Phase function effects on the retrieval of oceanic high-spectral-resolution lidar.

A semianalytic Monte Carlo model is developed to simulate oceanic high-spectral-resolution lidar (HSRL) signals with multiple scattering. The phase function effects on oceanic HSRL retrieval are studied, e.g., the effective particulate 180° volume scattering function (VSF) and lidar attenuation coefficient that describe characteristics of backscatter and attenuation, respectively. The results demonstrate that the particulate backward and forward phase functions both have a significant influence on δ1, which is the relative difference between the effective and true particulate 180° VSF. The values of |δ1| are typically quite small for all phase functions at the water surface and increase with depth up to ~17% for the Fournier and Forand (FF) phase function but up to ~40% for the two-term Henyey-Greenstein (TTHG) phase function and ~75% for the one-term Henyey-Greenstein (OTHG) phase function. The reason that δ1 is not zero is due to broadening of backscattering angles from 180° caused by multiple scattering and uneven backward phase function. Also, the reason that maximum TTHG and OTHG |δ1| are larger than FF is due to less sharply increasing feature of FF in the backward direction. In addition, the particulate forward phase functions are closely related to δ2, which is the relative deviation between the lidar attenuation coefficient and the sum of the absorption and backscattering coefficients. The values of δ2 are small for all phase functions at the water surface and increase with depth up to ~12% for TTHG but up to ~26% for FF and ~31% for OTHG, due to the less peaked forward phase functions that result in more angular spread of the beam with depth and therefore result in less photons within the field of view of the lidar.