The National Oceanic and Atmospheric Administration (NOAA-21) Visible Infrared Imaging Radiometer Suite (VIIRS) NASA Level-2 (L2) Cloud Mask is one of four continuity products designed to sustain the long-term records of both Moderate Resolution Imaging Spectroradiometer (MODIS) and VIIRS heritages. CLDMSK_L2_VIIRS_NOAA21 is the shortname for the NOAA-21 VIIRS incarnation of the Cloud Mask continuity product derived from the MODIS-VIIRS cloud mask (MVCM) algorithm, which itself is based on the MODIS (MOD35) algorithm. MVCM describes a continuity algorithm that is central to both MODIS data (from Terra and Aqua missions) and VIIRS data (from Suomi National Polar-orbiting Partnership (SNPP) and Joint Polar Satellite System (JPSS) missions). Upon becoming operational, the JPSS missions are renamed with a “NOAA” moniker followed by a series number, e.g., “NOAA-21” as in the case of this product. Please bear in mind that the term MVCM does not appear as an attribute within the product's metadata. Implemented to consistently handle MODIS and VIIRS inputs, the NOAA-21 VIIRS v1.0 collection products use calibration-adjusted NASA VIIRS L1B as inputs. The nominal spatial resolution of the NOAA-21 VIIRS L2 Cloud mask is 750 meters.

The L2 netCDF product, acquired and processed every 6 minutes, contains the following three data groups and Science Data Set (SDS) layers in each group:

Geophysical data

• VIIRS Clear Sky Confidence
• VIIRS Cloud Mask and Spectral Test Results
• VIIRS Integer Cloud Mask
• Quality Assurance for VIIRS Cloud Mask

Geolocation data

• Latitudes of pixel locations
• Longitudes of pixel locations
• Sensor azimuth angle at pixel locations
• Sensor zenith angle at pixel locations
• Solar azimuth angle at pixel locations
• Solar zenith angle at pixel locations

Scanline attributes

• Scan start time (TAI93)

Consult the MODIS-VIIRS Cloud Mask Product User Guide for additional information regarding the transition from the MODIS-MOD35 to the MVCM algorithm, inputs to MVCM, output files, file naming conventions, quality flags, and validation. Users are also encouraged to consult the MOD35 Algorithm Theoretical Basis Document (ATBD) and the MODIS Atmosphere QA Plan for relevant information that are applicable to the NOAA-21 VIIRS Cloud Mask product.