ACSPO Global SST from AVHRR GAC

AVHRR GAC Reanalysis 2 (RAN2) dataset is an improved and extended version of AVHRR GAC RAN1 (Ignatov et al., 2016). RAN2 is produced from 5 AVHRR/2s (onboard N07/09/11/12/14) and 5 AVHRR/3s (N15/16/17/18/19) using the NOAA Advanced Clear-Sky Processor for Ocean (ACSPO) enterprise system v2.81. At the time of release, it covers from 1 Sep 1981 to 18 Oct 2021 and will be periodically extended as new L1b data from N15/18/19 arrive. Out of ten satellites, seven (N07/09/11/14/16/18/19) were initially launched into afternoon orbits (1:30pm/am), two (N12/15) into early-morning orbits (7:30pm/am), and one (N17) into mid-morning orbit (10am/pm). All NOAA orbits are not controlled and evolve in time. The RAN2 dataset is documented in (Petrenko et al. 2020 and 2021; Pryamitsyn et al. 2020).

AVHRR FRAC onboard three Metop-First Generation (FG) satellites have been processed separately. AVHRR FRAC RAN1 dataset is available at  ACSPO Global SST from AVHRR FRAC.

The data are reported in GHRSST Data Specifications v2 (GDS2) format, in two projections: swath (L2P) and 0.02° gridded L3U (U=uncollated). Both products are aggregated into 10min granules, 144 per day, with a total daily data volumes of 0.27GB/L2P and 0.32GB/L3U. ACSPO retrievals are made in full AVHRR swath (~2,800 km). For data assimilation applications (such as production of L4 analyses, especially those blending satellite and in situ data), subtraction of the Sensor-Specific Error Statistics (SSES; Petrenko et al., 2016) biases reported in ACSPO files, is recommended.

In each valid water pixel (defined as ocean, sea, lake or river, up to 5km inland), the following layers are reported in both L2P and L3U: SSTs derived using Non-Linear SST (NLSST) algorithms (Petrenko et al., 2020); ACSPO clear-sky mask (ACSM; provided in each pixel as part of l2p_flags; Petrenko et al., 2010); SSES bias and standard deviation (Petrenko et al., 2016); MERRA-2 wind speed; and ACSPO SST minus reference SST (ESA CCI L4 SST from 1 Sep 1981 - 31 Aug 1991; switched to CMC L4 SST from 1 Sep 1991 to present time). In "invalid" pixels (defined as those with >5km inland), fill values are reported.

Only ACSM "confidently clear" pixels (equivalent to GDS2 "quality level"=5; QLs are also reported for each pixel) should be used. The ACSM also provides day/night, land, ice, twilight, and glint flags.

Both L2P and L3U SSTs are monitored and validated against in situ data  iQuam (Xu and Ignatov, 2014) in SQUAM (Dash et al., 2010) and ARMS (Ding et al., 2017) systems, and BTs are monitored in MICROS (Liang and Ignatov, 2011). The AVHRR sensors calibration and characterization, and orbital stability of NOAA satellites, are monitored in the NOAA 3S system (He et al., 2016)