Laser backscattering from dense water-droplet clouds

Abstract

Laser backscattering from water-droplet clouds of known size distribution has been studied. A polarization analysis of the backscattered power as a function of attenuation coefficient, a has been made for receiver fields of view between 2.5 and 20 mrad. a values between 0 and 5 m⁻¹ have been used. A linearly polarized argon laser was operated at 514 nm and a considerable amount of depolarization arising from multiple scattering was observed. The polarization information shows that there is no change in either the direction of the polarization or in the ellipticity of the backscattered light. Thus the linear depolarization ratio δ (the ratio of backscattered signals polarized perpendicular and parallel to the incident beam polarization direction) provides a direct and quantitative indicator of the amount of depolarization due to multiple scatter. Using polarization information, the contribution of multiple scattering to the cloud reflectance has been measured. The polarization characteristics of the pure multiply scattered component have been examined with the aid of aperture stops and receiver angles which block singly scattered radiation. This information is then used to analyze the reflected return from clouds and separate the contributions of single and multiple scattering. Comparison of the measured single scattering with theory shows good agreement. The contribution of multiple scattering to the total reflectance can be very large in dense clouds.