Polar stratospheric clouds (PSCs) which do not fit into the standard type Ia/Ib scheme were measured by the airborne lidar OLEX (Ozone Lidar Experiment) on board the Deutsches Zentrum f¿r Luft- und Raumfhart (DLR) Falcon during the Airborne Polar Experiment and Polar stratospheric clouds, Leewaves, Chemistry Aerosol and Transport (APE-POLECAT) campaign. In contrast, the standard classification is satisfied by almost all observations for four winters at Ny ¿lesund, Spitsbergen, which is one of the most comprehensive data sets of ground station lidar measurements presently available. The cloud observed 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 depolarization ΔA=15%, which is clearly distinct from the Ny ¿lesund 4 year record. Using a combination of microphysical and optical modeling, we investigate the possible evolution of this cloud assuming either in situ freezing of ternary HNO3/H2SO4/H2O droplets as nitric acid trihydrate, or the formation of the clouds in mountain waves over the east coast of Greenland, as suggested by a mountain wave model. Best agreement with the observations was obtained by assuming mountain--wave--induced cloud formation, which yields nitric acid trihydrate particles with much higher total mass than achieved by assuming 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 equilibrium. ¿ 1999 American Geophysical Union