Δημοσιεύσεις
2015 |
Drakopoulos, P. G.; Banks, A. C.; Kakagiannis, G.; Karageorgis, A. P.; Lagaria, A.; Papadopoulou, A.; Psarra, S.; Spyridakis, N.; Zervakis, V. Estimating chlorophyll concentrations in the optically complex waters of the North Aegean Sea from field and satellite ocean colour measurements Journal Article In: Proceedings of SPIE, vol. 9535, 2015. Abstract | Links | BibTeX | Ετικέτες: algorithm development, MODIS, ocean optics, reflectivity, remote sensing, satellites, water @article{Drakopoulos2015c,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. |
Papadopoulou, A.; Drakopoulos, P.; Psarra, S.; Banks, A. Clive; Karageorgis, A.; Lagaria, A.; Spyridakis, N.; V. Zervakis, Deriving euphotic zone depth in the North Aegean Sea from field and MODIS ocean colour measurements Conference Fifth International CEMEPE and SECOTOX Conference Mykonos Island, Greece , 2015. Abstract | Links | BibTeX | Ετικέτες: chlorophyll, euphotic zone, MODIS, ocean colour, remote sensing @conference{Papadopoulou2015,Euphotic zone depth (Zeu) is defined as the depth at which the photosynthetically active radiation (PAR) value is 1% of the surface value. In this work, three approaches were used to estimate Zeu from field data (in the North Aegean Sea): (a) the Chl-approach, where surface chlorophyll-a values were determined via the high performance liquid chromatography (HPLC) and the fluorometric technique, (b) the PAR-approach, where photosynthetically active radiation was measured with a hyperspectral irradiance meter and (c) the Reflectance-approach, in which surface reflectance values were produced from the in situ downwelling spectral irradiance and upwelling spectral radiance field measurements. Using the Seawifs Data Analysis System (SeaDAS), MODIS daily Level-2 Zeu datasets were produced with two different bio-optical algorithms, which were based on chlorophyll-a concentration (satellite Chl-approach). The comparison of field and satellite Zeu values showed that MODIS tended to underestimate the Zeu for all different approaches. In more detail, the two bio-optical algorithms were found to underestimate Zeu values by ~20-31% and ~18-21% when compared to field measured Zeu data via Chl-approach and PAR-approach, respectively. The percentage range of the underestimation of the Zeu satellite values compared with the one obtained from the field Zeu values via the Reflectance-approach, was found particularly small (~1-5%), a fact that explains the differences in the products as delivered by space agencies. Furthermore, the high uncertainties between field and satellite Zeu measurements require new approaches for ocean colour algorithms in the North Aegean Sea. |