Description: Polar stratospheric cloud (PSC) measurements have been taken by means of a lidar system at the German Koldewey Station in Ny-Ålesund, Spitsbergen [79°N, 12 °E], since 1988. Here we present the first PSC climatology for the high Arctic built on Ny-Ålesund lidar data collected from winter 1995/1996 to 2003/2004, thus avoiding the effects of earlierbackground aerosol enhancement by large volcanic eruptions. As in a similar study performed on the Antarctic McMurdo PSC dataset, a numerical code has been applied to distinguish PSCs based on their vertical structure displayed on the lidar profile. Two cloud categories showing low or high variability of backscattering ratio with altitude are individuated andaddressed as Large and Small Scale Variation PSCs (LSV and SSV, respectively). It is possible to make reliable assumptions concerning the relationship between the obtained PSCcategories and the conditions under which they are likely to form. Ny-Ålesund is typically situated in the centre of the northern polar vortex, where the majority of PSC observations can be linked to the synoptic temperature field. The present study not only provides a general description of PSCs occurring at Ny-Ålesund, but it also focuses on the temporal and spatialvariability of cloud types observed under both LSV and SSV categories. Finally, the comparison with the McMurdo climatology provides an overview of long term inter-hemispheric differences in PSC appearance as measured by ground based lidars.

Description: The stratospheric water vapour mixing ratio inside, outside, and at the edge of the polar vortex has been accurately measured by the FLASH-B Lyman-Alpha hygrometer during the LAUTLOS campaign in Sodankylä, Finland, in January and February 2004. The retrieved H2O profiles reveal a detailed view on the Arctic lower stratospheric water vapour distribution, and provide a valuable dataset for the validation of model and satellite data. Analysing the measurements with the semi-lagrangian advection model MIMOSA, water vapour profiles typical for the polar vortex interior and exterior have been identified, and laminae in the observed profiles have been correlated to filamentary structures in the potential vorticity field. Applying the validated MIMOSA transport scheme to specific humidity fields from operational ECMWF analyses, large discrepancies from the observed profiles arise. Although MIMOSA is able to reproduce weak water vapour filaments and improves the shape of the profiles compared to operational ECMWF analyses, both models reveal a dry bias of about 1 ppmv in the lower stratosphere above 400 K, accounting for a relative difference from the measurements in the order of 20 %. The large dry bias in the analysis representation of stratospheric water vapour in the Arctic implies the need for future regular measurements of water vapour in the polar stratosphere to allow the validation and improvement of climate models.

Description: Within the extremely cold and stable polar vortex of the winter 2004/2005, a polar stratospheric ice cloud was observed from Ny-Ålesund (Spitsbergen) on 26 January 2005. The observation of a cloud with backscatter ratios up to 23 and volume depolarization larger than 50% is unique in our 15-year ground based lidar data record. Simultaneous balloon-borne water vapor measurements indicate the presence of mesoscale ice clouds nearby. Normally, low horizontal wind speeds inside the inner vortex prevent vertical wave propagation. However, the rare coincidence of different meteorological processes occurring during a poleward breaking Rossby wave event caused favorable conditions for the vertical propagation of mountain waves excited by the flow past Spitsbergen. Detailed meteorological analysis shows that the ice particle formation processes on 26 January 2005 were most likely provoked by mesoscale stratospheric temperature anomalies, leading to a local reduction in water vapor.

Description: During the recent winters water vapour has been accurately measured from different Arctic sites using balloon-borne Lyman-alpha FLASH-B hygrosonde. Here we present the results of 18 balloon water vapour soundings conducted at Sodankylä (67 N) and Ny-Alesund (79 N) during 2003/04, 2004/05 and 2005/06 winters. The obtained data set allows case studies and detailed characterization of stratospheric water vapour vertical distribution within different conditions in the Arctic polar stratosphere.Water vapour vertical distribution in the Arctic lower stratosphere is primarily affected by the dynamical effects of polar vortex and by phase aggregation at the temperatures below ice PSC threshold. The measured H2O profiles carry the signatures of different processes occurring in the Arctic winter stratosphere and reveal a detailed view on the Arctic UT/LS water vapour distribution.The measurements clearly demonstrate typical differences in stratospheric water vapour concentration inside and outside the vortex; for example in the polar vortex of 2003/04 at 20 hPa the difference reaches 1.4 ppmv. Also it is pointed out that water vapour profiles obtained at the edge or close to the edge of vortex are characteristic by their laminated structure. As shown by the results of RDF-analysis this structure is not linked to dehydration but to differential advection of air masses originating from inside and outside the vortex. Thus water vapour is proved to be a valuable tracer for dynamical processes in the polar stratosphere. In the other case, the water vapour vertical profiles obtained at Ny-Alesund in January 2005 during the presence of PSCs clearly show the dehydration layers with reduced water vapour.Another focus is put here on the water vapour vertical distribution within the so called transition layer above the polar tropopause. The existence of this transition layer may be caused by diffusion of water vapour through the smeared polar winter tropopause, whereas the vertical structure of the H2O profile is affected by the dynamical processes.In addition the variability of water vapour at the hygropause and the distance between tropopause and hygropause are discussed.

Description: Within the extremely cold and stable polar vortex of the winter 2004/2005, a polar stratospheric ice cloud was observed from NyÅlesund (Spitsbergen) on 26 January 2005. The lidar measurement of a cloud with backscatter ratios up to 23 and volume depolarization larger than 50% is unique in our 15-year lidar data record. In addition, simultaneous balloon-borne water vapour measurements indicate the presence of mesoscale ice clouds nearby.During winter, Spitsbergen is commonly situated well inside the vortex where low horizontal wind speeds prevent vertical wave propagation. In this particular case, the rare coincidence of different meteorological processes occurring during a poleward breaking Rossby wave event caused favorable conditions for the vertical propagation of mountain waves excited by the flow past Spitsbergen. The detailed meteorological analysis shows that ice particle formation processes on 26 January 2005 were most likely provoked by mesoscale stratospheric temperature anomalies, leading to a local reduction in water vapour.