# Product Tutorial on Sea Surface Temperature (SST)

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## Chapter VI: SST from infrared/microwave radiometry

### Table of Contents

- SST from infrared/microwave radiometry

### SST from infrared/microwave radiometry

The radiation emitted by the ocean in the infrared (IR) and the microwave wavelengths can be measured by satellites operating in these two different bands. When SST is computed from measurements in the infrared band (3.7-10 μm), the skin sea surface temperature (SSTskin) is obtained; when microwave radiometers operating in the 6-11 GHz frequency range are used sub-skin sea surface temperature (SSTsub-skin) is obtained.

Although this tutorial focuses in the IR SST product, it is relevant to understand the main characteristics of microwave SST.

Fig. 10:The ocean emits radiation in the infrared (from the 10-20 μm layer) and in the microwave wavelengths (from the topmost 1 mm layer). The emissivity in the microwave band is around 0.5 whereas in the infrared it is close to 1.

Before 1997, SSTs were only available globally from IR satellite retrievals. Thermal infrared instruments that have been used for deriving SST include the Advanced Very High Resolution Radiometer (AVHRR) on NOAA Polar-orbiting Operational Environmental Satellites (POES) and on board the Eumetsat Polar Orbiting Metop satellite series, the Along-Track Scanning Radiometer (ATSR) aboard the European Remote Sensing Satellite (ERS-2), the Geostationary Operational Environmental Satellite (GOES) imager, and the MODerate resolution Imaging Spectroradiometer (MODIS) aboard NASA Earth Observing System (EOS) Terra and Aqua satellites, the EUMETSAT Meteosat Second Generation (MSG) series of Geostationary satelites, and the Advanced Himawari Imager (AHI) radiometer on board the Himawari geostationary satellite operated by the Japanese Meteorological Agency (JMA).

The TRMM Microwave Imager (TMI) that is on board the Tropical Rainfall Measuring Mission (TRMM) satellite launched in 1997, provided the first SST fields within the microwave band. Infrared radiation of the ocean comes from the top 10 μm of the surface. Microwave radiation results from the topmost 1mm layer.

The main differences between SST products derived from infrared and microwave sensors are presented in next table.

| IR | Microwave
--- | --- | ---
Spatial resolution | 250m - 4km | 25km
Number of gaps in data | High, because clouds disable the retrieval | Low, because only sunglint, rain or proximity to land prevents the retrieval; microwaves penetrate the clouds.
Data longevity | Good heritage (observations began in the 1970s). | Available since 1998.

Radiation in the microwave (MW) wavelengths isn't affected by clouds and it is generally easier to correct for atmospheric effects. SST microwave retrieval is prevented in regions with sunglint, rain, in high windspeed regions, and in the proximity to land (50-100km from land). This results in a smaller amount of missing data and enables an almost complete global coverage to be observed at daily frequency. The sparse spatial resolution of MW derived SST is a major disadvantage of this type of data. Yet, an SST product at a ~25-km resolution is ideal for research activities, as in this case a complete daily SST map is preferable to one with missing data due to orbital gaps or due to environmental conditions that prevent SST retrieval.

Figure 11 illustrates the difference between an SST field derived from IR measurements (top figure showing many white patches corresponding to cloud contaminated pixels where surface information could not be obtained) and a microwave derived SST (bottom image, more complete coverage).

Source: (NASA JPL/PO.DAAC).

Fig. 11: comparison between SST derived from IR measurements (top image) and a microwave derived SST (bottom image).