ACSPO Global SST from AVHRR GAC

AVHRR GAC Reanalysis 1 (RAN1) dataset was produced from AVHRR/3s using the NOAA Advanced Clear-Sky Processor for Ocean (ACSPO) v2.40 enterprise system, from 3 afternoon (NOAA-16, -18, -19) and 2 mid-morning satellites (NOAA-17, MetOp-A), two satellites at a time, from 30 Aug 2002 to 1 Nov 2018. The dataset  is documented in (Ignatov et al., 2016). Work is underway to extend the period covered by RAN1 using AVHRR/2s  to 1981.

The real AVHRR GAC which was produced until June 1 2019 by OSPO from several sensors, had been discontinued. Users interested in AVHRR SST are advised to use  ACSPO Global SST from AVHRR FRAC.

The RAN1 data are presented in hourly granule files in GHRSST Data Specifications v2 (GDS2) format, in swath projection (L2P) and 0.02° gridded L3U (U=uncollated), 24 granules per day, with a total data volume of 0.8GB/day for L2P and 0.2GB/day for L3U, respectively.

ACSPO retrievals are made in full AVHRR swath (~2,800 km). For data assimilation applications (such as production of L4 analyses, especially those that blend satellite and in situ data), correction for the Sensor-Specific Error Statistics (SSES; reported in ACSPO files; Petrenko et al., 2016) biases is strongly recommended.

In each valid water pixel, defined as ocean, sea, lake or river, up to 5km inland (note that in "invalid" pixels, defined as those with >5km inland, fill values are reported), the following layers are reported in both L2P and L3U: SSTs derived using multi-channel SST (MCSST; night) and Non-Linear SST (NLSST; day) algorithms (Petrenko et al., 2014); 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); NCEP wind speed; and ACSPO SST minus reference (Canadian Met Centre L4 SST). For L2P, brightness temperatures (BTs) in M12, 15, and 16 are also reported, for those users interested in direct "radiance assimilation" (e.g., NOAA NCEP, NASA GMAO).

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. Note that users of ACSPO data have the flexibility to ignore the ACSM, derive their own clear-sky mask, and use BTs and SSTs in those pixels. They may also ignore ACSPO SST, and derive their own SSTs from the original BTs.

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).