As part of the Integrated Marine Observing System (IMOS), the Australian Bureau of Meteorology produce high-resolution satellite sea surface temperature (SST) products over the Australian region, designed to suit a range of operational and research applications. All these products follow the latest International Group for High Resolution Sea Surface Temperature (GHRSST: www.ghrsst.org) file formats, assisting international data exchange and collaboration.
The highest spatial resolution (1 km x 1 km) data from Advanced Very High Resolution Radiometer (AVHRR) sensors on NOAA polar-orbiting 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 ground-stations located in Darwin, Townsville, Melbourne, Hobart, Perth and Alice Springs and in Antarctica at Casey and Davis Stations.
The Bureau of Meteorology, in collaboration with CSIRO Marine and Atmospheric Research, is combining raw data from the various ground-stations and producing real-time HRPT AVHRR skin (~ 10 micron depth) SST data files in the GHRSST GDS v2.0 L2P (single swath, geolocated), L3U (single swath, gridded), one and three day daytime/night-time L3C (single sensor, multiple swath, gridded) and one, three and six day daytime/night-time L3S (multiple sensors, multiple swath, gridded) formats. The L2P, L3U, L3C and L3S files for NOAA-15, 17, 18 and 19 satellite data are available through the IMOS FTP server (ftp://aodaac2-cbr.act.csiro.au/imos/GHRSST), IMOS AO-DAAC (http://www.marine.csiro.au/remotesensing/imos/aggregator.html# ) and IMOS Ocean Portal (http://imos.aodn.org.au/webportal/), and will eventually be available through the GHRSST Global Data Assembly Centre (http://ghrsst.jpl.nasa.gov). Archived raw HRPT AVHRR data from Australian and Antarctic ground-stations back to 1992 will be progressively reprocessed into skin SST L2P, L3U, L3C and L3S files and be available to GHRSST and IMOS by June 2013.
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 60 day match-ups with drifting buoy SST data. Note that the closer an SST pixel is to cloud, the higher the standard deviation. Therefore, the presence of these quality level flags and error information enable users to tailor the L2P, L3U, L3C or L3S files for their particular research application by trading SST spatial coverage for accuracy and vice versa. Users have the ability to access L3U, L3C and L3S 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 and the Bureau of Meteorology’s daily regional and global SST analyses (RAMSSA and GAMSSA).
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 0.3°C and during daytime of around 0.4°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 AVHRR GHRSST products see Paltoglou et al. (2010) (http://imos.org.au/srsdoc.html). Enquiries can be directed to Helen Beggs (h.beggs(at)bom.gov.au).
All the IMOS satellite SST products are supplied in GHRSST netCDF format and are either geolocated swath ("L2P") files or level 3 composite, gridded files that will have gaps where there were no observations during the specified time period. The various L3U (single swath), L3C (single sensor, multiple swath) and L3S (multiple sensors, multiple swaths) are designed to suit different applications. Some current applications of the various IMOS satellite SST products are:
HRPT AVHRR data:
L2P: Ingestion into optimally interpolated SST analysis systems (eg. RAMSSA, GAMSSA, G1SST, ODYSSEA);
L3U: Calculation of surface ocean currents (IMOS OceanCurrents);
L3C: Estimation of diurnal warming of the surface ocean (GHRSST Tropical Warm Pool Diurnal Variation (TWP+) Project);
L3S: Estimation of likelihood of coral bleaching events (ReefTemp II).
L3P: Legacy 14-day Mosaic AVHRR SST which is a weighted mean SST produced daily from multiple NOAA satellites in a cut-down GHRSST netCDF format. This product is still used in a coral bleaching prediction system run at CMAR. The product is produced using the legacy BoM processing system and is less accurate than the new IMOS L3S product.
Geostationary satellite MTSAT-1R data:
L3U: Hourly, 0.05 deg x 0.05 deg SST used for estimation of the diurnal warming of the surface ocean and validation of diurnal warming models (TWP+ Project).
