These interim 2007 data were collected during the operational period of the NOAA-18 polar orbiting satellite.
The SST data described in this metadata record were produced prior to the derivation of the most recent set of coefficients by using previous coefficients. They are therefore Interim Pathfinder Version 5.0 data and not part of the Formal Pathfinder Version 5.0 data set.
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 roughly 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.
In addition to SST values, the Pathfinder V5.0 Project makes six other parameters available, for a total of seven per time step: 1. All-pixel SST - The all-pixel SST files contain values for each pixel location, including those contaminated with clouds or other sources of error. The Overall Quality Flag values may be used to filter out these unwanted values. The SST value in each pixel location is an average of the highest quality AVHRR Global Area Coverage (GAC) observations available in each roughly 4 km bin. 2. First-guess SST - The Pathfinder algorithm uses a first guess SST provided by the Reynolds Optimally Interpolated SST Version 2 (OISSTv2) product. The OISSTv2 is also used in the quality control procedures. 3. Number of observations - This parameter indicates the number of AVHRR GAC observations falling in each approximately 4 km bin. 4. Standard deviation - This is the standard deviation of the observations in each 4 km bin. 5. Overall quality flag - The overall quality flag is a relative assignment of SST quality based on a hierarchical suite of tests. The Quality Flag varies from 0 to 7, with 0 being the lowest quality and 7 the highest. For more information regarding the suite of tests, see the Kilpatrick et al. (2001) paper cited above. 6. Mask 1 - These files contain a mask code, which along with Mask 2, can be used to determine the tests in the hierarchical suite that were passed or failed, resulting in the Overall Quality Flag. 7. Mask 2 - These files contain a mask code, which along with Mask 1, can be used to determine the tests in the hierarchical suite that were passed or failed, resulting in the Overall Quality Flag.
Note on Pathfinder Program: The Pathfinder program was jointly created by NASA and NOAA through the Earth Observing System (EOS) Program Office in 1990. The focus of the Pathfinder Program was to determine how existing satellite based data sets could be processed and used to study global change. The data sets were designed to be long time-series data processed with stable calibration and community consensus algorithms to better assist the research community. For more information, see: National Aeronautics and Space Administration (NASA). 1993. Earth Observing System (EOS) Reference Handbook, ed. G. Asrar and D. J. Dokken. Washington, D. C.: National Aeronautics and Space Administration, Earth Science Support Office, Document Resource Facility. The 4 km Pathfinder Project effort at the National Oceanic and Atmospheric Administration (NOAA) National Oceanographic Data Center (NODC) and the University of Miami's Rosenstiel School of Marine and Atmospheric Science (RSMAS) is an extension of and improvement on the sea surface temperature (SST) fields from that original NOAA/NASA AVHRR Oceans Pathfinder program.
- 5-Day Files: SYYYYDDD-EYYYYDDD, with: SYYYYDDD = starting 4-digit year and 3-digit day EYYYYDDD = ending 4-digit year and 3-digit day
- 7-Day Files: YYYYWW, with: YYYY = 4 digit year of observation WW = 2-digit week number between 01 and 52
- 8-Day Files: SYYYYDDD-EYYYYDDD, with: SYYYYDDD = starting 4-digit year and 3-digit day EYYYYDDD = ending 4-digit year and 3-digit day
- Monthly Files: YYYYMM, with: YYYY = 4 digit year of observation MM = 2-digit month between 01 and 12
- Yearly Files: YYYY, with: YYYY = 4 digit year of observation
BITCODE = Indicates the bit length of the pixel values in the file. "s" is for 16 bit files, "m" is for 8 bit files (see BITS below)
RESO = Approximate resolution in km. Set to "04" for 4 km files
AVGPERIOD = Indicates the averaging period used to create the file. AVGPERIOD may be in one of the following forms: Daily Files = "d" 5-Day Files = "5" 7-Day Files = "w" 8-Day Files = "8" Monthly Files = "m" Yearly Files = "y"
DAYNIGHT = Indicates nighttime, descending pass (1) or daytime, ascending pass (3) observations
VERSION = Indicates the version of the Pathfinder algorithm used to create the file. This can be one of the following: pfv50 = Formal Pathfinder Version 5.0 pfrt = Interim Pathfinder Version 5.0 (i.e., "Pathfinder Real Time") pfrtn17 = Interim Pathfinder Version 5.0 from NOAA-17 (these appear only in 2004 interim file names)
TYPE = Indicates the type of data stored in the file. TYPE may be one of the following: sst: Pathfinder all-pixel SST bsst: OISSTv2 first-guess SST field sdev: Standard deviation num: Number of observations qual: Overall quality value msk1: Quality mask 1 msk2: Quality mask 2 BITS = Number of bits in each pixel. Only present for 16-bit files (-16b) hdf = Indicates HDF-SDS Version 4 file format
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.
Additional notes: These data were collected through the operational period of the NOAA-18 Polar Operational Environmental Satellite (POES). A new series of polar orbiters, with improved sensors, began with the launch of NOAA-15 in May 1998 and NOAA-16 (lauched September, 2000). The newest, NOAA-17, was launched June 24, 2002 and NOAA-18 launched in May 2005. Detailed information on the POES satellites is available at the NOAA Office of Satellite Operations website at URL: <http://www.oso.noaa.gov/poes/index.htm>. A table listing which satellites were used in the generation of Pathfinder V5.0 data for specific dates is available online at: <http://www.nodc.noaa.gov/sog/pathfinder4km/userguide.html>
(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 OISSTv2 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.
NOTE: Due to an error with the AVHRR instrument on board NOAA-18, there was a problem with the daytime quality flags for NOAA-18 which resulted in removing some valid data within the sunglint region. All data from NOAA-18 were reprocessed to correct this error, including final data from day 156 of 2005 through 2006, and interim data for 2007. These new data have replaced the original data in the following accessions: 0043748 (2005 Daily), 0043749 (2005 Monthly), 0043750 (2005 5-day), 0043751 (2005 7-day), 0043752 (2005 8-day), 0043753 (2006 Daily), 0043754 (2006 Yearly), 0043755 (2006 Monthly), 0043756 (2006 5-day), 0043757 (2006 7-day), 0043758 (2006 8-day), and 0045498 (2007 interim).
Each of the seven parameter files listed for each time step contains a mapped array with 8192 elements in longitude and 4096 in latitude, plus a vector of length 8192 identifying the longitudes and a vector with 4096 values indicating the latitudes. There are also global tags describing the entire contents as well as tags describing each of the 2 vectors and 1 array. The seven parameters are stored either as 8-bit or 16-bit unsigned integers which may be converted linearly (y = mx + b) to geophysical units using a scale (i.e., slope=m) and offset (i.e., intercept=b) according to the following table: PARAMETER # BITS SCALE OFFSET UNITS "All-pixel" SST: 16 bit 0.075 -3.0 Deg C First-guess SST: 16 bit 0.075 -3.0 Deg C Standard Deviation: 16 bit 0.150 0.0 Deg C Number of Observations: 8 bit 1.000 0.0 Unitless Overall Quality Flag: 8 bit 1.000 0.0* Unitless Mask 1: 8 bit 1.000 0.0 Unitless Mask 2: 8 bit 1.000 0.0 Unitless * - Note that the offset parameter in the Overall Quality Flag files is incorrectly set to a value of 1. The value of 0 listed here is correct.
-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 ocean temperature (~10 m) is significantly influenced by local solar heating and typically varies with depth over a time scale of hours. Consequently "SST" measurements should always be referenced against a specific depth or an average over a depth range. The notation SSTdepth refers to any temperature within the water column beneath the SSTsub-skin where turbulent heat transfer processes dominate. The traditional "bulk" SST is related to this measure. SSTdepth should always be quoted at a specific depth in the water column; e.g., SST1m refers to the SST at a depth of 1m.
The SSTskin is the closest parameter actually measured by the AVHRR satellite radiometer. However, because the Pathfinder algorithm regresses the satellite-observed radiances against buoy temperatures to determine a "bulk" SST, the actual SST is akin to the SSTdepth where depth is about 1 m.