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- Effects of integration time on in-water radiometric profilesPublication . D'Alimonte, Davide; Zibordi, Giuseppe; Kajiyama, TamitoThis work investigates the effects of integration time on in-water downward irradiance E-d, upward irradiance E-u and upwelling radiance L-u profile data acquired with free-fall hyperspectral systems. Analyzed quantities are the subsurface value and the diffuse attenuation coefficient derived by applying linear and non-linear regression schemes. Case studies include oligotrophic waters (Case-1), as well as waters dominated by colored dissolved organic matter (CDOM) and non-algal particles (NAP). Assuming a 24-bit digitization, measurements resulting from the accumulation of photons over integration times varying between 8 and 2048ms are evaluated at depths corresponding to: 1) the beginning of each integration interval (FST); 2) the end of each integration interval (LST); 3) the averages of FST and LST values (AVG); and finally 4) the values weighted accounting for the diffuse attenuation coefficient of water (WGT). Statistical figures show that the effects of integration time can bias results well above 5% as a function of the depth definition. Results indicate the validity of the WGT depth definition and the fair applicability of the AVG one. Instead, both the FST and LST depths should not be adopted since they may introduce pronounced biases in E-u and L-u regression products for highly absorbing waters. Finally, the study reconfirms the relevance of combining multiple radiometric casts into a single profile to increase precision of regression products. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement.
- Collection, analysis and on-line experimentation of ocean color remote sensing data An appraisal off the Southwestern Iberian PeninsulaPublication . D'Alimonte, Davide; Kajiyama, Tamito; Sa, Carolina; Brotas, VandaThis document concerns the collection, analysis and on-line experimentation of ocean color data off the Western Iberian Peninsula. Field measurements have been acquired during the BIOMETORE field campaign in summer 2016 to evaluate and enhance Earth observation capabilities of the Copernicus program. Deliverables of the Ocean and Land Colour Instrument on board of the Sentinel-3 satellite of the European Space Agency are of specific interest. Preliminary evaluations confirm the quality of the in situ measurements to address the match-up future analysis of radiometric values and derived data products. On-line experimentation undertaken with the WebEnhanced Service To Ocean Color demonstrates the feasibility of enabling in a transparent way the user's access to complex functionalities such as neural network applications.
- Algorithms merging for the determination of Chlorophyll-a Concentration in the Black SeaPublication . Kajiyama, Tamito; D'Alimonte, Davide; Zibordi, GiuseppeTwo regional bio-optical algorithms are combined to retrieve the Chlorophyll-a (Chl-a) concentration in the Black Sea. The first is a band-ratio algorithm that computes Chl-a as a function of the slope of Remote Sensing Reflectance (R-RS) values at two wavelengths using a polynomial regression that captures the overall data trend, enhancing extrapolation results. The second algorithm is a Multilayer Perceptron neural net based on Rgs values at three individual wavelengths that features interpolation capabilities helpful to fit data non-linearities. A new merging scheme is then designed to benefit from the complementarity of the two approaches. Remote sensing data employed to demonstrate the merging of regional results for the Black Sea are those acquired by the Ocean and Land Color Instrument on board Sentinel-3A to acknowledge the need for data products of higher accuracy within the long-term Copernicus program.
- Effects of light polarization and waves slope statistics on the reflectance factor of the sea surfacePublication . D'Alimonte, Davide; Kajiyama, TamitoAbove-water radiometry depends on estimates of the reflectance factor rho of the sea surface to compute the in situ water-leaving radiance. The Monte Carlo code for ocean color simulations MOX is used in this study to analyze the effect of different environmental components on r values. A first aspect is examining the reflectance factor without and by accounting for the sky-radiance polarization. The influence of the sea-surface statistics at discrete grid points is then considered by presenting a new scheme to define the variance of the waves slope. Results at different sun elevations and sensor orientations indicate that the light polarization effect on r simulations reduces from similar to 17 to similar to 10% when the wind speed increases from 0 to 14ms(-1). An opposite tendency characterizes the modeling of the sea-surface slope variance, with r differences up to similar to 12% at a wind speed of 10ms(-1). The joint effect of polarization and the the sea-surface statistics displays a less systematic dependence on the wind speed, with differences in the range similar to 13 to similar to 18%. The r changes due to the light polarization and the variance of the waves slope become more relevant at sky-viewing geometries respectively lower and higher than 40 degrees with respect to the zenith. An overall compensation of positive and negative offsets due to light polarization is finally documented when considering different sun elevations. These results address additional investigations which, by combining the modeling and experimental components of marine optics, better evaluate specific measurement protocols for collecting above-water radiometric data in the field. (C) 2016 Optical Society of America
- Standard and regional bio-optical algorithms for chlorophyll a estimates in the Atlantic off the southwestern Iberian PeninsulaPublication . Cristina, Sónia; D'Alimonte, Davide; Goela, Priscila; Kajiyama, Tamito; Icely, John; Moore, Gerald; Fragoso, Bruno; Newton, AliceThis study investigates standard and regional algal pigment index 1 (API1) estimates in the Atlantic off the southwestern Iberian Peninsula. Standard API1 data are those delivered by the Medium Resolution Imaging Spectrometer (MERIS) orbiting sensor. Equivalent quantities are computed by applying a regional inversion scheme using as input both MERIS and in situ remote sensing reflectances (R-rs). Reference data for the development of the regional algorithm and for the analysis of tested products include field measurements of total concentration of chlorophyll a (TChla) and coincident R-rs values collected at different distances from the coast. Validation results, based on matchup analysis, identifies a systematic overestimation of standard API1 versus the reference TChla values. The additional comparison of product maps in selected regions of interest confirms this tendency and demonstrates the feasibility and relevance of using regional algorithms for investigating spaceborne products. Analogous applications are hence devised for the early-stage evaluation of the forthcoming Sentinel-3/OLCI data products.
- A high-performance computing framework for Monte Carlo ocean color simulationsPublication . Kajiyama, Tamito; D'Alimonte, Davide; Cunha, Jose C.This paper presents a high-performance computing (HPC) framework for Monte Carlo (MC) simulations in the ocean color (OC) application domain. The objective is to optimize a parallel MC radiative transfer code named MOX, developed by the authors to create a virtual marine environment for investigating the quality of OC data products derived from in situ measurements of in-water radiometric quantities. A consolidated set of solutions for performance modeling, prediction, and optimization is implemented to enhance the efficiency of MC OC simulations on HPC run-time infrastructures. HPC, machine learning, and adaptive computing techniques are applied taking into account a clear separation and systematic treatment of accuracy and precision requirements for large-scale MC OC simulations. The added value of the work is the integration of computational methods and tools for MC OC simulations in the form of an HPC-oriented problem-solving environment specifically tailored to investigate data acquisition and reduction methods for OC field measurements. Study results highlight the benefit of close collaboration between HPC and application domain researchers to improve the efficiency and flexibility of computer simulations in the marine optics application domain. (C) 2016 The Authors. Concurrency and Computation: Practice and Experience Published by John Wiley & Sons Ltd.
- OCPortugal consortium: activities and challengesPublication . Brito, Ana; Brotas, V.; Cristina, Sónia; D'Alimonte, D.; Goela, Priscila; Icely, John; Kajiyama, T.; Moore, Gerald; Newton, Alice; Sá, C.The newly-constituted Ocean Colour Portugal consortium (OCPortugal) aims at promoting the use of remote sensing (RS) products in the Atlantic off Portugal. In close collaboration with the European Space Agency (ESA), OCPortugal currently links the following institutions: Centre of Oceanography (IO), Faculty of Sciences of the University of Lisbon; CENTRIA and CITI from Faculty of Science and Technology , New University of Lisbon (FCTUNL); and Sagremarisco-Viveiros de Marisco Lda. Through a coordinated set of actions undertaken by these research groups, OCPortugal brings together expertise in different research areas such as marine biology, applied optics, satellite imagery, and parallel computing.
- Validation of standard and alternative satellite ocean-color chlorophyll products off Western IberiaPublication . Sá, Carolina; D'Alimonte, Davide; Brito, A.; Kajiyama, T.; Mendes, C. R.; Vitorino, J.; Oliveira, P. B.; da Silva, J. C. B.; Brotas, V.Chlorophyll a concentration (Chl) product validation off theWestern Iberian coast is here undertaken by directly comparing remote sensing data with in situ surface reference values. Both standard and recently developed alternative algorithms are considered for match-up data analysis. The investigated standard products are those produced by the MERIS (algal 1 and algal 2) and MODIS (OC3M) algorithms. The alternative data products include those generatedwithin the CoastColour Project and Ocean Color Climate Change Initiative (OC-CCI) funded by ESA, as well as a neural net model trained with field measurements collected in the Atlantic off Portugal (MLPATLP). Statistical analyses showed that satellite Chl estimates tend to be larger than in situ reference values. The study also revealed that a non-uniform Chl distribution in the water column can be a concurring factor to the documented overestimation tendency when considering larger optical depth match-up stations. Among standard remote sensing products, MODIS OC3M and MERIS algal 2 yield the best agreement with in situ data. The performance of MLPATLP highlights the capability of regional solutions to further improve Chl retrieval by accounting for environmental specificities. Results also demonstrate the relevance of oceanographic regions such as the Nazaré area to evaluate how complex hydrodynamic conditions can influence the quality of Chl products.