 2017.  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 [Wang et al., 2013].

An excerpt from the EOS article by Mikelsons et al., 2019 explains the approach to the Level-3, multi-sensor merged, global, daily data product served here by CoastWatch at ~4 km spatial resolution in NetCDF format.  "SNPP and NOAA-20 operate along the same Sun-synchronous polar orbit that crosses the equator at about 13:30 local time—both satellites travel around Earth from pole to pole in such a way that they observe the same areas at about the same time of day, no matter the season. There is about a 50-minute delay between the paths of NOAA-20 and SNPP, so NOAA-20’s path runs between two adjacent SNPP orbital paths and vice versa. Thus, the overlap of the spatial coverages in the two VIIRS sensors automatically fills each instrument’s data gaps [Mikelsons and Wang, 2019]. In addition, ocean color data from the VIIRS SNPP and NOAA-20 have the same spatial and temporal resolution, and these data are processed using the same algorithm and software package (i.e., MSL12). Therefore, the statistics of their ocean color products are very similar, and the data can be merged into a global 9-kilometer resolution data set directly without adjustment [Liu and Wang, 2019]." 

These multi-sensor daily merged products are derived from MSL12 v1.3 for both SNPP plus NOAA-20.  The chlorophyll-a algorithm is OCI [Hu et al., 2012; Wang and Son, 2016].