The total non-water absorption coefficient of seawater, πππ(π) (πβ1 ) (light absorption coefficient after subtraction of pure water contribution) provides information about the amount of light absorbed by various optically significant substances in natural waters (π is wavelength). Partitioning πππ(π) into phytoplankton, ππβ(π) and colored detrital matter, πππ(π) is useful to understand the light interaction with the distribution and variability of constituent matter, light availability at various depths, and ecological and biogeochemical cycles, as these constituents represent pools of carbon and other elements. Some of the existing partitioning methods either require ancillary inputs or assume limited shapes for constituents absorption in deriving the ππβ(π) and πππ(π) from πππ(π). In this study, we propose a decomposition method of πππ(π) using a spectral optimization routine utilizing a spectral library consisting of various shapes for both phytoplankton and colored detrital matter absorption components. The proposed method does not require any ancillary inputs in deriving absorption of the constituent subcomponents. Performance of the proposed method is evaluated using two dataset compilations covering a very wide range of water types with sampling locations. Among various parameterizations tested in the decomposition method, the parameterization with the phytoplankton shape model of Ciotti et al. (2005) combined with exponential, stretched exponential, and hyperbolic shapes for colored detrital matter resulted in lower Mean Absolute Percentage Errors (MAPE) consistently across all sites. The good performance of the proposed method is characterized by average MAPE values of 17% and 13% and average percentage absolute errors (%AE) of 15.5% and 11.5% for the derived ππβ(443) and πππ(443) respectively. The proposed method exhibited better performance with 7 - 10% lower average spectral MAPE (MAPE averaged over all wavelengths) values compared to two other existing partitioning algorithms in optically complex waters. The proposed method can be used for deriving the constituents absorption from input data of πππ(π) collected from various oceanographic and remote-sensing platforms. Since apg is a core product of several semi-analytical ocean color inversion algorithms, this approach has relevance to the future hyperspectral NASA PACE ocean color imager, as it is directly adaptable to hyperspectral reflectance data.
Optimization techniques are used in inversion of ocean color remote sensing reflectance measurements, where the error between forward modelled spectra and observed spectra is minimized. In this study, NASA Bio β optical Marine Algorithm Dataset (NOMAD) is used to test the performance of global optimization technique based on Multi-Verse Optimization (MVO) for retrieval of Bulk and Individual Inherent optical properties (IOPs) from Remote sensing reflectance (Rrs). The results are compared with other global optimization algorithms such as Particle Swarm Optimization (PSO) and Genetic algorithms (GA) in terms of their statistical goodness of fit and computational time requirements. MVO (743.82 secs) offered computational fastness over both PSO (1261.8 secs) and GA (3818.8 secs). The RMSE values in log space, obtained for bulk IOPs, i.e., total absorption coefficient at 440 nm and total backscattering coefficient at 555 nm using MVO (0.264,0.265), PSO (0.264,0.265) and GA (0.264, 0.274) respectively show that MVO performed either better or similar to PSO and GA. In case of individual IOP retrieval i.e., log scale RMSE values obtained for absorption due to phytoplankton at 440 nm (MVO β 1.038, PSO β 1.200, GA β 1.215), absorption due to gelbstoff at 440 nm (MVO β 0.272, PSO β 0.272, GA β 0.273) and backscattering due to particulate matter at 555 nm (MVO β 0.228, PSO β 0.227, GA β 0.238) showed similar performance as in bulk IOP retrieval. MVO can thus be used effectively on satellite imagery data for retrieval of IOPs owing to its faster computational capability and comparable or better performance to existing global optimization algorithms.
State of the art Ocean color algorithms are proven for retrieving the ocean constituents (chlorophyll-a, CDOM and Suspended Sediments) in case-I waters. However, these algorithms could not perform well at case-II waters because of the optical complexity. Hyperspectral data is found to be promising to classify the case-II waters. The aim of this study is to propose the spectral bands for future Ocean color sensors to classify the case-II waters. Study has been performed with Rrsβs of HICO at estuaries of the river Indus and GBM of North Indian Ocean. Appropriate field samples are not available to validate and propose empirical models to retrieve concentrations. The sensor HICO is not currently operational to plan validation exercise. Aqua MODIS data at case-I and Case-II waters are used as complementary to in- situ. Analysis of Spectral reflectance curves suggests the band ratios of Rrs 484 nm and Rrs 581 nm, Rrs 490 nm and Rrs 426 nm to classify the Chlorophyll βa and CDOM respectively. Rrs 610 nm gives the best scope for suspended sediment retrieval. The work suggests the need for ocean color sensors with central wavelengthβs of 426, 484, 490, 581 and 610 nm to estimate the concentrations of Chl-a, Suspended Sediments and CDOM in case-II waters.
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