Ice particle formation and sedimentation in the tropopause region: A case study based on in situ measurements of total water during POLSTAR 1997

During the Polar Stratospheric Aerosol Experiment, total water was measured using an airborne Lyman-alpha fluorescence hygrometer. The inlet is design ed for anisokinetic sampling, thus large ice particles are detected sensiti vely. During the flight on January 24, 1997, temperatures of 200 K occurred close to the tropopause. Within this cold area, the existence of large par ticles with radii greater than or equal to 4 mu m containing large amounts of water is observed. At the edge of this cloud, total water mixing ratios of 5-6 ppmv and close to saturation were measured at and below the tropopau se. This observation together with analysis of the correlation of the H2O m ixing ratio with other long-lived tracers indicates that the air masses had been dried by sedimentation of large ice particles.