On 1 October 2010 the International Group for High Resolution Sea Surface Temperature (GHRSST: www.ghrsst.org) released its new GHRSST Data Specification Document (“GDS v2.0”), defining the format of satellite SST products to assist international data exchange and collaboration.  The Integrated Marine Observing System (IMOS) Satellite Remote Sensing Facility has adopted this format for its newly released SST products from Advanced Very High Resolution Radiometers (AVHRR) aboard NOAA polar-orbiting meteorological satellites.

The highest spatial resolution (1 km x 1 km) data from AVHRR sensors on NOAA satellites can only be obtained through receiving direct broadcast “HRPT” data from the satellite.  In Australia, HRPT data is received by a number of agencies (Bureau of Meteorology, Geoscience Australia, AIMS and CSIRO) and consortia (WASTAC and TERSS) at groundstations located in Darwin, Townsville, Melbourne, Hobart, Perth and Alice Springs and in Antarctica at Casey and Davis Stations. 

As part of the IMOS Project the Bureau of Meteorology, in collaboration with CSIRO Marine and Atmospheric Research, is combining raw data from the various groundstations and producing real-time HRPT AVHRR skin (~ 10 mm depth) SST data files in the new GHRSST GDS v2.0 L2P (single swath, geolocated), L3U (single swath, gridded) and single day/single night L3C (single sensor, multiple swath, gridded) formats.  These GHRSST files for NOAA-17, 18 and 19 satellite data are now available through the IMOS FTP server (ftp://aodaac2-cbr.act.csiro.au/imos/) and will eventually be available through the IMOS AO-DAAC (http://www.marine.csiro.au/remotesensing/imos/aggregator.html# ), IMOS Ocean Portal (http://imos.aodn.org.au/webportal/) and GHRSST Global Data Assembly Centre (http://ghrsst.jpl.nasa.gov).  Archived raw HRPT AVHRR data from Australian groundstations back to 1992 will be progressively reprocessed into skin SST L2P, L3U and L3C files and be available to GHRSST and IMOS by June 2011.

For the user, there are several advantages to using GHRSST-format SST products.  For each SST value the GHRSST files contain a quality level flag (based on proximity to cloud, satellite zenith angle and day/night) and bias and standard deviation error estimates based on match-ups with drifting buoy SST data.  The presence of these quality level flags and error information enable users to tailor the L2P, L3U or L3C files for their particular research application by trading SST spatial coverage for accuracy and vice versa.  Users now have the ability to access L3U and L3C SST products through IMOS OPeNDAP servers, greatly simplifying data access and extraction.  Providing real-time HRPT AVHRR SST files in GHRSST-L2P format enables them to be incorporated into global and regional, gap-free, analyses of L2P SST from multiple satellites such as NASA’s G1SST global 1 km daily SST analysis. 

The new IMOS AVHRR L2P SSTs exhibit approximately 75% the error of the Bureau’s pre-existing HRPT AVHRR level 2 SST data, with standard deviations compared with drifting buoys during night-time of around 0.25°C and during daytime of around 0.35°C for quality level 5 (highest). This significant improvement in accuracy has been achieved by improving cloud clearing and calibration, using regional rather than global drifting buoy SST observations and incorporating a dependence on latitude.

For further details on the new AVHRR GHRSST products see Paltoglou et al. (2010) (http://imos.org.au/srsdoc.html).

1. Select the required quality level or error threshold For best quality SST data (lowest errors) select sea_surface_temperature values where quality_level is 5.  For greater spatial coverage, but still generally cloud-free pixels, select SST values for quality_level > 2.  Alternatively, select SST values for the required sses_bias and sses_standard_deviation thresholds for your application.
   
2. If required, convert from skin to buoy depth temperatures Users can convert the AVHRR skin SST product to a drifting buoy depth (~20 cm to 30 cm) temperature via the addition of 0.17 K. The skin SST is valid at all wind speeds, but the conversion to buoy-depth SST is less accurate at low wind speeds (< 6 m/s during the daytime and < 2 m/s at nighttime) but the effect is uncertain and possibly quite small.  Refer to Paltoglou et al (2010) at http://imos.org.au/srsdoc.html for more details. 
3. Mask out land To mask out land/ice apply the l2p_flag.  Note that the Bureau uses NAVOCEANO's 1/100 degree resolution land/water mask and have no information regarding whether water is ocean, lake or river.