The derivation of Sea Surface Temperature (SST) from satellite measurements has been a focus of numerous studies since the early 1970s. The Advanced Very High Resolution Radiometer (AVHRR) onboard the National Oceanic and Atmospheric Administration (NOAA) series of Polar-orbiting Operational Environmental Satellites (POES) is primarily designed for SST retrieval and cloud detection. POES satellites known as Advanced Television Infrared Observation Satellites (TIROS-N or ATN) operate as a pair to ensure that the data, for any region of the earth, are no more than 6 hours old. AVHRR has five channels, two visible channels (channels 1 and 2 at 0.6 and 0.9 µm, respectively), one short-wavelength infrared channel (channel 3 at 3.7 µm), and two long-wavelength infrared channels, the split window channels (channels 4 and 5 at 11, and 12 µm, respectively). The wavelengths of the three infrared channels are selected in a range of the electromagnetic spectrum in which the radiation from the earth's surface and clouds is only weakly attenuated. To determine the actual SST from the AVHRR radiation measurements, one must correct for absorption and reemission of radiation by atmospheric gases, predominately water vapor. The split window method, which uses the channel 4 and 5 brightness temperatures to calculate SST, is widely used for atmospheric correction.
NOAA's National Environmental Satellite, Data, and Information Service (NESDIS) produces two main types of SST products; i.e., global SST and CoastWatch SST. Operational AVHRR multichannel SST algorithms have been used to generate global SST measurements at NESDIS since November 1981. The global SST suite of products are generated from AVHRR Global Area Coverage (GAC) 4 km data recorded on-board the POES satellites and downlinked to NESDIS acquisition stations at Wallops Station, Virginia and Fairbanks, Alaska. Global SST measurements are produced at 8 km resolution with variable spacing from 8 km to 25 km in cloud-free areas twice per day from each of the two operational POES satellites.
CoastWatch SST products are generated from a different data stream, the AVHRR High Resolution Picture Transmission (HRPT) data, broadcast continuously by the POES satellites. These data are received at the Wallops station for the East Coast, Great Lakes, and Gulf of Mexico CoastWatch Regional Nodes. The West Coast, Hawaii, and Alaska CoastWatch Regional Nodes all have their own HRPT reception stations and acquire their own data. The HRPT data have a resolution of 1.1 km at nadir and are mapped to almost full resolution in the production of CoastWatch AVHRR visible, infrared, and SST images.
NLSST=A1 (T11) + A2(T11-T12)(MCSST) + A3(T11-T12)(Secq -1)-A4
MCSST= B1 (T11) + B2(T11-T12) + B3(T11-T12)(Secq -1) - B4
Where, T11 and T12 are the AVHRR 11 and 12 µm channel temperatures in Kelvin; Secq is the secant of the satellite zenith angle q; NLSST and MCSST are the non-linear and linear multi-channel SST, respectively, in Centigrade; A1-A4 and B1-B4 are constant coefficients.
The CoastWatch equations differ from the global SST equations in three respects:
(1)The CoastWatch equations use the MCSST value in the non-linear term rather than an priori SST estimate obtained from an analysis of past satellite SST data. This means that there is somewhat more noise in the CoastWatch observations. Both the global operation and CoastWatch constrain the value of the a priori SST or the MCSST to the range 0oC to 28oC.
(2) In the Great Lakes, the MCSST value is used as the final SST value during the day; i.e., a linear equation is used as the operational equation rather than a non-linear equation.
(3) The NLSST split-window equation is used for CoastWatch rather than the triple-window equation (employing all three infrared channels) which is used in the global operation.
The National Centres for Environmental Predication (NCEP) provides the buoy data used in the matching procedure. These data are placed in the buoy data file four times a day. The buoy data file also gives the current NOAA moored buoy locations, so an analyst can overlay the buoy positions on the AVHRR imagery. AVHRR imagery is in the CoastWatch format and images are archived at the National Oceanographic Data Centre (NODC). The main input for this long-term validation system is the Target Match File (TMF). The TMF is generated by extracting 15 by 15 pixel array targets of the CoastWatch imagery (i.e., mapped full-resolution AVHRR HRPT imagery including all five channels, cloud masks, and SST) centred at NOAA moored buoy positions on the NOAA/NESDIS CEMSCS main frame computer. The corresponding buoy data are appended to each target. The long-term validation system enables an analyst to:
(1) preview AVHRR images (both infrared and visible channels) to see whether an image contains cloud-free SST measurements at any buoy location.
(2) overlay coastlines, grids, buoy locations, and AVHRR imagery header information on the images.
(3) renavigate the imagery by remaping the image to agree with selected ground control points.
(4)display cloud masks.
(5) extract clear 3 by 3 arrays of CoastWatch SST values centred on each of the buoys in each coastal region.
(6) create an output SST match file which contains satellite and buoy SSTs, air temperature, wind and wave information, solar and satellite zenith angles, and navigation information.
(7) calculate statistics and make graphic output. Further, cloud screening is done by examining SST and, when necessary, visible imagery. If fog or cloud is suspected, a matchup is not made.