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Instruments rigorously tested at sea to assess accuracy of satellite ocean colour

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3 image montage of radiometers, the RRS Discovery ship and additional radiometers
AMT4OceanSatFlux researchers have undertaken a comprehensive evaluation of two frequently used ship-mounted radiometer systems that measure ocean colour. The research, which was recently published in the journal Remote Sensing, tested the instruments across a wide range of environmental conditions and water types. This will enhance confidence in radiometric data used to validate ocean colour measurements from sensors on satellites.
 
The colour of the ocean gives us insight into the health and functioning of the ocean. Using measurements of light absorbed and reflected it is possible to calculate phytoplankton biomass, as the chlorophyll pigment in the plankton absorbs blue light and reflects green, giving a green appearance to the water. Primary production by these marine plankton is one of the largest fluxes of carbon on our planet. Plankton draw down carbon dioxide from the atmosphere to the sea and are responsible for almost half of the total global net primary production. This forms the basis for the marine food web, influences biodiversity and fisheries and plays an important role in the global carbon cycle and consequently climate regulation. Increasingly, primary production is being measured from satellite and gives unprecedented coverage of plankton dynamics across the global ocean.

Accurate and widespread measurements of ocean colour are therefore essential to understand the dynamics of the ocean system, but it is not possible to view the whole ocean with in situ sensors.  Satellites offer a cost effective means to see the global picture. To have confidence in this satellite data we need reliable measurements at the ocean’s surface to provide verification or validation of these satellite-mounted sensor measurements. These measurements need to be trustworthy and of the same high quality as those taken by the satellites - this entails rigorous intercomparisons of carefully calibrated instruments and uncertainty assessments for each of the in situ sensors.

In this study two instrument types commonly used in the ocean colour community,  TriOS-RAMSES and Seabird-HyperSAS radiometer systems, were deployed simultaneously on an Atlantic Meridional Transect cruise, which covered a track of over 13,000km from the UK to the Falkland Islands in the South Atlantic.  This gave an opportunity to test and compare the systems under a huge range of conditions and evaluate the inherent uncertainties associated with taking measurements from a moving vessel.

Simultaneous measurements were taken at over 30 stations along the Atlantic. The agreement between the two systems was found to be good for visible wavelengths thus demonstrating the reliability of these in situ measurements. The average uncertainty of the data was close to that needed for the Sentinel-3 satellite mission requirements (5%) and the consistency between in situ and satellite data was good with uncertainties of less than 7%. These measurements are vital for estimating errors and uncertainties resulting from environmental variability, as well as instrument deployment methodology, instrument specifications and calibration.

This work will have a long-standing legacy. It will be used to evaluate the state-of-the-art in ocean colour radiometry and underpin recommendations to optimise processing methodologies, instrument deployment techniques and calibration processes and also achieve community consensus on the practical limitations and applications of the technique. Crucially, the results will be used to calibrate and improve accuracy of data from the Ocean and Land Colour Instrument (OLCI) mounted on the Copernicus Sentinel-3 satellite, a programme which will continue for at least two decades and provide a long-term view of changes in the ocean.

Acknowledgements

Related Information

Alikas, K., Vabson, V., Ansko, I., Tilstone, G., Dall’Olmo, G., Nencioli, F., Vendt, R., Donlon, C. and Casal, T. 2020. Comparison of above-water Seabird and TriOS radiometers along and Atlantic Meridional Transect. Remote Sensing, 12(10), art:1669. doi:10.3390/rs12101669