Monte Carlo simulation of light propagation in stratified water

Abstract

The spectral reflectance of the sea surface contains information about light absorption and scattering properties of water. Most of the existing algorithms implicitly assume that the water column is vertically homogeneous while oceanographic observations have shown the existence of vertical inhomogeneity of the sea water constituents. The aim of this thesis is to study the link between the remote sensing reflectance and the vertical structure of the oceans optical properties.The tool developed for this purpose is a Monte Carlo code for the simulation of the penetration of light in sea water. The code worked well for the ideal case of homogeneous waters when compared to the results obtained by the Ocean-Colour Algorithms working group of the International Ocean Colour Coordinating Group.The influence of vertical stratification on the reflectance of a water column was studied. Stratifications are included in the water column by using a Gaussian function that describes a depth dependent chlorophyll profile superimposed on a constant background. This Gaussian function describing the vertical chlorophyll profile was then used to simulate a relatively broad range of open ocean conditions characterized by the presence of this subsurface maximum at depths greater than or equal to 20m. The comparison with a homogeneous ocean (with the background chlorophyll concentration of the stratified case) was carried out and it was seen that the magnitude of the above surface remote sensing reflectance of the stratified cases differed significantly from the reference values of homogeneous oceans, specially in the case of low surface chlorophyll concentrations and shallow pigment maximum. The analysis of how the depth varying optical constituents contribute to the overall reflectance was then carried out by using a multiband quasi analytical algorithm (QAA) developed for the retrieval of the absorption and backscattering coefficients, as well as the absorption coefficients of phytoplankton pigments and gelbstoff and based on the remote sensing reflectance models derived from the radiative transfer equation. For the case of a homogeneous ocean, the retrieved values compared very well with the actual values found in the water column (the linear error being in the range of 5-8%). This retrieval algorithm was also applied to an inhomogeneous water column. The QAA retrieved absorption and backscattering values were found to have a good correlation with their vertically weighed average values. It was also analysed whether the reflectance of a stratified ocean is identical to that of a hypothetical homogeneous ocean having a pigment concentration that is the depth weighted average of the actual depth varying pigment concentration. It is seen that the case where both the absorption and scattering coefficients covary with the depth dependent chlorophyll concentration, this hypothesis shows less error than when only the absorption coefficient is made to covary with the chlorophyll concentration.Field trips were carried out in Singapore waters in June and August 2004 and in situ measurements of reflectance and the depth dependent backscattering coefficient. The data for the backscattering coefficients and the QAA estimated absorption coefficients were used to obtain the reflectance from the Monte Carlo code set up and this reflectance was compared to the measured one for all the 12 locations covered. The QAA retrieved values of the backscattering coefficient were also compared to the measured values .