The SST climatologies are available as Hierarchical Data Format (.HDF) data files for 1985-2001 and include daily, 5-day, 7-day, 8-day, monthly, yearly, and seasonal time periods which are filed as separate NODC accessions. Four corresponding graphics (climatologies) in GeoTIFF format are provided for the seasonal climatologies. Although the data are represented using 16-bit in the .HDF files, 8-bit GeoTIFF files were developed to facilitate access and use of these data by the widest variety of users. As a result, the GeoTIFF representations of the .HDF files are not a complete representation of the data in the .HDF files.
The v4.2 algorithm offered marked improvement over operational retrieval algorithms such as MCSST and was applied to AVHRR data to maximize accuracy and to minimize artificial fluctuations arising from the sequence of AVHRR instruments flown on NOAA's polar-orbiting satellites during the past 2 decades. The 9 km v4.2 Pathfinder SSTs have already been shown to be the highest quality product currently available for the construction of global climatologies (Casey and Cornillon, 1999) and longer-term SST trend determination (Casey and Cornillon, 2001), and have been demonstrated to be accurate within about 0.3 degrees C under optimal conditions (Kearns et al., 2000). Relative to the older 9 km v4.2 Pathfinder data, the new, ~ 4 km resolution Pathfinder Version 5.0 global SSTs increase detail by a factor of four simply by virtue of the increased resolution. The increase in detail over widely used but relatively coarse SST datasets such as Optimally Interpolated SST Version 2 (OISSTv2; Reynolds et al., 2002) and the Hadley Centre's Global Sea Ice and SST (HadISST1; Rayner et al., 2003) is far greater.
In addition to the increased resolution, significant improvements have been made in the Version 5.0 which enhance the usefulness of the SST fields. Currently, these enhancements include the use of sea ice in the quality level determination scheme, inclusion of many inland water bodies, and the use of a greatly improved land mask. The greatest improvements are seen in coastal zones, marginal seas, and boundary current regions where SST gradients are often large and their impact on operational or research products is greatest. Separate SST products for daytime and nighttime AVHRR retrievals are made to better understand the differences in skin and bulk temperatures, since mean differences between AVHRR-measured skin temperatures and bulk temperatures of 0.1 to 0.2 degrees C (Schluessel et al., 1990) and locally varying differences of up to 1.8 degrees C (Minnett et al., 2000) have been observed.
Geotiff_Information: Version: 1 Key_Revision: 1.0
Tagged_Information: ModelTiepointTag (2,3): 0 0 0 -180 90 0 ModelPixelScaleTag (1,3):
0.0439453 0.0439453 0 End_Of_Tags.
Keyed_Information: GTModelTypeGeoKey (Short,1): ModelTypeGeographic GTRasterTypeGeoKey (Short,1): RasterPixelIsArea GTCitationGeoKey (Ascii,17): 'LONG/LAT E005' GeographicTypeGeoKey (Short,1): GCS_WGS_84 GeogAngularUnitsGeoKey (Short,1): Angular_Degree ProjLinearUnitsGeoKey (Short,1): Linear_Meter End_Of_Keys. End_Of_Geotiff.
GCS: 4326/WGS 84 Datum: 6326/World Geodetic System 1984 Ellipsoid: 7030/WGS 84 (6378137.00,6356752.31) Prime Meridian: 8901/Greenwich (0.000000/ 0d 0' 0.00""E)
Projection Linear Units: 9001/metre (1.000000m) Corner Coordinates: Upper Left (180d 0' 0.00"W, 90d 0' 0.00"N) Lower Left (180d 0' 0.00"W, 90d 0' 0.00"S) Upper Right (180d 0' 0.00"E, 90d 0' 0.00"N) Lower Right (180d 0' 0.00"E, 90d 0' 0.00"S) Center ( 0d 0' 0.00"E, 0d 0' 0.00"N)
Resource Description: NODC accession numbers 0006682 (daily), 0009534 (5-day), 0009535 (7-day), 0009536 (8-day), 0009537 (annual), 0009538 (monthly), 0001658 (seasonal)
Browse Graphic(s): Browse_Graphic_File_Name <http://www.nodc.noaa.gov/archive/arc0002/0001658/2.2/data/0-data/Combined/season1_combined.tif> Browse_Graphic_File_Description 1985-2001 averaged SST(January to March)
Browse_Graphic_File_Name <http://www.nodc.noaa.gov/archive/arc0002/0001658/2.2/data/0-data/Combined/season2_combined.tif> Browse_Graphic_File_Description 1985-2001 averaged SST(April to June)
Browse_Graphic_File_Name <http://www.nodc.noaa.gov/archive/arc0002/0001658/2.2/data/0-data/Combined/season3_combined.tif> Browse_Graphic_File_Description 1985-2001 averaged SST(July to September)
Browse_Graphic_File_Name <http://www.nodc.noaa.gov/archive/arc0002/0001658/2.2/data/0-data/Combined/season4_combined.tif> Browse_Graphic_File_Description 1985-2001 averaged SST(October to December)
- Although the data are represented using 16-bit in the .HDF files, 8-bit GeoTIFF files were developed for the seasonal climatologies to facilitate access and use of these data by the widest variety of users. As a result, the GeoTIFF representations of the .HDF files are not a complete representation of the data in the .HDF files.
- The .HDF files are 16-bit files, and pixel values can range from 0 to 65535 (2 to the 16th power). However, realistic pixel values for SST will always be less than 600 or so. Land has a value of 1. SST in degC = 0.075 x pixel value - 3. Temperatures are represented in 0.075 degC increments
- Differences in scale and offset between .HDF and .GeoTIFF files: Derived GeoTiffs (8-bit files): Pixel values can range from 0 to 255. Land has a value of 255. SST in degC = 0.15 x pixel value - 3. Temperatures are represented in 0.15 degC increments.
(a) Clock Correction To minimize error in the along track position estimated by the orbital model, a satellite a clock correction factor is applied to the time code imbedded in each piece. The method used to determine these clock correction factors is presented below. The clock aboard a given satellite drifts continually at a relatively constant rate (e.g., for NOAA-14, ~9msday-1) compared to the reference clock on Earth. Because of this drift, the NOAA/NESDIS Satellite Operation Control Center periodically sends a command to the satellite to reset the on-board clock to a new baseline thereby eliminating the accumulation of a large time offset error between the Earth and satellite clocks. To correct for clock drift between these resets, correction factors were determined from a database of satellite clock time and Earth time offsets collected at the RSMAS High Resolution Picture Transmission (HRPT) receiving station. During HRPT transmission, both the satellite clock (used to create the embedded time code in each piece) and the Earth clock are simultaneously available. The clock correction bias was determined by (1) visual examination of the Earth/satellite clock differences collected in the database to locate the precise magnitude and timing of clock resets performed by the Satellite Operation Control Center and (2) recorded time differences between the identified reset periods were then filtered to remove spurious noise, and regressed against the corresponding satellite time to determine the clock drift correction. These drift corrections were then applied to all data time-stamped during a given reset period. Refer to Sea Surface Temperature Global Area Coverage (GAC) Processing Appendix A: Calibration and Navigation Correction Factors for a list of clock offsets for each NOAA spacecraft (<http://www.rsmas.miami.edu/groups/rrsl/pathfinder/Processing/proc_app_a.html>).
(b) Attitude Corrections After clock correction, a nominal attitude correction is then applied to minimize the uncertainty in regard to the direction in which the spacecraft is pointing. The nominal attitude correction applied was determined by averaging the absolute attitude of the spacecraft over many geographic locations and times along the orbital track. The method used to determine the absolute attitude of the spacecraft involves matching a digital coastal outline to a given image and recording the amount of pitch, yaw, and roll required to make the outline and land coincide. This method has the advantage that it can be performed over small geographical distances and is similar to other techniques which rely on widely separated geographical control points to anchor the navigation. The resultant navigation information, output by the SECTOR procedure for each piece, provides the mapping parameters needed to convert between the satellite perspective of pixel and scan line, and Earth-based latitude and longitude coordinates. Refer to Sea Surface Temperature Global Area Coverage (GAC) Processing Appendix A: Calibration and Navigation Correction Factors for attitude correction factors for each NOAA spacecraft (<http://www.rsmas.miami.edu/groups/rrsl/pathfinder/Processing/proc_app_a.html>).
AVHRR Pathfinder SST Processing Steps A. Ingestion, calibration, and navigation of Global Area Coverage (GAC) data a. Calibrate and convert AVHRR digital counts for channels 1 through 5 to radiances i. Obtain AVHRR channels 1 through 5 radiometer count data. ii. Channels 1 and 2 require pre-launch calibration coefficients for linear counts-to-radiance conversion, followed by a correction for temporal changes using sensor decay rate data and then a correction for inter-satellite differences using inter-satellite standardization data to the NOAA-9 reference, both of which use Libyan desert target area data. iii. Channels 3, 4, and 5 require both the above pre-launch calibration data and onboard blackbody (space view and sensor base plate) data for non-linear counts-to-radiance conversion.
b. Navigation, Clock, and Attitude Corrections i. Satellite clock corrections need Earth time offset data based on RSMAS High-Resolution Picture Transmission data. ii. Attitude corrections are made using coastline comparison data. iii. At this point, navigated, calibrated albedos/brightness temperatures are available for all five channels. Note that channels 1-2 are not used in the Pathfinder SST algorithm, and channel 3 is used only in assignment of a quality indicator (see step B.d.i.).
B. SST Calculation a. Channel 4 and 5 brightness temperatures are converted to SST in degrees C using the Pathfinder algorithm, which requires a set of monthly coefficients. b. These coefficients are derived using the Pathfinder Buoy Matchup Database. This is a set of in situ buoy SST observations and collocated AVHRR data. c. In addition, a first-guess SST field is needed by the algorithm. This first-guess field is the Reynolds Weekly Global Optimally Interpolated SST version 2 (OISSTv2) product. Note: the older 9km Pathfinder used OISST version 1. d. Quality Flag Assignment i. A Channel 3, 4, and 5 brightness temperature test is performed. These data are already available from step A.a.iii. ii. The viewing angle is evaluated using a satellite zenith angle check. iii. A reference field comparison check is made against the Reynolds OISST used in step B.c. iv. A stray sunlight test is performed which requires information on whether the data in question are to left or right of nadir. v. An edge test is performed which checks the location of the pixel within a scan line and the location of the scan line within the processing piece (a piece is a subset of an entire orbit file). vi. A glint test is performed which requires a glint index calculated according to the Cox and Munk (1954) formulation. vii. A sea ice mask is used to identify pixels falling on areas of sea ice. The ice mask is based on weekly SSM/I data and the ice information contained in the Reynolds OISSTv2. (Note: this step was not present in the 9 km Pathfinder reprocessing and is used only in the 4km Version 5.0 Pathfinder product.) viii. These steps are all combined into an overall quality flag assignment for each pixel.
C. Spatial Binning a. An equal-area is grid is defined into which GAC pixels are binned. No external data are needed, only information on the equal-area binning strategy itself. b. A data-day is defined following a spatial data-day definition. See <http://www.nodc.noaa.gov/sog/pathfinder4km/Data-day.pdf> for a description of the spatial data-day definition, written by Guillermo Podesta, University of Miami RSMAS. c. A land mask is applied to the dataset, identifying pixels that fall on land. This land mask was based on an old CIA database in the 9 km Pathfinder (no citation or further information is known). In the 4 km Version 5.0 Pathfinder, a new and improved land mask based on a 1 km resolution MODIS dataset derived by the USGS Land Processes Distributed Active Archive Center is used (see <http://edcdaac.usgs.gov/modis/mod12q1.html> for more info.)
D. Temporal Binning a. The spatially binned pieces from step C are accumulated into a single ascending (daytime) or descending (nighttime) file for each day. In case of overlapping satellite passes, only the best pixels of equivalent quality are binned. No external information is needed, only information about the accumulation procedure itself. Note: the new 4 km Version 5.0 Pathfinder also generates temporal averages on 5-day, 7-day, 8-day, monthly, and yearly periods. b. A final comparison is made to an internal 3-week Pathfinder comparison field. No external data are required, only knowledge of the Pathfinder reference check. c. Fields are reformatted from equal-area to equal-angle for distribution in HDF format. Note: the old 9 km Pathfinder data were distributed in HDF4 Raster format, while the new 4 km Version 5.0 Pathfinder data are distributed in HDF4-SDS format, with tiling (internally compressed chunks) enabled. d. The result of all these steps is the high-level Pathfinder SST product.
E. Additional processes required for SST climatologies: 1. The individual files generated by the Version 5.0 Pathfinder Project for 1985-2001 are averaged to create a set of initial climatologies. For example, January of 1986, January of 1987, ...., January of 2001 are averaged to create a climatological January. Only the highest quality data (overall quality flag=7) are used.
2. Then, following the steps described in Casey and Cornillon, 1999 (Journal of Climate, vol 12, pp 1848-1863): a. Apply a 7x7 median-filling which fills most of any remaining gaps in the initial monthly climatology. No already present data are modified. b. Linearly fill any remaining gaps using the previous and following monthly climatological values if they are available. No already present data are modified. c. Apply another 7x7 median-filling in case any gaps remain. No already present data are modified. d. The final step, described in Casey and Cornillon (1999), of applying a 7x7 median-filter to smooth the entire field, is not performed. Steps are modified to produce daily, -5, -7, -8, seasonal, and yearly climatologies as required.
-The interface SST, SSTint, is the temperature of an infinitely thin layer at the exact air-sea interface. It represents the temperature at the top of the SSTskin temperature gradient (layer) and cannot be measured using current technology. It is important to note that it is the SSTint that interacts with the atmosphere.
-The skin SST, SSTskin, is a temperature measured within a thin water layer (<500 micrometer) adjacent to the air-sea interface. It is where conductive, diffusive and molecular heat transfer processes dominate. A strong vertical temperature gradient is characteristically maintained in this thin layer sustained by the magnitude and direction of the ocean-atmosphere heat flux. Thus, SSTskin varies according to the actual measurement depth within the layer. This layer provides the connectivity between a turbulent ocean and a turbulent atmosphere.
-The sub-skin SST, SSTsub-skin, is representative of the SST at the bottom of the surface layer where the dominance of molecular and conductive processes gives way to turbulent heat transfer. It varies on a time scale of minutes and is influenced by solar warming in a manner strongly dependent on the turbulent energy density in the layer below.
-The near surface oce