GHRSST GDS2 Level 2P Global Subskin Sea Surface Temperature version 2.60 from the Visible Infrared Imaging Radiometer Suite (VIIRS) on the NOAA-20 satellite created by the NOAA Advanced Clear-Sky Processor for Ocean (ACSPO) (GDS version 2) for 2019-02-18 (NCEI Accession 0185762)

NOAA-20 (hereafter, N20), also known as JPSS-1 or J1 prior to launch, is the second satellite in the US National Oceanic and Atmospheric Administration (NOAA) latest generation Joint Polar Satellite System (JPSS). N20 was launched on November 18, 2017. In conjunction with the first US satellite in JPSS series, Suomi National Polar-orbiting Partnership (S-NPP) satellite launched on October 28, 2011, N20 form the new NOAA polar constellation. NOAA is responsible for all JPSS products, including SST from the Visible Infrared Imaging Radiometer Suite (VIIRS). VIIRS is a whiskbroom scanning radiometer, which takes measurements in the cross-track direction within a field of view of 112.56 degrees using 16 detectors and a double-sided mirror assembly. At a nominal altitude of 829 km, the swath width is 3,060 km, providing global daily coverage for both day and night passes. VIIRS has 22 spectral bands, covering the spectrum from 0.4-12 microns, including 16 moderate resolution bands (M-bands).

The Level 2 Preprocessed (L2P) SST product is derived at the native sensor resolution (~0.75 km at nadir, ~1.5 km at swath edge) using NOAA's Advanced Clear-Sky Processor for Ocean (ACSPO) system, and reported in 10 minute granules in netCDF4 format, compliant with the GHRSST Data Specification version 2 (GDS2). There are 144 granules per 24hr interval, with a total data volume of 27GB/day. In addition to pixel-level earth locations, Sun-sensor geometry, and ancillary data from the NCEP global weather forecast, ACSPO outputs include four brightness temperatures (BTs) in M12 (3.7microns), M14 (8.6microns), M15 (11microns), and M16 (12microns) bands, and two reflectances in M5 (0.67microns) and M7 (0.87microns) bands. The reflectances are used for cloud identification.

SSTs are derived from BTs using the Multi-Channel SST (MCSST; night) and Non-Linear SST (NLSST; day) algorithms (Petrenko et al., 2014). An ACSPO clear-sky mask (ACSM) is provided in each pixel as part of variable l2p_flags, which also includes day/night, land, ice, twilight, and glint flags (Petrenko et al., 2010).  Fill values are reported at all pixels with >5 km inland. For each valid water pixel (defined as ocean, sea, lake or river, and up to 5 km inland), four BTs in M12, 14, 15, and 16 are reported (included for those users interested in direct "radiance assimilation", e.g., NOAA NCEP, NASA GMAO, ECMWF), along with derived SST. Other variables include NCEP wind speed and ACSPO SST minus reference SST (Canadian Met Centre 0.1deg L4 SST; available at https://podaac.jpl.nasa.gov/dataset/CMC0.1deg-CMC-L4-GLOB-v3.0).  Only ACSM confidently clear pixels are recommended (equivalent to GDS2 quality level=5; note that QL is reported for each pixel). Per GDS2 specifications, two additional Sensor-Specific Error Statistics layers (SSES bias and standard deviation) are reported in each pixel with QL=5. Note that users of ACSPO data have the flexibility to ignore the ACSM and derive their own clear-sky mask, and apply it to BTs and SSTs. They may also ignore ACSPO SSTs, and derive their own SSTs from the original BTs.

The ACSPO VIIRS L2P product is monitored and validated against in situ data (Xu and Ignatov, 2014) using the Satellite Quality Monitor SQUAM (Dash et al, 2010), and BTs are validated against RTM simulation in MICROS (Liang and Ignatov, 2011). A reduced size (1GB/day), equal-angle gridded (0.02 deg resolution), ACSPO L3U product is also available at https://podaac.jpl.nasa.gov/dataset/VIIRS_N20-OSPO-L3U-v2.60, where gridded L2P SSTs with QL=5 only are reported, and BT layers omitted.

Tags: earth-science--atmosphere--atmospheric-winds--surface-winds--wind-speed, earth-science--oceans--ocean-temperature--sea-surface-temperature, earth-science--oceans--sea-ice--ice-extent, geographic-region--global-ocean, nodc.noaa.gov, oceanography
Organization: Ocean Data Partners