On Depolarization Lidar-based Method for the Determination of Liquid-cloud Microphysical Properties

Under single scattering conditions, water droplets clouds do not depolarize the backscattered light. However, backscattered light from multiple scattering will be depolarized. The level of depolarization is a function of the droplets size, the cloud extinction coefficient value and profile; it has also an important dependency on the lidar field-of-view (FOV). The use of depolarization information to retrieve cloud microphysical properties, using Multiple-FOV has been the object of studies, [1], [2]. Recently the use of the depolarization, at a single FOV, has been studied for cloud with linear liquid water content profiles, [3], [4]. In this paper we present the mechanism leading to depolarization and identify the FOV values for which the information on particle size is high. Also Monte Carlo simulations for cloud with constant and ramp up profiles are presented. The degree of linear depolarization as a function of cloud penetration is significantly different for both cloud profiles. This suggests that the use of the degree of linear depolarization at a single FOV should be used with caution to determine clouds micro-physical parameters. 1. DEPOLARIZATION RATIOS Backscattered light from spherical particles does not change the polarization orientation state of a linearly polarized incident light. This particularity allows the discrimination of spherical particles from non-spherical particles. However, under multiple scattering conditions, even spherical particles generate depolarized signals. The linear depolarization ratio (δlin) at a scattering angle β is defined as the ratio of the power integrated over the azimuthal angles φ in the plane of polarization orthogonal ( ⊥ I ) and parallel ( // I ) to the linearly polarized source: ∫ ∫ = > < > < = ⊥ π π φ φ β φ φ β β δ 2 0 2 0 d ) , ( I d ) , ( I I I ) ( x y // lin , [1] Using Mie theory, it can be shown [1] that: ) 3 cos 2 cos 3 ( ) cos 2 cos ( ) (