An in situ optical dataset for working towards fiducial reference measurements based satellite ocean colour validation in the Eastern Mediterranean

The societal benefits of satellite ocean colour include aiding the management of the marine ecosystem, helping understand the role of the ocean ecosystem in climate change, aquaculture, fisheries, coastal zone water quality, and the mapping and monitoring of harmful algal blooms. Ocean colour is also designated as an essential climate variable by the Global Climate Observing System (GCOS). However, in order to have confidence in earth observation data, measurements made at the surface of the Earth, with the intention of providing verification or validation of satellite mounted sensor measurements, should be trustworthy and of the same high quality as those taken with the satellite sensors themselves. In order to be trustworthy, in situ validation measurements should include an unbroken chain of SI traceable calibrations and comparisons and full uncertainty budgets for each of the in situ sensors used. This metrological traceability is beginning to be demanded by the space agencies for satellite validation measurements and, for ocean colour, should follow the guidelines and protocols of the ESA Fiducial Reference Measurements for Satellite Ocean Colour (FRM4SOC) project (www.frm4soc.org). Until now, this has not been the case for most measurements used for validation, including those taken in the Aegean and Eastern Mediterranean. Subsequently, the Hellenic Centre for Marine Research (HCMR), in cooperation with the Laboratory of Optical Metrology (LOM), has started to follow the FRM direction by ensuring that the radiometers of its optical suite underwent SI-traceable absolute radiometric calibration. This included an estimate of the radiometry calibration uncertainty budget and was performed at the marine optical laboratory of the European Commission’s Joint Research Centre prior to their deployment on the recent PERLE-2 oceanographic cruise in the Eastern Mediterranean (Feb-Mar 2019). As well as irradiance and radiance sensors, the HCMR optical suite also houses instruments for measuring inherent optical properties (IOP) of the water column. Therefore, this paper presents the in-water radiometry matchups from PERLE-2 with Sentinel-3 Ocean and Land Colour Instrument (OLCI) measurements, and investigates their validation potential. It also presents the PERLE-2 cruise profile chlorophyll and backscatter measurements that aid this effort through characterizing the light scattering and absorbing constituents that contribute to the signal detected by satellite ocean colour sensors during validation matchups.