Toward the sea surface temperature climate data record from space: 4km AVHRR Pathfinder SST

With today's modern satellite sensors, many oceanic parameters are capable of being observed from space. Of these, only sea surface temperature (SST) from the Advanced Very High Resolution Radiometer (AVHRR) series on board the NOAA polar-orbiting satellites enjoys an unbroken, nearly 30-year long history of observation from the same class of instrument. These AVHRR observations date back to 1981, when the first five-channel AVHRR flew on board the NOAA-7 platform. The AVHRR series continues to fly today, and will continue perhaps another ten years into the future on board the remaining NOAA polar orbiting satellites and the European MetOp platforms.

The 4 km AVHRR Pathfinder Version 5 SST Project is a reanalysis of the AVHRR data stream developed by the University of Miami's Rosenstiel School of Marine and Atmospheric Science (RSMAS) and the NOAA National Oceanographic Data Center (NODC). This reanalysis is targeted at producing an even more accurate, consistent, and finer resolution SST data set from the AVHRR sensor series. Production, distribution and long-term stewardship of Pathfinder Version 5 data is a joint RSMAS/NODC effort. RSMAS processes the data from Level 0/Level 1b to Level 3, including a transformation from the integerized sinusoidal grid to the regularly gridded data. These data are then transferred to NODC and subjected to additional, rigorous quality assurance routines, involving browse image generation, visual inspection of every image, and comparison against the UK Met Office's Hadley Centre SST Version 2 (Rayner et al., 2006). Once cleared through these steps, the data are then archived at NODC and provided to users through http, ftp and OPeNDAP services. Access to the entire Pathfinder Version 5 collection is available at http://pathfinder.nodc.noaa.gov. Currently, the Version 5 Pathfinder collection consists of 4 km data for the entire AVHRR record, from September of 1981 through the end of 2009.

Due to its fine spatial and temporal resolution, global coverage, and long record, Pathfinder Version 5 is used by a wide array of applications. These include scientific studies in climate science, oceanography, and meteorology, as well as the specific needs of industries such as tourism and environmental consulting. One notable application in marine ecology is the Coral Reef Temperature Anomaly Database (CoRTAD). NODC partnered with the University of North Carolina Chapel Hill to use Pathfinder data to address gaps in the understanding of the relationship between coral reef temperatures, thermal stress, and climate change. The CoRTAD contains a collection of SST and related thermal stress metrics, developed specifically for coral reef ecosystem applications but relevant to other ecosystems as well. It uses the global Pathfinder Version 5 collection on a weekly timescale to quantify thermal stress patterns on the world's coral reefs from 1982-2008. NODC makes this and other value-added products developed from Pathfinder data available online as well.

Despite the improvements implemented over the years, problems still remain in Pathfinder Version 5. These errors are being corrected, and implemented in a new, Version 6 of Pathfinder. Version 6 improves upon Version 5 in several notable ways. First, known errors in the land mask are corrected, and Version 6 utilizes the higher resolution Reynolds Daily ¼-degree Optimum Interpolation SST (DOISST) product as a first guess and reference SST field for the entire record. Use of the finer resolution DOISST is resulting in improved ability of the quality procedures to properly identify cloudy pixels and to retain good pixels in the vicinity of coasts and strong gradient regions. Sea ice information is also used to a greater extent in Version 6, and is provided as part of the data set. Many spatial and seasonally varying biases evident in the Pathfinder Version 5 comparisons with HadSST2 are also minimized through the use of new coefficients generated using a latitudinal band scheme.

Significantly, Version 6 will also become a fully operational Group for High Resolution Sea Surface Temperature product. GHRSST is an international group of scientists striving to produce the highest quality, highest resolution sea surface temperature products in a standard, well-documented format for use in a wide variety of applications by the operational oceanographic, meteorological, climate and general scientific community. Pathfinder Version 6 will serve that community in delayed mode, with a product spanning the longest time period in the GHRSST program (along with the DOISST). Pathfinder Version 6 will conform to the newest GHRSST data format, data content, and metadata requirements, known as the GHRSST Data Specification Version 2 (GDS v2). The GDS requires each SST pixel value to have an associated bias and uncertainty error, as well as several other auxiliary fields relevant to the quality of the SST retrieval. It also requires the use of netCDF with Climate and Forecast metadata attributes. For the first time, Pathfinder SST data will be made available in not just collated L3 files, but also at the L2 swath and uncollated L3 processing levels.

Another ongoing addition to Version 6 is the inclusion of a large collection of High Resolution Picture Transmission (HRPT) and Local Area Coverage (LAC) data collected at stations around the world. These full-resolution, 1 km data sets will provide key inputs to ultra-high resolution analysis systems and other fine scale applications. HRPT passes covering the east and west coats of the United States, the western Pacific off of Japan, and the waters around Australia, have already been identified for inclusion into the Pathfinder Version 6 processing system, and other HRPT station data are being actively sought.

These improvements will enable an even greater range of science and applications, enhance the compatibility of Pathfinder data with numerous other GHRSST-compliant data streams, and will result in a more accurate, consistent, and useful climate data record for SST.