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Changes in the temperature of the ocean have a major impact on the planet and society. They are a key indicator of climate change as well as providing near term inputs into weather forecasting. Warmer water leads to the increased melting of sea ice, the expansion of water volume and ultimately in land loss. Increases in sea temperature also impacts ocean biology, with coral bleaching and killing one of the early consequences.
NPL’s Optical Radiometric Metrology and Climate and Earth Observation groups, in collaboration with the Temperature and Humidity group hosted participants at NPL to undertake laboratory-based comparisons, with subsequent activity conducted on Boscombe pier in Bournemouth. This international comparison project was undertaken to ensure harmonisation in validation of satellite measurements of climate critical sea surface temperature. The comparison was organised under the auspices of the Committee on Earth Observation Satellites (CEOS) Working Group for Calibration and Validation(WGCV), through a project sponsored by the European Space Agency (ESA).
In the laboratory stage the participants* compared each other’s radiometers with the NPL radiance source serving as the reference. Participants own radiance sources (blackbodies) used to calibrate their radiometers were also measured and compared with a transfer radiometer traceable to NPL’s standard radiometer, Absolute Measurements of Blackbody Emitted Radiance (AMBER). Both the reference blackbody and the AMBER are traceable to the International Temperature Scale of 1990 (ITS-90) through linkage to the scale realised by NPL’s temperature and humidity group.
Ocean temperature has been routinely measured for more than a century and over the last three or four decades from space, providing the necessary global coverage. The most direct validation of satellite derived water temperature is to compare the remote observation with surface measurements using similar instrumentation, i.e. infrared radiometers. It is essential that the surface radiometers, usually deployed on ships, make consistent measurements across the globe, no matter the manufacturer or the user.
Comparisons are critical to ensuring that measurements made using one instrument can be accurately compared with measurements made using another. However, no such comparisons of primary radiometry have been conducted on a global scale in the thermal infrared. As such, the team brought together their skills and knowledge in radiometry, thermometry, and climate science, to deliver the measurement campaign of instruments measuring radiometric temperature in the infrared specific to climate science. For the climate we are looking to detect change of a few tenths of a degree per decade and therefore robust SI traceability is essential to ensure trust in the satellite observations and conclusions. The team’s work in infrared will continue as they support the pre-flight calibration of the satellites.
With increasing interest in measuring changes in land surface temperature at relatively high spatial resolution, it is anticipated that there will be further comparison activities in this domain. In addition, the group are also currently active in running comparisons of satellite measured surface reflectance, as well as more general studies to improve the linkage of surface-based observations to those at the satellite level i.e. how to optimally account for differences in the satellite view and the comparatively small sample size made with ground instrumentation.
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Yoshiro Yamada, Principal Research Scientist said: “Measurement requirements for climate science are extremely challenging for metrologists. How well we are managing to meet the level of accuracy can only be verified by such comparisons, and what we find will indicate to us where our next effort needs to be focussed”
Subrena Harris, Higher Research Scientist said: “It has been a great experience to be a part of this comparison and to see directly how measurements from the laboratory support measurements of instruments that will be deployed to further our knowledge of the environment and our climate. The collaboration and cooperation among the scientist who came from across the world to participate in the comparison was key to the success of the campaign.”
Nigel Fox, NPL Fellow said: “Satellite-based sea surface temperature is one of the more mature remote sensing measurements hence this being the fifth formal comparison. However, the demanding uncertainty and practical challenges, waves, ambient temperature, impact of the sky and sun make it critical to undertake and evolve the comparison methodologies. Over the last two comparison cycles it is notable how practitioners/participants have significantly improved their understanding and assessment of uncertainty which consequently provides policy makers with greater confidence in the results allowing evidence-based action”
*The comparison participants, in addition to NPL, comprised the University of Southampton, STFC Rutherford Appleton Laboratory, the Danish Meteorological Institute, CSIRO / Australian Bureau of Meteorology, IMK-ASF / Karlsruhe Institute of Technology and the University of Valencia, unfortunately on this occasion COVID limited participation from some countries.
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22 Sep 2022