Lidar Observations of Polar Stratospheric Clouds over Ny-Aalesund in the Winters of 1994/95-1996/97: Impact of the Temperature and the Temperature History on the PSC Structure

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Lidar observations of polar stratospheric clouds (PSCs) were made during three winter campaigns from 1994/95 to 1996/97 at Ny-Aalesund, Svalbard. PSCs composed mainly of solid particles were found at the beginning of most PSC events. They appeared when the temperature became lower than the assumed equilibrium temperature of nitric acid trihydrate (T NAT ) but sometimes appeared even at temperature a little higher than T NAT . As the stratospheric temperature became very low, close to the frost point of ice (T ice ), PSCs composed mainly of liquid particles appeared. The behaviors of liquid PSCs observed are consistent with the expected ones, which were estimated based on the current formation theory of super cooled ternary solution (STS) particles. Results of backward trajectory analysis show that some solid PSCs experienced the temperature below T NAT at least for more than 10 hours after passing through the melting point of sulfuric acid tetrahydrate. The solid PSCs which appeared at temperatures higher than T NAT experienced the temperature below T NAT for more than 1 day in the past. Almost no PSCs experienced the temperature lower than T ice during 10 days prior to the observation. Results of trajectory analysis also suggest that the increase of the depolarization ratio depends strongly on the degree to which air parcels of PSCs cooled below T NAT and on the period during which the air parcels experienced the temperature lower than the condensation point, but not on the experience of lower temperature than T ice . Some liquid PSCs with a large scattering ratio (4-5) and a low depolarization ratio (0-0.005) appeared after they experienced the temperature close to T ice for a few days prior to the observation.