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‐α fluorescence hygrometer. The inlet is designed for anisokinetic sampling, thus large ice particles are detected sensitively. During the flight on January 24, 1997, temperatures of 200 K occurred close to the tropopause. Within this cold area, the existence of large particles with radii ≥ 4 µm 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 tropopause. This observation together with analysis of the correlation of the H2O mixing ratio with other long‐lived tracers indicates that the air masses had been dried by sedimentation of large ice particles.