Observations of the stratosphere were carried out at Thule (76.5°N, 68.8°W), Greenland, in the period 1990-1997 with an elastic backscatter/depolarization lidar. Polar stratospheric clouds (PSCs) were never detected during the winters 1991, 1992, 1993, and 1994. Small nondepolarizing PSCs could have been masked, however, by the large volcanic aerosol cloud that was in the stratosphere during 1992, 1993, and 1994. During 1995, 1996, and 1997, PSCs were observed on a total of 32 days. The evolution of the PSCs during each winter was mainly modulated by the behavior of the polar vortex. PSC signatures were often detected in regions where, assuming a 5 ppmv concentration of water vapor and a 10 ppbv concentration of nitric acid, condensation was not possible. Thus larger concentrations of these gases, or significant differences between the true and the radiosonde/analysis temperatures, were experienced. All observed PSCs could be classified as type I PSCs. Type Ia PSCs, sometimes found as stable homogeneous layers, were common. These layers formed when the air mass temperature reached the frost point during the days preceding the passage over Thule. Type Ib PSCs were generally associated with an articulated cloud structure, consisting of several layers of type Ib and type Ia particles. Circumstantial indications suggest that the development of these articulated clouds is associated with oscillations in the temperature profile, and with rapid temperature changes along the isentropic trajectories. The behavior of the depolarization ratio appears to depend on the minimum temperature experienced by the air mass during the past few days, and possibly on the condensation of ice prior to the PSC observation. The properties of a large type Ib PSC have been studied by means of a ternary solution model, and theoretical calculations of the backscattering. The measured cloud properties are not satisfactorily described by the ternary solution model, and a droplet composition with a water fraction larger than expected may be implied. The type Ib PSC properties are, however, consistent with a cloud composed of a large number of small particles, with a narrow size distribution. In a single case a cloud exhibiting relatively large backscatter ratio and depolarization, classified as type Ic, was detected; the isentropic trajectory of the air mass shows that the particles probably reached the frost point a few days before arriving at Thule. Theoretical calculations of the backscatter ratio from this cloud indicate that it was probably constituted of a relatively large number of submicron to micron-sized crystals. Copyright 2002 by the American Geophysical Union.

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