Topics 2016.02.01

The progress of the satellite sea surface temperature observation

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A water temperature measured at near sea surface is called as "sea surface temperature (SST)". ツ黴€The above image is an example of SST map around Japan. ツ黴€SST represents the thermal state of the upper ocean and it is used asツ黴€an index of the climate change. ツ黴€SST is a boundary condition in a numerical model for weather prediction. ツ黴€The fine map of SST distribution is also available for finding eddies and currents in the ocean.

SST is not uniform and not steady. ツ黴€Therefore we need the data assemled from all corners of the world by the repeated observation. ツ黴€Remote sensing by artificial satellites is the most effective tool to collect SST data. ツ黴€ It is because that the mobility of satellite is superior to that of in-situ observation objects, such as ships and buoys.

The technique has been developed to retrieve SST from the infrared or microwave radiation emitted from sea surface. ツ黴€The Advanced Very High Resolution Radiometer (AVHRR) is a representative satellite sensor of the infrared SST observation. ツ黴€ The AVHRRs aboard the US's NOAA polar orbiting satellite series have lead the infrared SST observation from about 1980s. ツ黴€The AVHRR provides us SSTツ黴€with the spatial resolution of 1-7km.

Around 2000s, the microwave SST observation has started into practical operation. ツ黴€The representative sensor is the Advanced Microwave Scanning Radiometer (AMSR) developed in Japan. ツ黴€The AMSR-E aboard the US's Aqua satellite had observed SST from 2002 to 2011. At present, the AMSR2 aboard the Japanese satellite "Shizuku" (Global Change Observation Mission-Water, GCOM-W) has made the microwave SST observation.

A spatial resolution of the infrared observation is higher than that of the microwave observation. ツ黴€The microwave observation can be used to measure SST under clouds, where the infrared cannot. ツ黴€To be utilized the respective advantages of the infrared and microwave to SST observation, the technique has been developed to produce "the merged SST".

Today, a system works to observe global SST daily, with the spatial resolution of 1 to 50km, by the merging technique to SSTs from the in-situ observation and satellite infrared and microwave observation. ツ黴€ In the near future, a close observation by the new generation sensors/satellites will be made. ツ黴€For example, the Advanced Himawari Imager (AHI) / Himawari-8 has made a full disk observation with a 10-minutes interval. ツ黴€JAXA (Japan Aerospace Exploration Agency) has a new operation plan of the S-GLobal Imager(GLI)/GCOM-Climate (GCOM-C), which will observe SST with a spatial resolution of 250-500m. ツ黴€It must be that such new comers will make a large contribution to the study of the ocean, especially the research of the small scale and/or rapid changed phenomena in the ocean.

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The image of the "foundation SST", which is the merged SST after the application of the diurnal variation correction, produced in the CAOS, Tohoku University. ツ黴€The date is January 1, 2014. ツ黴€The opening image of SST around Japan is produced from the same data.

CAOS_O_20160201_03.gifThe images of Himawari-8 Band-13 infrared observation. ツ黴€The SST variations from 09 to 21UTC of March 26, 2015 are shown at the ocean off Tohoku Area.ツ黴€

(by Futoki Sakaida, Satellite oceanography laboratory, CAOS)ツ黴€

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