Aircraft lidar observations of an enhanced type Ia polar stratospheric clouds during APE-POLECAT

Polar stratospheric clouds (PSCs) which do not fit into the standard type I a/Ib scheme were measured by the airborne lidar OLEX (Ozone Lidar Experimen t) on board the Deutsches Zentrum fur Luft- und Raumfhart (DLR) Falcon duri ng the Airborne Polar Experiment and Polar stratospheric clouds, Leewaves, Chemistry Aerosol and Transport (APE-POLECAT) campaign. In contrast, the st andard classification is satisfied by almost all observations for four wint ers at Ny Alesund, Spitsbergen, which is one of the most comprehensive data sets of ground station lidar measurements presently available. The cloud o bserved by the Falcon south of Spitsbergen on December 31, 1996, was a 400- km long type I cloud with backscatter ratio S = 2.5 and aerosol depolarizat ion delta(A) 15%, which is clearly distinct from the Ny Alesund 4 year reco rd. Using a combination of microphysical and optical modeling, we investiga te the possible evolution of this cloud assuming either in situ freezing of ternary HNO3/H2SO4/H2O droplets as nitric acid trihydrate, or the formatio n of the clouds in mountain waves over the east coast of Greenland, as sugg ested by a mountain wave model. Best agreement with the observations was ob tained by assuming mountain-wave-induced cloud formation, which yields nitr ic acid trihydrate particles with much higher total mass than achieved by a ssuming synoptic-scale freezing. Our analysis suggests that this rare type of PSC, which we term type Ia-enh, is characterized by nitric acid hydrate particles rather close to thermodynamic equilibrium, while the more common type Ia PSCs appear to contain much less mass than representative of equili brium.