Semi-analytic Monte Carlo radiative transfer model of laser propagation in inhomogeneous sea water within subsurface plankton layer

Current Monte Carlo simulation methods of laser propagation in sea water are mostly based on the assumption of homogeneous water. In this study, a semi-analytic Monte Carlo radiative transfer model is developed to study laser propagation in inhomogeneous sea water within subsurface plankton layer. It is based on analytical estimate of the probability of collection by a remote receiver of scattered or emitted photons; in particular, the water optical input parameter of vertical profile is no longer simply set as a constant. In the new transfer model, the input optical vertical profile can vary with a resolution as high as 0.11 m. Using this model, we study the influences of water optical properties, multiple scattering, wind-driven sea surface condition, and lidar incident angle. The effective lidar attenuation coefficient is found to depend on the field of view of the lidar system: it approaches attenuation coefficient with a narrow field of view, and approaches absorption coefficient with a wide-enough field of view. We also find the lidar transmittance through the air-sea interface decreases significantly as the wind speed increases to greater than 7 m/s, and the transmittance can even be reduced by half when the wind speed reaches 18 m/s. Finally, we compare the simulations by our new model with airborne lidar measurements. We find that the simulation curve is quite similar to that of the lidar measurements. The absolute error of normalized simulation signals compared with the measurements is within 0.05, suggesting the effectiveness and applicability of our approach for inhomogeneous water.