The Mixing State of Polar Stratospheric Cloud Particles in “Sandwich Structure” Observed by Lidar 1. Determination of the Mixing State of PSC Particles

The mixing state of polar stratospheric cloud (PSC) particles in the vertical sandwich structure observed simultaneously by lidar, and by Optical Particle Counter (OPC) above Ny Ålesund on 6th January 1996, is investigated. The vertical sandwich structure of PSC (observed by lidar) is characterized by one layer that exhibits large backscattering and low depolarization enclosed between two layers that exhibit low backscattering and high depolarization. The assumption that the observed optical properties of the cloud can be explained by an external mixture of solid and liquid particles is used as a basis for developing a new scheme for interpreting the lidar data. The backscatter and depolarization ratios observed by lidar are employed in equations, that allow for the separate calculation of the backscattering coefficients of the liquid and the solid particles, and for clear distinction of the mixing state of the cloud. Utilization of the developed scheme to analyze the vertical profile of the investigated PSC observed by lidar reveals that no purely liquid or solid layers exist, and that the entire altitude of the cloud can be represented by an external mixture of solid and liquid particles. Comparison of the vertical profiles of the calculated backscattering coefficients of the solid and liquid particles observed by lidar, and the number concentrations of the particles in different size categories observed by the Optical Particle Counter (OPC) implies radiuses of greater than 1.8 mm, and number concentrations of around 10-4-10-3 cm-3 for the solid particles.