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.
The main aim of the bio-optics experiment was to collect optical data in the northwestern Levantine Sea, south of Crete Island, to be used in climatology analysis and remote sensing products. All measurements and sampling were conducted on board the R/V Aegaeo during LEVECO cruise from 9 to 18 April 2016 within a grid of 9 stations. The experiment included the measurement of apparent and inherent optical properties of sea water by several optical sensors deployed in and above water (TriOS radiometers, ac-s absorption-scattering meter, LISST particle size distribution profiler, Eco-bb backscattering meter, fluorometers, JAZ spectroradiometer). Penetration depth estimations were on the average around 30 m while a well-developed deep chlorophyll maximum was recorded at 70-90 m depth. The horizontal distribution of chlorophyll-a, shows that the eastern part of the study area is affected by the cyclone of Rhodes resulting in slightly elevated values (typically 0.07 vs 0.04 mg m-3). The particulate matter field was very weak throughout the water column (range: 19-255 μg/l), underpinning the oligotrophic nature of the area and the overall scarcity of suspended particles. Particle size was rather high (D50<70 μm) suggesting the predominance of micro-aggregates. All reflectance band ratio derived chlorophyll-a concentrations were compared against the HPLC derived ones. Between global OC4v6 and MedOC4.2018 versions, the latter performed better predicting values very close to field observations. On the contrary, the locally tuned algorithm for Kd retrieval, gave values much lower than field observations.
Oceanographic investigations have significantly benefited from multispectral satellite products that simplify monitoring in coastal regions thanks to their high spatial-temporal resolution. The surface Suspended Particulate Matter (SPM) is an important water quality parameter which can be derived from empirical or analytical algorithms by using the atmospherically-corrected remotely sensed reflectance Rrs(λ) retrieved from satellite imagery. In this study, in situ SPM and Rrs(λ) data were collected in the Gulf of Alexandroupolis, Northeastern Aegean Sea in Greece (Eastern Mediterranean Sea) during low discharge period (June 2016). We attempt to compute remotely sensed reflectance from Landsat OLI8 imagery, in order to quantify surface SPM concentrations via both a Semi-Analytical and a Multi-Band Empirical Algorithm. When comparing the satellite estimations against the field measurements, both algorithm approaches provide a non absolute correlation with in situ Rrs (~20-30 % offset). As a result, a generic semi-analytical equation for Alexandroupolis Gulf is developed, following algorithm calibration in low turbidity waters. The proposed algorithm can be then equally implemented to the new Sentinel-2A sensor, in order to assess its variations against Landsat 8 and to determine the applicability extend of our approach.
Within the framework of Perseus and AegeanMarTech projects, multidisciplinary bio-optics experiments were conducted in the optically complex, permanently stratified waters of the NE Aegean Sea. We were able to obtain the particle size distribution (PSD) slope, using different optical sensors: (a) WET Labs ECO-B3B backscattering sensor measuring VSF at three wavelengths (470, 532, and 650 nm); (b) WET Labs C-Star transmissometer (660 nm) and Chelsea ALPHAtracka MKII (470 nm); and (c) Laser In Situ Scattering and Transmissometry – LISST-Deep. Values of the PSD slope estimated by all three methods were found to be within the ranges predicted by Mie theory and the literature. The optical backscattering ratio, bbp, was calculated from bbp(660) and cp(660) and subsequently the bulk index of refraction (np) was estimated as a function of the backscattering ratio and the PSD slope. In July 2014, the values of np varied between ~1.01 and >1.24 (mean 1.12±0.08), which fall between phytoplankton- and mineral-dominated waters. According to the spatial distribution of np two water layers could be identified, associated with different particle composition: (a) Black Sea water (BSW) and Levantine waters (LW) (from the surface to ~65 m depth) that appear to be dominated by material with mean index of refraction 1.13; and (b) the near-bottom layer which exhibited high np, >1.24, the latter attributed to mineral particles with high bulk index of refraction resuspended from the sea floor.
In the Aegean Sea and Eastern Mediterranean there are large discrepancies between in situ and satellite ocean colour
derived chlorophyll concentrations. The quantity that is monitored by ocean colour satellites and that can be used in the
estimation of chlorophyll concentration is the remote sensing reflectance, defined as the ratio of the water leaving
spectral radiance to the downwelling spectral irradiance. It can be determined in the field, with either above or in-water
radiance and irradiance measurements. The complex optical properties of the North-East Aegean Sea, including radiance
and irradiance, were studied during the AegeanMarTech project. Chlorophyll concentration estimates were derived from
simultaneous above and in-water radiometric measurements. These were validated against chlorophyll concentration
field data and compared against concurrent MODIS data from which chlorophyll was derived using two simple empirical
algorithms. It was found that the MedOC3 algorithm outperforms the operational OC3M-547 algorithm and produces the
least bias when compared against HPLC derived in situ chlorophyll. It is concluded that the greatest uncertainty in the
inversion arises due to CDOM absorption below the 488 nm band. The reflectance ratios indicated that there is always an
excess of yellow matter present in the study area and the water type could not be characterized optically as ‘’typical open
ocean” Case 1.
An absolute sea level monitoring and altimeter calibration permanent facility is being established on the isle of Gavdos, south of the island of Crete, Greece. This calibration facility has been chosen because Gavdos is under a crossing point of the ground-tracks of TOPEX/Poseidon and Jason-1, and adjacent to an ENVISAT pass.
Satellite altimeter missions will be evaluated at that site using external measurements from tide gauges, GPS, a DORIS beacon, meteorological sensors, wave height sensors, airborne campaigns for gravity and sea surface topography, water vapor radiometry, solar atmospheric spectrometry, GPS buoys, altimeter transponder, Satellite Laser Ranging, etc. The mean sea level and the earth's tectonic deformation field in the region will be also determined accurately.
The GAVDOS project has started in December 2001 and has been in the context of an international calibration/validation effort of the Jason-1 Science Working Team.
This paper describes the objectives, current status and future plans for the establishment of the GAVDOS calibration facility for satellite altimeter missions.
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