A lidar system was installed at Ny-Ålesund, Svalbard in January 1994 to observe polar stratospheric clouds (PSCs). Observation campaigns were performed in the following seven winter seasons. The backscattering at 1064 and 532 nm, and the depolarization at 532 nm were observed by the lidar system. The results of observations in each winter season are summarized. A vertical sandwich structure of type I PSCs, in which layers of depolarization are above and below a scattering layer, has frequently been observed when the stratospheric temperature decreases to near the frost point of ice. An enhancement in scattering was observed at the altitude of the liquid PSC layer, but not at the altitudes of the solid layers. There tended to be a negative correlation between scattering ratio and depolarization ratio. The small increase in the scattering ratio over time at the altitude of the depolarization maximum implies a slow nucleation of solid particles. The lidar observations including temperature histories suggest that the sandwich structure arises from the external mixing of two different types of particles.
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