Calibration of an optical equation to analyse the atmospheric turbidity and water quality of an estuarine environment

Abstract

Mapping of oceanic productive zones through an optical sensor on board a satellite requires a detailed knowledge about the interaction of solar flux with the atmosphere and water column. Hence the radiative transfer on February 14, 2002 over estuaries of Goa has been examined through a couple radiative transfer model (CRTM). This has been carried out in the context of an optical sensor when it looks down at nadir. The forcing of extraterrestrial solar irradiance with atmosphere results in its absorption and scattering due to aerosol particles and gaseous constituents and rayleigh particles. The optical spectrum has been subjected to atmospheric forcing and radiation has been calculated for every 1 nm. Special reference is given to two wavelengths (443 and 510 nm), which are important for ocean optics. The atmospheric components pertaining to these wavelengths are 0.04193 w/m2/nm/sr and 0.0225 w/m2/nm/sr (rayleigh) and 0.0121 w/m2/nm/sr and 0.015 w/m2/nm/sr (aerosol). Within the optical frame work (400 –700 nm), the effect of aerosol overrides that of rayleigh from 545 nm onwards. The specular reflectance at the sea surface contributes 1.725 x l0-12 w/m2/nm/sr and 1.947x 10-12 w/m2/nm/sr while the respective signature from water column is 0.418x 10-3w/m2/nm/sr and 0.536x 10-3 w/m2/nm/sr. The simulated water signature has been compared with radiances measured by a radiometer (Satlantic radiometer). The correlation (R2) is found to be around 0.97. The sensitivity analysis further reveals the respective wavelengths of the optical spectrum affected by atmospheric and oceanic constituents. Moreover, the effect of change in solar zenith angle on water signature has also been carried out. This could be applied to invert satellite data so as to retrieve coastal water constituents and productivity.