16 The Past, Present, and Future of the AVHRR Pathfinder SST Program
279
measurements recorded at the University of Miami Domestic Communication
Satellite (DOMSAT, the communication satellite NOAA used to transfer AVHRR
data) station with the time information in HRPT passes received at Wallops Island,
VA, and Fairbanks, AK. Averaging over multiple geographic locations and times
along the orbital track mitigated attitude errors.
Further improvements introduced in Pathfinder Version 1 were implemented during the next phase of reprocessing, in which SST values are calculated. Three sets
of algorithm coefficients (a, b, c, and d in Equation 16.1) were generated using the
satellite-in-situ matchup database: one for high atmospheric water vapor regimes
where the T 4 − T 5 brightness temperature difference is greater than or equal to
1.8 ◦ C, one for low water vapor regimes where the T 4 − T 5 brightness temperature
difference is less than 0.7 ◦ C, and one for moderate levels of water vapor where the
T 4 − T 5 brightness temperature difference is greater than or equal to 0.7 ◦ C and less
than 1.8 ◦ C. In Version 1, the coefficients were calculated on an annual basis, using
12 months of matchup data.
Finally, an extensive set of quality tests were implemented in Version 1, including
checks for gross cloud contamination, uniformity tests, satellite zenith angle tests,
stray sunlight tests, and a check against a reference SST field based on the Reynolds
Optimally Interpolated SST (OISSTv1; Reynolds and Smith, 1994). Pixels on the
edges of a scan line or in the first or last scan line of an orbital piece were excluded,
and those subject to sun glint were also identified. Based on the combined results
of these many tests, Version 1 Pathfinder SSTs were then assigned a quality level of
between 0 (worst) and 4 (best). Pathfinder Version 1 data spanning 1987–1993 were
released to the public beginning in 1995.
16.3.2 Version 2
A second version of Pathfinder was developed to better correct for temporal changes
observed in the match-up statistics. The time-dependent term was removed and coefficients were generated on a monthly instead of annual basis. The coefficients were
calculated using a temporally weighted 5-month running window and were found
to better correct the temporal changes than the time-dependent term used in the
NLSST equation in Version 1. The central month is given a 100% weight, the adjacent months an 80% weight, and the months at the ends of the window a 50% weight.
This version of the Pathfinder data set was never released to the public.
16.3.3 Version 3
In the third version of Pathfinder SST, two sets of algorithm coefficients (a, b, c,
and d in Equation 16.1) were generated using the satellite-in-situ matchup database
instead of the three sets used in Version 1. The two sets were found to better reduce
the overall bias between satellite and buoy SST. One set was for high atmospheric
water vapor regimes where the T 4 − T 5 brightness temperature difference is greater
than or equal to 0.7 ◦ C and the other was for low water vapor regimes where the
279
measurements recorded at the University of Miami Domestic Communication
Satellite (DOMSAT, the communication satellite NOAA used to transfer AVHRR
data) station with the time information in HRPT passes received at Wallops Island,
VA, and Fairbanks, AK. Averaging over multiple geographic locations and times
along the orbital track mitigated attitude errors.
Further improvements introduced in Pathfinder Version 1 were implemented during the next phase of reprocessing, in which SST values are calculated. Three sets
of algorithm coefficients (a, b, c, and d in Equation 16.1) were generated using the
satellite-in-situ matchup database: one for high atmospheric water vapor regimes
where the T 4 − T 5 brightness temperature difference is greater than or equal to
1.8 ◦ C, one for low water vapor regimes where the T 4 − T 5 brightness temperature
difference is less than 0.7 ◦ C, and one for moderate levels of water vapor where the
T 4 − T 5 brightness temperature difference is greater than or equal to 0.7 ◦ C and less
than 1.8 ◦ C. In Version 1, the coefficients were calculated on an annual basis, using
12 months of matchup data.
Finally, an extensive set of quality tests were implemented in Version 1, including
checks for gross cloud contamination, uniformity tests, satellite zenith angle tests,
stray sunlight tests, and a check against a reference SST field based on the Reynolds
Optimally Interpolated SST (OISSTv1; Reynolds and Smith, 1994). Pixels on the
edges of a scan line or in the first or last scan line of an orbital piece were excluded,
and those subject to sun glint were also identified. Based on the combined results
of these many tests, Version 1 Pathfinder SSTs were then assigned a quality level of
between 0 (worst) and 4 (best). Pathfinder Version 1 data spanning 1987–1993 were
released to the public beginning in 1995.
16.3.2 Version 2
A second version of Pathfinder was developed to better correct for temporal changes
observed in the match-up statistics. The time-dependent term was removed and coefficients were generated on a monthly instead of annual basis. The coefficients were
calculated using a temporally weighted 5-month running window and were found
to better correct the temporal changes than the time-dependent term used in the
NLSST equation in Version 1. The central month is given a 100% weight, the adjacent months an 80% weight, and the months at the ends of the window a 50% weight.
This version of the Pathfinder data set was never released to the public.
16.3.3 Version 3
In the third version of Pathfinder SST, two sets of algorithm coefficients (a, b, c,
and d in Equation 16.1) were generated using the satellite-in-situ matchup database
instead of the three sets used in Version 1. The two sets were found to better reduce
the overall bias between satellite and buoy SST. One set was for high atmospheric
water vapor regimes where the T 4 − T 5 brightness temperature difference is greater
than or equal to 0.7 ◦ C and the other was for low water vapor regimes where the