Impacts of laser beam divergence on lidar multiple scattering polarization returns from water clouds

Abstract

Theoretically, laser beam divergence can redistribute multiple scattering lights by spreading laser illumination and thus alter lidar polarization measurements. To study multiple scattering effects of laser divergence on lidar polarization signals, we apply the Monte Carlo polarized radiative transfer model MSCART to simulate lidar Stokes vector signals from a uniform water cloud with and without taking account of laser divergences. The comparison analysis shows that for lidar receiver footprints with roughly the same sizes of the multiple scattering regimes, laser divergence has almost no effect on ground-based lidar signals, but can have significantly large and much complex polarization effects on spaceborne lidar signals. An increase in laser divergence on one hand can greatly enhance spaceborne lidar multiple scattering depolarization when the divergence is larger than the FOV, and on the other hand can also weaken it slightly when the divergence less than the FOV. The weakest multiple scattering depolarization occurs at the divergence equal to the FOV. Furthermore, laser divergence can significantly reduce the sensitivity of FOV-resolved polarization measurement of spaceborne lidar to non-diagonal elements of phase matrix and make the measurement at different receiving polar angles only sensitive to diagonal elements of phase matrix at almost the same scattering angles. The spaceborne MFOV and CCD polarized lidar thus might not provide more information about phase matrix than the single FOV lidar does.