T. Suresh

Secchi depth analysis using bio-optical parameters measured in the Arabian Sea

Secchi depth provides the oceanographer with the first hand information about transparency and penetration of light in the water. Here we present results of the Secchi depth and the optical properties measured in the Arabian Sea. Our analyses show spatial and temporal variability of Secchi depth and their dependence on the optical properties beam attenuation and diffuse attenuation the biological parameter of Chlorophyll. The in-situ measured inherent and apparent optical properties have been used to understand the underwater light properties and their relations to the Secchi depth in various water types. The Secchi depth model is validated using the measured optical properties. We also present an empirical method to determine Secchi depth from the satellite ocean color sensor, and the application of the same to the IRS-P4 OCM is found to provide comparable results to the measured values.

A simple method to minimize orientation effects in a profiling radiometer

Marine optical parameters required for ocean color satellite applications must be measured with high accuracy and errors within the permissible limits. These stringent requirements demand careful measurements of optical parameters. Though the free-fall radiometer is found to be a better option for measuring underwater light parameters as it avoids the effects of ship shadow and is easy to operate, the measurements demand profiling the radiometer vertical in water with minimum tilt. Here we present the results of our observations on the tilts of the radiometer from the measurements in the Arabian Sea. Since there is hardly any study carried-out on the tilt of the profiling radiometer, the result of this study will help in the better design of such marine instruments. The tilt of the radiometer near the surface of the water is attributed to the mode of deployment and environment parameters, while the tilt at depth of the water is influenced by the density variations of the water. Here we also demonstrate a method of deploying the instrument that minimizes the tilt of the instrument at the surface layer of the water.

Cross-calibration of IRS-P4 OCM satellite sensor

We present here the cross calibration of ocean color satellite sensor, IRS-P4 OCM using the radiative transfer code, with SeaWiFS as a reference. Since the bands of IRS-P4 OCM are identical to those of SeaWiFS and SeaWiFS has been continuously and rigorously calibrated, SeaWiFS is used as a reference for the cross calibration of IRS-P4 OCM. Calibrations coefficients for each band of IRS-P4 OCM are derived by comparing the actual radiances detected by the satellites at top of the atmosphere and those obtained from the radiative transfer simulations of IRS-P4 OCM and SeaWiFS. The chlorophyll a values derived using the calibrated IRS-P4 OCM are found to be comparable with those derived from SeaWiFS and in close agreement with the measured values. The relative root mean square error (RRMSE) between measured chlorophyll a and those derived from the satellites are found to be 0.28 and 0.26 for SeaWiFS and IRS-P4 OCM respectively.

An algorithm to determine backscattering ratio and single scattering albedo

We present here algorithms to determine the inherent optical properties of water, backscattering probability and single scattering albedo at 490 and 676 nm from the apparent optical property, remote sensing reflectance. We have used the measured scattering and backscattering coefficients and the remote sensing reflectance to obtain a relationship for the backscattering ratio, which is defined as the ratio of the total backscattering to the total scattering in terms of the remote sensing reflectance of two bands. Using the empirical relationship for the total backscattering ratios, we have also computed single scattering albedo, which is defined as the ratio of the scattering to the beam attenuation coefficient. The values of single scattering albedo obtained from measured values and those obtained from the empirical method are found to be comparable. The values of single scattering albedo derived using the algorithm are found to be comparable to the measured values obtained from the eastern Arabian Sea, with the root mean squared error of 0.078 and the mean percentage error of 9.5% for the 490 nm and root mean squared error of 0.043 and the mean percentage error of 7.5% for the 676 nm.

An empirical method to estimate bulk particulate refractive index for ocean satellite applications

An empirical method is presented here to estimates bulk particulate refractive index using the measured inherent and apparent optical properties from the various waters types of the Arabian Sea. We have used the empirical model, where the bulk refractive index is modeled in terms of the backscattering ratio and the hyperbolic slope of the particle size1. Empirical models are obtained to determine the slope of the particle size and backscattering ratio as a function of the remote sensing reflectance. We have used these algorithms with the IRS-P4 OCM satellite data and derived the refractive index image. The values of indexes are found to be lower for the open ocean and relatively higher for the coastal waters. Distinct features are observed even in the in the waters with similar chlorophyll concentrations. This algorithm provides scope for the classification of water types and detection of blooms.