Ocean-color radiometry across the Southern Atlantic and Southeastern Pacific: Accuracy and remote sensing implications

Ocean color radiometry (OCR) provides valuable data for biogeochemical oceanography. In situ OCR measurements are used in the development and validation of bio-optical models and vicarious calibration of satellite ocean-color sensors. It is thus crucial to obtain accurate in situ OCR measurements, which is a challenge, especially in regions subjected to adverse environmental conditions and where waters are optically complex. In the present work, the accuracy of in situ OCR is analyzed with data acquired in a wide range of bio-geographic provinces across the Southern Atlantic and Southeastern Pacific during the R/V Melville MV1102 cruise. Varied techniques employed to measure above-water remote sensing reflectance (Rrs) are inter-compared. Measured Rrs is also compared with modeled Rrs in a closure experiment. The impact of Rrs uncertainties on the retrieval of chlorophyll a concentration (Chla) and inherent optical properties (IOPs) is evaluated using operational bio-optical algorithms. The relative percent difference (RPD) between Rrs measured by the various techniques ranged from 12 to 26% for the ocean-color bands (412-555 nm), and 3-12% for the ratios (412-510/555). A merged Rrs obtained by averaging the different types of measurements, INS, is recommended to reduce uncertainties. The coefficient of variation of INS and reflectance ratios was 11-13% and 3-5%, respectively. The RPD between INS and modeled Rrs and the corresponding ratios ranged from 18 to 34% and from 13 to 17%, respectively. Complete closure could not be obtained due to both measurement and modeling uncertainties. The impact of INS uncertainties on retrieved Chla and IOPs was generally smaller than the intrinsic errors of the inversion schemes. The results suggest that even though more accurate ocean-color radiometry is desirable, improving retrieval algorithms is essential to properly describing and furthering our understanding of bio-optical variability in the world's oceans.