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GOES Sea Surface Temperature Products: Background

Background

The National Oceanic and Atmospheric Administration's Office of Satellite Data Processing and Distribution are generating operational sea surface temperature (SST) retrievals from the Geostationary Operational Environmental Satellite (GOES) 11 and 12 satellite imagers. They are situated at longitude 135oW and 75oW, respectively, thus allowing the acquisition of high-temporal-resolution SST retrievals.

A new cloud masking methodology based on a probabilistic (Bayesian) approach has been implemented for improved retrieval accuracy. This new GOES SST Bayesian algorithm provides SST retrievals with an estimate of the probability of cloud contamination. This indicates the confidence level of the cloud detection for the retrieval, which can be related to retrieval accuracy.

The GOES-11 and 12 imagers observe both northern and southern hemisphere every half an hour. These 5-band (0.6, 3.9, 6.7, 10.7, 12 or 13.3 µm) and 4-band (0.6, 3.9, 6.7, 10.7. or 13.3 µm) images are processed to retrieve SST retrievals at 4-km resolution. The window infrared channels determine the SST, and all channels (except the 6.7 and 13.3 µm) determine the cloud contamination. These retrievals are remapped, averaged and composited hourly and posted to a server for user access. The retrievals are available approximately 90 minutes after the nominal epoch of the SST determinations. 3.hour and 24.hour averages are also made available. CoastWatch Regional Imagery is generated every three hours by combining the 1hourly SST images for these areas.

Algorithms

The NOAA Office of Satellite Data processing and Distribution is generating sea surface temperature (SST) retrievals on an operational basis from the GOES-11 and 12 satellite imagers. The algorithm retrieval schemes are based on Radiative Transfer Modeling (RTM), generating skin temperatures not bulk temperatures. The Imager channels for GOES-11 and 12 are depicted in Table 1 and Table 2.

The GOES-12 satellite imager has only two channels (3.9 and 11 µm) available to generate SSTs. The former channel is difficult to use during the day because of solar contributions to the signal that derive from surface reflection and atmospheric scattering. The current scheme for GOES-12 consists of (1) screening out areas of significant sun glint prior to the application of a daytime retrieval that applies a solar correction to the 3.9 µm channel by assuming a typical aerosol loading and accounting for variability in sun-pixel-satellite geometry; (2) a new cloud masking methodology based on a probabilistic (Bayesian) approach to detection using thermal infrared radiances estimated from the Community Radiative Transfer Model (CRTM) together with a spatial variance predictor.

Radiative-transfer-based SST retrieval algorithms are used to generate the GOES-11/12 SST retrievals. The form of the current GOES operational SST equation is:

where: i is GOES-Imager channel number (2, 4, 5)

S = sec (satellite zenith angle) - 1 and Tiis channel brightness temperature in Kelvin

Coefficients (for Kelvin brightness temperatures) and modeled RMS error are:

  ao ao' a2 a2' a4 a4' a5 a5' modeled RMS error
GOES-11 (day) -18.01 -6.52 -0.0 -0.0 3.3188 0.1466 -2.2588 -0.1174 0.68364262
GOES-11 (night) -5.46 -2.93 9.449 -0.0384 0.5698 0.3328 -0.4905 -0.2775 0.30877404
GOES-12 (day and night) -2.1 -1.15 -1.177 -0.073 -0.162 -0.069 0.0 0.0 -

As shown in Table 2, channel 5 of the Imager on the GOES-12 platform is centered at 13.3 microns (i.e. is primarily sensitive to temperatures in the low-to-mid troposphere, rather than the surface) and is therefore not used in the SST retrieval. While SSTs can still be retrieved at night using the 3.9 and 11 micron channels (2 and 4), daytime retrievals are complicated by the contribution of reflected and scattered solar radiation to the channel 2 brightness temperature. Three extra steps are performed: i) the region affected by sun glint is estimated using NCEP model winds, the Cox and Munk (1954) slope distribution and the satellite-solar geometry; ii) the clear-sky scattered solar radiation contribution is estimated for a typical value of aerosol optical depth; iii) channel 2 brightness temperatures are adjusted to compensate for the solar contributions estimated in i) and ii), with the exception that sun glint corrections >1 deg. K are flagged as insufficiently reliable to be used for SST retrieval. SSTs are retrieved for the remaining clear-sky pixels using the adjusted channel 2 brightness temperatures along with the channel 4 data, using the same retrieval equation form described above.

Bayesian Cloud Mask

The previous cloud detection scheme was based on a series of threshold tests. The new methodology applies Bayes. theorem to estimate the probability of a particular pixel being clear of cloud given the satellite-observed brightness temperatures, a measure of local texture and channel brightness temperatures calculated for the given location and view angle using NCEP GFS surface and upper air data and the CRTM fast radiative transfer model. The method is described in detail in a paper by Merchant et al. (2005).

Validation Methodology

NOAA/NESDIS Office of Research and Applications generates a matchup data base for validation of the GOES-SST retrieval algorithms. This is important for the maintenance and improvement of the GOES-SST products.

The global drifting buoys and the TOGA TAO moored buoy array are matched with GOES-SST retrievals within one hour and 5 km. The buoys used are extracted by the Climate Prediction Center (CPC), which is one of the National Weather Service.s National Centers for Environmental Prediction (NCEP). The buoys are quality controlled using the Reynolds.s Optimum Interpolation Sea Surface Temperature (OISST) Analysis and NCEP Atmospheric Analysis Fields before being matched with the GOES-SST retrievals. Matchup files are stored in the NOAA Satellite Active Archive (SAA) for user access.

An automated validation system has been in place since the inception of the operational GOES-SST retrievals. This system computes the GOES-SST accuracy statistics (monthly) from the satellite-buoy matches. The program uses the match up file as an input to calculate the number of matches, bias, and standard deviation. The statistical results are continually updated with the latest matches.