Physical state and composition of polar stratospheric clouds inferred from airborne lidar measurements during SESAME

The airborne lidar LEANDRE was deployed from Kiruna (Sweden) in January 1995, during Phase III of the Second European Stratospheric Arctic and Mid-latitude Experiment (SESAME). Polar Stratospheric Clouds (PSC) were detected on several flights. Four Lidar retrievals, on two different days, are presented together with the thermal history of air masses, derived from 5-day backward trajectories calculated from the TOPCAT model, using ECMWF analyses. The first PSC case was observed at the synoptic scale and at a temperature close to the ice frost point, with no significant depolarization, suggesting that particles were in the Liquid phase, probably liquid ternary H2SO4/HNO3/H2O solution forming a Type 1b cloud. Locally, on the same day, a Type II PSC (ice cloud) was detected above the Scandinavian Mountains and was related to mesoscale processes. On another day, the third lidar retrieval only showed background aerosols signature, whereas the last one was performed at the NAT equilibrium temperature and its measured depolarization ratio was consistent with few solid state Type Ia particles, probably composed of NAT. From the thermal histories, the different behaviour of the two Type I clouds is explained in terms of cooling rates and compositional hysteresis, driven by temperature.