Arctic Stratospheric Water Vapor Observations by Frostpoint Hygrometers During the Winters of 2002/2003-2004/2005

Water vapor in the lower stratosphere plays an important role in the climate system and it has an influence on ozone depletion by affecting polar stratospheric cloud formation. There are only few in situ measurements of water vapor profiles from polar stratosphere. Here we report balloon borne water vapor measurements from Arctic lower stratosphere obtained during the winters of 2002/2003, 2003/2004 and 2004/2005 by using NOAA-CMDL frostpoint hygrometers. Hygrometer data obtained by balloon launches from Sodankylä in Northern Finland (67.4°~N, 26.6°~E) indicate strong gradients in water vapor mixing ratio at the vortex edge region (by comparing data obtained during vortex movement above the station) and fine scale anomalies with vertical extent of 1-3 km. We show that the observed anomalies in water vapor profiles can be attributed to filamentary structures near the vortex edge region and in two cases to dehydration in the cold vortex air due to formation and sedimentation of particles containing water vapor. Selected cases of large anomalies in water vapor profiles either due to differential airmass advection or particle formation are discussed in detail. On January 26, 2005 we observed strong water vapor loss in several layers between 18 and 23 km of altitude with up to 2 ppmv water vapor loss at peak altitude of 22.5 km. This observation is highly unusual in the Arctic stratosphere and can be explained by additional wave-induced cooling under very low synoptic-scale temperatures found in the Arctic vortex in January 2005, thus allowing formation and sedimentation of particles. Aerosol backscatter sonde measurements during the same event indicate the presence of water ice particles just below the layer of maximum loss of water vapor.